CHARLES W. ERIKSEN and ROBERT L. COLEGATEUniversity of Illinois, Urbana, Illinois 61803

Selective attention and serial processingin briefly presented visual displays*

Displays of eight capital letters were presented in a circular arrangementaround a central fixation point at tachistoscopic durations. Under differentexperimental conditions, one or two indicators, presented either 250 msecbefore, simultaneously with, or after the display, designated the letters that Swas to report. The arrangement of conditions permitted inferences as to theorder in which S encoded the letters from the display. The results supported aninterpretation in terms of a serial process by which the letters were encoded ortransferred from iconic representation to a short-term memory.

If a human S is presented eightletters simultaneously in a brief visualexposure, he is able subsequently toreport only a few of the letterspresented, typically four or less.However, if simultaneously with thedisplay an arrow or similar indicator isused to designate one of the letters,the S reports this letter with a highlevel of accuracy. To accomplish this,S apparently brings into play sometype of selective mechanism thatenables him to effectively choosewhich of eight equally potent stimulihe will note or encode into hisshort-term store. This selective processor mechanism would seem to embodymost, if not all, of the characteristicsthat are embodied in the concept ofselective attention.

In prior work we have determinedsome of the characteristics of thisselective process, as well as some ofthe variables of which it is a function.Of major interest is the time requiredfor the selection to occur. Thepioneering works of Averbach andCoriell (1961) and Sperling (1960)have shown that a visual display maybe available in the form of an icon forseveral hundred milliseconds beyondits exposure duration. To be effective,selectivity would seem to have tooccur during the stimulus presentationor the duration of the icon, althoughslight performance gain might beattributable to the preferred order ofrecall from short-term memory.

Research indicates that the selectiveprocess requires between 200 and300 msee (Eriksen & Collins, 1969;Averbach & Coriell, 1961). In theEriksen and Collins experiment,individual S performance was adjustedto approximately 75% correct with amultiitem display and a simultaneousindicator. Then the experimental

*This investigation was supported byUnited States Public Health ServiceResearch Grant MH-1206, United StatesPublic Health Service Research CareerProgram Award K6-MH-22014, andPredoctoral Fellowshipl-F01-MH-47201-01.

manipulation consisted of presentingthe indicator at various lead timesbefore the occurrence of the display.Performance was found to increaseuntil the indicator led the display byapprox.imately 200 m sec , Thisasymptotic value would indicate themaximum time required for preciseselection to take place.

Averbach and Coriell (1961)obtained estimates of approximately270 msec for the selectivity process inan experiment using masking. Again amultiitem display with simultaneousindicator was presented, but a maskwas applied to the indicated item atvarious times following termination ofthe display. The assumption was thatif the item had already been processedinto a short-term store, it could not bemasked.

In other experiments the numberand physical spacing of irrelevant ornoise elements of the display werefound to effect the efficiency ofselective attention. Keeley (1967) hadhis Ss report a target Landolt C(gapped either at bottom or at top)embedded among noise Landolt Cs(gapped right or left). Even when anindicator wac simultaneouslypresented, designating the target, S'sperformance was impaired when thenumber of noise elements in thedisplay was greater than three or four.Similarly, Eriksen and Rohrbaugh(1970) found, in displays consisting ofcapital letters, that accuracy inreporting the designated or indicatedletter decreased as the number ofletters in the circular display increasedfrom 4 to 12. They further found thatthis effect of number was independentof the physical spacing betweenadjacent stimuli. When spacingbetween adjacent stimuli was heldconstant, performance still declined asa function of the number of irrelevantstimuli.

However, the Eriksen andRohrbaugh results also showed thatspacing in itself was a variable thateffected the efficiency of the selectiveprocess. The closer the stimuli were

spaced to each other, the poorer wasperformance in identifying theindicated target. Further investigationfound this spacing effect to have atleast two different components. If thestimuli were within a 1{4 of a degreeof angle of each other, interferencewas due apparently to impairment ofacuity. Effects of adjacent contours onacuity at brief exposures hadpreviously been found by Flom,Weymouth, and Kahneman (1962)when adjacent contours were 1{3 of adegree of angle or less apart. However,Eriksen and Rohrbaugh found nocontour interference (acuity) effectswith stimuli spaced .38 deg of angle orfurther, a result that agrees withCollins (1970). But, since they alsofound spacing effects, independent ofnumerosity at distances of .38 deg andgreater, it would appear that physicalspacing of stimuli in a visual displayhas an effect on selectivity over andabove that attributable to contourinterference and acuity.

The present experiments weredesigned to provide furtherinformation on the process by whichone or two of a set of equally potent

.stimuli is selected from a brieflypresented visual display. A within-Ssdesign involving five experimentalcon ditions was employed inExperiment 1. In all conditions,displays consisting of a circulararrangement of eight letters werebriefly presented. There were twoconditions that involved a singleindicator and required a singleresponse. In one, the S was required toreport the letter designated by a blacklin e indicator that occurredsimultaneously with the display. In theother single-response condition, he wasinstructed to report the letterdiametrically opposite on the circulararray to the letter designated by thesimultaneous indicator.

The other three conditions requiredS to report two of the display letters.In one of these conditions, two letterswere designated by two separateindicators; in another condition, onlya single indicator was employed, but itfell midway between two adjacentletters and S was instructed to reportboth of these. In the final condition,two indicators were again employed,but one indicator occurred 250 msecbefore the onset of the letter displayan d the second occurredsimultaneously with the display. Hereagain S was required to report bothdesignated letters.

The rationale for the choice of theseparticular conditions was as follows:The single indicator with the singleresponse may be viewed essentially asa base-level condition. By settingperformance at 85% to 90% correctunder this condition, through

Perception & Psychophysics, 1971, Vol. 10 (5) 321

adjustment of exposure duration ofthe display, the effects of the otherconditions can then be determined.The performance obtained with thetwo simultaneous indicators/tworesponses condition when comparedwith this base level providesinformation relative to the difficultyof locating two indicators in thedisplay and noting or encoding twoletters. If a performance decrement isfound for this double-indicatorcondition, it could be attributable tothe increased time required in locatingthe second indicator and encoding theresponse. However, any performancedecrement also could reasonably beattributed to a serial encoding of theinformation. Locating the positiondesignated by an indicator may consistof structuring in a spatial manner thedisplay; once structured, no furthertime is required to go selectively toany position in the display. If thiswere the case, then any decrement inperformance under this conditionmight be attributable to serialencoding of the letters. During thetime required for encoding of the firstletter, the icon of the display wouldhave deteriorated sufficiently to leadto greater inaccuracies in the encodingof the second letter.

Information relative to these twopossibilities can be obtained from thecondition requiring two responses, butwith a single indicator that fallsmidway between the two letters to bereported. Here S has only oneindicator to locate or apprehend. If

,performance under this condition iscomparable to that obtained in thetwo simultaneous indicators/tworesponses condition, it would suggestthat impairment relative to the basecondition was due to the serialencoding of the display letters.

Further evidence on this point isprovided in the condition where thefirst indicator occurs 250 msec priorto the display. On the basis of priorresearch (Eriksen & Collins, 1969),this should allow sufficient time forthe location process for one of theletters to have occurred. Performanceon the second letter in the displayin this condition should then be afunction only of the location time forthat letter and its encoding;'

TIie condition -where the S reportsonly the- letter diametrically oppositethe position where the indicatoroccurs was included to provide someevidence on whether the internalscanning or encoding process hasspatial limitations. Shaw (1969) haspresented evidence suggesting thatspaces or blank areas require a finiteprocessing time.

EXPERIMENT 1Subjects

Six female undergraduate students

at the University of illinois served aspaid Ss. All had normal or corrected tonormal vision.

Apparatus and StimuliStimuli were displayed with a

three-field Model G·A ScientificPrototype tachistoscope, which hadbeen modified by replacing themanufacturer's bulbs with SylvaniaF4T5/CWX fluorescent lamps. Theluminance of each of the three fieldswas set at 1 mL.

Thirty-two stimulus displays wereconstructed by using the capital lettersA, T, H, and Y twice each to fill eightpositions on each display card. Thearrangements of the letters in acircular array on the display cardswere constrained in that the sameletter could not appear in adjacentpositions and that letter-positioncombinations occurred equally often.The letters were black and wereobtained from Paratipe 1131618Futura Bold. (Each letter sub tended.2 deg of visual angle.) They weremounted on white vinyl cards. Theletters were equally spaced, .6 deg ofvisual angle apart, around an imaginarycircle 1.5 deg of visual angle indiameter. All cards had the 12-, 3" 6·,and 9-0'clock positions filled, plus thefour intermediate positions.

Indicators consisted of black lines,.5 deg of angle long and .1 deg of anglewide, mounted on clear plastic cards.Each indicator was located withreference to the center of the displayon an imaginary ray extending beyondthe designated letter position, with thenearest end .2 deg of angle from theletter. One set of eight singleindicators was constructed. A secondset of 28 double indicators consistingof all possible combinations ofpositions taken two at a time wasconstructed. A third set of eight singleindicators was constructed such thateach indicator fell midway betweenadjacent positions at the same distancefrom the center of the display as theother indicators. The adaptation fieldcontained a black fixation cross,subtending .7 deg of visual angle,mounted on the same white vinylcards that were employed for thestimulus displays. The adaptation fieldwas on at all times except whenstimulation was provided in one of theother fields of the tachistoscope.

The procedure of mounting theindicators on clear plastic cardsreduced the number of stimulusdisplays that had to be constructed.The clear plastic card with theindicator was inserted in the same fieldas the stimulus display but in front ofit. In the condition where oneindicator preceded the display by250 msec, the clear plastic indicatorcard was inserted in front of a blankvinyl stimulus card and presented in

the other field of the tachistoscope. Inorder to maintain comparability ofluminance and viewing conditions inall three tachistoscopic fields, a clearplastic sheet without indicator wasalso present in the indicator andadaptation field.

ProcedureEach S was given two practice

sessions. The first was devoted toidentifying letters when a singleindicator occurred simultaneouslywith presentation of the display. Thesecond practice session was used toestablish an exposure duration suchthat the S could identify the indicatedletter with approximately 90%accuracy under a single simultaneousindicator condition. In this secondsession, S was also given 16 trials ofpractice on each of the fiveexperimental conditions. Feedbackwas provided after each trial. Prior toinitiating a trial, S fixated on the crossin the adaptation field and when it wasin clear focus pressed a switch toinitiate the stimulus presentation.

Ten experimental sessions of fiveblocks of 16 trials each (one block ineach condition per session) were run incounterbalanced order. Each sessionwas preceded by 5 min of darkadaptation and warm-up employing asingle simultaneous indicator.Feedback was given on each trialduring warm-up, but once theexperimental trials began feedback wasprovided only at the end of the trialblock in the form of telling the Showmany correct responses he had made.For each block of trials, each letter ineach position occurred equally often.

The five experimental conditionswere: single simultaneousindicator/single response, in which theindicator occurred with the displayand S was required to report the letteroccupying the indicated position; twoindicators/two responses, in which twoindicators occurred with the displayand S reported the letters occupyingthe designated positions with order ofreport in clockwise order; early andsimultaneous indicators/two responses,in which one indicator appeared250 msec before the display for aduration equal to that of the displayand a second indicator occurred withthe display and S reported the lettersin the order that they were indicated;single indicator/two responses, inwhich the indicator occurred with thedisplay but was located midwaybetween two letters and S wasrequired to report the letter on eachside of the indicator in clockwiseorder; and opposite/single response,which was identical to the singlesimultaneous indicator/single responsecondition, except that the S wasinstructed now to report only the

322 Perception & Psychophysics, 1971, Vol. 10 (5)

Table 1.\lean Percent Correct Identifications of Indicated Letters for the Five Condit•..;u.

Single Single Two Early and Simul-Indicator. Indicator. Indicators. taneous Indicators. Opposite.

ringle Two Responses Two Responses Two Responses SingleR, sponse R1 R2 R1 R2 R1 R2 Response

87 81 83 81 76 88 72 79

Iecter diametrically opposite the letterindicated.

On the basis of the second practicesession, an exposure duration wasdetermined for each S that yieldedapproximately 90% report accuracyfor the single simultaneous indicatorcondition. That exposure duration wasthen used throughout the experimentfor all conditions for that S. The meanexposure duration was 38 msec, with arange of 30 to 50.

Results and DiscussionIn Table 1 the mean percent correct

identifications of the indicated letterare shown for each of the fiveexperimental conditions. In the threeconditions where S reported twoletters, R. and R z in the tabledesignate these responses in the orderthat S was instructed to make them.The overall significance of meandifferences in Table 1 was evaluatedby a two-way classification analysis ofvariance (Ss by Condition), in whichthe second response in each of thedouble response conditions wastreated as a separate condition. Thus,there were eight conditions in theanalysis of variance. The analysisshowed a significant variation betweenconditions [F(7,35) = 15.3, p < .001]as well as a significant S effect[F(5,35) =12.4, p < .001].

The effect of having to process twoindicators and two letters from thedisplay can be seen by comparing thetwo simultaneous indicators/tworesponses condition with the singlesimultaneous indicator/single responsecondition. In the former condition, theSs were instructed to give theirresponses in a clockwise order.Although the 8's first response underthis condition is significantly moreaccurate than the second response, it isstill significantly poorer thanperformance for the single responsecondition.I

The decrement in performanceunder the two indicators/tworesponses condition could be due tothe added load of processing twoindicators, of encoding two letters, ora combination of these. However,comparison of the data of thiscondition with the singleindicator/two responses conditionsuggests that the decrement is mainlyattributable to encoding two lettersfrom the display. In this lattercondition, only a single indicator isinvolved, and S reports the two letters

designated by it. Average performancefor the two responses is about equal tothe average performance when twoindicators are used. Neither the firstnor the second response under thislatter condition is significantly orappreciably better than the firstresponse under the two indicators/tworesponses condition, and both arelower than for the singleindicator/single response.

In the early and simultaneousindicators/two responses condition,the early indicator occurred andterminated 250 msec before thedisplay appeared, time for the S toh ave processed the indicator orstructured the spatial organization ofthe display (Eriksen & Collins, 1969).The S was instructed to report theletter designated by the early indicatorfirst and, as can be seen from thetable, the accuracy of this report iscomparable to that obtained under thesingle indicator/single responsecondition; in fact, the performance isslightly better, although notsignificantly so. However, the accuracyof the second response, the letterdesignated by the simultaneousindicator, is the poorest obtained forthe second response under anycondition.

Although the manner used todesignate the two letters to bereported differed quite markedlybetween the three two-responseconditions, the average accuracy of aresponse is quite comparable across allthree. When first and second responsesare averaged, the accuracy with whichletters are reported in the twoindicators/two responses condition is79%, which compares with 80% forthe early and simultaneous conditionand 82% for the single simultaneousindicator/two responses condition.These differences are well withinsampling error. Thus, while accuracyof first and second responses can differqui te markedly across theseconditions, the three conditionsviewed in terms of the totalinformation they yield about thedisplay are quite comparable. Thissuggests that there may be some kindof "tradeoff" process operating bywhich accuracy for the first responsecan be increased but only at theexpense of a decrease in accuracy ofthe second response. (It is to be notedthat the performance obtained in theopposite condition, where only asingle response is required of S, is also

right at this average level, 79%.)The above pattern of results can be

explained reasonably well in thefollowing manner. Let us assume thatencoding from the display or thetransfer from the icon to a short-termmemory is carried out serially, letterby letter, and that encoding itselfrequires a finite period of time. Duringthe time encoding of a letter is takingplace the display icon is deterioratingso that by the time the second letter isencoded the legibility of the icon isreduced. These assumptions wouldlead to the prediction that the secondresponse would be less accurate thanthe first, which is contradicted by thedata obtained in the singleindicator/two responses condition.However, it is to be noted that theorder in which S reports the letters isnot necessarily the same order that heencoded them.

If the three two-response conditionsare compared, it seems reasonable thatthe encoding order would havefollowed the report order mostfrequently in the early andsimultaneous indicators condition.Here the 8 has 250 msec to process theindicator and to locate position beforethe display and the second indicatorappear. It seems most likely that the Swould encode the letter in the positionhe had already located before turningto the second indicated letter. Onlywhen the second indicated letter wasin the immediately adjacent positionof the display would there seem to bea significant probability that this letterwould be encoded first.

The most frequent deviation ofencoding order from report ordermight be expected to occur in thesingle indicator/two responsescondition, where the indicatordesignates a position between twoadjacent letters. Although instructedto report the letters in a clockwiseorder, the close adjacency of the lettersmight well lead to variation in theorder in which they are encoded,particularly for bottom and left halfdisplay positions.

In the two simultaneous indicatorscondition with clockwise order ofreport, order of encoding wouldprobably follow response order exceptfor deviation again when the twodesignated letters occupy immediatelyadjacent positions or when, throughrandom variation in the clarity of theletters in the display, the letter to bereported second stands out more inclarity (Eriksen, 1966).

This account fits the differenceobserved between first and secondresponses in the three conditions quitewell. The biggest differences inaccuracy between first and secondresponses occurred in the early andsimultaneous indicators and twosimultaneous indicators conditions.

Perception & Psychophysics, 1971, Vol. 10 (5) 323

Conditions

Table 2Mean Percent Correct Letter Identificationsby Response and Condition as a Functionof Whether the Two Indicated Letters Werein Adjacent or Separated Display Positions

SubjectsOne male and four female

undergraduate students at theUniversity of lllinois served as paid Ss.All had normal or corrected to normalvision.

ProcedureThe procedure was identical in

terms of practice and experimentalsessions to that of Experiment 1 wit\tthe exception that only eightexperimental sessions were involved.During these experimental sessions,seven blocks of 12 trials each (oneblock of each condition per session)were run in counterbalanced order.Each session was preceded by 5 min ofdark adaptation and warm-up with asingle simultaneous indicator.Feedback was given on each trialduring warm-up. During experimentalsessions, feedback was given by telling

Apparatus and StimuliThe apparatus and stimuli were the

same as those employed inExperiment 1, with one slightmodification. A capital V wassubstituted for the capital Y in thefour stimulus letters. There had been asuggestion in the previous data thatthe Y was different in discriminabilityfrom the other three letters.

control condition was a delayedindicator/single response condition inwhich an indicator appeared 700 msecfollowing the termination of thedisplay and S was required to reportthe letter that had been in the positiondesignated by the indicator. While theadvantages of an indicator presentedduring the exposure of the displayrelative to one presented after the iconhas deteriorated would seem to havebeen well established by now(Averbach & Coriell, 1961, andEriksen & Collins, 1969), wenonetheless deemed it desirable tomake certain that the effect wasoperating in our present experimentalarrangement.

In addition to these controlconditions, a modification was madein the opposite condition ofExperiment 1. In the previousexperiment S was required to reportonly a single letter, the one occupyingthe position diametrically opposite theletter designated by an indicatoroccurring simultaneously with thedisplay. In the present experiment, Swas instructed to report the letterimmediately designated by theindicator as well as the one occupyingthe diametrically opposite position.Thus, this condition became atwo-response condition. The rationalefor this modification will be madeclear in the results and discussionsection.

markedly, whereas that of the secondreported letter shows a markeddecrease. A somewhat similar effect isobtained in the data for the twosimultaneous indicators condition.Accuracy for the second reportedletter drops rather pronouncedly whenthe letters are separated by at least oneintervening position, although the gainin accuracy with separation for thefirst reported letter is not aspronounced as in the other condition.

The above explanation of the resultscan be extended to account for thedata obtained in the opposite/singleresponse condition. Here the S wasinstructed not to report the letterindicated but the one diametricallyopposite on the circular display.Although this condition involved S'sgiving only one response, accuracy wassignificantly poorer than that obtainedin the single indicator/single responsecondition. However, the Ss ran theexperimental sessions in which thiscondition was intermixed with blocksof trials under the other conditions. Ifwe assume that, due to thepreponderance of their experience,they developed the habit of encodingfirst, on a large proportion of trials,the letter immediately designated bythe indicator and then of encoding thediametrically opposite letter they wereto report, the performance of theopposite/single indicator condition isexplicable. Although required toreport only a single letter, Ss wereactually encoding two letters on asignificant proportion of the trials.This would account for whyperformance under this condition for asingle-letter response was at the levelof the two-response conditions whenthe average performance on bothresponses for these conditions wasconsidered.

EXPERIMENT 2Since an evaluation of the data in

the above experiment involvedmultiple comparisons within the samesample of Ss, there are problemsassociated with determining just howreliable obtained differences are.Accordingly, it was desirable toreplicate the conditions ofExperiment 1 on a new sample of Ss.

Two new control conditions wereadded in this experiment. The earlyindicator/single response conditionconsisted of an indicator's appearing250 msec before display onset. Thiscondition was included to assist ininterpreting the data from the earlyand simultaneous indicators/tworesponses condition and to insure thatthe gain in performance obtained withan early indicator in previousexperiments was operating in thepresent experimental arrangements(Eriksen & Collins, 1969). The other

8373

8081

7869

8090

Early andSimultaneous

Indicators,Two Responses

R1 R2

TwoIndicators,

TwoResponsesR1 R2

-----------Position

AdjacentSeparated

The least difference occurred for thesingle indicator/two responsescondition.

However, a further check can bemade upon the assumption concerningthe deviation of encoding order fromreport order when the letters to bereported are in immediately adjacentpositions. The data from the twoindicators/two responses and the earlyand simultaneous indicators/tworesponses conditions were analyzed todetermine whether the accuracy of thefirst and second responses varied as afunction of whether the two indicatorsdesignated immediately adjacentletters in the display as opposed toletters removed by one position ormore.

The data in Table 2 show the resultof this analysis. Percent correctidentifications are shown for each ofthe two conditions and for each of thetwo responses as a function of whetherthe two indicators designated adjacentpositions on the circular display orpositions separated by at least oneintervening position. A four-wayanalysis of variance (conditions, firstand second responses, positions, andSs) was performed on these data. Themain interest was the significantinteraction between first and secondresponses and position (adjacent orseparated) [F(1,5) = 29.36, p < .01),

If the letter to be given in the firstresponse has a greater likelihood ofhaving been encoded second when theother letter to be reported is in animmediately adjacent position, itwould be expected that the accuracyfor the first response would increasewith the separation between thestimuli and the reverse would occurfor the second reported letter. Thiseffect is confirmed quite clearly in thedata for the early and simultaneousindicators condition. If the two lettersto be reported occur in adjacentdisplay positions, there is littledifference in the accuracy of report asa function of the order in which Sreports the letters. However, if theletters are separated by at least onedisplay position, accuracy of the firstreported letter increases very

324 Perception & Psychophysics, 1971, Vol. 10 (5)

Table 3Mean Percent Correct Identifications of Indicated Letters for the Seven Conditions of Experiment 2. Also shown are

the values averaged across the common conditions of Experiments 1 and 2.

Single Single Two Early and Simul- Single Indicator, Early Delayed

Indicator, Indicator, Indicators, taneous Indicators, Opposite, Indicator, Indicator,Single Two Responses Two Responses Two Responses Two Responses Single Single

Response Rl R2 Rl R2 Rl R2 Rl R2 Response Response

87 85 83 &3 79 89 77 89 73 95 62

Values Averaged Across Conditions of Experiments 1 and 287 83 83 &2 77 89 74

S the total number of correct for eachblock of 12 trials. For each of theseven experimental conditions, eachletter and each position weredesignated by the indicators equallyoften.

ResultsIn Table 3 the percent correct letter

identifications are shown under eachof the seven experimental conditions.For the four two-response conditions,R, and R2 in the table designate firstand second responses in the order thatSs were instructed to give them. Thetable also contains, by condition, the .data averaged through Experiments 1and 2. An analysis of variance of theExperiment 2 data, in which thesecond response data for the fourtwo-response conditions were treatedas four additional conditions, gave anF value for between conditions of17.0, which, with df = 10/40, issignificant beyond the .01 level.

If the four conditions that are thesame across Experiments 1 and 2 arecompared, it is seen that a satisfactoryreplication has been achieved. In bothexperiments, the simultaneousindicator/single response conditionyielded 87% correct (which by itself isnot too remarkable since an attemptwas made to adjust the exposureduration for individual Ss to yieldperformance under this base conditionbetween 85% and 90% correct). Forthe three two-response conditions,performance is also comparable. Forthe two indicators/two responsescondition, there is lower accuracy forthe second response as compared tothe first, and both responses arepoorer than those obtained for thesingle indicator/single responsecondition. The early and simultaneousindicators/two responses conditionagain shows slightly betterperformance for the first response andan appreciable deficit in secondresponse accuracy as compared withthe single indicator/single responsecondition. This is the same result aswas obtained in Experiment 1. Highcorrespondence across experiments isalso obtained in the singleindicator/two responses conditionwhere again both responses differ littlefrom each other in accuracy and both

are lower in accuracy than thatobtained with the base condition,although in this experiment thedifference between these conditions isnot quite as great as it was inExperiment 1.

The opposite/two responsescondition was new with Experiment 2.It was employed in order to test theexplanation for the data obtained inthe opposite/single response conditionin Experiment 1. In Experiment 1 thiscondition, although involving only asingle indicator and a single response,had given significantly poorerperformance than the singleindicator/single response condition.The interpretation advanced for thisresult had been that the Ss, althoughrequired to report only the letter inthe position diametrically opposite tothe position where the indicatoroccurred, were nonetheless, on a largeproportion of the trials, also encodingthe letter immediately designated bythe indicator before encoding the oneopposite. The decrease in accuracy forthe reported opposite letter was thusattributed to decay in the icon thatoccurred during the time the otherletter was being encoded.

In the present experiment, therequirement that the S report bothletters checked upon thisinterpretation in the following way. Ifthe S by instruction is actuallyencoding the letter closest to theindicator first on all or nearly all trials,we would expect little or nodeterioration of performance on thisletter relative to the base condition.The spatial separation of the twoencoded letters would almost alwaysassure that this letter would beencoded first. There should, however,be an appreciable drop in the accuracyof report of the letter in the oppositeposition inasmuch as S would encodethis letter second. As can be seen fromthe data in Table 2, this is the result.Under conditions where we can bereasonably confident in assuming thatthe order of report is also the order ofencoding, there appears to be nodecrement for the letter reported firstas compared with the base condition.

If the four two-response conditionsare compared on the average accuracywith which indicated letters are

reported, all four conditions are foundto be very comparable, with averageaccuracy varying from 81% to 84%.This is so despite the differences in theway the two letters to be reported areindicated in these four conditions.This result is consistent with the"tradeoff" interpretation advancedabove, namely, that Ss are capable ofprocessing two letters from a displayat a given level of accuracy, andvariation in accuracy between lettersreported first and second can beattributed to the order in which theywere encoded into short-termmemory. The more consistently Ss usethe prescribed order for encoding, thegreater difference there will bebetween the two letters. Thecomparable average accuracy acrossconditions also suggests that themethod of attempting to program ordirect selective memory within theoperations of these four experimentalconditions is relatively ineffective inthe sense that it failed to improve theS's information handling capacity.This, of course, implies that havingtwo separated indicators eachdesignating a separate element, havinga single indicator designating twoadjacent display elements, or even asingle indicator designating twoelements separated by the maximumdistance in the display can be equallyeffective in directing selectiveattention.

Since the performance differencefor the two responses in thesimultaneous and opposite/tworesponses condition can be accountedfor adequately in terms of the order inwhich the stimuli are encoded, itwould appear that there is no basis for8118uming that time is required for theselective encoding process to movespatially from one display position toanother. The relatively lower accuracyof the second responses of thiscondition is most likely notattributable to the time required for acentral processing or encodingmechanism to go diametrically acrossthe display, but is most readilyexplained in terms of the increasedprobability that the opposite item willbe encoded second on nearly all trials.

In this connection, there is a slighttendency for the single indicator that

Perception & Psychophysics, 1971, Vol. 10 (5) 325

occurs between two letters to showsuperiority consistently over the othertwo response conditions in terms ofthe average accuracy of the tworesponses. While the consistentsuperiority of this condition is notsignificant. it is suggestive-it might beattributable to decreased locationerrors due to the two letters to bereported being immediately adjacentto one another.

The result obtained with thedelayed indicator/single responsecon dition confirms that theexperimental arrangements that wereemployed in these two experimentswere sufficient to produce an overloadof information and that asimultaneous indicator did indeedshow selective effects on encodingfrom iconic memory. When theindicator occurs 700 msec aftertermination of the display, it can bepresumed that the icon hasdeteriorated beyond any usable level.Thus the 62% accuracy obtained underthis condition represents theprobability of having tapped one ofthe few items that S was able toencode from the display during itspresentation and the duration of theusable icon.

The early single indicator/singlerespoD8e condition is found to lead toa n a ppreciable increment inperformance over that obtained for asingle simultaneous indicator. This isconsistent with previous (mdings(Eriksen & Collins, 1969). Itsimportance for the presentexperiments, however, is in terms ofint e rpretation of the early andsimultaneous indicators/two responsescondition. In both Experiments 1 and2, the first responses obtained underthis experimental condition were atthe same level of accuracy as that forthe single indicator/single responsecondition. In interpreting this effect asattributable- to the S's sometimesencoding the second indicatedresponse first. particularly on trialswhere it was adjacent to the ill8tindicated response, we were assumingthat there should have been a gain forthe imt or early indicated responserelative to the single indicator/singlere&poD8e condition. If S had time tolocate the display position or tospatially structure the display forencoding prior to the onset of thestimulus letters, performance for theprior designated letter would be betterthan that obtained with the singlesimultaneous indicator. The analysiscontained in Table 2 supported thisinterpretation, but it seemed desirable

to determine for sure that the gainfrom an early indicator that wasassumed to underlie this explanationwas indeed obtained in the presentexperimental arrangements. Thefi n dings for the early indicatorcondition in Experiment 2 areconfirmatory.

Both Collins (1970) and Gardiner(1970) have found that averageresponse accuracy decreases when Ssare required to report two or moreletters from visual displays. AsGardiner points out, this reduction inaccuracy per se cannot unequivocallybe attributed to a serial processing orencoding of the display informationsince limited capacity 1 parallelprocessing models such as Rumelhart's(1970) predict such an outcome. Thereduction in performance with two ormore items can be attributed to thedistribution of the limited capacityover all the items which aresimultaneously, but less adequately.processed.

The present results provide muchless equivocal evidence for serialencoding. To the extent that ourinferences are sound as to the Ss'encoding order of the display letters, aserial model is indicated. The fll8titem, or letter, encoded on a multipleitem encoding task is as efficiently andaccurately processed as when the taskrequires encoding only a single item.This is contrary to what would beexpected from a limited capacityparallel model such as Rumelhart'swhen extended to encompass theCollins (1970) and Gardiner (1970)results.

While a serial encoding of thedisplay letters is supported, thepossibility remains that indicatorprocessing and position location in thedisplay can occur simultaneously withletter encoding. In view of theevidence that apprehension of theindicator and location of displayposition requires between 200 and300 msec (Eriksen & Collins, 1969;Averbach & Coriell, 1961)performance much worse than thatobtained would have been expectedfor the second response in the twoindicators/two responses condition. If200 msee was required to locate thesecond letter after the first letter hadbeen selected and encoded,appreciable deterioration in the iconwould have occurred. Yet, averageperformance across the two responsesin this condition was not appreciablylower than that obtained for tworesponses when only one indicator andlocation were presented (single

indicator/two responses condition).An alternative to simultaneousprocessing of indicator and positionwhile encoding letters is to considerthe possibility that the 200 or moremilliseconds required to apprehend anindicator and determine positionreflects a spatial structuring process.Once this structuring is accomplished,encoding can move about thestructured field with no further timerequired for position location. Thepresent results do not rule out apossibility of this sort.

REFERENCESAVERBACH. E.. Ie CORIELL. A. S.

Short-term memory in vision. Ben SystemTechnical Journal. 1961. 40. 309-328.

COLLINS. J. F. CorrelaUona in sensitivityon different foveal ueu. Unpublisheddoctoral dissertation. University ofDUnois. 1970.

ERIKSEN. C. W. Independence ofsuccessive inputs and uncorrelated eD'Orin visual form perception. Journal ofExperimental Psychology. 1966. 72.26-35.

ERIKSEN. C. W.. Ie COLLINS. J. F.Temporal course of selective attention.Journal of Experimental PsychololY.1969. 80. 254·261.

ERIKSEN. C. W•• Ie ROHRBAUGH. J. W.Some factors determining efficiency ofselective attention. American Journal ofPsychology. 1970.83,330-342.

FLOM. M. C.. WEYMOUTH. F. W.. IeKAHNEMAN, D. Visual resolution andcontour interaction. Journal of theOptical Society of America. 196. 53.1026-1032.

GARDINER. G. T. Spatial processingcharacteristics in the perception of briefvisual lI!l'ays. Unpublialled doctoraldissertation. University of Michigan.1970.

KEELEY. S. M. Visual detection as afunction of attentional demand andperceptual system error. Unpublisheddoctoral dissertation. University ofDlinois.1968.

RUMELHART. D. E. A m11lticomponenttheory of the perception of brieflyexposed visual displays. Journal ofMathematical Psychology. 1970. 7.191-218.

SHAW. P. Processing of tachistoseopicdisplays with controlled order - ofcharacters and spaces. Perception ,.Psychophysics. 1969. 6. 257·266.

SPERLING. G. The informaUon available inbrief visual presentaUons. PsycholoeicalMonographs. 1960. 74(11. WholeNo. 498).

NOTE1. Evaluation of the statistical

significance of difference betweenpercentages is complicated by the fact thatm11lUple comparisons ue being made on thesame sample of SL As a rough rule adifference will be considered "sIgni1ieant" ifit equals or exceeds the mean difference(5'J1l) for the .01 level on a two-tailed t test.The alarmed statistical conservatives maY bereassured by tumin& to Experiment 2.where the important differences have beenreplicated on an independent sUDple.

(Received for publicotion December 12,1910.)

326 Perception &:Psychophysics, 1971, Vol. 10 (5)