the forest revisited: more on global precedence

Psychol Res (1981) 4 3 : 1 - 3 2

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The Forest Revisited: More on Global Precedence

David Navon

University of Haifa, Haifa, Israel

Summary. Several issues pertaining to the notion of global precedence, its testing, and its ecological validity are discussed. Global precedence is presented as a claim that, other things being equal, global structure is available in the percept earlier than local features are. It does n o t state that local processing starts only after global processing is terminated. Since global precedence is a hypothesis about temporal development of percepts, a natural way to test it is to limit exposure duration. Global precedence predicts that local features will be more sensitive to stimulus duration than global ones.

An experiment is reported in which subjects were presented with 150 ms exposures of large letters made up of small ones, and had to discriminate either just the large letters or just the small ones. In addition, they had to respond to the presence of a concurrent tone. Local letters were responded to more slowly than global ones, and were associated with more tone errors. The variation of the global level as well as the consistency between the identities of the levels affected the latency to the local letter, but not vice versa. A single small- letter control-condition was used to rule out an explanation in terms of relative size. The effects, albeit weaker than some previous results, are compatible with the hypothesis tested.

The results of this experiment are interpreted in the context of an extensive discussion of findings in the paradigm and the generality of the phenomenon assumed to underly them. Possible mechanisms which may mediate global precedence are evaluated. Among them are relative size, lateral masking, diffuse attention, and spatial uncertainty. The magnitude, or even the presence, of global precedence depends on some factors, most prominent of which are visual angle and retinal position. Ecological considerations suggest that proximal sizes and eccentricities which favor global features may be very frequent. Some further subtleties of, as well as problems with, the logic of compound letters are discussed.

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2 The Forest Revisited

Introduction

The stimulus fo r this paper is a paper by Navon (1977) as well as some reactions it pro- voked. Navon reported some experimental findings which he argued to support the hypothesis that perceptual processing proceeds from the global structure to analysis of more local details. Since then several authors (e.g., Hoffman 1980; Kinchla and Wolfe 1979; Martin 1979; McLean Ref. Note 1) have raised doubts about the generality of this phenomenon, have proposed some interpretations of it, or have collected data to refute what they believed to be his view.

The purpose of this paper is to clarify some points about the hypothesis of global precedence and about its empirical testing, to present some relevant new data, to discuss several mechanisms which could produce results claimed to support the notion of global-to-local processing, and finally to comment on the ecological validity of global precedence. Since the paper is meant to add and darify rather than to restate and to review, some familiarity with previous papers is assumed. I begin with the darifications.

In his thesis (Ref. Note 2), Navon made a distinction between two aspects of the direction of processing, the source of processing initiative (input vs concept) and the level of analysis which is completed first (local vs global). Navon's claim about the or- thogonality of these aspects, which was also graphically represented in a figure reprinted by Broadbent (1977), probably did not stand out in his later paper (Navon 1977), to judge by the fact that these aspects are still sometimes confused. Thus, an overview of the meaning of globality and global-to-local processing is in order.

What is Globality?

First, 'global' and 'whole' should not be equated. A whole comprises both global and local features. Similarly, the structure of a part may also contain features that are more global than others.

For example, consider the representation of a visual stimulus whose spatial structure can be captured by the following Polish notation:

Whole = R (Unit 1, Unit 2) = R (P(a, b), Q (c,d)), where R, P, and Qare spatial relations and a, b, c, d are primitive elements, say line segments;Whole, Unit 1, and Unit 2 are concepts defined by these relations and elements. (They are concepts not in the sense of being associated with any extra-visual knowledge, but rather in the sense of being familiar visual patterns.)

P is formally more global than b: it will also most often withstand better sensory degradation such as filtering out high spatial frequency components. Still, b is not a subpart or a feature of P; both are features of Unit 1. By the same token, R is more global than P, and if the stimulus were to be described by a hierarchical network, such as the one in Fig. 1 it would be located more at its top. Yet P is not a feature of R; it is a feature of Unit 1, which is in turn a constituent of the whole just as R is. If this argument seems too abstract, it may help to concretize it by regarding, say, a human face as some spatial relationship among facial features which are, in turn, defined by some relationships among their subparts.

It would not be right to equate global spatial relationships with configurations, if by the latter term we mean that the parts being configured do not constitute perceptual

D. Navon 3

L E V E L S OF G L O B A L I T Y W H O L E

_ , ,Nf

I ~ _ I z

L E G E N D

C O N S T I T U E N T O F

F I R S T A R G U M E N T ( - - - -

S E C O N D A R G U M E N T 4¢ .. . . . . . .

Fig. 1. An illustration of a network representation of a simple visual stimulus designed to clarify the relationship between globality and whole-part relations

units in themselves (Navon 1977, pp 354, 380). Global relationships hold among fairly separable elements that do not configure, at least not in the sense above. A similar distinction is suggested by Pomerantz (in press).

To complete the picture, it should be emphasized that global features need not be spatial relationships; they may be pattern envelopes (Bouma 1971), symmetry properties (Gibson 1969; Fox 1975), relatively low spatial frequency components (Broadbent and Broadbent 1977; Ginsburg 1976), size relationships (Palmer 1975), etc. For convenience of notation, spatial relationships such as R, P, or Q may be viewed as subsuming all that information.

Active Processing

To discuss the direction of processing I borrow terminology used by Navon (Ref. Note 2 ;and more thoroughly in Ref. Note 3). Similar notions have been proposed by several authors (e.g., Broadbent 1977; Levin Ref. Note 4; Norman and Bobrow 1976; Palmer 1975). I am presenting a view rather than a factual account.

The percept of a totally unfamiliar stimulus depends solely on sensory excitation. In other words, its construction is completely input-driven. To the extent that its components are stochastically independent, the course of its construction is determined by the relative latency of detectors reporting the presence of the components.

The case is different when the scene is familiar or contains familiar parts. Concepts can evoke each other. A constituent, be it global or local, can evoke its superordinate (namely, the node to which it points by a bold arrow in Figure 1). In this case we may say that the superordinate is suggested; its presence is just anticipated. To be verified it will have to set up an inquiry, in other words to see that its constituents are actively probed, and perhaps to see that they have sufficient amount of resources to be able to do so. Thus, an inquiry, namely the activation of a constituent by its superordinate, typically drives perceptual activity which terminates by confirmation (or disconfirma- tion).

Analysis of a familiar scene may be concept-driven, namely initiated by concepts such as Whole, Unit 1, or Unit 2 (or by other concepts semantically related to them), and propagated upwards by suggestions and downwards by inquiries. It may also be


input-driven and concept-facilitated s namely, initiated by the detected presence of a, b, c, d, P, (2, or R, and then propagated up by suggestions and down by inquiries.

Underlying this view is the postulate that some perceptual processing is active, in the sense that it involves probing constituents by their superordinates, and that it depends on some input from the system, be it a command, resource investment, or sustained control. There is no need to review the arguments in favor of an active perception, since others have done so quite successfully (e.g., Broadbent 1977; Hochberg 1970; Norman and Bobrow 1976; Rumdhar t 1977). I would just like to emphasise the fact that when confronted with stimulation that cannot be totally accounted for by a single schema, we seem to attend only to portions of it even when it is presented for long enough. Some anecdotal evidence is provided by the difficulty of performing 'difference detection' tasks, such as the one in Fig. 2. I leave the reader to judge the level of the aspects that are hard to notice. It is to this active type of processing that I refer in my discussion of the order of processing. The view of concept-induced facilitation presented here is that it serves in determining and channeling the course of active processing.

An alternative prevalent approach is that a processed feature does not affect the processing of other features, but just biases the system towards interpretations that are compatible with it. According to the latter view, the course of processing is solely determined by sensory factors; facilitation affects just the output of processing: if features are anticipated, perhaps less sensory evidence will be required to determine their presence.

(A)

(B) Fig. 2. Two almost identical drawings containing seven differences to be detected

D. Navon 5

However, the view of facilitation presented here regards it rather as the way in which processing is initiated or intensified. Sensory evidence for the presence of suggested stimuli is actively sought, in much the same way that a datum received from an information source which is diagnostic of a certain hypothesis will not only sensitize and bias an intelligence officer towards the incidental flow of other information which is indica- tive of the same hypothesis, but will presumably also lead the officer to seek such information.

Global Precedence

Nothing in this conceptualization entails that the order of processing is in any way related to globality of constituents.

Actually, traditional feature analysis models (e.g., Gibson 1969; Rumelhart and Siple 1974; Selfridge 1959) may be seen as a special case of input-driven and concept- facilitated processing in which initiation is reserved for the lowest-order elements, and only suggestions exist.

Alternatively, one could conceive of models in which some local features arrive first and drive suggestions which, in turn, evoke inquiries for all relevant features (e.g., Lindsay and Norman 1972).

Another possibifity is that more global features are registered more quickly. This may cause active perceptual processing, namely inquiries, to concentrate at the global level before more emphasis is put on more local levels (Navon 1977). The natural course of concept-driven processing (namely perceptual processing that is not initiated by a suggestion from any detector, but rather by some instructions or expectancies) may also be global-to-local in the same sense.

Alternatively, global-to-local processing may be coupled with the assumption that perception is passive, and that the availability of various features is exclusively determined by the relative speed of their detection (Kinchla and Wolfe 1979; McLean, Ref. Note 1). Thus, in principle, the source of processing initiative and the order (or relative speed) of various levels of globality are orthogonal.

One may wonder how global is global enough to start from. A strong assumption is that it applies to the entire visual field or to the novel stimulation in it. A weaker, and more plausible, claim is that the processing system may be fairly free to decide which part of the visual field it attends to (although there is a limit to the ability to restrict attention to a very small region and to filter out at will all information outside it; see Eriksen and Hoffman 1972; Kahneman and Henik in press; Keren, O'Hara, and Skelton 1977; Purcell, Stanovich, and Spector 1978), and that global-to-local applies to 'perceptual analysis of whatever is attended to' (Navon 1977, p 355). In other words, the span of attention may be bound neither to any extent of visual angle nor to the bound- aries of stimuli. But once one level of globality is tuned to, the next one to be inquired is likely to be more local rather than more global. This may be due to the relative strength of suggestions from local and global detectors, or to a policy of allocating resources over time, or to both.

To illustrate, suppose you look out of the window and see a car. Let us skip the issues of what enables you to segregate the car from the background and what caused


your attention to be attracted to the car. What happens as your attention is focused on some region containing an object that you have to identify? The global precedence hypothesis states that its global features will be available 1 before its local ones, and that they may suffice for recognition, but if not, then more local features will be processed.

Thus, global precedence should best be regarded as a claim about a disposition of the microgenesis of percepts rather than as a model of a specific mechanism which gives rise to that microgenesis. The claim is that the confirmed presence of the global features of an attended stimulus and of its local features is not equally available in the percept (or to the system that generates it): processing of the local level tends to be later or slower and perhaps more optional than processing of the global one. Hence, its impact on the early percept is likely to be smaller.

There is no claim that the whole precedes its parts; such a claim would be quite meaningless, because if the whole is known, why bother about the parts? The claim is rather that global features of the whole precede local ones.

Seriality or Relative Speed?

Does global analysis strictly precede local analysis, as illustrated in Fig. 3a, Or does it just yield its output earlier, as in Fig. 3b?

It is not made completdy dear by Navon (1977) which model he is proposing. He frequently uses phrasing that connotes seriality (e.g., 'perceptual processes proceed from global structuring', p 354), yet does not exclude the possiblity of some concurrence (e.g., 'processes of local analysis may already have begun before the global analysis is completed,' p 372; 'global processing is done before local analysis is completed,' p 382).

A careful look at Navon's papers might show that he describes the perceptual system as a parallel system limited in resources (see Navon, Ref. Note 2, p 3; Navon 1976, pp 137-138) , and the apparent order of levels of globality, as the 'outcome of a continuous self-regulated change in the allocation of resources' (Ref. Note 2, p 10; see also p 3, lines 23 -24 , and Navon 1977, p 354, lines 38--42), caused by the relative activation of global and local detectors over time (Ref. Note 2, p 10).

TIME (A)

GLOBAL <

LOCAL

> Fig. 3. Two possible understandings of global pre- t o t l t 2 TIME cedence

(B)

The convention in this paper is to refer to differences that may be due to globality per se as differences in 'availability' or 'accessibility' rather than 'visibility' or 'discriminability'. The main reason.is to emphasize their status as a dependent variable rather than as a nuisance factor

D. Navon 7

In other words, the operations of global and local detectors were seen to be presumably concurrent and to differ in rate or strength. The allocation of resources to inquiries evoked by that input-driven activity (or to the evaluation of the output of detectors) dictates that local analysis is completed later or not at all. Whether it also starts later is difficult to decide at this stage. Since there is probably no way at present of decid- ing between the two theoretical possibilities in Fig. 3, it seems reasonable to phrase the global precedence notion without specific commitment to either of them.

Is the relative rate of (or the order of completion of) processing global and local features invariable and unaffected by any other factor aside from globality? Judging by Navon's explicit attempt to equate the global and local levels with respect to com- plexity, salience, familiarity, recognizability, and relative diagnosticity for determining the identity of the whole (Navon 1977, p 358), it seems that he probably regarded globality as one factor that would be critical, if other things were equal, and would account for some of the variance if they were not.

Since globality is presumably one of a number of factors involved in determining the order of availability of various features, it is not surprising that it can be overridden by other relevant factors, such as brightness or pattern goodness, when these are manipulated effectively enough, as by McLean (Ref. Note l ) and Hoffman (1980). These other factors may change the order of arrival of suggestions or the relative rate of evaluating the output of detectors, or even the relative impact of early and late analyses on the percept or on the response. The challenge for anyone who wants to study this issue is, thus, not to show that globality is or is not the only factor, but rather to provide adequate controls for all other factors. I return later to the issue of whether this has been properly done in previous studies.

Experimental Testing: The Choice of a Paradigm

[tow can the hypothesis of global precedence be tested? Since it is a claim about the development of the percept, a straightforward way to

test it is to try to capture the percept before it has developed fully. Atime-honored way to do it (albeit not uncontroversial; see Neisser 1976), is to limit the availability of visual information for analysis by exposing the stimulus for a short duration. A widely used way to ensure that availability of stimulus information does not outlast exposure duration (which is not risk-free either; see, e.g., Schuhz and Eriksen 1977), is to replace the stimulus by a mask.

If global precedence is valid, under very brief exposure local features will be perceived inaccurately, if at all. Under longer exposure they will be perceptible, but less acces- sible than global features in the sense that they require longer latency to be responded to with perfect accuracy. In that case, it is reasonable to expect them to be influenced by, but not to exert influence on, output of processing which is accomplished sooner.

What does the global precedence hypothesis predict about perception of stimuli presented for a duration that permits both aspects to be fully processed (say, t 2 - t 0 in Fig. 3b)?

One could conjecture that global structures tend to be more salient than equally complex local features, that is to say they dominate our awareness or our response


system. Such a finding is, o f course, compatible with the notion of global precedence. However, note that rdais is not a necessary outcome of global precedence which is a hypothesis about microgenesis. In other words, global precedence is a claim about what Js present :an the ea r~ percept rather than about what is salient in the final one.

It may ;be that ~oba] analysis is sufficient to account for the data, and if it is com- Ifleted sooner, some local details may not be noticed at all. This may have been the case wich most o f the subjects in one o f Navon's experiments (Experiment II in Navon 1977), ha which chey viewed a letter made out of smaller ones, without having been told ~ M n g before.hand about the structure of stimuli, if their post-experimental reports are to be relied o n .

ORe may corvecdy contend that processing that is more optional is not necessarily slower or 'later. However, a decision whether to materialize an option typically follows some dissatisfaction w~th the result of previous processing which is either mandatory or of hi# ~o-~y.

On ~ e other h~nd, £ ~ ` l ~ even to notice the elements is clearly not the rule in the percep~,,'on o f r ~ c sdmuli. Furthermore, it will certainly not occur if the subject is made aware o f the elemen¢~, as James (1890, pp 442-444) so compellingly noted. Asking a sab~e~t ~ iigno~ them may be as effective as asking him not to think of pink elephant . Volma~ry fi~.tefi~g cannot .be as efficient as natural overlooking, even when activation is no't inpu,~-driven but just concept-driven.

I~ is hardly starprisi, ng'that focused attention fails when three harmful conditions conjoha: ~hen the re~,egant and irrelevant aspects are not spatially separated (see Pomer~ntz and Schwait~berg, 1975), when they are to be analyzed by the same proces- ~sing r n e e h ~ (shape analyzers in this case), and when the irrelevant aspect is activated by cxpt ldt :instructions to ignore it.

But this seems :to be the case with the global and local levels of a visual stimulus presented for a long duration in a focused attention paradigm. Can one help seeing a dimple on a face whose e~ress ion is to be judged once its possible presence has been indicated? The tree-top coracealing part of the righthand building in panel A of Fig. 2 may be hard to aofice, ba t once it !is, its presence has to be filtered out actively, not always successfully.

One co u|d argue Chat people should nevertheless succeed in focusing at an aspect which is processed earlier. This seems to be predicated on the assumption that a response based on a fast perceptual process cannot be interfered with (or facilitated) by a slower or a later pereep!tua`l process. Is this assumption a reasonable one?

Order o f ;processing ks not a necessary interpretation of failures of focused attention. For example,, the finding that written color names affect the latency of response to the color o f ~ by which they are written (Stroop 1935) is not taken to indicate that word r e c o g ~ i o n ,is accomplished sooner than color recognition. Furthermore, the response to a target ~ can be ~ i b i t e d by the appearance of another digit 50 ms after the onset of~he t a~e t ((Hoffman 1975 ; a similar effect is reported by Eriksen and Schultz 1979); this . s~dy does no t indica~ that the later digit was processed earlier or faster! Hence, a good deal of local processing might take place before the response to the global level is im'ti~ted, and therefore might interact with it. Focusing may still be reasonably success- fal, bu t for reasons that have nothing to do with processing precedence.

D. Navon 9

It seems that one way to observe the information available to the system at the early phases of microgenesis is to instruct the subject to put a high premium on speed. This is reasonable as long as it is assumed that the subjects can initiate a response as early as, say, t l , or that response selection based on information accrued by t 1 is immune to interference from information that emerges later. I shall not discuss the plausibility of these assumptions here. Yet, plausible as they may seem, they are still assumptions.

A more straightforward way to investigate microgenesis without being entangled in strong assumptions about the temporal course of encoding and response preparation is to look for interactions of perceptual phenomena with exposure duration. We shall have to capture the percept before the possibly transient dominance of the output of early analyses is undermined by ongoing activity of later or slower ones. Say at t 1 rather than at t 2 (see Fig. 3B).

Needless to say, having no a priori knowledge of what t 1 might be, we are confronted with a delicate methodological problem. If the predicted outcome fails to occur with a particular exposure duration, it is still possible that it would with a shorter one. Recom- mended research strategies are, thus, either a systematic manipulation of the exposure duration, or the establishment of the expected effect for a short duration, and later an

extension of the duration in order to determine the range within which the effect holds.

Experimental Testing: The Choice of Stimulus Material

One way to tackle the issue of global precedence is to present subjects with stimuli in which the global and local levels are a) statistically independent, in the sense that the identi ty of neither can be predicted from the identi ty of the other one, b) equally recognizable, in the restricted sense that they are matched as much as possible in appearance, so that they are equally good as instances of their category, and presumably equally suggestive of it. One example is a set of letters made out of identical small letters of the same set (see, e.g., stimuli in Fig. 5). Let them be called compound letters.

The experimental task may call upon the subject to process both levels, or one, or neither. In the first case, speed and/or accuracy of response to the two levels can be compared. In the second, one can also study the extent to which the irrelevant level affects the response to the relevant one. In the third case, indirect methods can be con- trived to ascertain which of the two levels is more effective despite the irrelevance of both to the experimental task.

Tasks involving the processing of compound letters have been used by several researchers. Navon (1977, Experiment II) asked subjects to respond to an auditorily presented name of a letter while looking at a compound letter presented right in front of them for 80 ms and not followed by a mask. The subjects' auditory discrimination responses were interfered or facilitated by the global level of the visual stimulus but not by the local one. Furthermore, it was found that a single small letter and a single large one were equally effective in producing the same interaction with responses to auditory stimuli. Thus, it was not let ter size per se that was responsible for the effect.

Martin (1979) similarly found that auditory responses were affected only by the global level, but that the local level could be made effective by a considerable reduction in the number of elements making up the global letters. This result is hardly surprising, since


letters made out of few elements may bear very small resemblance to their prototypes. Thus, global analysis may be started early, yet fail to activate the letter code early. I discuss this issue below.

A more controversial paradigm is one in which the relevance of the two levels of globality for processing is manipulated. Navon (1977 Experiment III) presented compound letters at one of four locations (centered 2.5 ° away from fixation) for 40 ms followed by a mask. When subjects were asked to respond to the global letter, the identity of the local one had no effect on their response latencies. However, the identity of the global letter appeared to interfere with the latency to the local level. In a similar paradigm, McLean (Ref. Note 1) found that this asymmetry still held when stimuli subtending 3 ° visual angle were presented for unlimited time, but not when the stimuli subtended 12 °. When presentation was at the center of the field, the same was true for stimuli subtending 1.5 ° and 15 ° respectively. Recently, Miller (in press) reported global precedence with 3 ° stimuli presented centrally for unlimited duration. The emergence of the effect under unlimited exposure duration may suggest that some size parameters may cause processing precedence to be reflected also in relative salience. Such a conclusion appears to be inconsistent with the findings of Stirling and Coltheart (1977) and Pomerantz and Sager (1975). Stirling and Coltheart had their subjects name the global level of a compound letter presented foveally for an unlimited duration, and found that the local letter interfered considerably with a response which was incompatible with it. Pomerantz and Sager used a different logic. They inferred the presence of interference from the incre- ment in time to sort a deck of cards when the irrelevant level was made to vary ortho- gonally with the relevant one (rather than held constant). Their sets of local and global stimuli were not identical, but Pomerantz and Sager argued that they were matched on discriminability because they were sorted equally quickly. The result was that both levels could not be successfully ignored, and it was harder to ignore the local level. Interference in both directions was also reported by Hoffman (1980) who asked his subjects to search for a designated target or targets at either or both of the levels of a compound letter presented for 200 or 90 ms and not masked.

As stated above, such results are not particularly surprising for anyone who does not interpret global precedence as entailing global dominance as well. However, they might be viewed differently, depending on how one interprets the requirement that the local and global levels be equated with regard to recognizability.

For anyone interested in comparing the levels on their salience, the natural criterion for equal recognizability is equal baseline latency (namely, when the irrelevant level is assumed not to interfere). However, those latencies may be affected by the order (or relative speed) of processing of the levels. This is a reasonable prediction from the hypothesis of global precedence. So, if one is looking for such an order, and is trying to diagnose it by the effects of variation of the irrelevant level on latency, equating baseline latencies is most likely to eliminate the desired effect! A basic principle in experimental design is that if factor A is expected to affect variables x and y, one should seldom try to partial out x when testing the effect of y. Accordingly, the criterion set by Navon (1977) for equal recognizability was some formal equivalence in structure.

It is clear that stimuli such as those used by Pomerantz and Sager (1975) did not satisfy this criterion. The stimuli used by Martin (1979) or by Stirling and Coltheart (1977) do not seem to fare much better; to judge by the authors' description, local let-

D. Navon 11

ters were much more standard-looking and dense than the corresponding global letters.

My main concern is, however, with sets of stimuli that do formally meet this criterion, for example when the global letter is constructed from a 5 x 5 matrix of letters, and the local one is similarly constructed from a 5 x 5 matrix of dots (see Hoffman 1980). Is this sufficient to make the local and global patterns equally good instances of their stimulus- category? Consider, for example, panels A and B in Fig. 4. The F in panel B is much more recognizable. That this is not just due to the fact that the elements in Panel A are letters

L L L L L Q Q Q Q Q L • L L L 0 0 Q L • L •

CA1

I-- i - I - l - i - H H H H H F H I - i - l - H H H F H U H

(C) . (D)

Fig. 4. Four displays each making up the configura- tion of an F. The elements affect the goodness of the pattern

is evidenced by Panels C and D: an F made of upside down Ls appears still worse than an F made of Hs.

But note that Panel B is a magnification of a local letter. Thus, global patterns are inferior when it comes to equating the goodness of their shape with the goodness of shape of the elements comprising them. They will presumably be more so the fewer elements there are (see Martin 1979) and the more blurring is required to turn their elements into some biaxially symmetrical closed forms. Therefore, since compound letters are used just as a vehicle in studying order of processing, the letters used should not be sampled randomly out of the alphabet but rather selected judiciously, so that they minimize the disadvantage of global patterns. H, S, X, and O seem to be a better choice than, say, L, T, F, and Y. A small number of elements probably aggravates this disadvantage (see section 'number of elements'), hence it seems better to construct letters from 5 x 7 or 6 x 7 matrices as done, e. g., by Kinchala and Wolfe (1979), McLean (Ref. Note 1), Miller (in press), or Navon (1977). Another precaution is to make the local letters out of t iny rectangles whose size relationship to the whole local letter is the same as that of the local letter to the global one. In this way, they are matched not only in shape but also in their internal structure, as was done in the experiment reported below. Though this precaution may in the future prove to be superfluous, it is still a prudent step to take at this stage.

One might contend that, on the other hand, local letters suffer other disadvantages,


for example their smaller size. The issue is whether those disadvantages are inherently correlated with globatity or can be eliminated by proper methodology. Disadvantages of the first kind may be considered as intervening variables. That means that their role in mediating the phenomenon should be studied, but they should not be treated as if they were nuisance variables, because by trying to control for them one may end up throwing out the baby along with the bath water. I elaborate on this issue in the discussion.

The Rationale of the Experiment

The purpose of the experiment to be reported was fourfold: 1) to see whether Navon's results (1977, Experiment III) would be replicated if higher contrast, a longer exposure duration, and a more appropriate mask were used; 2 2) to provide a direct size control, namely to have the subject respond to a single letter of the same size and shape as the local letter; 3) to test asymmetry of interference by the logic of inhibition due to variation of irrelevant dimension (Garner 1974; Pomerantz and Sager 1975); 4) to test

whether there is a difference in the amount of resources required to process either of the levels (or to ignore the other one) by means of a subsidiary task: simple reaction time to the presence of a concurrent tone. Since, as stated above, global precedence is assumed to apply to perceptual processing of whatever is attended to, it does not predict that when required to respond to the local letter subjects will not be able to focus on a spot that contains just one small letter. To overcome this difficulty, I presented the stimuli, as Navon (1977) did, at one of four possible locations, so that subjects had to distribute their at tention over a large area of the visual field. The single letter meant to control for the possibility that the parafoveal presentation that resulted (about 0.4 ° - 0.6 ° visual angle for the element closest to the fixation point) can account for the results. It certainly cannot, and is not meant to, control for other variables that are inherently correlated with globality, such as number of neighbors, number of same-size characters, etc.

Method

Apparatus. Visual stimuli were presented by a three-field Gerbrands Harvard type tachistoscope Model T-3B-1. Viewing distance was 76 cm. The luminance of the fields was approximately 11.0 cd/m 2, and it was ascertained that the stimulus could not be seen when presented simultaneously with the masking field. An electronic tone gene- rator served to produce a moderately loud 2800 hz tone stimulus. The subjects responded to the visual stimulus by pressing one of two response keys. The response to the presence of the tone was given by pressing a foot pedal. Latencies were measured by means of a digital millisecond clock, and recorded by means of a printer.

2 The mask used by Navon was presumably biased in favor of the global letters, because it was made of a grid of dots spaced less densely than the dots making up the letters

D. Navon 13

Stimuli. The set of compound patterns is presented in Fig. 5A-5D. Stimuli were pre- pared in the following way: large letters of the type presented in Fig. 5E were drawn,

H H H H H H FIHHHHH H H H H H H

(A

S S S S S S SSSSSS S S S ~:::

( B )

H HHH ~i-i H

' F ~ H H H H

HHHHHH

( c )

sSSSSs S

-~SSSq ~ S

S ~ ,-, S S "~

o)

ooooo

OOOOOo ODoDDD (E)

H

(F)

;ffl; ~- ~ : .',~'~ ~..,.

Fig. 5. Panels (A)-(D) present the compound letters used in this experiment. Panel (E) presents an original stimulus used for making the compound letters. Panel (F) presents an example of a stimulus in the single letter condition. Panel (E) presents the texture of the masking stimulus

reduced by photography by a factor of 8, duplicated, and then positioned as local letters at the appropriate locations of the global letters. The rectangles of the original letters would precisely circumscribe the local letters in the compound patterns if super- imposed on them. Thus, a local letter was an exact small replica of a global letter made of rectangles. One of each of the small letters was used for preparing single-letter stimuli as the one in Fig. 5F. The resulting stimuli were further reduced by photography to the size used for the experiment, and then printed on white cards. The small letters


measured about 4.6 mm vertically (0.35 ° at viewing distance of 76 cm), and the whole stimulus measured 37.5 mm (2.820). The average reflectance of the strokes of a small letter matched approximately the greyness level of a standard Letratone sheet (produced by Letraset Company in London), composed of dots with density of 12.75 per cm and percentage of black area 60-70%.

A compound letter appeared at any one of the four quadrants of the card, such that its center was at a 31 mm (2.34 °) distance of the fixation point. A single small letter appeared in any of the four quadrants, and was located where the most foveal element of the global H was, viz., its center was at a 6 mm (0.45 °) distance of the fixation point. A black point of 1 mm diameter drawn on a white card was used for fixation. The masking stimulus consisted of fragments of the small letters, covering the entire stimulus field as in Fig. 5G. Design and Procedure. Every trial consisted of a verbal ready signal from the experi- menter, followed by an 800 ms presentation of the fixation field, followed by a 150 ms presentation of a visual stimulus (see Fig. 5 A - S D or 5F), followed in turn by a 150 ms presentation of the masking stimulus. In a randomly determined half of the trials a ton e was presented simultaneously with the visual stimulus. The primary task of the subject was to respond to the presence of an H in the relevant dimension (namely, the level of globality to which he was instructed to attend) by pressing t h e k e y touched by his dominant hand, and to the presence of an S by the other hand. The secondary task of the subject was to indicate the presence of the tone by pressing the foot-pedal after the manual response. Latencies were measured from the onset of the stimuli.

Each subject was tested individually in two sessions. The total number of trials was 480. There were five blocked conditions of 96 trials each. The subject was allowed a short rest after each consecutive run of 48 trials. One condition was used for presenting the single small letters. The other four conditions constituted a factorial design of two variables: at tention condition (global-directed or local-directed), and variability of irrelevant dimension (variable or fixed). The conditions in which the irrelevant dimension was held fixed were further divided in two according to the letter that served as the fixed value. The first 16 trials out of the 48 in each of those subconditions were considered as practice. In the other conditions, the first 32 trials out of 96 were considered as practice. Each type of stimulus, each quadrant in which it appeared, and the presence of the tone occured randomly, independently, and equally often within the set of experimental trials of each condition. Thus, in half of the trials in which a compound letter was presented, the two levels of globality were consistent, whereas in the other half they conflicted. Attent ion conditions were administered in different sessions to minimize the chance of confusions as to what the relevant dimension was. Order of at tention conditions was counterbalanced across subjects. Within each group the order of conditions manipulating the variability of the irrelevant dimension was counterbalanced. The single-letter condition was administered at the beginning of the first session, at the beginning of the second one, or at its end. Thus, there were 12 different sequences of conditions. An average session lasted about 75 rain. Subjects. One subject in each sequence was tested, making a total of 12 subjects. A second subject was disqualified because of too many errors (more than 4%), and another was eliminated at random because two subjects were mistakenly given the same sequence. All subjects were undergraduates at the University of Haifa who participated

D. Navon 15

in the experiment as part of their course requirement. They were not paid but were

promised participation in a lottery in which the winner would win a sum that depended

heavily on accuracy and slightly on speed. All the subjects had normal or corrected-to- normal vision.

Results

As was stated earlier, the goal of the interference paradigm employed here is to establish

the existence of some epoch t during the course of processing at which local information is poorer, but still can be responded to with perfect accuracy. 3 Thus, since subjects who committed many errors might have done so because they did not see the local letters very well, I decided to eliminate their data from analysis. Of course, such subjects might have simply opted to emphasize speed more than accuracy, or had more intrusions from the irrelevant channel, but there was no way of ascertaining that.

Accordingly, the data of the 12 subjects who did not make more than 4% errors were

analyzed. The accuracy of these subjects was very high in all conditions: 97.0% and 95.3% correct in the local-directed conditions when global letter was fixed or variable

respectively, 98.7% and 97.9% in the corresponding global-directed conditions, and 98.0% in the single-letter condition. A repeated measures analysis of variance performed on the

arcsine square root transforms of the accuracy data did not yield a significant difference among the five conditions: F(4,44) = 2.26; P > 0.05. A further comparison between the

two conditions in which the irrelevant dimension varied also yielded a nonsignificant result: F(1,11) = 1.84; P > 0.10. Thus, single small letters as well as local letters were

visible enough to be discriminated quite accurately. Mean latencies of correct responses are plotted in Fig. 6 as a function of attention condition, variability of the irrelevant

dimension, and consistency of letters in relevant and irrelevant dimensions. A repeated

775 "

750 .

A 725 "

700

675 .

z 650

,~ 625-

600 .

575

5 5 0 -

IRRELEVANT ATTE NT IO N D IMENSION CONDIT ION

/ V A R I A B L E

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F IXED GLOBAL

. . . . . . VAR IABLE SMALL

CONSISTENT CONFL ICT ING

CONSISTENCY

Fig. 6. Mean latency of correct responses to the visual stimuli as a function of attention condition, variability of the irrelevant dimension, and consistency. Mean latency to small single letter is represented by a dashed and dotted line

3 Although researchers do not typically expect to find perfect accuracy in responses to masked stimuli, that is beacuse they set their experimental conditions to measure error rates. It is not logically impossible to have two different speed-accuracy curves, both asymptoting at 100% accuracy, and it is not unreasonable to look for them.


measures analysis of variance performed on the latency data indicated a highly signi-

ficant difference among the five conditions: F(4,44) = 17.05 ;P < .001. A five-factor repeated measures analysis of variance was performed on the latencies to compound letters only (since most of the factors did not apply to the single letter condition). Factors were variability of irrelevant dimension, at tention condition, consistency, type of response (H or S) and quadrant. Only at tention condition and type of response were found to have significant main effects: F(1,11) = 27.81: P < .001, and F(1,11) = 17.40; P < .005 respectively. Thus, the response to the local letter seems to be inhibited by the mere fact that it is embedded in a global pattern rather than by its size, as indicated by the short mean latency to the single small letter.

Variability and at tention condition were found to interact significantly: F(1,11) = 5.83 ;P < .05. The variability of the local le t ter did nor mat ter when at tent ion was directed to the global one, whereas variability of global let ter did matter in the local- directed condition. This t rend was apparent in the data of 10 out of 12 subjects (bi-

nomial probabil i ty of P = .019). According to the logic proposed by Garner (1974) and used, e.g., by Pomerantz and Sager (1975), this suggests that the globyl level is prior in some sense.

The pairwise interaction of type of response and quadrant was found significant, as were four more triple or quadruple interactions involving at least one of those factors. No other interactions were found significant. The interaction of at tention condit ion and consistency just approached statistical significance: F(1,11) = 3.44; P < .10. An inspection of the data of the single subjects showed that this was probably due to the high variability of this interactive effect within the conditions of fixed irrelevant dimension. Since the hypothesis we set out to test predicted the presence of this inter- ~ction within the conditions of variable irrelevant dimension, a further analysis was performed just on the latency data from those conditions. This analysis revealed a significant interaction between at tention condition and consistency: F(1,11) -- 4.90; P ~ .05. The effect of inconsistency was inhibi tory on the response to the local (729 vs 702 ms) but not to the global (558 vs 566 ms) letter. This trend predicted by global precedence was apparent in the data of 10 out of 12 subjects (P = .019). A similar analysis on the accuracy data yielded a nonsignificant result: F(1,11) = 1.08.

The interaction of consistency with type of response and the interaction of these two factors with at tention conditions were also found significant in this analysis (P .001 and P < .05 respectively). This is due to the differential effect of consistency on the response to the two local letters. A local H was more rapidly responded to when embedded in a global H than in a global S (667 vs 741 ms), whereas the effect of consistency on the local S was even slightly reversed 4 (738 vs 717 ms). This trend characterizes the data of eight subjects (p = .194). Does this indicate that the effect of consistency is not due to a response conflict, but can rather be wholly explained as an outcome of differential discriminability of local letters in the various configurations?

Several arguments can be raised against this possibility. 1) In Navon's original experiment (1977), the effect of consistency on the local S was even somewhat greater than its

4 The special difficulty with a local S embedded within a global one is also reflected in an excep- tionally high error percentage: 10.9%

D. Navon 17

effect on the local H. So the effect cannot reside in the configurations. 2) An effect that is due to differential discriminability should recur in the conditions in which the global let ter was fixed throughout a block, ye t it does not: neither is there an effect of consistency on responses to the local letter (t(11) = 1.037), nor does consistency interact with the type of response (F(1,11) = .15). The only noteworthy observation is that six subjects made more errors in response to the S embedded in an S than in response to the three other stimuli, which suggests that perhaps it was less discriminable, a fact which must have worked against the hypothesis. Also, if the effect had been caused by differences in reflectance, then some trace of this trend should have been apparent in the response to the global let ter; ye t latencies to the four configurations are quite random across subjects (Kendall coefficient of concordance is .11 ; P > .20). 3) Ident i ty of local letter, ident i ty of global letter, and consistency were obviously not completely orthogonal, but they may have had their independent contributions. For example, the data are accounted for fairly well by assuming that stimulus or response factors favor the H response by 18 ms (which is about the corresponding difference in the singleqetter condition), that the ident i ty of the local le t ter is quickly discriminated within a global H by 44 ms, and that the effect of consistency per se is 36 ms. Thus, perhaps the consistency effect that existed in the case of the local S as well was just overridden by the debilitating effects of the presence of the let ter S in both the local and global levels,

or by some degradation in the S made of Ss. Analysis of tone detection latency data (calculated for only 11 subjects, since, be-

cause of a technical problem the data of one subject were not printed) yields a fairly similar picture, but with somewhat weakened effects which sometimes fall short of significance. This is not surprising, since a manipulation of a task is expected to affect performance on it more than performance on a concurrent task 5 (Navon and Gopher 1979). An effect on the latency to the tone could be produced even if the tone was re- trieved from echoic memory after manual response had been initiated, so that tone processing was not interfered with by concurrent visual processing, but just delayed. This would cause the audi tory latency to correlate with visual latency quite artifactually. However, that this is not the whole story is suggested by a significant, albeit small, effect of at tention condit ion on accuracy of tone detection. In 5.1% of the trials in the local-directed variable condition tone-errors were made (most of them false alarms), whereas just 0.8% tone errors were committed during the corresponding global-directed condit ion: F(1,10) = 6.03; P < .05. Accuracy during the single-letter condition was almost error-free as well (1.3% errors). This suggests that attending to the local level is more resource-consuming.

Discussion

It was found that local letters were responded to more slowly than global ones and were associated with some failures to react to a concurrent tone. It was also found that variation of the global level, as well as the ident i ty of the global letter, did affect the latency to the local level, but not vice versa. This is compatible with the notion of global prece-

5 On the other hand, the dual-task situation might have contributed to the error variance of letter processing, and thereby might have been responsible for the weakness of some effects


dence. The latter effect is considerably smaller than the one reported by Navon (1977), probably because of the longer exposure duration, and the possibility that Navon's re- suits were inflated because of the inappropriate mask he used. It seems that a duration that is not much longer than the one employed here would make the relative availability of global and local features practically indistinguishable (say, point t 2 in Fig. 2). Thus, the reported finding gives an idea of the temporal range of the phenomenon.

The results cannot be due to unequal recognizability: care was taken to match the levels in appearance. That they are not just due to the smaller size of the elements is indicated by the finding that a single letter as small as the local letter was responded to as fast as the global one. That is not to say that size or eccentricity may not have a differ- rential effect on the visibility of the two levels, but merely that whatever effect they might have is probably not responsible for the finding reported here.

That brings us to the issue of the relevance of various factors for interpreting results of experiments designed to test the notion of global precedence. The discussion that follows is intended to be general, rather than limited to the results of our experiment.

Possible Explanations

As stated earlier, global precedence is just a claim that, other tbings being equal, global features are available in the percept earlier than local features are. It makes no commitment to any specific mechanism that might give rise to it. Even if we accept the speculation developed in Navon's thesis (Ref. Note 2) and restated in the introduction to this paper, we may proceed to ask what sensory mechanism mediates the order of arrival of global and local features which sets perceptual processing in a global-to-local mode when it is data-driven or facilitates global precedence when processing is concept-driven.

The question now arises whether any given factor is to be considered as one of those 'other things' that 'should be equal', or as a candidate for mediating global precedence. In other words, is a lack of balance of tevels of globality on factor x to be taken as a methodological bias, or as a faithful reflection of a natural relationship?

I suggest the following criterion. Some factors are by the nature of the things confounded with globality: e.g., elements are smaller than global structures and have more neighbors. It is hard to think of any laboratory setting that would eliminate this confounding. These factors should be considered as possible mechanisms or intervening variables. One may try to evaluate their relative impact by means of various procedures for partialling their contributions out. Any positive result of such a test does not invali- date global precedence as an empirical relationship, but rather accounts for part of the variance.

On the other hand, factors that can be manipulated with regard to each level inde- pedently (e. g., shape, stimulus ensemble) should be considered as nuisance variables to be controlled for.

Factors manipulated with respect to the entire stimulus, such as visual angle or eccentricity, may nevertheless interact with the effect of globality. These factors, thus, specify the conditions for the occurrence of the phenomenon. It is then advisable to recruit some ecological considerations to determine how often the conditions are met, hence how prevalent the phenomenon is.

I shall now discuss the relevance of various factors for explaining global precedence or for limiting its applicability.

D. Navon 19

Relative Size. The phenomenon might be explained as a consequence of the different size of the global and local letters (Kinchla and Wolfe 1979); or (McLean, Ref. Note 1) in terms of the relative speed of different spatial frequency channels (Breitmeyer 1975; Vassilev and Mitov 1976). According to this view, a tree would be processed more slowly (or less accurately) than a forest simply because it is smaller, regardless of whether it is embedded in a forest or stands alone.

Size is certainly a relevant factor; within a certain range, large letters will be discriminated more rapidly than smaller ones (Schultz and Eriksen 1978). The issue is whether it is sufficient to account for the phenomenon. The results of the present experiment as well as of some previous studies (Hoffman 1975; Navon 1977) indicate that it is not: subjects ' performance on stimuli as small as the elements resembled their performance on the global level rather than on the elements. Thus, being an element renders a stimulus much less available than it would have been, given its size, had it stood alone.

It is still possible to argue that differences in accessiblity due to size, which are too small to be detected when stimuli are presented separately, are magnified under concurrent presentation. Perhaps it is just a manifestation of a more general phenomenon which

may be called ' the advantage of being easy'. Suppose that facing two perceptual tasks, the processing system directs more resources to the easier one. The outcome would be attentional accentuation of sensory differences: the task that is inferior in the first place will suffer from the concurrent presentation much more than the superior task. The global level may be slightly easier to process because it is always larger than the local level, and this small difference will be accentuated by the concurrent presentation of two types of letters embedded in each other. This may be mediated via inhibition exer- ted by global detectors on the local ones (Petersik 1978) or by a larger share of capacity consumed by the global analysis. But if this explanation is correct, the effects demon- strated with stimuli embedded in each other should hold also for separate stimuli presented simultaneously. McLean (Ref. Note 1) obtained precedence of large letters drawn as hollow frames over small letters presented within the frame. Although the small letters are circumscribed by the frame, and thus in a sense are more local, they are not dominated by it in the same way that they are by a global letter of which they constitute the elements. It is unclear, then, whether the large and the small letters should be regarded as separate stimuli or as different levels of the same stimulus. A side-by-side presentation would be more instructive as to the validity of the notion of ' the advantage of being easy'.

LateralMasking. The perceptibil i ty of a stimulus may be impaired by the concurrent presence of neighboring stimulation (e.g., Wolford and Hollingsworth 1974). This, of course, cannot harm the global structure, but it may inhibit the processing of elements that are close to each other. However, there are several reasons to doubt that the infer- iori ty of a ' tree in a forest ' compared with a 'single tree ' is accounted for by such lateral interference.

First, lateral masking predicts that the effect will be very sensitive to retinal separa- t ion of the elements, much more than is evidenced, for example, by the emergence of global precedence with the five widely separated elements used by McLean (Note 1). Second, it seems difficult to explain by lateral masking the finding that small letters are responded to more slowly when embedded in global letters than when arranged in


three rows (Regan 1981). Third, lateral masking does not degrade the outermost stimuli in a string (Townsend, Taylor and Brown 1971), so if all the elements are the same, the observer can use the undegraded ones. Fourth, any possible inhibitory effect of masking could have been compensated for, or even overridden by, the presence of many identical elements. They could facilitate each other, or at least race against each

other so that latency of response is determined by the element that is processed most rapidly (cf. Pollatsek and Digman 1977).

Finally, by mentioning these points I do not wish to convey that retinal proximity of contours is unrelated to the strength of global precedence. I just doubt that it plays a decisive role. Furthermore, caution is needed when at tempting to give a causal

interpretat ion which construes global precedence as the effect. The term 'lateral masking' is associated with lateral inhibition, which is a neurophysiological phenomenon first discovered in the eye of limulus (Hartline and Ratliff 1957), but the former term is quite loosely used as a label for a class of empirical facts about human perception. These facts are in great need of explanation in themselves, and several such explanations have been offered (e.g., by Estes 1972; Estes, Allmeyer and Reder 1976). One of them states prior registration of configurations as the cause of some proximity effects (Prinzmetal and Banks 1977). So the most that can be said is that the unknown mechanism that mediates the effects usually classed under the label 'lateral masking' may be partly responsible also for the phenomena that are claimed to demonstrate global precedence.

Diffuse At tent ion. One such mechanism may be related to a tendency of visual atten- t ion to be distributed evenly or haphazardly over the field during the first 'pre-atten- t i re ' phase of perceptual processing (Kahneman and Henik, in press; Neisser 1967). What we loosely call 'a t tent ion ' may be embodied in sensory mechanisms that sample spatial information randomly or judiciously (see the notion of logarithmic scan, Navon, Ref. Note 2), or in higher-order processes that analyze the extracted features. The share of attention apport ioned to an element may not suffice for a thorough processing, yet the global structure may get along with whatever is allocated to the region it occupies.

Actually, this view resembles the model described in Navon's thesis (Ref. Note 2) in its emphasis on regulation of at tention. However, unlike that model, it does regard attention as a sort of a magic fluid which can decipher the visual code written on a retinal area once poured over it in sufficient quantity. In my opinion, we allocate resources to processes, no t to locations. If at tention is involved in the effect, it is hard to see how the mere spread of at tention over a group of elements would automatical- ly yield the recognition of its overall shape; a process of analyzing that shape must be activated, and in principle it may compete for resources with processes that analyze the elements.

If the stimulus consists of many different elements whose processing depends on the same pool of resources, i t is clear that many of them are likely to be missed. How- ever, a fact that seems somewhat disturbing for this model is the finding of global precedence with patterns made of many identical elements. It is not unreasonable that even when attention is diffuse, some elements receive more at tention than others. Now, even if the likelihood that an element gets resources sufficient for its recognition is as small as .10, the probabil i ty that at least one of 18 identical elements (as in

D. Navon 21

the large H; Fig. 5) will be recognized, assuming stochastic independence, is .85. Thus, once again, we see that the multiplici ty of identical elements in stimuli used for study-

ing global precedence poses a problem for any explanation that resorts to some inferi- ori ty of the elements due to the presence of other elements.

Spatial Uncertainty. Once the observer attends to the region where the stimulus is located, then he naturally does not have to search for the global features. This is usually not true for the elements whose precise location is to be determined in the course of processing. It was pointed out to me that this might account for the effect of the variability of the global letter on the latency to the local letter (see Fig. 6). How-

ever, one drawback in this explanation is that the features of two alternative global letters fell in considerably overlapping regions; an effective strategy in the local-directed at tention condition would be to orient towards the most foveal 2 x 2 corner of the ima- ginary matrix subsuming the global patterns: in both patterns it contained at least one element.

Nevertheless, in more naturalistic situations the need to locate the local features may be one of the causes of their inferiority as well as one of the possible functions of prior global processing. Global analysis may not only give a rough clue as to what the element may be, but also indicate where in the visual field it is to be searched.

Texture Perception. It has been suggested (Pomerantz, personal communication) that because of their mult ipl ici ty and regularity, the elements tend to be perceived as molecules of matter, as motifs of a texture. In my view, this is just a restatement of the phenomenon. Granted, elements are perceived as elements, not as separate stimuli. The problem is why, and it becomes more intriguing when the elements may constitute stimuli in their own right, as in the case of compound letters. The answer must be in terms of global precedence or in terms of greater salience of the global structure. I return to this issue at the end of this discussion.

An Interim Conclusion. It seems to me that the effect of global precedence is probably due to several mechanisms. I believe that some of them are sensory and some others are cognitive. I venture to guess that Nature would not have let a functionally important proper ty of perceptual processing (see Navon 1977, pp 355--356) hinge on the satis- faction of just one antecedent condition.

Regardless of how valid this position is, the dependence of global precedence on each of its conceivable mediating mechanisms is yet to be determined.

Generality

Global precedence is not exhibited under certain conditions. In other words, its presence or magnitude depend on some stimulus- or situation-variables.

Visual Angle. As Navon pointed out, 'if the perceiver is close enough to the forest, he will probably see a tree rather than a forest ' (1977 p 380). What is the range of stimulus size that is subject to global precedence?


Kinchla and Wolfe (1979) asked their subjects to search for a designated target at the two levels of a compound letter, and showed that when the global pattern subtended more than 6 0 - 7 ° visual angle, it was responded to more slowly than the local letter. They reported that over the whole range of visual angle they used, both levels were recognized with almost no errors. This appears to conflict with Navon's speculation (1977, pp 354, 380) that failure to observe global precedence would occur only when the global level is not as visible as the local one. Using a paradigm more akin to the one reported in this paper, McLean (Ref. Note 1) found that global precedence did not hold for patterns larger than about 10 ° , even though it did not seem to be superseded in the sense that global features never went completely unnoticed.

Although the fact that forests should be seen from a certain perspective to be perceived as forests is indisputable and can be explained in several ways (see McLean, Ref. Note 1), some problems arise with regard to how to assess the critical perspective.

Those problems stem from the confounding of globality and eccentricity in many stimuli. That confounding is aggravated the larger the stimuli are.

Eccentricity. Letter stimuli are responded to more slowly, the more distant their retinal position is from the fovea (Eriksen and Schultz 1977). The extraction of gross features fails off more slowly than the extraction of finer ones (Hilz and Cavonius 1974), but it also suffers from peripheral presentation. As McLean (Ref. Note 1) pointed out, it makes little sense to ask whether a local feature measuring 2 ° that falls at the center of the retina is available earlier or later than a global feature located 10 ° off center, because the effects of globality and eccentricity are confounded in the answer. McLean a t tempted to solve this problem by presenting stimuli of all sizes in the periphery, hold- ing constant the eccentricity of the most foveal edge. Note, however, that in McLean's mode of presentation, if the stimulus measures 12 °, the center of its global structure is 9 ° off fovea. One could estimate the impact of differential conditions of size-eccentricity by administering single-letter conditions, as in the experiment reported in this paper, ye t there is a more straightforward way.

Eccentricity is not necessarily confounded with globality in the same manner that relative size is; it is confounded in some stimuli but not in others. 1 suggest, therefore that there is only one sensible way to pose the question about the effect of globality: Which features of a centered stimulus are processed faster, local ones located k visual

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D. Navon 23

angles off center or global ones located at the same position? This calls for employing patterns like the one illustrated in Fig. 7. Of course, to the extent that eccentricity fa- vors low spatial frequencies (namely gross features), local features will be at a disadvantage; but that is part and parcel of being local.

One may contend that most real-world stimuli do not possess this nice property. To this my answer is threefold. 1) There is no way to gauge the validity of this contention before an ecological s tudy is made of the spatial distributions of global and local fea-

tures at various radii off the centers of real-world stimuli. 2) Compound letters are not very frequent in natural environments either. The rationale behind employing them is that they serve to equate the levels on some aspect. By the same token, patterns of the type illustrated in Fig. 7 are used to equate the levels with respect to a variable that is not inherently confounded with globality, thus should be regarded as a nuisance variable to be controlled for, especially when its potential impact is large, namely in the case of very large stimuli. 3) The use of stimuli made up of identical elements accentuates the problem of unequal eccentricity of the global and local levels. In such a case, identifying just the most foveal element of a centered stimulus is sufficient to 'process the local level' as defined by the demands of the particular experiment° However, is local processing attained so easily in more naturalistic stimuli?

I suspect that very rarely can the whole be identified by just one or two elements. So, although it is not unreasonable that one ore two local features are available at the

same time as, or even before, the global structure is, still local processing is not completed until more elements, of which some must be less foveal, are analyzed. Further- more, processing of elements is at a disadvantage not only because there are many of them. If you doubt that the global structure is much more likely to be processed than the average element over a wide range of visual angles, retinal positions, etc., you are in- vited to carry out the following experiment: Briefly flash a let ter made of n different element letters, then ask the subjects to report as many letters as they can, and caIcu- late the probabil i ty that the global let ter is included in the report. If a subject reports on the average (n+ 1)/2 letters, would you expect that probabil i ty to be 1/2, as should happen if the global structure is just another feature?

This again indicates the bias of stimuli having identical elements in favor of ' local processing': if the global structure was as likely to be perceived as any of the n elements (say, with probabil i ty P), then it would be reported much less accurately than the local level, since the probabil i ty that any of the elements is perceived (consequently that the local level is reported accurately) would be 1-( l -P) n. To illustrate, if n=l 8 and P=.10, then 1-( l -P) n = .85.

Finally, it is useful here to add one comment about the importance of spatial uncertainty with regard to the entire stimulus. As stated earlier, because what is claimed to be processed in a global-to-local fashion is whatever is at tended to, focused at tention paradigms require such spatial uncertainty. Unfortunately, this cannot be attained without peripheral or parafoveal presentation. Thus, the only remedy seems to be single- le t ter control conditions, as used here.

Number o f Elements. Martin (1979) showed that when a global let ter is made out of few elements (e.g. 11 local elements composing the S and H), i t seems no longer to precede, but rather to be preceded by them. Although in Martin's s tudy the number of

24

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Hb t H

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The Forest Revisited

Fig. 8. A letter S made out of 11 characters (after Martin, 1979)

elements was not experimentally dissociated from size and spacing, it is nonetheless quite to be expected both that global precedence must break down with a sufficiently small number of elements (where the breakdown point may vary from one stimulus to another) and that this might occur more readily the larger the element spacing is. The reason is simple, and was actually pointed out by Martin herself. A let ter made out of few elements can hardly match people's internal representation of that letter. For example

it takes some effort to see that the 11 widely spaced Hs in Fig. 8 are laid out in a way that bears some resemblance to the letter S. This is less true of the elements that may be made of dots (see Fig. 4B) let alone of elements made of solid lines, as Martin has

used. But apart from being an oberservation about the letter S, how does this illuminate

the issue of global precedence? As stated earlier, global precedence is testable when the global forms and the local ones are equally recognizable, namely equally suggestive of their categories. This requirement can be partly satisfied in experimental t reatment, for example by making the local letters out of the same number of elements. To the extent that it is not satisfied, we might have an inherent dependence of global precedence on number of elements, which is conceivably determined by the dependence of pattern goodness on the internal symmetry of each of its elements (see above). Does that mean that in Nature global precedence is not manifested in recognition of objects that have few elements? This issue is taken up later in the section 'Goodness of pat tern ' .

Ecological Considerations

Most empirical statements are true only within certain limits. However, their conditional nature is neglected in contexts in which the limits are seldom met. For example, in most contexts it takes more than scientific prudence to refrain from regarding water as a liquid, though actually it is a liquid only within the quite restricted temperature range of O°C-100°C under pressure of one atmosphere. Thus, the generality of a phenomenon should not be evaluated by the range of parameters for which it holds relative to their maximal range of variation, but rather by the frequency of its occurence in reality. In other words, if a phenomenon occurs only in certain conditions, it becomes useful to ask how likely those conditions are in our environment.

Proximal Size of Stimuli. Suppose global precedence applies only to stimuli not larger than 7 ° - 1 0 ° of visual angle, as can be inferred from the results of Kinchla and Wolfe (1979) and McLean (Ref. Note 1). ls this a narrow range? I think not.

First, there is no doubt that in most experimental work in cognitive psychology the stimuli employed subtend not more than 5 0 - 6 °. Second, I believe that in natural sett- ings 5asually the objects we at tend to subtend fairly small visual angles' (Navon 1977,

D. Navon 25

p 380). For example, the mean extent of a saccadic eye movement during reading

is about 2 ° (see, e.g., Rayner 1977), so even if the regions of effective processing during

successive fixations overlap as much as to have every feature processed during two consecutive fixations, they do not cover more than 4 °. The width of a 19" TV screen viewed from 3 m subtends 7.50; watching TV from a closer distance would probably be judged

as inconvenient. A human face subtends about 6 ° vertically when viewed from a distance of 2 m. It is not unsafe to assume that usually when we meet a person, the distance from which we do the act of face recognition is at least 2 m.

What happens when the global structure is larger than that? One possibility is that rather than processing the whole scene, the observer will process just a subscene. Some- times this will even be enough to get a good idea of the whole scene, so even though more global information is present, it may not be utilized and processing may not pro-

ceed. This may be the case, for instance, when we identify a person by his face and perhaps do not even at tend to visual information in the region of his body. The presence

of the person may be suggested, but this does not entail that global features, such as outline of body, are verified by inquiry to confirm their presence. They might be inferred in much the same way that the existence of completely invisible objects is inferred (cf. Neisser, 1976), and presumably quite validly.

But often a subscene does not provide us with all the information we need. Do we then go on to process more global information as is claimed for the 'middle-out ' model (Kinchla and Wolfe 1979)? We presumably do, but perhaps not in the same sense that we go on to process more local information.

First, a part is usually much less useful for anticipating the overall structure than the overall structure is for anticipating the part. So, to the extent that further global processing takes place, it does not benefit very much from the output of previous processing.

Second, when the whole scene subtends 1 0 - 2 0 ° of visual angle, i ts overall structure is often identifiable with one fixation, although more slowly than the subscene fixated at (see Kinchla and Wolfe). However, most often the resolution of subscenes not yet fixated is too low to enable much information to be extracted from them. So, if the conjunction of the relatively rich information about the fixated part, the slightly poorer information about the overall structure, and the much poorer information about non- fixated parts is not enough for interpretat ion of the whole scene, eye movements will probably be initiated. But perceptual processing that involves aggregating information from a number of fixations is not the kind of processing that the global-to-local model is meant to describe. Global precedence is claimed about the microgenesis of a single percept obtained from one fixation. Thus, small objects may be processed in a global-to- local fashion; larger objects will often not be fully recognized from one fixation any-

way, so that a more sophisticated model for their processing is yet to be developed.

Proximal Size of Elements. Figure 2 may serve to remind us that even in scenes that en- compass a fairly large visual angle, many details are much toosmal l to be readily access- ible. A little exercise in exploration which any reader can do is the following: sit com- for tably in front of your TV set, stretch your arm out and count how often significant parts of at tended objects can be covered by your thumbnail (less than 1 ° at arm's


length). Also, a regular printed lower-case English letter with no ascenders or descen- ders subtends about .2 ° when viewed from 40 cm.

Eccentricity of Stimuli. Not every stimulus we look at in our normal environment is preceded by a ,ready' signal and a 500 ms fixation point. The surrounding stimulation that is worth processing is too rich to concentrate always around the fovea. We tend to foveate what we attend to, but a lot of processing can be done during the latency of a saccade (150-170 ms; Yarbus 1967, p 153), namely while the stimulus to be foveated is still viewed peripherally.

Eccentricity of Elements. Every fixated stimulus whose proximal size is k degrees of visual angle necessarily contains some elements that are located about k/2 degrees off fovea. Without really investigating the spatial distribution of elements in real-world stimuli, we can calculate the probabilities that they fall within circular regions of various radii around the center of the fovea, if they were scattered randomly. For example, just about a quarter of the elements are expected to fall within a central region of a diameter twice as small as the size of the stimulus. Unless these are diagnostic enough to provide sufficient evidence, elements residing in the stimulus periphery will have to be processed. The discriminability of these elements suffers willy-nilly; if the stimulus spans a large visual angle, they are large but peripheral; if it is viewed

from a distance, they are more foveal but may be very small.

Goodness of Pattern. The poor recognizability of global patterns made of asymmetrical elements was pointed out earlier. Another obvious determinant of the goodness of global patterns is the number of elements comprising them.

Does this counteract global precedence in real-world situations? Note that these factors may make global patterns less recognizable qua unitary patterns, yet the relationship they form may be quite transparent, and this may suffice as a cue for recognition of the whole. For example, the pattern in Figure 9 may not be readily categorized as a 'rec- tangle' but quite easily diagnosed as 'four elements arranged in a rectangular structure', which may precisely match the schematic encoding of one property of the face of a familiar playing card, thereby suggesting its presence. In other words, global precedence is a claim about order (or relative speed) of the processing of features. Features are not necessarily codable, but to enable experimental testing by search or choice RT one has to resort to the use of codable features. The problem is that the findings in the test are affected not only by the relevant characteristics of the features (i.e., globality) but also by their codability, which is irrelevant from a theoretical point of view. Thus tests that do not require coding may be more pertinent in some cases. Experiment IV

Fig. 9. An illustration of the salience of an arrangement that may not be apprehended as a unitary pattern

D. Navon 27

in Navon (1977) bears on this question, but i t is still open because of the possible bias

introduced by the mask he used (see Note 2).

Elements and Features

Should the local letters in Figure 5 be considered as local features? Two arguments can be raised against this, and at least one was raised by McLean (Ref. Note 1).

First, Navon pointed out that, strictly speaking, global precedence cannot be tested unless i t is known what the perceptual units are (1977, pp 354, 380). This is true for compound letters as well as for forests, words, human faces, etc. Does a local let ter constitute a perceptual unit of the compound letter? To answer this question we have to rely on our common sense reinforced by our knowledge of Gestalt laws of organization. These tell us that sometimes line-segments from neighboring stimuli configure, thereby destroying the separate perceptual existence of the stimuli. For example, several Hs one above the other may be perceived as a ' ladder ' when they are very close. If no local letter stands out as a separate enti ty, the inferiority of the response to the local level follows. However, an inspection of Figure 5 reveals that in most of the cases (except, perhaps, for the pattern in Panel A), a considerable number of elements are dear ly perceived as such. This is even more true of the stimuli used by Kinchla and Wolfe (1979) and McLean (Ref. Note 1) in which the elements were more spaced.

Second, one may contend that the local letters, albeit perceptually distinct, are not the features of the global letter; the features are horizontal and vertical bars, open curves, symmetries, etc. This is obviously true, and so i t is m e a n t to be. To explain this, we have to rephrase the logic underlying the usage of compound letters for testing global precedence, which was not spelled out in Navon (1977).

A stimulus is defined by its elements and their spatial relationship; e.g.,

Whole = R(Unit 1, Unit 2).

The issue of global precedence is whether while recognizing an instance of 'Whole', the global feature R is available before the local features, Unit 1 and Unit 2. Unfortunately, most natural stimuli are not suitable for testing global precedence for three reasons: 1) in such a stimulus the global structure may not be equally complex and recognizable as the elements ; 2) since all the features define the same object, a feature may be used to predict the ident i ty of another one; 3) the features may not be equally codable. Therefore, we have to use artificial stimuli which do not have these three shortcomings. But for the global structure to be as codable as the elements, it has to constitue a stimulus in its own right, regardless of what the elements are. In other words, there must be some category of stimuli A whose definitive description is

R (anything, anything). So the question of which level of features of a natural stimulus is available first is tested by devising a potential ly ambiguous 6 perceptual situation, whose alternative interpretations are

6 I say potentially, since it is not clear whether both interpreations can or cannot be entertained at once


A = R (anything, anything)

and

B = any-relation (a, a).

Of course, the a's are not the features of the A. The A is independent of the a, and so are their defining features. For example, the global H in Fig. 5A is characterized by a gross horizontal bar, the local H by a thin bar, If you wish, global precedence can be reformulated as the precedence of the features of a high-level unit over the features of

a lower-order one. The assumption of the experimental test is that the dominating interpretat ion will be

determined by the relative availability of the two levels at a given exposure duration: the a's will be taken to be faceless building blocks of an instance of A if the R is more available than the a's. From this oberservation it may be inferred that for a hypothetical observer who is not familiar with stimuli A and B, R will be available earlier than the a. If the observer is familiar with stimulus C whose definition is R (a,a), the R will be an earlier recognition cue. For example, if in the hypothet ical culture of that observer the digits of every large number were written as an S-shaped array, that arrangement would be perceived before the digits are, and might even drive an ,inquiry' for the digits.

It is quite understandable why in a culture in which the only familiar S-shaped pattern is the let ter S itself, the elements will not be ' inquired' once the S is detected; they

will be considered as molecules of material (Goldmeier 1972) rather than as distinct parts whose ident i ty matters. Thus, global precedence does not just state that elements ar-

ranged in certain ways are grouped and that their identi ty is defaced within the group, but rather explains why this should happen.

Finally, let me briefly comment about some shortcomings of the paradigm used in the experiment reported here. This seems like one of those cases in which an experimental paradigm that at tempts to control for several nuisance variables and to undo correlations that exist in nature ends up introducing other variables and correlations which may be quite problematic in themselves. For example, identical elements are used in order to equate the burden of processing at the local and global levels. Yet, this may have created mutual facihtation of the elements on the one hand, or a tendency to per- ceive the pattern as more unitary because of the homogeneity of the elements (eft. Garner 1978) on the other hand 7. Another problem is the delicacy of timing involved in testing global precedence by supra-threshold techniques. As stated above, if exposure duration is long, we might be testing attentional preferences rather than processing order. On the other hand, if it is short enough to render the local let ter legs perceptible than a single small letter and a global one (namely, less accurate even when premium is put on accuracy), then the use of other tests such as response interference is some- What gratuitous. When one also remembers that tasks involving focused at tention show an endemic sensitivity to practice, one must realize the limited usefulness of such tasks. More promising procedures are incidental perception tests (such as Experiment II in Navon 1977) or studies of how exposure duration interacts with the effect of globality

However, recent results (Miller, in press) indicate that global letters that are made of many different letters can be focused as successfully, and responded to as rapidly, as when they are made of identical letters

D. Navon 29

on accuracy or speed of responses to one of the levels, when the other one is neither

compatible nor conflicting with the response.

Summary

Global precedence was seen to claim that, other things being equal, global structure is available in the percept earlier than local features are. Whether this is due merely to sen-

sory factors or is regulated or augmented by top-down dispositions, such as resource-

allocation policy, is an open question. A partial temporal overlap between global and local processing is quite reasonable,

though hard to prove. Since global precedence is a hypothesis about microgenesis, it does not commit it-

self to a statement about relative salience in the final percept, but rather about the development or creation of the percept. Thus, a natural way to test it is to limit exposure

duration. Other ways, such as manipulating speed emphasis by instructions, necessitate

more assumptions. A direct prediction from global precedence is inferiority of local features with regard

to speed and/or accuracy relative to the same features presented in isolation or to equi- valent global features. To attempt to control for discriminability by equating latencies is

to treat a dependent variable as if it was a nuisance variable. It is recommended, instead, that special care be taken to equate possible stimuli in the two levels on their appearance.

The experimental results reported here lend some support to the hypothesis of global precedence. The small effects are attributed to the relatively long duration employed: 150 msec.

Possible mechanisms that may mediate global precedence are evaluated. Among them are relative size, lateral masking, diffuse attention, and spatial uncertainty.

The magnitude, or even the presence, of global precedence depends on some factors,

the most prominent of which are visual angle and retinal position. It is suggested that

the two were confounded in some studies, so that the effective range of global precedence

is yet to be determined.

Ecological considerations suggest that proximal sizes and eccentricities (of the entire

stimulus or of the elements relative to it) which favor global features are probably very frequent. On top of that, real-world stimuli contain many different elements. Thus, it may be concluded that the local level has a much better chance of emerging in the percept

early in some laboratory tests than it has in natural environments°

Finally, it is explained why the logic of employing compound letters requires the

use of elements that are not the features of the global letter; but some dissatisfaction with the external validity of this paradigm is expressed.

This research was supported in part by the Israeli Academy of Sciences and by the Faculty of Social Sciences of the University of Haifa. The manuscript was reviesed while the author visited the Cog- nitive Psychophysiology Laboratory at the Department of Psychology and the Center for the Study of Reading, University of Illinois ar Urbana Champaign. Its preparation was supported by AFOSR grant No. F 49620-79C-0233. I am indebted to Abraham Papo for his technical assistance, to Tami Eitan, Baruch Margalit, and Michal Sukenik for their assitance in conducting the research, to Joel Norman and two referees for helpful comments on a draft, and to many colleagues, too many to be exhaustively listed here, for stimulating talks or useful ideas and comments.


Reference Notes

1. McLean J P (1978) Perspectives on the forest and trees: The precedence of parts and wholes in visual information processing. PhD Thesis, University of Oregon

2. Navon D (1975) Global precedence in visual recognition. PhD Thesis, University of California, San Diego

3. Navon D (1976) On activation, evaluation, cursory verification, and other processing principles in visual recognition. Unpublished masucript, University of Haifa, Israel

4. Levin JA (1976) Proteus: An activation framework for cognitive process models. Working paper ISI/WP-2. Information Sciences Institute, University of Southern California, Los Angeles

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Received June 3, 1980/Accepted January 31, 1981

the forest revisited: more on global precedence

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