semantic category priming in the left cerebral hemisphere

~ Pergamon 0028 3932(95)00144-1

Neurop.sTchologia, Vol. 34, No. 5, pp. 33%350, 1996 Copyright (~ 1996 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0028-3932/96 $15.00 + 0.00

Semantic category priming in the left cerebral hemisphere

MARJORIE ABERNETHY* and JEFFREY CONEY

Murdoch University, Murdoch, 6150, Western Australia

(Received 20 December 1994; accepted 26 August 1995)

Abstract--The representation of semantic codes in the cerebral hemispheres and the interhemispheric communication of these codes, was investigated in two priming experiments where prime and target words were independently projected to the left or right visual fields (LVF or RVF). Nonassociated category exemplars were employed as related pairs in a lexical decision task and separated by a stimulus onset asynchrony of 250 msec in Experiment 1 and 450 msec in Experiment 2. Both experiments obtained priming effects when primes and targets were both projected to the RVF, but not the LVF. Semantic category primes projected to the RVF also facilitated responses to LVF targets, but no LVF-RVF priming was obtained. This suggests that semantic category information is relayed from left to right hemisphere, but not vice versa. The results are consistent with the view that semantic categories are represented in the left hemisphere.

Key Words: hemispheres; priming; semantic category.

Introduction

There has been a good deal of recent interest in the problem of how internal representations of inter- relationships between real-world entities are structured in the two hemispheres of the brain. Recent research has focused upon hemispheric differences in the use of two fundamentally different ways of organizing information: associative and semantic category relationships. The present study addresses the question of the representation of semantic categories in each cerebral hemisphere.

The particular problem which motivated the present study involves the question of which hemisphere is primarily concerned with the representation of links between entities related within the same semantic category. Sev- eral studies have suggested that the representation of semantic categories may be confined to memory structures in the left hemisphere. For example, it has been reported that projection of stimuli to the right visual field results in superior performance in semantic category matching [19, 36]. Klein and Smith [23] also found a right visual field advantage for category matching, although this did not emerge until the second block of trials when categories were repeated. A similar trend was observed

* Address for correspondence: Dr Marjorie Abernethy, Psy- chology Department, Murdoch University, Murdoch, 6150, Western Australia; e-mail: [email protected].

by Urcuioli et al. [44]. In a recent experiment, Nieto et al. [33] replicated Klein and Smith's study using a larger number of categories, but did not observe an interaction between right visual field advantage and repetition of category. Their general conclusion was that semantic category judgements were restricted to the left hemisphere. Even though the right hemisphere could recognize concrete words, their results suggested that this hemisphere was unable to judge whether they were members of the same semantic category.

Priming procedures have also been used to probe the differences between the hemispheres in relation to the representation of semantic categories. For example, Abernethy and Coney [1] used a priming procedure to investigate hemispheric differences in the properties of the lexicon and concluded that semantic category relationships prime the left, but not the right, hemisphere. Right visual field priming for semantic category relationships has also been reported by Hines et al. [20] and by Drews [16].

The studies cited above clearly support the notion that it is the left hemisphere which specializes in semantic category relationships. As we have argued previously [1], this finding is consistent with the conceptualization offered by Drews [16]. On the basis of her investigations of 'priming' in judgements of word relatedness, Drews has put forward the view that the hemispheres differ qualitatively in the organization of information. She pro-

339

340 M. Abernethy and J. Coney/Hemispheric priming

posed that the left hemisphere is organized according to an hierarchy of logical relationships described as 'intra- conceptual' (e.g. BUS-TRAIN). The right hemisphere, on the other hand, is organized in accordance with 'inter- conceptual' relationships (e.g. COFFIN EARTH). Levy and Trevarthen [27] have previously expressed a similar view, arguing that the left hemisphere is specialized for detecting conceptual-functional similarities (e.g. HAT- GLOVES), while the right hemisphere is able to derive associative meaning for words [26]. Finally, Urcuioli et

al. [44] have argued on the basis of their results that it is only in the left hemisphere that category members can be related to adequate superordinate terms.

The empirical and theoretical consistency which emerges from the above would seem to point to a power- ful consensus that it is the left hemisphere which is the natural site of semantic category relationships. However, Chiarello et al. [9] recently examined hemispheric priming for semantic category relationships and have reported results which contradict the view outlined above. For lexical decisions with lateralized prime and target, no priming was evident in either visual field for pairs related by association only. Priming was observed, to an equal degree in each visual field, for associates that happened to be members of the same semantic category. However, of particular interest from the present perspective was the finding that when word pairs were members of the same semantic category, but not associated with one another, priming was present only in the left visual field.

These findings were confirmed in a subsequent study by Chiarello and Richards [10] which investigated the possibility that category dominance of primes was instru- mental in influencing the direction and magnitude of lateral differences in priming. Chiarello and Richards systematically varied the category dominance of pairs projected to the left and right visual fields. They were also careful to ensure that their prime-target pairs were as free from associative links as possible. Their results showed no significant effects of category dominance, but priming was once again reliably obtained only in the left visual field. They interpreted this result as confirming their previous finding that semantic category priming was confined to the right hemisphere.

These results clearly contradict the view that the left hemisphere is primarily responsible for representing relationships between entities which belong to the same semantic category. In seeking an explanation for this contradiction, we looked more carefully at the idea, suggested by the results of Chiarello et al. [9], that the hemispheres may respond differently to semantic category and associative relationships. This idea is in agreement with observations in split brain patients of a right hemisphere capacity for generating associative, but not categorical responses to stimulus words [18]. It is also noteworthy, in this respect, that Neely [31] has suggested that a clear distinction needs to be made between the representation of semantic category and associative relationships in the lexicon, since priming effects may be highly dependent

upon the precise status of the relationship between two linguistic entities. These considerations are relevant to the present problem, because studies of priming in the hemispheres have frequently confounded categorical and associative relationships. For example, in our own inves- tigation of semantic priming in the hemispheres [1], we failed to control for degree of association between semantic category pairs. Consequently, the priming effects we observed may have reflected priming facilitation engendered by an associative relationship of the pairs in the stimulus set, rather than categorical priming per se.

The exemplar pair provided by Chiarello [8] as an example of her semantic category set also appears associated (INCH-YARD), which suggests that association was not controlled in that study either. It is possible that inad- equate control of the distinction between associative and semantic category relationships may have distorted the outcome in other priming studies as well [e.g. 7, 11, 17, 20, 28, 45].

A further factor that may be relevant to the difference between the results reported by Chiarello et al. and other studies involves the temporal separation between prime and target. Abernethy and Coney [2] found that associative priming in the hemispheres varied significantly as a function of the interval separating prime and target. When associative pairs were projected to the left or right visual field, and the stimulus onset asynchrony (SOA) between prime and target was 250 msec, it was observed that priming occurred in the right visual field, but not in the left. However, when the SOA was increased to 450 msec, significant levels of priming were observed in both visual fields. We interpreted this result to mean that lexical representations are activated more slowly in the right hemisphere than in the left.

It is interesting, in this context, to note that Chiarello and Richards [10] employed an SOA of 600 msec, while Chiarello et al. [9] used 575 msec. Our study of semantic category priming [1], on the other hand, utilized an SOA of only 250 msec. Although we have not investigated priming in semantic category pairs as a function of SOA, it is clearly not unreasonable to ask whether the right hemisphere responds in a manner similar to that observed for associative priming. Given the magnitude and nature of the changes we observed in associative priming over a range of 250450 msec and the range under consideration at the moment (250-600 msec), it might be too much to expect that timing differences can entirely account for the disparity between our results and those obtained by Chiarello et al. However, it is possible that timing might explain part of the difference between the two sets of results.

We thus set out in the present study to determine whether eliminating associative links from semantic category pairs and manipulating stimulus onset asynchrony, would reveal results that could point the way to a resolution of the conflicting findings observed in this area. We conducted two separate experiments to resolve these questions.

M. Abernethy and J. Coney/Hemispheric priming 341

Experiment 1

The first experiment was designed to examine categorical priming in the left and right hemispheres using a stimulus set comprised o f nonassociated category exemplars, where the number o f pairs drawn f rom each semantic category was minimized. The latter methodological control was implemented for two reasons: repetition o f category may interact with hemispheric superiority [23], and it has been suggested that categorical priming restricted to the R V F may occur when a small number o f categories are repeated [10]. Al though our previous study [1] drew 128 related pairs f rom a large number o f categories (i.e. 58) we did not consistently control the number o f pairs per category. Twenty-seven pairs were drawn f rom unique categories, but one category had eight exemplars drawn f rom it and another had seven. In the present experiment, no more than four exemplar pairs were drawn f rom any one category.

Several steps were also taken to ensure that any priming effects could be interpreted in terms o f the automat ic spread o f activation through the hemispheres. Using Seidenberg et al.'s [38] method of calculating stimulus set proport ions,* related word pairs comprised a low propor t ion o f the total stimulus set (i.e. 25%), a short SOA of 250 msec was employed and instructions were specifically framed to avoid the possibility o f subjects drawing the conclusion that categorical relationships were relevant to their performance of the task. A G O - N O G O response procedure was adopted in which no overt response was required for nonword targets. This curtails subjects' use o f a semantic matching strategy and ensures that response measures reflect lexical access mechanisms rather than post-lexical meaning integration [31]. This procedure also negates problems that may otherwise be associated with the relatively high proport ion o f related stimulus pairs in the positive set [31].

The additional data limitations imposed when stimuli are lateralized [39] were also expected to slow lexical access, increase task difficulty and constrain controlled processing. Taken together, these design constraints should minimize contextual effects associated with postlexical processing [31,34, 35, 38].

Method

Subjects

Twenty-eight undergraduate psychology students were selected as subjects. Data for six of these subjects were discarded as one moved his eyes from fixation on several trials and error

*Two methods of calculating proportion are commonly adopted in priming research. Seidenberg et al. [38] calculate the proportion of related pairs relative to the total stimulus set, while deGroot [ 15] measures proportion relative to the positive set only. The former method was adopted in this study.

rates for five exceeded 30% in either the word or nonword condition. Twelve female and 10 male subjects remained, their mean age being 21.5 yr. All had normal or corrected-to-normal vision, were predominantly fight-handed according to Bryden's [6] simplified hand preference questionnaire (mean handed- hess quotient: +0.743; S.D.: 0.249) and English was their first language.

Apparatus

Subjects were tested in a well-lit cubicle room containing a modified CBM 4032 microcomputer system which controlled trial sequencing, stimulus presentation, timing and data collec- tion. The onset and offset of all stimuli was controlled with a circuit that allowed the screen to be written while blank and then 'flashed' on (or off) within a single raster scan. Screen intensity was diminished to the minimum level of the factory capability to decrease phosphor persistence. Reaction time (RT) was measured to a resolution of I msec via a centrally positioned microswitch response box connected to the user port of the microcomputer. The stimuli were printed in green capital letters on a high-bandwidth monitor. Each letter was 3 mm wide and 4 mm high, with 1 mm spacing. Eye movements were monitored by a Sanyo video camera which was connected to a video monitor which provided a magnified view of the subject's eyes. A chin rest was used to stabilize the subject's head in the correct position relative to the screen. EMH12 high performance ear- muffs were used to minimize any noise interference.

Des&n

RT was chosen as the principal dependent variable as it is a more sensitive and direct measure of hemispheric processing than accuracy [3, 37, 46]. Errors were also recorded and analysed. A lexical decision task was used which required subjects to discriminate words from nonwords using a GO -NOGO response procedure. The first experimental variable was stimulus pair relationship, incorporating four prime-target conditions. The related-word condition consisted of 128 exemplars from the same semantic category (e.g. OAK-MAPLE; LEMON-PEAR). Exemplars were chosen from 57 different categories in published category norms [4, 40]. No more than four exemplar pairs were chosen from any one category. Associ- ative relationship was controlled by selecting only those pairs that did not appear as associates in association norms [29, 41, 42]. For prospective exemplar pairs that were not included in these norms, associative responses were collected from 30 psychology students. Only those pairs which were never given as associates were included in the stimulus set. All related word pairs used in this experiment can be found in the Appendix. A further 128 pairs formed the baseline condition. This condition comprised the prime 'BLANK' paired with each of the word targets from the related set (e.g. BLANK-MAPLE).

For the negative set, 128 additional primes were each paired with an orthographically legal, pronounceable nonword target which was matched in length with the targets in the positive set (e.g. FROG-TEVE; CHAPEL-WOTUB). These primes were matched with the primes in the related-word condition with respect to word length, grammatical category and frequency. The nonword targets were also paired with the word 'BLANK' to form 128 pairs which mirrored the neutral condition in the positive set (e.g. BLANK-TEVE).

All pairs were phonemically dissimilar and none began with the same phoneme. Orthographic similarity was minimized by ensuring no pairs had more than two letters in common and this was never the first letter. All words were concrete, imageable


nouns. All words and nonwords were within a range of three to seven letters in length, most being five letters. The mean frequency of primes in the positive and negative sets was 31.6 and 31.4 words per million respectively [24].

Visual field of presentation was also manipulated, and comprised four levels: (i) prime and target to the RVF (henceforth referred to as RVF-RVF); (ii) prime to the LVF and target to the RVF (LVF-RVF); (iii) prime and target to the left visual field (LVF LVF); (iv) prime to the RVF and target to the LVF (RVF-LVF). Each stimulus pair was presented in only one visual field condition. Selection of visual field for each pair was randomly determined, as was order of presentation of word pairs in each condition. Thus, each subject was exposed to a unique distribution of pairs in each visual field, and a different sequence of prime-target conditions. Each stimulus pair was presented once only.

Procedure

Subjects were seated in front of the display monitor with their heads positioned by a chin-rest directly in front of, and 45 cm distant from, the centre of the screen. Task instructions emphasized the necessity of maintaining fixation on the central fixation cross during trial presentation. Subjects were given 64 practice trials with the same structure as the experimental trials. No stimuli from the experimental set were used. Feedback relating to accuracy was provided after each practice trial. If the video monitor revealed any deviation of eyes from fixation during the practice trials, subjects were reminded of the import- ance of maintaining fixation at all times.

Each trial began with a central fixation cross which remained on throughout the trial. After 750 msec, a prime word was displayed in the LVF or RVF for 150 msec. Two hundred and fifty msec after the prime appeared, the target was presented to the LVF or RVF for 150 msec. The target appeared 5 mm beneath the display location of the prime to minimize masking effects. The fixation cross then disappeared and the entire screen remained blank for 1500 msec, during which the subject sig- nalled a response. All stimuli were presented 2 degrees of visual angle from the central fixation and subtended a horizontal visual angle of between 2 and 6 degrees. Randomization of trials ensured that subjects were unable to predict the visual field of presentation of either prime or target.

Subjects responded in accordance with a GO-NOGO procedure. When the target was a word, they were required to respond by simultaneously depressing two centrally positioned microswitches with both index fingers, the faster of the two responses being taken as RT for that trial. When the target was not a word, they were required to withhold their response. Subjects were permitted 1500 msec after the disappearance of the target to respond. Failure to respond within 1500 msec was treated as a NOGO response. Following trials in which an incorrect response was made, the word 'ERROR' appeared directly above the fixation. Subjects were permitted to rest after each block of 64 trials and were given feedback regarding their accuracy and overall speed for the preceding block. They were also encouraged to maintain an error rate of less than eight per block. Sessions required approximately 25 min to complete, excluding rest periods.

Results

Statistical analyses were carried out on mean correct reaction time (RT) for the positive set (no RT data were available for the negative set as the response procedure

was G O - N O G O ) . A two-way analysis of variance ( A N O V A ) ( 2 x 4 ) was computed on stimulus pair relationship and visual field o f presentation. The main effect for stimulus pair relationship was significant IF( I , 21) = 10.43; P = 0.004] revealing an overall facili- tat ion o f 19 msec for related pairs relative to baseline pairs. The main effect for visual field of presentation [F(3, 6 3 ) = 11.36; P < 0.001] reflected the consistently faster responses to R V F targets (see Fig. 1). This is consistent with previous findings o f a left hemisphere advantage in lexical decision [e.g. 1,2, 12, 25].

The interaction between stimulus pair relationship and visual field of presentat ion [F(3, 63) = 8.68; P < 0.001] is of pr imary interest here, as it reflects priming differences in the hemispheres. Four related samples t tests were used to carry out planned comparisons between each related condit ion and its baseline. Two of these comparisons were significant. RT to R V F targets was 43 msec faster when preceded by a category exemplar in the same visual field [t(21) = 5.22; P < 0.05]. Likewise, a 34 msec facilitation was evident for R V F - L V F presentations [t(21) = 3.4; P < 0.05]. However, the 12 msec advantage favouring related pairs did not reach significance for L V F - R V F presentations [t(21) = 1.22; P > 0.05], nor did the 15 msec inhibition in the L V F - L V F condit ion It(21) = 1.57; P > 0.05]. It must be noted here that the stability of latencies for the neutral baseline conditions were similar to those in the related conditions (see Table l). Chiarello et al, [9] in contrast , reported that latencies

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Fig. 1. Mean correct RT for nonassociated category exemplars and neutral word pairs, as a function of visual field, at SOA

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M. Abernethy and J. Coney/Hemispheric priming

Table 1. Standard deviations associated with RT means at SOA 250 msec

Visual field of presentation

343

LVF-LVF LVF RVF RVFRVF RVF LVF

Stimulus pair Related 81.6 75.89 69.35 89.65 Neutral 84.39 62.28 85.6 100.2

for their neutral condition were more variable. This may be attributable to the small number of observations in their neutral condition [i.e. 14].

These results illustrate that projection of nonassociated category exemplar primes to the left hemisphere facilitated responses to targets subsequently projected to the left or to the right hemisphere, whereas projection of these primes to the right hemisphere did not facilitate responses to targets subsequently projected to either hemisphere. It appears that under the present experimental conditions, the left hemisphere is primed for semantic category relationships, whereas the right hemisphere is not primed for this type of relationship between words (refer to Fig. 1).

Error rates for both the positive and negative sets were also analysed by means of a two-way ANOVA (4 x 4). The two main effects reached significance. The main effect for stimulus pair [F(3,63) = 17.13; P < 0.001] reflected the greater accuracy for the word target conditions relative to the nonword target conditions (12.59 and 19.74% errors respectively). The main effect for visual field was also significant [F(3, 63) = 15.82; P < 0.001]. As with the RT data, it is apparent that projection of the target to the RVF results in better performance than projection to the LVF (see Table 2). In contrast to the interaction between these factors in the RT data, there was no interaction between stimulus pair and visual field of presentation [F(9, 189) = 0.77; P = 0.64]. No speed- accuracy tradeoffs were evident in the data.

Discussion

No support was found for Chiarello et al.'s findings [9, 10] of categorical priming restricted to the right hemisphere. Rather, priming facilitation was obtained when semantic category pairs were projected to the right visual field, but not when they were projected to the left visual field. This is consistent with our own previous findings [1] even though categorical and associative relationships were confounded therein. Notably, the magnitude of facilitation for RVF-RVF presentations in this experiment is very similar (at 43 msec) to the 50 msec facilitation we observed previously.

As in our previous study, cross-hemisphere facilitation was also found for RVF-LVF presentations. Apparently, a prime received by the left hemisphere can activate categorically related words in the right hemisphere, even though a prime projected directly to the right hemisphere cannot activate these exemplars. Of particular interest in the present experiment is the absence of priming for LVF- RVF presentations. The projection of a prime to the right hemisphere failed to facilitate responses to categorically related targets subsequently presented to the left hemisphere. Abernethy and Coney [1], in contrast, found facilitation for this condition. In that case, the right hemisphere was able to activate semantic representations in the left hemisphere, although it was unclear whether the source of this activation was the categorical or associative relationship between stimulus pairs. The present experi-

Table 2. Percentage error for word and nonword pairs in each visual field at SOA 250 msec


LVF-LVF LVF-RVF RVF-RVF RVF-LVF

Stimulus pair Related word 15.05 10.08 6.68 14.49 Neutral word 17.897 12.07 7.81 16.76 Word/nonword 22.58 20.45 17.89 22.44 Neutral/nonword 20.45 17.61 15.06 21.45

Mean error 18.99 15.05 11.86 18.78


ment included only nonassociated category exemplars, which has resulted in LVF-RVF facilitation being elim- inated. Evidently, the right hemisphere is unable to activate categorically related words in the left hemisphere. This is in spite of the fact that the left hemisphere is capable of generating semantic exemplars from a prime, as observed by the facilitation found for the RVF RVF condition. The indication is that the right hemisphere activates semantic exemplars in the left hemisphere only when they are also associates. This is quite remarkable, considering the left hemisphere's superiority in linguistic processing and the consistent RVF advantage observed in our previous research [1,2]. Evidently, a prime projected to the right hemisphere activates associative, but not categorical information in the left hemisphere, as previously suggested [2].

Chiarello and Richards' [10] suggestion that categorical priming restricted to the RVF occurs only when "repeatedly presenting items from a small number of categories" (p. 389) is not supported by the present experiment either. We used a large number of categories, with only one or two exemplar pairs drawn from most categories, but no more than four pairs selected from any one. The effects of category repetition were reduced even further by ensuring that each stimulus pair was only presented once, and selection of the sequence of stimulus pair presentation was randomized so that each subject was exposed to a unique combination of pairs in each visual field as well as a different sequence of stimulus pair conditions. Taken together, these factors minimize any effects of category repetition. This procedure is com- parable to that adopted by Chiarello and Richards [10] wherein only one stimulus pair from a given category was selected, but each category was repeated by presenting pairs once in each visual field. Even though the number of category repetitions is similar in the two studies~ no hint of LVF-LVF priming was evident in the present experiment whilst it was in the latter. Therefore, it seems unlikely that differences in the number of category repetitions can account for the discrepancies between our research and Chiarello et al.'s [9, t0]. This parallels Nieto et al.'s [33] failure to replicate Klein and Smith's [23] interaction between RVF advantage and repetition of category.

In view of this, it appears more feasible that one salient factor contributing to the discrepancy between these studies is the difference in temporal separation between prime and target. Right hemisphere priming may have failed to occur in the present experiment as activation within this hemisphere did not have sufficient time to spread to category exemplars within the short SOA used. Burgess and Simpson's [7] findings suggest a similar explanation may also be relevant to left hemisphere priming effects. For word pairs associated through their sub- ordinate meaning, they found left hemisphere facilitation at a short SOA of 35 msec, whereas an inhibitory effect was present at an SOA of 750 msec. However, left hemisphere priming for pairs associated through their domi-

nant meaning was equal at the two SOAs. This suggests that for certain types of word pair relationships, left hemisphere priming differs for short and long SOAs. It is possible that we obtained left hemisphere priming for category pairs as we used a shorter SOA than Chiarello et al. [9, 10].

Experiment 2

Perhaps right hemisphere priming for category exemplars becomes evident at longer SOAs, while left hemisphere priming decreases. The second experiment investigated this using nonassociated category exemplars separated by an SOA of 450 msec. This SOA was chosen for two reasons. First, Abernethy and Coney [2] dem- onstrated that priming within the right hemisphere changes significantly between 250 and 450 msec. At an SOA of 250 msec, we failed to find right hemisphere priming for associatively related pairs. However, right hemisphere priming was evident when the SOA was increased to 450 msec. Secondly, SOAs less than 500 msec are recommended when interest is focused upon automatic processing [e.g. 15, 32], as in the present study.

Method

A second group of 27 undergraduate psychology students acted as subjects. Data for four subjects were discarded as their error rates exceeded 30% in either the word or nonword condition. Thirteen females and 10 males remained, their mean age being 20.5 yr. The selection criteria used in the first experiment were also applied in this experiment. All subjects were predominantly right-handed (Bryden's [6] mean handedness quotient: +0.68; S.D.: 0.176). Apart from the SOA of 450 msec employed to separate prime and target presentation, this experiment was identical in all other respects to the first.

Results

A two-way ANOVA (2 x 4) was computed on mean correct RT for stimulus pair relationship and visual field of presentation. The main effect for stimulus pair relationship [F (1 ,22)= !1.67; P = 0.002] revealed an overall facilitation of 27 msec for related pairs relative to baseline pairs. The main effect for visual field of presentation [F(3, 66) = 24.89; P < 0.001] reflected, as in the first experiment, the consistently faster responses to RVF targets (see Fig. 2). However, there was no interaction between stimulus pair relationship and visual field of presentation [F(3, 66) = 1.51; P = 0.22].

Consistent with the first experiment, four related-samples t tests were used to carry out planned comparisons between each related condition and its baseline. Sig- nificant priming facilitation of 39 msec was found for RVF-RVF presentations [t(22) = 3.36; P < 0.05], and a 29 msec facilitation was obtained for the RVF-LVF con-


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Fig. 2. Mean correct RT for nonassociated category exemplars and neutral word pairs as a function of visual field, at SOA 450

msec.

dition [t(22) = 2.08; P = 0.050]. However, for LVF-RVF presentations the 23 msec advantage favouring related pairs did not reach significance [t(22) = 1.74; P > 0.05] nor did the 3 msec advantage favouring related pairs for LVF-LVF presentations [t(22)= 0.33; P > 0.05]. Thus, like the first experiment, the RT data do not show any evidence of priming within the right hemisphere for nonassociated category exemplars (see Fig. 2).

Error rates for the positive and negative sets were also analysed by means of a two-way ANOVA (4 × 4). Unlike the first experiment, there was no main effect for stimulus pair [F (3 ,66 )= 1.81; P=0 .153] . Although responses were most accurate for the related word condition (at 16.36% errors), accuracy for the neutral word condition (at 19.8% errors) was similar to that in the word-nonword condition (at 19.2% errors), whilst 17.36% errors were made in the neutral-nonword condition. Like experiment 1, there was a main effect for visual field of presentation [F(3, 66) = 20.9; P < 0.001]. Fewer errors occurred when the target was presented in the RVF. There was an interaction between stimulus pair condition and visual field of presentation [F(9, 198) = 4.11; P < 0.01]. For RVF- RVF, RVF-LVF and LVF-RVF presentations, accuracy appeared greater in the related word conditions than the baseline word conditions. However, in the LVF-LVF condition, responses were more accurate for the nonword than the word conditions (see Table 4). These data are generally consistent with the RT data. No speed- accuracy tradeoffs were evident.

General discussion

The results of the second experiment were consistent with those of the first. One question addressed here was whether priming for nonassociated category exemplars would be evident within the right hemisphere at an SOA of 450 msec, in view of the fact that it was absent at SOA 250 msec. Presentation of a prime to the LVF did not facilitate responses to categorically related targets subsequently presented to the LVF 450 msec later. This suggests that under the present experimental conditions, projection of primes to the LVF does not activate categorical exemplars in the right hemisphere. Indeed, the absence of facilitation for this condition strongly sup- ports the view [1,44] that the right hemisphere is not sensitive to the relationship between category exemplars. In contrast, priming facilitation for RVF-RVF presentations was evident at both SOAs. Moreover, the magnitude of this facilitation in the second experiment (39 msec) was very similar to that found in the first (43 msec). This suggests that in the left hemisphere, the absolute magnitude of priming facilitation for category exemplars remains the same for intervals spanning 250-450 msec. This parallels our previous observations with associated pairs [2].

No support was found for Chiarello et al.'s [9, 10] findings of categorical priming restricted to the LVF. Rather, categorical priming was restricted to the RVF at SOAs of 250 and 450 msec. This is consistent with studies indicating that the left hemisphere specializes in semantic category relationships [e.g. 20, 33]. Hence, it is more likely that procedural differences other than the temporal separation between prime and target account for the discrepancies in priming research. The choice of a baseline is one candidate. The present study used a neutral baseline against which to compare facilitation produced by related pairs, whilst Chiarello et al. [9, 10] used an unrelated baseline. Although there was no significant difference between RTs for neutral and unrelated trials in their earlier study [9], it is difficult to ignore the fact that their priming effects change dramatically if calculated using their neutral conditions as the baseline: for nonassociated categorical pairs, the 55 msec facilitation for LVF-LVF presentations is reduced to a mere 14 msec, whilst for RVF-RVF, the 9 msec inhibition becomes 31 msec facilitation. In the latter condition, it is noteworthy that RTs for the unrelated condition were 40 msecfaster than those for the neutral condition. These 'recalculated' values are more consonant with our results, particularly for RVF- RVF presentations. It is not possible to make the same comparisons for the later study, as no neutral condition was used. It appears, then, that the choice of baseline may be critical in the interpretation of hemispheric priming effects.

The appropriate baseline for priming research has been debated for some time: unrelated primes can never be truly neutral, yet they alert subjects equally across trials; neutral primes such as 'BLANK' are neutral with respect


Table 3. Standard deviations associated with RT means at SOA 450 msec


LVF LVF LVF-RVF RVF-RVF RVF-LVF

Stimulus pair Related 99.07 93.59 65.84 98.48 Neutral 83.76 78.83 87.89 109.47

Table 4. Percentage error for word and nonword pairs in each visual field at SOA 450 msec


LVF-LVF LVF-RVF RVF RVF RVF-LVF

Stimulus pair Related word 24.87 I 1.82 11.23 17.53 Neutral word 27.03 15.08 13.04 24.05 Word/nonword 22.15 16.03 18.88 19.84 Neutral/nonword 19.7 16.44 15.76 17.53

Mean error 23.45 14.84 14.73 19.74

to the prime, yet their encoding time and ability to alert subjects decreases as the number of repetitions increases [22, 31]. After extensive consideration, Neely [31] con- cludes that if the focus of a priming study is the relative magnitude of facilitation or inhibition across different experimental conditions, as in the present study, then neutral is acceptable as a baseline. He notes that at short SOAs, this may mean that inhibition estimates are over- estimated whilst facilitation is underestimated. Con- sistent with Neely's suggestions and our previous observation [1] that there were no significant differences between RTs for unrelated and neutral conditions, we adopted the latter as a baseline. In that study, priming facilitation for RVF RVF presentations was 50 msec relative to neutral and 46 msec relative to the unrelated condition; for L V F - L V F presentations, facilitation was 14 msec relative to neutral and 10 msec relative to unrelated. The same trend was obtained in the cross-hemisphere conditions: R V F - L V F facilitation was 22 msec and 27 msec relative to neutral and unrelated baselines respectively, and 21 msec compared with 24 msec for LVF RVF presentations. The close correspondence in priming estimates when assessed relative to neutral or unrelated baselines encouraged us to omit the latter from the present design, in order to reduce the number of trials from 768 to a more manageable 512. In contrast, Chiarello et al. [9] used an unrelated baseline, as latencies for their neutral condition were more variable than those in the related and unrelated conditions. We have not experienced the same difficulties in our research: RTs were equally stable in the related and neutral conditions (see Tables 1 and 3). This may be attributable to the number of observations per condition in these studies. Consistent with Bradshaw and Nettleton's [5] recom-

mendations for obtaining reliable data in divided visual field studies, we have used 32 observations per condition in our research. This is more than twice as many observations per condition than Chiarello et al. It is feasible that this resulted in more reliable data for our neutral condition in particular. Even so, although choice of baseline may be an important factor in priming research, the similarity we previously observed between priming when estimated relative to neutral or unrelated baselines [1] indicates that this factor alone cannot account for the discrepancies in this area.

Another pertinent factor may be the proport ion of related pairs in the stimulus set. Chiarello [8] examined semantic category priming under automatic and controlled conditions, wherein related pairs comprised 10 and 30% of the total stimulus set respectively. For the automatic condition, the magnitude of RT priming was significantly larger when prime and target were both projected to the LVF (61 msec) than when projected to the RVF (17 msec). For errors, automatic priming was equivalent in each visual field. In contrast, the magnitude of semantic priming for RTs was equivalent across the hemispheres for the controlled processing condition, while for errors a significantly larger priming effect was obtained for R V F - R V F presentations. Clearly the conclusions drawn from these results will differ according to the dependent variable selected. Even so, Chiarello based her conclusions for the automatic condition upon the RT data, whereas the error data were used for the conclusions pertaining to controlled processing. This is dubious: the choice of dependent variable should be consistent across comparisons, particularly where there are inconsistencies between the two. This anomaly, in conjunction with methodological differences (e.g. always projecting prime


and target to the same visual field) preclude a direct comparison with the present findings. Further, Chiarello did not ascertain whether the 17 msec facilitation for RVF-RVF presentations was significant. Rather, the statistical analysis compared the relative magnitude of priming for RVF and LVF presentations.

Nonetheless, the hemispheric priming differences obtained by Chiarello [8] when varying the proportion of related pairs in the stimulus set may provide a key to unravelling discrepancies in priming research. In her study, larger right hemisphere priming for semantically related pairs was associated with a smaller proportion of related pairs. Other studies which have found priming restricted to the right hemisphere have also used very small proportions of related pairs: 13.33% [10] and 9% [9]. Categorical priming restricted to the left hemisphere has been obtained in studies using a larger proportion of related pairs: 16.66% [1], 25% [current study] and 30% [8]. This raises the possibility that the proportion of related pairs in the stimulus set may differentially influ- ence right and left hemisphere categorical priming. How- ever, it seems likely that proportion interacts with another variable to produce these differences. After all, in the study using the smallest proportion of related pairs [i.e. 9], if priming is measured relative to neutral rather than the unrelated condition, a larger facilitation is present for RVF-RVF presentations. In view of the interaction between SOA and the proportion of related pairs in a stimulus set [15], and the observation that either of these factors can be manipulated to produce controlled or automatic processing, it is possible that they jointly contribute to the differences observed in these priming studies. Even so, it must be noted that Chiarello et al. [9] have the smallest proportion of category pairs, and a relatively long SOA of 575 msec, yet they have left hemisphere priming if calculated relative to the neutral baseline. This is the same outcome as the present study which used a larger proportion of related pairs (25%) and SOAs of 250 and 450 msec. We are currently investigating whether priming in each hemisphere interacts with SOA and the proportion of related pairs in the stimulus set.

Another pertinent factor that may contribute to the differences between the studies is that Chiarello et al. [9,10] did not include cross-hemisphere priming conditions, but presented related pairs only to the same visual field. Crossed visual field conditions received only unrelated word pair filler and nonword trials, while an earlier study presented all pairs in the same visual field [8]. Our research [1,2] clearly underlines the significance of interhemispheric communication in this task, and the present results provide further insight into the nature of interhemispheric priming for category exemplars. The priming facilitation observed for RVF-LVF at SOAs 250 and 450 msec demonstrates that semantic category information is relayed from left to right hemisphere. Evidently, a word presented to the left hemisphere activates semantic category information in the right hemisphere. The absence of any facilitation for LVF-RVF presentations

at these SOAs indicates that a word presented to the right hemisphere is unable to activate categorically related words in the left hemisphere. This is consistent with our previous studies which suggest that associative, but not semantic category information is relayed from right to left hemisphere [1,2]. Furthermore, it suggests that the LVF-RVF priming observed by Marcel and Patterson [28] arose because of their use of word associations.

In conclusion, the results of the present study further emphasize the need for a distinction to be made between the hemispheric representation of semantic category and associative relationships. The facilitation found in the present study when prime and target were both projected to the left hemisphere provides further corroborative evidence in support of those studies which have indicated that the left hemisphere is organized in a format sensitive to semantic category information [e.g. 1, 16, 20, 33]. Taken in conjunction with our previous finding of left hemisphere representation of associative information [2], it appears that the generally superior verbal capacity of the left hemisphere allows it a versatility which permits it to encompass both semantic category and associative relationships within the lexicon. In view of unresolved discrepancies in hemispheric priming research, it is premature to draw stronger conclusions pertaining to the characteristics of left and right hemisphere lexical representation. It must be noted here that inconsistencies often occur in hemispheric research. For instance, findings of a right hemisphere capacity for processing concrete, imageable words, but not abstract, low imagery words [e.g. 13, 14] have proved difficult to replicate [30]. Inconsistencies are also prevalent in studies of the lateralization of Chinese characters (Ref. [2l] compared with [43]) and emotion [6].

The results of these experiments do permit some com- ment upon the character of interhemispheric communication. Semantic category information appears to be relayed between the hemispheres. The cross-hemisphere effects observed in Abernethy and Coney [I, 2] suggested that semantic/associative information is relayed from both left to right, and from right to left hemisphere. The present study allowed a closer examination of whether the salient aspects of interhemispheric communication relate to the categorical or the associative aspects of the prime. After eliminating associated category exemplars from the related set, relay of semantic category information was found to occur from left to right hemisphere, but not from right to left. It appears the left hemisphere can transfer both semantic category and associative information relating to a prime, whilst the right hemisphere transfers only associative information [2].

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A P P E N D I X

Related word pairs used for Experiments 1 and 2

Related primes F Target

Part of a living room COUCH 12 RUG CARPET 13 FLOOR

Part of a bed PILLOW 8 SHEET

Part of a building ATTIC 16 HALL WINDOW 119 ROOF

An alcoholic bererage BEER 34 GIN RUM 3 ALE

A ,fish TROUT 4 COD SHARK 1 EEL

A tree OAK 15 MAPLE CEDAR 1 GUM FERN 1 PALM WILLOW 9 ELM

A wild animal JAGUAR 5 LION TIGER 7 WOLF BEAR 57 MOOSE

An article of clothing JUMPER 1 VEST GLOVE 9 SCARF JACKET 33 PANTS

A musical instrument PIANO 38 FLUTE TUBA 1 DRUM


A type ofseaJood OYSTER 6 CLAM LOBSTER 1 PRAWN SHRIMP 2 CRAB

An eating utensil KNIFE 76 SPOON GLASS 99 CUP

An electrical appliance CLOCK 20 RADIO IRON 43 MIXER

An instrument of war TANK 12 BOMB RIFLE 63 CANNON

A fruit CHERRY 6 PLUM RAISIN 1 APPLE MANGO - PRUNE GRAPE 3 MELON

A building material WOOD 55 BRICK PLASTER 23 CEMENT

A weapon DAGGER 1 GUN SWORD 7 PISTOL

A kitchen utensil CUP 45 DISH SAUCER 2 BOWL

A type of building HOUSE 591 GARAGE

A rodent HAMSTER - MOUSE RABBIT 11 MOLE

A part of the face EYE 122 NOSE MOUTH 103 CHIN EAR 29 CHEEK

Part of a kitchen OVEN 7 SINK STOVE 15 FRIDGE

A pet CAT 23 BIRD

A musical string instrument GUITAR 19 CELLO VIOLIN 1 HARP

A flower PANSY 6 TULIP DAISY 3 LILAC ROSE 86 LILY

An article of furniture DESK 65 LAMP CHAIR 66 STOOL

An article of male clothing SHIRT 27 COAT SUIT 48 TIE

A kind of juice APPLE 9 LEMON

A liquid SODA 3 COKE JUICE 11 MILK

A part of a boat STERN 23 KEEL

A kind of meat BEEF 32 PORK

A cleaning instrument CLOTH 43 RAG BUCKET 7 SPONGE



An item used by smokers C I G A R 20 PIPE

A milk product C R E A M 20 M I L K C H E E S E 9 B U T T E R

A ,]bur-footed animal Z E B R A 1 M U L E D E E R 13 ELK G O A T 6 SHEEP STAG 8 DOE

Part of the human body ELBOW 10 K N E E H A N D 431 TOE A N K L E 8 HIP H E A R T 173 L U N G

A type of human dwelling CABIN 23 T E N T C O T T A G E 19 H U T S H A C K 1 H O M E

A dessert C A K E 13 PIE C A N D Y 16 F U D G E

An item used to fasten clothing B U T T O N 10 ZIP H O O K 5 PIN

A religious article BIBLE 59 CROSS

A kind of money C E N T 158 D I M E

An item o['of, fice supplies PEN 18 E R A S E R

A weather phenomenon SNOW 59 H A I L SLEET 1 FOG F R O S T 6 R A I N

An article of['emale clothing BLOUSE 1 DRESS

A kind of seasoning G A R L I C 4 O N I O N S U G A R 34 N U T M E G

A vegetable PEA 24 C O R N BACON 10 P O R K PIE 14 T A R T C A R R O T 1 BEAN

A O'pe oJi[botgear S A N D A L 5 BOOT C L O G 2 T H O N G


A carpenter's tool H A M M E R 9 AXE CHISEL 4 SAW PLIERS l N U T D R I L L 33 BOLT

A type o f vehicle T R A I N 82 JET P L A N E 114 SHIP JEEP 16 TAXI BIKE - C A R

An insect BEETLE 1 FLY L O C U S T 6 G N A T SNAIL 1 W O R M T E R M I T E -- A N T

A nonalcoholic beverage C O C O A 2 TEA

A bird ROBIN 6 W R E N P A R R O T 1 DOVE STORK - G U L L RAVEN - OWL

A precious stone J A D E 1 OPAL PEARL 9 R U B Y

A type o f reading material BOOK 193 C O M I C

A natural earth Jbrmation L A K E 54 P O N D G O R G E 1 CAVE C L I F F 11 O C E A N SWAMP 5 R I V E R

A toy T E D D Y 4 D O L L P U Z Z L E 10 C A R D

A O'pe o[ship Y A C H T 4 C A N O E BOAT 72 F E R R Y D I N G H Y 5 R A F T

A snake P Y T H O N 14 C O B R A

F = frequency, derived from Kucera and Francis [24]. The semantic category for each exemplar pair is denoted in italics.

semantic category priming in the left cerebral hemisphere

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