Perception & Psychophysics1973. 1'0/. 13. So. ::.344-348

Concerning the role of perceptual factorsin the serial position effect*

GEOFFREY UNDERWOODUniversity ofNottingham. Nottingham NG 7 2RD. Eng/and

The experiment described here addresses itself to the serial position effect of recall, and in particular to the cause ofprimacy. Two of the three theories discussed here (selective rehearsal and proactive interference) postulate competitionbetween items during the storage interval as a cause of primacy, acquisition being assumed to be equal for all items. Thethird theory (differential perceptual processing) places the locus of the effect prior to the storage stage, and does nothold interactions between items to be essential to the effect after encoding. The experiment used this distinctionbetween the two classes of theories. storage of the whole presentation (and hence interactions during storage) not beingrequired. Only one word from each list was recalled, and this item was indicated to S on presentation, therebyeliminating the necessity to attempt to encode all items. The storage-interaction theories predict no primacy for therecall of individual items in this experiment, but the initial member of the list was recalled more often than the thirdmember. This example of primacy lends support to the argument that the trace strengths of items are not always equalimmediately after presentation.

Serial position effects in recall have beendemonstrated using a number of techniques-by ordered,anticipated. and free recall and by the various proberecall and recognition methods-and all of thesetechniques can depend upon a single serial presentationof a list for the effect to be observed. It has been arguedthat the recall advantage gained by the initial membersof a sequential presentation of items (the primacyeffect) is a function of selective rehearsal of early itemsduring and after presentation of the list, or a function ofproactive interference from early items encouraging theforgetting of later items, or a function of perceptualprocessing differences within the sequence during theacquisition stage.

One variant of the selective rehearsal hypothesismaintains that more information about early items isstored because the rehearsal strategies employed directthe S's attention to. these items (Waugh & Norman,1965; Crowder, 1969). The other variant considers thatearly items enter a relatively empty rehearsal buffer andare not displaced as quickly as are central or latemembers of the list. More information about early itemsin the presentation is therefore transferred to thelong-term memory store and their recall probabilityincreased (Atkinson & Shiffrin, 1968; Bruce & Papay,1970).

An alternative explanation of the increased recall ofearly items relies upon the construct of proactiveinterference (PI). This explanation can be developedfrom the observation of a decrement in recall as aconsequence of the presentation of previous items

*This work was supported by a Research Studentship fromthe U.K. Science Research Council and is based upon a thesissubmitted to the University of Sheffield. I am grateful toProfessor Harry Kay for advice and encouragement and toProfessors Judith Reitman and Doris Aaronson for theircomments on a previous draft of this paper.

(Keppel & Underwood, 1962; Murdock, 1964). Thedecrement, said to be due to PI from earlier itemscausing forgetting or response confusion for new items,can be eliminated by presenting a different class ofmaterial as the new items (Wickens, Born. & Allen,1963). Primacy may then be due to PI from the earlyitems in the list affecting the recall of succeeding items(Glanzer & Cunitz, 1966). Early items would not bevulnerable to PI (except from words in other lists), andso recall would not be depressed.

It has also been argued that the recall advantagegained by the initially presented members of a sequenceis at least partly a function of intraserial differences inthe perceptual processing of items (Aaronson, 1968;Aaronson, Markowitz, & Shapiro, 1971; Underwood.1972). The perceptual processing theory considers theintervals between presentation of items and theirsemantic decoding, and the degree of analysis of items,to be critical variables in the encoding of serialpresentations. This theory will now be described indetail.

The work of Efron (1970), Massaro (1970a),Treisman and Rostron (1972), and others indicates theexistence of sensory storage of auditory informationprior to identification, with similar storage functions tothe visual memory described by Sperling (1960, 1967).The function of such preperceptual storage would be tomaintain the sensory representation of the stimulusbeyond the duration of presentation. Aaronson (1967)points out that perceptual analysis can continue aftertermination of the stimulus presentation, and with shortpresentation durations, or complex material, suchpostpresentation analyses of echoic or iconic images maybe essential for correct perceptual identification.Preperceptual images of auditory and visual stimuli aresubject to rapid decay, estimates of the period beyondwhich information is irretrievable varying between a few100 msec and 2 sec (depending largely upon the types of

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ROLE OF PERCEPTlJAL FACTORS Il\; THE SERIAL POSITION EFFECT 345

information presented). So. an item can be stored as asensory representation and analyzed some time afterpresentation. but if the identification delay exceeds acertain critical value the representation will havedecayed and not be available for analysis. Similarly. anitem which has partially decayed may be imperfectlyanalyzed and hence an imperfect representation may bestored in postperceptual memory. For example, if theitem "P" is spoken to a S who does not perform animmediate analysis because he is engaged in anothertask. then only gross acoustical features may be availableby the time he gets around to analyzing the word. Whatis stored in this case may not be the item presented, butany of the numerous other items with the sameacoustical features. With both the active and postponedidentification strategies described by Aaronson (1968),the items presented later in the sequence are subject togreater identification delays than are items presentedearly. and so would be in a more decayed state whenanalyzed. Thus. late items in the list are argued to havegreater identification delays than do early items. and soless perfect representations are available for recall.

The second variable relevant to the perceptualprocessing theory of primacy is that of the time availablefor analysis. Massaro (1970b) asserts that memorystrength increases exponentially with an increase in theperceptual processing of the item, and so recalldifferences may be accountable to differences in thetim e available for initial inspection. With thepresentation of a list, serial position curves may be dueto longer processing of early items than of succeedingitems. The initial members of the list may have such aprocessing advantage due to strategical factors. The Smay choose to attend selectively to the beginning of thelist because of the change in stimulus presentation: thefirst item followsa period of nonstimulation, but thesecond item is preceded by a similar stimulus (Kintsch,1970). Alternatively, the early items may demandattention due to their distinctiveness (Murdock, 1960) incomparison with preceding stimulation. Such differencesin the strategical distribution of attention within a listmay correspond to the selection of "anchor points" inthe serial learning situation, as described by Feigenbaumand Simon (1962).

The perceptual processing theory of primacy statesthat initial items may be more efficiently analyzed thancentrally presented items, resulting in stronger memorytraces and hence higher recall probabilities of early thanof central and late items. Selective rehearsal andproactive interference theories, however, consider thelocus of the primacy effect to be after initial encoding:they assume that all items are encoded equallyefficiently and that primacy is a function of selectiveprocessing or differential forgetting during storage. Toeliminate the possibility of interactions between itemsduring the storage period. and hence to test these twocategories of theories. the method used in theexperiment described here required Ss to retain only one

item from each list and to respond with this item at theend of the presentation. If perceptual processing is afactor which contributes to the primacy effect, and ifsequential presentations result in less efficient processingof central items than of initial items, then a primacyeffect should be observed with this technique, asretention is still a function of the efficiency ofperception. Rehearsal theory should not predict such aprimacy effect. because there is no reason why Ss shouldselectively rehearse the first item of the series whenindicated as the recall item rather than rehearse a centralitem when it is required. As only one item from the listhad to be retained from each list. storage interferenceeffects should also be minimal. but incidental retentionof noncritical items may lead to a certain amount of PIinfluencing the recall of central items more than that ofinitial items. However, in an experiment reported byBruce and Papay (1970), presentation of itemssubsequently cued as "to be forgotten" had little effecton primacy, suggesting that intraserial PI effects fromincidental material are minimal.

The item for recall was designated at the instant ofpresentation by the occurrence of a tone in the oppositeear to that of presentation of the set of verbal items.This method differs from the usual probe recall methodof testing for partial recall of the list. With the proberecall paradigm of Waugh and Norman (1965), outputinterference effects are eliminated, but acquisition,storage, and retrieval processes still remain as possiblecauses of the (reduced) primacy effect. With the probetechnique, Ss are required, ideally, to store the whole ofthe presentation until the probe itself is presented. Thepresent technique of indicating the critical item to the Sat the time of presentation eliminates the possibility ofinteractions between items during the storage interval:retention is a function of perceptual analysis andencoding and of subsequent forgetting of a single item.

To prevent retention of the cued item by selectiverehearsal, Ss were required to shadow (overtly rehearse)the whole list. Underwood and Moray (1971) criticizedthe employment of the shadowing response inexperiments seeking an absolute measure of thedetectability of target items, but in the present situationthe attention-directing and capacity-consuming featuresof the response were of advantage. Simply asking Ss notto rehearse selectively. or telling them to think only ofthe last item presented would not control theirstrategies: these instructions. used by Waugh andNorman (1965), can only be effective when S does notknow in advance which item will be critical. Levy andJowaisas (1971) found recall decrements when itemssemantically similar to the target item were shadowedduring the interval between p-esentation and recall ofthe critical item. Kroll. Parks. Parkinson. Bieber. andJohnson (1970) similarly found that the failure of recallof a single spoken item was a function of the amount ofretroactive interference (RI) from the interpolatedshadowing task. Semantic interference alone should

346 UNDERWOOD

depend only on the number of interpolated items.according to this finding, with no increase in theprobability of recall of initially presented items. Tracedecay theory would also predict a recall probabilityfunction which varies inversely with the time betweenpresentation and recall, provided that Ss were notrehearsing the critical item. Again, no primacy should beexpected if this is the only factor operating.

METHOD

Apparatus and Stimulus Materials

shadQwing Scores

Position Effects

RESULTS

Fig. 1. Shadowing and critical-item recall scores.•--.correct recall of critical item; 0--0 recall of critical item +adjacent recall errors.

As illustrated in Fig. 1, recall of items presented inSerial Position 1 (63.75%) was greater than recall ofitems in Serial Position 3 (46.25%), thus confirming theprediction derived from the perceptual processing theoryof primacy (t =2.77, df = 19, p < .01, one-tailed).

Recall errors were classified into two general types:omissions and guesses, and recall of items adjacent to thecritical item. Ss who offered an adjacent item as therecall response appeared unaware that they were in errorand gave this item immediately and confidently. Theother error category was characterized by the Seitherfailing to recall at all or hesitating for a few seconds andthen offering an incorrect letter, which was occasionallya letter which had been presented towards the end of thelist. These errors may have been caused by interferenceor decay effects, whereas adjacent recall errors suggest afailure of perceptual integration. This view is supportedby the absence of any systematic interaction betweenthe number of these errors over time. The relationshipsbetween adjacent recall errors and correct recall areillustrated in Fig. 1. Ladefoged and Broadbent (1960)reported an experiment in which clicks weresuperimposed over lists of digits, one click during eachlist of 10 digits, and the S's task was to remember thedigit occurring at the same time as the click. Thetendency was for Ss to report the click as occurringearlier in the list than was the case, and the authorsdiscussed the result in terms of the theory of prior entry,which states that we perceive events which we are

,..,....1074

SERIAL POSITION

-"CI

40

Subjects

Twelve females and eight males, between the ages of 18 and25 years. were drawn from a volunteer S pool composed ofundergraduate and research students from the University ofSheffield. All Ss claimed to be right-handed.

Procedure

Half of the Ss were presented with verbal material to theirright ears (and therefore tones to their left ears), and half werepresented with the lists of digits and letters to their left ears.Lists of letters were first presented for shadowing practice until acriterion of three successive errorless lists was achieved. Thethree practice trials of shadowing and remembering the criticalitem were then given. To minimize selective rehearsal of thememorized letter, emphasis on perfect shadowing was made inthe instructions: Ss were told that only those trials in whicherrorless shadowing was obtained would be scored on thememory task. Ss were instructed that they should remember theletter that occurred at the same time as the tone and to reportverbally that letter only after completing the shadowing task.The 40 experimental trials were prerented with only a shortbreak between trials.

Forty lists. each composed of four randomly selected digits (1through 10, excluding 7) followed by 10 randomly selectedletter names (excluding W). were recorded on Channell of aSony Ie-500a stereo tape recorder, with a presentation rate of1.5 items/sec in each list. Repeats of items within each list wereeliminated. The digits and letters were read, by a male speaker,as a continuous string in each list, with equal pauses betweeneach of the 14 items. The purpose of the digits was to ensurethat shadowing had commenced at the time of presentation ofthe first letters. Without the digits at the beginning of the lists,the first few letters would have been. perceived more clearly thanthe remaining letters because of the absence of stimulus maskingfrom the shadowing voice. The letter for recall was designated bythe recording of a 10Q-msec tone (approximately 900 Hz) onChannel 2 of the tape recorder at the onset of the appropriateitem. The tone accompanied a letter in each of the 10 serialpositions four times. with random selection of the criticalposition within this constraint. A relatively slow presentationrate was used because perfect shadowing performances, collectedon a second tape recorder, were demanded from Ss. In practice,all lists were scored, regardless of whether or not perfectshadowing had been observed. Several more lists were recordedto give practice at the shadowing task, and three lists similar tothe experimental lists were prepared for pratice at the combinedtasks. Presentation of stimuli was over Eagle SE-l stereoearphones.

ROLE OF PERCEPTUAL FACTORS IN THE SERIAL POSITION EFFECT 347

expecting more quickly than we do other events. Hence,the 5 would be predisposed towards, or have a "set"towards. hearing the click and so may associate thisevent with an item earlier than the item which issimultaneously presented but which is still beingprocessed. In the present experiment, however, thetendency to offer earlier items was not so pronounced incomparison with the offer of later items (t = 1.25, df =19, n.s.). The effect may have been confounded by theshadowing activity. The shadowing may. for instance,have reduced the subjective set for the tone by directingthe S's attention towards the verbal material.

Ear Differences

From the groups of Ss listening to the stimuli withdifferent ears came a curious, though nonsignificant,trend. The work of Kimura (I 961, 1964), Bryden(1967), Darwin (1971), and others indicates a right-earadvantage (REA) for the perception of verbal materialand a left-ear advantage (LEA) for the perception ofnonverbal material such as clicks, tones, and melodies. Inthe present experiment, in which verbal and nonverbalmaterial were presented simultaneously to opposite ears,it might be expected, therefore, that Ss who werepresented with verbal material to their right ears andtones to their left ears would perform better than Ss inthe reverse condition, who would not be given theindependent-ear advantages for perception defined bythe previous asymmetry work. This was not 'the case.When letters were presented to Ss' left ears, critical itemswere recalled on 77.5% of all occasions, whereas whenthe critical item was presented to the right ear, the recallrate was 66.25% (F =3.18, df =1,18, n.s.).

DISCUSSION

Position Effects

The experiment reported here did find a deviationfrom the exponential decay or cumulative interferencefunction which might be expected if each item had anequal initial representation in memory. Whereas theprobability of recall decreased as the time and numberof items between presentations and retrieval increasedup to a point, the items presented in Serial Position 1were recalled more often than those presented in SerialPosition 3, despite the extra decay and/or interferenceto which the earlier items were subject. This resultcreates a difficulty for accounts of primacy based onrehearsal and on proactive interference, for both of thesetheories postulate interactions between the items of thelist during the storage processes tobe the cause of theeffect. Such interactions were not present in thisexperiment. as only one item per trial was stored forsubsequent retrieval. This assertion is dependent.however. upon two basic assumptions: that theshadowing task effectively prevents the S from attending

to material other than that being shadowed, and that PIfrom items not designated as critical, but which may beincidentally retained, is minimal. Time/item decay andRI predict the decrease in recall with earlier presentationposition, but similarly cannot account for the increase inrecall over the initial serial positions. If items receivingattention gain differential processing according to theirinput serial positions, however, with early items beingprocessed more efficiently because they are subject toshorter delays of perceptual analysis than are later items,then the resultant stronger traces of initial items mightoverride any decay/interference influences moreeffectively than those items whose traces are weakthrough insufficient processing.

It is further possible that the primacy effect wouldhave been magnified in the absence of the four digits,which might have reduced the perceptual distinctivenessof the initial letters. These digits were redundant to thememory task, but were necessary to avoid differentialmasking by the shadowing voice within the series ofletters. The primacy here, therefore, reflects theseman tic distinctiveness of the initial letters: they arefrom a different semantic category to their precedingitems, and this distinctiveness may have enhanced theirprocessing and encoding when indicated as critical.However, while the listener was analyzing the final digit,a refractory period effect may have impaired theperception of the following item, the first letter, andhence reduced the primacy.

Ear Differences

Apart from the difficulty of explaining a result whichis so inconsistent with the literature-that of a LEA forverbal material-two further problems are present. First,independent groups of Ss yielded the data for the earscomparison, whereas within-S comparisons arepreferable when investigating slight differences whichshould be present for each individual but which haveconsiderable between-S variation. Second, the Ss wererequired to shadow the material presented. Springer(1971) found that the REA for verbal material is aperceptual phenomenon and not simply a function ofthe asymmetry of verbal output, but this does notexclude the possibility of the continuous verbaltranscription of a message influencing the lateralizationof the processing stages.

Rosenzweig (1951) and Kimura (1961) consider theasymmetry of auditory perception to be due to thesuperiority of contralateral pathways over ipsilateralpathways. The left hemisphere is generally dominant inthe perception of verbal material and hence suchmaterial is perceived better when presented to the rightear than when presented to the left. The "split brain"experiments (Gazzaniga & Sperry, 1967: Sperry. 1968)indicate that both hemispheres are capable ofmanipulat ing verbal rnaterial. but tha t only thedominant hemisphere is able 111 control the speech

348 UNDERWOOD

mechanisms. Shadowed material would then have to beprocessed by the left hemisphere, at least at the outputstage, although it is possible that the two types ofmaterial may have been processed by either of thehemispheres in the present experiment. Treisman andGeffen (1968) also consider the possibility of speechmessages being monitored by the right hemisphere undercertain circumstances. The necessity for the integrationof the two sources of information for the S to tag oneletter as being critical argues against the total separationof processes, but one highly speculative explanation ofthe curious effect observed here is that the mostefficient way of handling simultaneous verbal andnonverbal 'information presented to different ears is toavoid multiple interhemispheric transfers by distributinginformation arriving in each ear along the ipsilateralpathways. This would be of particular benefit in theshadowing situation where verbal material presented tothe left ear would be perceived and output without anyinterhemispheric transfer of information.

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(Received for publication July 17, 1972;revision received December 11, 1972.)