Output Loss or Rehearsal Loop - Output-time Versus Pronunciation-time Limits in Immediate Recall for Forgetting-matched Materials

7/26/2019 Output Loss or Rehearsal Loop - Output-time Versus Pronunciation-time Limits in Immediate Recall for Forgetting-

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Journal of Experimental Psychology:Learning, Memory, and Cognition1998,Vol . 24,No.2,316-335

Copyright 1998 by the American Psychological Association, Inc.0278-7393/98/$3.00

Output Loss or Rehearsal Loop? Output-Time VersusFtonunciation-Time Limits in Immediate Recall

for Forgetting-Matched Materials

Barbara Anne DosherUniversity of California, Irvine

Jian-Jiang MaColumbia University

Forgetting during recall may be one limit on memory span. Output time and accuracy ofimmediate serial recall using spoken and keypress responses were measured for digit, letter,and word sets approximately matched in phonemic discriminability and in immediaterecognition memory. Nonetheless, the materials differed from one another in recall span, inoutput time during recall, and in pronunciation time (speech rate). Recall output timesaccounted precisely and completely for the measured memory span for these matchedmaterials. Pronunciation times are correlated with recall output times, but output time gives aslightly better ac count of recall accuracy. The output time equivalent t o the rule that short-termmem ory span corresponds to the num ber of items that can be said in about 1.5-2 s is that spancorresponds to the number of items that can be recalled in about 4- 6 s. Additional variations inspan reflect differential item interference.

Forgetting During Imm ediate Recall

In traditional measures of short-term or w orking m emory,items are recalled sequentially. Ordered recall is a tempo-rally extended act. Loss of information from memory mayoccur not just during explicitly labeled retention intervals,but during the process of recall itself. In this article, wedocument the relation between accuracy and output time inrecall and model the impact of materials-induced variationsin output time on forgetting during recall.

Opinions on the importance of temporal limits in short-

term memory (STM) are in strong opposition. Lewan-dowsky and Murdock (1989), following Waugh and Norm an(1968), stated that "relative to interference, decay has beenshown to be of little importance in forgetting" (p. 28). TheTOD AM model (Murdock, 1982) and its application toserial recall (Lewandowsky & Murdock, 1989) include notime-dependent forgetting. Similarly, in the feature model ofNairne (1990; Neath & Nairne, 1995), "forgetting . . . is . . .a particular form of retroactive interference and is notdependent on time per se" (Neath & Nairne, 1995, p. 430).In contrast, Baddeley (1986; Baddeley, Thomson, &Buchanan, 1975) views time-depend ent forgetting as the keyto explaining STM span. Items are forgotten over time, but

Barbara Anne Dosher, Department of Cognitive Science andInstitute of Mathem atical Behavioral Scienc e, University of Califor-nia, Irvine; Jian-Jiang Ma, Department of Psychology, ColumbiaUniversity.

This work was supported by Grant SBR-9396076 (NSF89-19498/AFOSR89-NL295). Data collection was carried out at ColumbiaUniversity; modeling and w riting were carried out at the Universityof California, Irvine.

Correspondence concerning this article should be addressed toBarbara Anne Dosher, Department of Cognitive Science, 3151Social Science Plaza, University of California, Irvine, California92697-5100. Electronic mail may be sent to [email protected].

can be revived if they can be rehearsed within a criticalinterval. In the perturbation model of Estes (Lee & Estes,1977, 1981), cyclic reactivation of memory codes occursover time, and hence time is a key component to forgettingin this model as well.

Item-related interference is defined here as forgetting thatdepends only on the kind and number of items processedduring a retention intervalmemory is decremented foreach item processed (Lewandowsky & Murdock, 1989) oras a result of adjacent subsequent items (Nairne, 1990;Neath Nairne, 1995). Encoding of each new item (whetherin input or in response) results in a decrement of memory.Time-related loss is defined here as forgetting that dependson the amount of time taken to process those items. It neednot, and probably does not, reflect passive decay. It mayreflect interference from mental processes carried out duringthe interval. Another label for this might be nonspecificinterference.

Our assumption is that both item-related interference andnonspecific time-related loss contribute to forgetting inimmediate memory. This is consistent with an elegant earlytreatment of items and tim e as joint contributors to forgettingin short-term recognition (Wickelgren, 1970). This articlefocuses on the contributions of time-related loss, but this

should not be taken to imply that item-related interference isnot centrally important. Phenomena such as the lexicalityeffect (e.g., G. D. A. Brown & Hulme, 1995) or thephonological similarity effect (e.g., Schweickert, Guentert,& Hershberger, 1990) on span almost certainly reflectdifferences in item encoding and interference. Our strategyhere is to control or match for item interference to revealclearly the impact of time-related forgetting and, specifi-cally, of time-related forgetting over the output intervalduring immediate recall.

We begin with a review of the temporal correlates ofmemory span. Limits on output recall time are related to

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FORGETTING MOD EL OF SHORT-TERM MEMORY 317

Baddeley's (1986) idea that recall span (that list length thatis correctly repeated half the time) is limited by the speed ofsubvocal rehearsal. Temporal limits on recall due to forget-ting are trea ted first at the level of the entire list. A simplifiedforgetting model is developed for span results. Extensions toserial-position da ta are also briefly considered.

Time and Decay in Recall Sp an

Early conceptions of the limits on im mediate serial recall,or span, emphasized the item-limited capacity of STM(Miller, 1956). However, a purely item-limited STM cannotaccount for the now well-documented relation of immediateserial-recall spans to time limits on speech rates. Forexample, the measured spans for certain materials arerelated to the time required to say the items (i.e., Baddeley etal., 1975; Schweickert & Boruff, 1986; Zhang & Simon,1985). These observed relations of span to speaking timeprompted Baddeley et al.'s (1975) influential proposal thatrecall is limited to the number of items that can be pronounced inapproximately 1.5-2 s. This article examines whether similartime limits apply during (he act of ordered recall.

The Articulatory Loop

Baddeley (1986) proposed that the time limits on orderedrecall reflected the operation of an articulatory (rehearsal)loop. Items in STM or working memory were assumed todecay to the point of loss unless they could be reactivated bymeans of rehearsal (Baddeley et al., 1975; Reitman, 1974).Rehearsal presumably requires the repeated subvocalizationof list items "som ewh at analogous to a closed loop on a taperecorder" (Baddeley, 1986, p. 82). Hence, speech rate wasconsidered critical to the measured spans for different

materials (Schweickert & Boruff, 1986), languages (Ellis &Hennelley, 1980; Zhang & Simon, 1985), and individuals(Hulme, Thomson, Muir, & Lawrence, 1984; Hulme &Tordoff, 1989).

Initial observations of the temporal limits on recall weredesigned to mimic subvocal rehearsal times. Most of theseearly observations of speech rates and span measuredpronunciation times in a variety of tasks, but did not measurepronunciation time during the span task itself. For example,Vallar and Baddeley (1982) measured pronunciation timeper item by asking participants to cyclically repeat smalllists of three items 10 times. Other studies (e.g., Baddeley etal., 1975) measured the time for participants to read aloudlong lists of items as quickly as possible. Estimated time p erword was calculated from these reading times. More recentstudies (Schweickert & Boruff, 1986; Schweickert et al.,1990) measured speeded reading time for lists of the samelength and com position as those tested in the span task itself.

Time Limits During Recall Production

Temporal decay of memory items should occur not justduring periods of cyclic rehearsal, but during the output ofthe items in the act of recall. Theoretical simplicity suggeststhat the limits on decay during rehearsal and during recallshould be similar (see also G. D. A. Brown & H ulme, 1995).

Schweickert and Boruff (1986) very clearly discussedapplying the decay limits to recall, although they did notactually m easure the time taken to recall lists of span length.They suggested that ordered recall would be successfulwhenever the entire list was rehearsed before the tracedecayed to a low criterion level or whenever the entire listcould be spoken during recall prior to the time of criticaltrace decay. They formalized the latter by the equation P c =P(tT < fv), where P c is the probability of correct recall, tr isthe time to reproduce (speak) the list, and tv is the time bywhich the verbal memory trace has decayed. The temporallimit tv corresponds to the hypothesized limit of about 2 s,and this can be related to the span, s , by the speech rate, r, initems per second: s = rfv.

Cowan et al. (1992) pursued the idea of temporal limitsoperating during recall output. They manipulated the posi-tion in to-be-recalled lists of words that were long or short tosay, but they did not measure recall output times. Theimportance of output time during recall was inferred bynoting that recall accuracy was most damaged by placing

words that took longer to say early in the list. Cowan et al.estimated the delay during recall by calculating fromseparately measured speaking times for short and longwords, and they found that error rates were higher for itemsthat were calculated to occur after a longer output time. Avons,Wright, and Pammer (1994) also reported that the output processis critical; they compared probed and serial recall and showedthat word-length effects are related to output delays.

Several studies have reported the relation of output timeduring recall to span performance. These studies docu-mented the time to recall span-length lists for certainmaterials, languages, or subgroups. Stigler, Lee, and Steven-son (1986) found that the time to recall a span-length list ofdigits was 2.91 s for their native English speakers and 2.42 sfor their native Chinese speakers. These values were similarto each other and to the 1.5-2.0-s limit of Baddeley (1986)despite the larger item spans for Chinese digits. Schweick-ert, McDaniel, and Riegler (1994) showed that both wordlength and a study manipulation (generation vs. reading)affect the duration of the recall response. Cowan (1992)measured the duration of spoken recall for span-length listsfor 4-year-olds and found that children w ith larger spans didnot necessarily recall at a faster rate than those w ith shorterspans. Cowan, Keller, Hulme, Roodenrys, and Rack (1994)found that word length affected the duration of articulationin recall output, but that other factors controlled pause andinterword intervals.

Perhaps the strongest evidence for forgetting duringoutput comes from Cowan et al.'s (1992) research, in whichoutput time was not actually measured. The current ap-proach was specifically designed to document the forgettingduring recall directly attributable to output delays.

Recognition-Matched M aterials in Recall

In this article, we report accuracy and duration of recallfor three selected materials sets. The goal was to constructsets that were matched in interference propertiesforgetting-matched setsy et were spoken at different rates. Ordinarily,


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318 DOSHER AND MA

encoding or interference differences between materials mightdominate o ther factors in determ ining span. If item interfer-ence is controlled with our matched materials, then theimportance of output delays will be simply revealed.

In the classic materials effects in STM or workingmem ory, ordered recall span has varied from 2 or 3 items fornonsense syllables to 7 or 8 items for digits. In Cavanagh's(1972) summary of span estimates from the literature, theaverage span for digits was 7.7 items, for unselected letterswas 6.35 items, and for larger word sets was 5.5 items.Digits may yield high spans because digits are nearlyoptimally spaced for discriminability in phonemic space(Sperling, Parish, Pavel, & Desaulniers, 1984; Sperling &Speelman, 1970).

However, digits were compared in our experiments, notwith unselected letter and word sets as in Cavanagh'sreview, but with sets that were approximately equated forphonemic confusibality. On the basis of auditory confusibal-ity measured by Tetzlaff (1988), a subset of discriminableletters closely approximates digits in overall discriminabil-

ity. A third materials set was developed from the digit-matched pseudodigits of Sperling et al. (1984) by choosingwords that sounded almost like each of their (nonsense)pseudodigits. The pseudod igits were constructed to have thesame phonemic relations to one another as a set as do theEnglish digits. These m aterialsdigits, selected letters, andmatched words are listed in the Method section.

These sets were chosen to match in phonem ic confusibal-ity and were tested with immediate recognition to verify thatthey were forgetting matched (recognition matched). Inimmediate recognition, we could exactly control the timefrom study to test, so any differences between materials setswould necessarily reflect item-related interference. Therelevant literature on immediate recognition can be summa-rized briefly. Various materials sets have been tested forimmediate item recognition for lists of lengths up to aboutsix items in the standard recognition-retrieval task ofSternberg (1966, 1969, 1975). Materials with larger spanstend to have smaller slopessmaller increases in responsetime with increasing list length (H. L. Brown & Kirsner,1980; Cavanagh, 1972; Puckett & Kausler, 1984). Forexample, Cavanagh (1972) listed Sternberg's slopes fordigits, all letters, and larger word sets as 33 .4,4 0.2 , and 47.0ms per item, respectively.

Although the unmatched materials sets considered byCavanagh (1972) differed in immediate recognition, ourselected materials lists did not. The sets of digits, selected

letters, and selected words used here were tested in Stern-berg's (1966, 1969) immediate-item-recognition paradigm(Dosher, 1991, 1994). Item recognition was evaluated forsingle test items within 1 s of the end of list presentation.Our matched materials sets showed nearly identical serial-position profiles for list lengths of two through five items,measured in both the speed and accuracy of item recogni-tion.1

In general, materials sets differed in both encoding andinterference p roperties, and these differences may have beenlarge with respect to the impact of delays during output.However, our materials sets were approximately matched in

phonem ic confusability and recognition. The working hypoth-esis of the current study is that by extension, these sets werealso forgetting-matched sets with respect to the item andorder information necessary to support ordered recall. Theworking hypothesis was verified b y the results reported h ere.

With item-related interference matched or controlled, theimpact of time during recall output (or of retention time)could be evaluated without the complication of differentialencoding efficiency or differential forgetting rates. Thematched materials allowed us to ask the following questions:With item forgetting controlled, (a) is the recall span relatedto the time taken to produce the recall response, and (b) isthis relation the same as the relation between span andpronunciation rate?

Main Experiments

Ordered recall or span performance was evaluated for thethree sets of selected m aterials: digits, discriminable letters,

and matched words. There are a number of ways to measurememory span: Span is often measured by using a series oftrials of increasing list length with two or three trials at eachlength (i.e., Stigler et al., 1986). More stable estimates basedon a larger number of trials near span have been collectedusing staircase methods (Schweickert et al., 1990). How-ever, stringent tests of the relation between recall accuracyand recall time require measurem ents of accuracy and outputtime for different materials at several list lengths, not just atthe point of span. The reason for this is that comparisonsacross the entire range of performance are stronger thancomparisons at a single point. Therefore, like Schweickertand Boruff (1986), we collected enough data to evaluateperformance at a range of list lengths.

The output times during recall were measured with bothspoken and keypress output modes. The spoken mode is insome sense a standard mode of recall, but the keypress mod eyields timing data very simply,2 and there is some reason tobelieve that the keypress mode is essentially equivalent tospoken recall in important properties. In the keypress modeof recall, each item in the response corresponded to a singlekeypress. Although this equates the motor output demandsof the three types of materials, we nonetheless expectedkeypress recall to be quite similar to spoken recall in itssensitivity to materials factors on the basis of the results ofSchiano and Watkins (1981). Schiano and Watkins foundthat ordered recall with a pointing response yielded effectsof phonological similarity and word length equivalent tothose of standard spoken recall. For spoken recall, we

1 A simple forgetting model related to earlier models by Mur-dock (1985), Norman and Wickelgren (1969), and Wickelgren(1970) accounted very well for the data of all three selectedmaterials sets, using nearly identical initial acquisition levels andidentical forgetting rates (Dosher, 1991,1996).

2 The timing of each keypress was automatically recorded, butthis did not provide a measurement of the duration of the responseitself (see Sternbeig, Monsell, Knoll, & Wright, 1978; Cowan,1992).


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FORGETTING MOD EL OF SHORT-TERM MEMORY 31 9

provide a less detailed analysis of output time, one that givesa good estimate only of overall time to completion of theresponse. The information is, however, sufficient to yield astrong comparison to the keypress recall data. Finally, forcomparison to prior studies of Baddeley and others (e.g.,Baddeley et al., 1975), we collected a separate measure ofparticipants' pronunciation speeds while reading from thestimulus lists.

Overall, three types of sessions were run in the experi-ment: ordered recall with spoken output (Version A),ordered recall with keyp ress output (Version B) , and orderedlist reading for the measurement of pronunciation times(Version C). Because of similarities in design, the methodsfor the three versions are described jointly.

Method

Participants

Four native English speakers with normal or corrected-to-normalvision and hearing were tested extensively. Each participated in 14one-hour sessions, 7 sessions each of spoken recall and keypressrecall. First sessions were considered practice and were not furtheranalyzed. Participants also took part in 2 or more additionalsessions for the measurement of pronunciation (reading) speed andadditional informal controls.

Stimuli

The three materials sets with approximately matched discrim-inability (see Dosher, 1991, 1994) were as follows: the set of 10digits (0-9); a subset of 10 discriminable letters based on thediscriminable set of Tetzlaff (1988; F, H, M, O, P, Q, R, W , X, Z);and a set of 10 short words based on the pseudodigits of Sperling etal. (1984; HOW, GEM, KEY, FLU, TALL, SAYS, CUTS, VISIT,UKE, MAIN). Because all materials pools consisted of 10 items,explicit stimulus pool size was eliminated as a confounding factor(although the implicit pool may, of course, have been larger forsome sets). Data from recognition indicated that the same presenta-tion (study) rates could be used for these stimulus typ es.

Design

Each of the three materials sets was tested in separate blocks oftrials. Each set was tested in 4 blocks each per session. Theresulting 12 blocks per session were presented in random order.Each block, which presented only a single materials type, consistedof 15 trials. The list length on different trials was va ried from fourto nine items. There were two retention delays of 0.333 and 1.5 s(the time from the offset of the last stimulus to the onset of a tone

cue to begin recall). Participants produced items during recall withone of two methods or output modes: spoken recall (Version A) orkeypress recall (Version B). These were alternated across sessions.Pronunciation time, or list-reading time, (Version C) was measuredin separate sessions.

Procedure

At the beginning of each block, a message indicating thestimulus type for that block was displayed at the center of thecomputer screen until the participant pressed a key to initiate theblock of trials. The sequence of events for each trial, as illustratedin Figure 1, was as follows: A warning stimulus appeared at the

Output Time

Terminator Key

Participant Responses

Cue to Recall

Retention Interval

Mask

0.75 s per item

List Items

Warning Signal

Figure I. Illustration of a trial sequence for the letter materials.The definition of output time and output duration a re indicated.

center of the screen for 1.0 s. Following that, items of the listappeared in succession at the center of the screen for 0.75 s each.The last list item was followed by a brief (0.167-s) masking string(#########). After a retention delay of 0.333 or 1.5 s, a tonesounded briefly to cue the beginning of recall. A recall productiontime of 12 s was allowed. Recall productions that were notcomplete within this interval for keypress recall were terminated.The intertrial interval was approximately 0.5 s.

Participants were instructed to reproduce the memory list in thecorrect order and to terminate their response by pressing one of themarked termination keys on the keyboard. During spoken recall,participants spoke their answers into a tape recorder and indicatedtheir completion with a single keypress of a termination key. Tape

recorded responses were subsequently transcribed for analysis bycomputer. During keyboard response sessions, participants wereinstructed to press single-token keys in succession with the indexfinger and to end with a termination keypress. Single keys werelabeled to correspond to one item from each stimulus set, with thelayout shown in Figure 2.

What Baddeley (1986) and others have called pronunciationduration, perhaps better labeled list-reading duration, was mea-sured in a design identical to the recall sessions by simultaneouslydisplaying an entire list arrayed from top to bottom on the com puterscreen. The participant was instructed to take a breath and then readthe list quickly without pausing. The experimenter pressed a key atthe beginning and the end of the reading production. For a subset ofthe data, these measures were cross-checked by remeasuring theduration from tape recorded records of the session. The average

measu res differed by less than 5 ms from the original values.

Analyses

The data of each participant were summarized by computingmeasures on the accuracy and timing of responses. A trial wasscored as correct only if all the items on that trial were recalled inthe correct order and with no extra items. For other analyses, theproportion of correct responses per position (correct response andin the correct position) were also calculated.

For spoken output, only one measure of time during recall wasavailable: the output time, or the time from the cue to recall to thetermination key. For keypress recall, additional measures of output


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320 DOSHERANDMA

Keyboard Layout

TT TT TT

Q M W SAYS KEY [VISIT

'GEM LIKE JCUTS

FLU TAL Ll H OW. 'M AIN i

Figure 2. The keyboard layout used in the keyboard m ode of recall. Ma terials conditions wereblocked, so the digit, letter, and word portions we re used at separate times. The symbol TT indicates apossible terminator key.

durationthe time from the first keypress response to the termina-

tion key, as well as the timing of each keypresswere available.Pronunciation-duration averages were based on correctly pro-nounced trials, after eliminating one trial from each tail of the timedistribution. The measure of average output time during recall wascalculated by averaging over trials on which the correct number ofitems was produced. This definition of output time was selectedbecause we wished to measure the limits on recall for a typical list.Output time calculated only on correctly rec alled lists was notappropriate for these purposes: It would ha ve reflected increasingselection artifacts in trial tim es of increasingly longer list lengthsbecause it would have been selected for trials wh ere productionwas sufficiently rapid to escape delay-related losses. In thecalculation of output tim e, three trials from each tail of the timedistributions were eliminated. The elimination of outliers from theoutput-time data was based on an analysis of the distributions of

output times. It served the function of smoothing the data, but didnot fundamentally change the resu lts. Informal exploration of rulesthat filtered lower and slightly higher pe rcentages of trials from thetime calculations indicated that the results were not stronglydependent on these details of the analysis.

Span values estimate that point where performance correspondsto 50% completely correct rec all. Spans as a function of eitheritems or time were estimated by fitting an inverse cumulativeGaussian function (cf>) on the probab ility of correct recall. That is,P c = 1


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FORGETTING MODEL OF SHORT-TERM MEMORY 321

A1.0-

o

e

| 0 . 4 -

Q_

0.2-

o.o-

Spoken Recall^ Q "" Q DDigits

^ X \ OLeliers\ o \ AWords

Av \

4 6 8List Length

10

B1.0-

0.8-

lo0.6-O

^ 0 . 4 -CD

C L

0 .2 -

0 .0 -

Keypress Recall

g \ \\o

A\

\

DDigitsO LettersAWords

\ D

x \A \

A

6 8List Length

10

Figure 3 . A: Percentage of completely correct recall as a function of list length and materials forspoken recall; B: this percentage for keypress recall.

though smaller than, those usually seen for unselectedstimulus samples of letters and words. The spans differedacross materials for each recall mode and participant.Inverse cumulative Gaussian model fits of the percentagecorrect data, which allowed separate span estimates for eachmaterial, yielded significantly superior ^s compared w ithfits that assume a common span (cf. a test for additionalregressors, Wannacott & Wannacott, 1981). For spoken

Table 1Estimated Recall Spa ns (in Items) Across M aterials

Participantand source

SLSpokenKeypress

LMSpokenKeypress

JWSpokenKeypress

UJSpokenKeypress

AverageSpokenKeypress

Cavanagh

Digits

8.237.05

8.348.61

9.8910.00

9.048.43

8.818.60

7.70

Materials

Discriminableletters

7.546.07

7.907.62

8.929.36

7.937.24

8.007.57

6.35

Matchedwords

6.966.04

7.247.23

9.149.25

8.316.92

7.807.34

5.50

Note. The values for the average data refer to spans calculatedfrom the average proportion of correct recall data, not the averageof the estimated spans. Cavanagh1 s (1972) estimates av eraged ov ervalues from the literature that included full letter sets and large,randomly selected word sets.

recall mode, F(2, 13) = 33.41, 68.69, 16.75, and 27.06 forParticipants SL, LM , JW, and UJ, respectively; for keypressrecall mode, F(2 ,13 ) = 34.19, 24.11, 20.13, and 27.25,respectively; allps < .001.

Because of the variability in span from one participant toanother, the spans estimated from spoken and keypressrecall did not differ in an ANOVA across subjects: F( l, 3) =1.69, ns , on percentage correct data, and F( l, 3) = 1.89, ns9on estimated item spans. Nonetheless, keypress recall spans

were somewhat lower than spoken recall spans. The datafrom spoken and keypress recall could not be jointlydescribed with a single span function without loss in qualityof fit, when each participant's data were considered sepa-rately. We argue that the slightly lower keyp ress recall spansat least partially reflect longer output times for keyp ress thanfor spoken recall (see the Discussion section).

Recall Production Times

This section reports the effects of list length and materialson the time taken to recall the lists. We note that recall outputtime has a quadratic relationship to list length.

Figure 4 shows how the time for recall productiondepends on list length. Output timetime between the tonecue to recall and the termination keyis shown averagedacross subjects for spoken recall (Figure 4A) and keypressrecall (Figure 4B). Keypress output duration, which excludes thelatency to the first response, closely tracks keypress output time.These data are averaged across subjects; individual participantdata are consistent with (his graph.

Output times and d urations were fastest for digits, longestfor words, and intermediate for letters. This was so even forkeypress recall, where the physical demands of the response(press of a single key by the index finger) were identical forthe three types of m aterials. Digits were significantly faster


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322 DOSHER AND MA

1 0

3C D

E8 -

B 6Q-

2 -

Spoken Recall B Keypress RecallD DigitsO LettersA Words

'to" '

S

U

i

Q_

o

10-

8 -

6 -

4 -

2

D DigitsO LettersA Words

/

A /

A DA D/A/ /

/ o n

A

D/

4 6 8List Length

10 4 6 8List Length

10

Figure 4. Recall output time in seconds as a function of list length. A: Output time for spokenrecall; B: output time for keypress recall. Recall output times are a quadratic function of list length.

than letters, and letters were significantly faster than words:For spoken output times, r(3) = 2.92, p < .05, and t(3) =4.03, p < .025; for keypress output times, /(3) = 7.31, p < 01 .

the data. Inspection of Figures 5 and 7 and of the individualparticipant data in the Appendix (Figures A1-A4) indicatesgreater deviation between materials curves as a function ofpronunciation duration than as a function of output time. Thisrelation in the data for output duration is even cleaner.

A secondary analysis was performed that also supports theimportance of output time in memory span. If forgettingindeed occurs during the act of recall output, then trials thatare recalled more slowly should be associated with loweraccuracy. We divided trials at each list length for eachmaterials set for each participant into the fastest half and theslowest half with respect to output time and found that,indeed, trials associated with the fastest half of the outputtimes yielded higher accuracies than did trials associatedwith the slowest half. We also replicated, for each halfseparately, the relationship between accuracy and outputtime for the different materials shown in Figure 5. However,the output-time functions from the slower half and the fasterhalf, although they are close to one another, differ systematically.This issue is considered again in the Discussion section.

In summary, although both pronunciation duration andmeasures of recall output du ration do quite well in account-ing for the variations among these materials, the residualdeviations are smaller for the output-time measures. Spansare quite well described in terms of limits on output time

during recall; pronunciation duration may be effective inpredicting span primarily because of its correlation withoutput time. It should be recalled, how ever, that the ability ofoutput times (or pronunciation durations) to fully accountfor span performance applies only when the materials arecontrolled for differences in item interference. Materialswith substantial differences in item interference would notbe equated by consideration of the differences in output duration.

The Relationship Between Span and Production Rate

One side question is whether other commonly evaluatedrelationships between span and pronunciation duration arealso found between span and output time. One such finding

CO

CO

Spoken Recall

0.5 1.5 2.5 3.5Pronunciation Duration s)

B Keypress Recall

0.5 1.5 2.5 3.5Pronunciation Duration s)

Figure 8. Reca ll output times as a function of pronunciation duration for the same list lengths andmaterials. A: Output time for spoken recall; B: output time for keypress recall. Recall output timesare related quadratically to pronunciation duration and are much longe r than pronunciation duration.


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FORGETTING MODEL OF SHORT TERM MEMORY 327

8 -

705

I 6

5 -

4 -

Spoken Recall

Output TimeA Pronunciation Duration

B

9 -

8-

7 -

6-

5 -

4

Keypress Recall

I I I 1II 1

Output Timeo Output DurationA Pronunciation Duration

0 1 2 3 4Production Rate (WPS)

0 1 2 3 4Production Rate (WPS)

Figure 9. A: Item span shown as a function of average production rate in words per second (WPS)for output time (or duration) during recall and for pronunciation duration for spoken recall; B: theseitem spans for keypress recall. The linear relationships between item span and production rate foroutput-time measures are similar to the standard observation of a linear relationship between itemspan and production rate in pronunciation duration (e.g., Brown Hulme, 1995).

is a linear relationship between memory span and speechrate over various materials sets (G. D. A. Brown & Hulme,1992, 1995; Cowan et al., 1994; Hulme, Maughan, & Brown,1991; Hulme, Roodenrys, Brown, & Mercer, 1995). Brown andHulme (1995) focused their model of memory span withoutrehearsal around its prediction of this form of relationship.

Figure 9 shows the relationship of estimated m emory spanin items to mea sures of production rate in words per secondover the three materials sets for spoken recall (Figure 9A)and keypress recall (Figure 9B). Both panels show therelationship between span and pronunciation duration andspan and recall output time; the relation between span andoutput duration is also shown for keypress recall. Forpronunciation duration, on e production rate characterizes alllist lengths equally, because pronunciation du ration is linearin list length. For output-time m easures, one production rateignores slight differences for different list lengths, becauseoutput time is quadratic in list length. The often-reportedlinear relationship between item span and pronunciation durationis replicated in our data (r = .997 and .994 for spoken andkeypress, respectively). A similar, albeit quantitatively different,linear relationship holds between item span and output time(r = .998 and .998 for spoken and keypress, respectively, andr = .992 for keypress duration). To the extent that this commonlyevaluated relationship is a valid one, it holds for measures ofoutput time as well as for measures of pronunciation duration, thestandard measure of speech rate.

Discussion

Models of Tempo ral Limits in Recall

The articulatory rehearsal loop model. Baddeley's(1986) principle of time-related forgetting asserts that forget-

ting occurs during a cyclic rehearsal loop, where the speedof the rehearsal loop is measured in the classic way bypronunciation duration. Extensive prior evidence has docu-mented the relationship between speech rate measured bypronunciation duration and ordered recall span measured initems (Baddeley, 1986; Ellis & Hennelley, 1980; Schweick-ert & Boruff, 1986; Standing, Bond, Smith, & Isely, 1980;Zhang & Simon, 1985). The span length in items multipliedby the calculated average speech time per item has oftenbeen approximately 1.5-2.0 s. For our materials sets, theestimated pronunciation-duration spans across subjectsranged from 1.65 s to 2.46 s (Table 3). The observed relationbetween speaking rate for an item set and item span has beeninterpreted as evidence for a subvocal rehearsal loop.

The recall-output-time model. An alternative explana-tion of temporal limits on recall is an output-time model inwhich forgetting occurs during the act of recall itself. Recentstudies by Cowan (1992; Cowan et al., 1992), Stigler et al.(1986), and Schweickert et al. (1994) began the investiga-tion of the temporal limits on span that operate during recall.The current study extended these investigations with a

systematic analysis of output time and its relation to theaccuracy of recall over a range of list lengths, differentmaterials, and different modes of recall output. Our findingsoffer compelling evidence for a strong relation betweenrecall span and recall output times. Temporal limits duringthe act of recall can account for limitations previouslyattributed to temporal limits on a rehearsal loop .

However, the first principle of ou r analysis is that differentmaterials may differ significantly not just in the temporalproperties of rehearsal or output, but in item encoding anditem interference as w ell. It is precisely because we believeitem interference to be important that we approached this


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328 DOSHER AND MA

problem by defining sets of materials that were approxi-mately matched. Only when item interference is controlledcan the effects of output delays be revealed clearly.

Current Findings

Output times as an account of span. Differences inrecall accuracy for our matched materials, within eitherspoken or keypress output, are well accounted for bydifferences in output time for these materials. Wide varia-tions over materials in the probability of correct recall as afunction of list length were essentially equated when graphedas a function of recall output time or duration. Recall outputtimes account for limits on span better than, but certainly atleast as well as, classic measures of rehearsal speed such aspronunciation duration. We conclude that temporal limitsduring recall offer an alternative, and possibly superior,explanation of span performance that accounts for many ofthe phenomena previously attributed to an articulatoryrehearsal loop. In conflict with our view are two reports

(Smyth & Scholey, 1992, 1994) that questioned the impor-tance of output durations. However, these experimentsspecifically apply to spatial memory span and the "visuospa-tial sketchpad," and output times may be more important inverbal than in visuospatial STM .

Output times exceed the 2-s speech limit. Estimatedtime limits corresponding to span-length lists are substan-tially longer when calculated for output times or durationsthan when calculated for pronunciation times (classic speechrates). For our participants, recall span limits range from3.49 s to 5.24 s for spoken recall output times, from 5.92 s to7.13 s for keypress output times, and from 5.15 s to 6. 21 sfo rkeypress output duration.5 Although these estimates differsomewhat, they represent values that are approximately tw oto three times as long as the values estimated from pronun-ciation duration: Across subjects, output-time spans forspoken recall range from 1.65 to 2.48 times the pronunciation-duration spans (calculated for the same accuracy data). Forkeypress recall, the output-time spans range from 2.42 to3.59 times the pronunciation-duration spans, and the output-duration spans range from 1.77 to 3.12 times the pronuncia-tion duration spans.

Output time in different response modes. Perhaps themost interesting observation in compa ring the results for thespoken and keypress response modes is the similaritybetween the patterns of results for the two output modes.Although neither the recall accuracies nor the output timesare exactly identical for keypress and spoken modes, thepattern of effects is equiva lent In particular, output times inkeypress recall are slower for the slower-to-say letter andword stimuli, despite the fact that the m otor requirements ofthe responses in keyp ress recall are identical. The sensitivityof keypress recall to variations in pronunciation highlightsthe importance of articulatory phonemic coding in short-term working memory (see also Avons et al., 1994; Bishop& Robson, 1989; Caplan, Rochon, & W aters, 1992).

Another interesting aspect of the relationship betweenkeypress item spans and spoken item spans is that the spokenspans are similar to, but slightly higher than, the keypress

spans. This finding sheds light on the role and properties ofspeech-output interference in span limitations (Cowan et al.,1992). Under the output-interference view, one might haveexpected the spoken spans to be lower than the keypressspans because they include interference from the speechitself. One possible explanation is that output interferencetakes place at the level of articulatory planning codes(Caplan et al., 1992; Longoni, Richardson, & Aello, 1993;Waters, Rochon, & Caplan, 1992).

The average output time for span-length lists is slightlyshorter for the spoken than for the keypress output mode.The finding that the speech spans are slightly higher thankeypress spans is generally consistent with this observationunder the recall-output-time mod el. However, differences inoutput times do not account completely for variations inrecall accuracy over output mode even in our matchedmaterials. Spoken and keypress recall accuracy versusoutput-time functions do not overlap. One speculation is thatthese modest differences between mode of recall reflectdifferential interference characteristics during output Time

differences may also be introduced by the necessity of visualreorientation from screen to keyboard in the keyboard outputmode. Furthermore, paradigms mat explicitly manipulate outputrate would almost certainly introduce compensatory rehearsal.

Output time acc ounts for materials variation not partici-pant variation. Output time is expected to be the primaryfactor in limiting span for different materials only ifitem-encoding and interference factors are controlled. Addi-tionally, our analysis deals only with materials variationseither within a single participant or in the average over a setof participants. Our analysis does not imply that output timeis the determining factor in participant to participant varia-tion in spans; the output time corresponding to spanperformance varies widely from participant to participant.This is generally consistent with the observation that aver-age speech rates predict group differences, but individualspeech rates do not necessarily predict individual memoryspans (e.g., Henry, 1994). It is perfectly consistent with our

5 Stigler et al.'s (1986) estimate of the average span-lengthoutput duration was 2.91 s for American college students, notice-ably shorter than our average of 5.88 s. This difference may reflectmethodological differences in estimation: (a) Stigler et aL calcu-lated span from two (rials at each increasing list length until bothtrials were incorrect. Span (50% accuracy) is expected to yield zeroof two correct trials 25 % of the time . Hence, their estimates of spanlength may have been biased to be low. (b) Stigler et al. reportedduration of correct span-length trials only. Our distribution analy-

ses showed that correct trials are associated with shorter outputtimes, (c) Stigler et al.'s participants may have been tested on asfew as six to eight lists. Our participants were tested on manyhundreds of lists. All of these factors may have lowered estimatesof span in Stigler et al.'s data relative to ours, (d) Our estimateswere lengthened by 0.1-0.3 s by the inclusion of the terminatorresponse. Keypress output times, but not spoken output times,could have excluded the time for the termination response, but thiswould have made the two noncomparable and would have underes-timated output duration. Perhaps m ost importantly, our mea sures ofpronunciation-duration spans, output-time spans, and output-duration spans were collected within subject and can be compareddirectly.


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FORGETTING MODEL OF SHORT-TERM MEMORY 32 9

view that different participants, different conditions, or bothmay exhibit different levels of both item interference andtime-related interference.

Are output times a cause or an effect? Another issue iswhether output times are the cause or the consequence offorgetting. Is performance worse for words because they aremore difficult to remember and therefore take longer to

output during recall? Or is performance worse for wordsbecause they take longer to output during recall and, hence,more is forgotten? Some part of each explanation may havecontributed to the relationship between item span and outputtime during recall. We believe, consistent with the findings ofCowan et al. (1992), that delays in recall output of earlier itemsresult in more forgetting of later items (see model below).

Relation to Existing Views

In this section, we consider several different explanationsfor the current set of findings and their relation to existingviews or models of span tasks. One argument is that if

span-length lists can be remembered through a 4-6-s outputinterval during recall, then the 2-s limit in rehearsal speedestimated from pronunciation durations clearly cannot becorrect as an estimate of trace duration. Under this account,the success of pronunciation duration in predicting span is aby-product of the correlation between output time andpronunciation time. Pronunciation time indexes the articula-tory rate for particular sets of words. Although in recall theactual time spent in articulation is only a part of the timespent in production (Cowan, 1992; Sternberg et al., 1978),limits on speech rate for different items also slow recalloutput. Recent work suggests that the critical factor rests,not in articulation, but in phonemic representations (Caplanet al., 1992; Longoni, Richardson, & Aiello, 1993). The

slowing for certain materials seen in pronunciation duration,whether based in articulation or in phonemic planning, may bepredictive of recall accuracy because it is a good index of (at leastpart of) the materials-induced differences in recall output times.

Another possibilityone we feel is far less parsimoni-ousis that 2 s is the true limit on trace duration, but thatparticipants maintain performance for longer durations whilerecalling by integrating rehearsal into the output interval.Baddeley (1986, p. 82) proposed that participants use sub-vocal rehearsal during the recall process. More recentlyCowan (1992) proposed that memory scanning occurs inbetween the production of items in recall to refresh remain-ing list eleme nts. It is our opinion that a complex secondaryprocess of rehearsal during output is unlikely to yield theextremely precise relationship of accuracy to output times shownin the current studies. However, explicit introduction of delays inpaced output paradigms would force rehearsal and likely breakthe strong relation between output time and accuracy.

Yet a third possibility is that temporal limits occur both ina rehearsal process and during recall output. Rehearsalstrategies during encoding and the retention interval, as wellas limitations on output time, clearly should affect whichinformation is still in memory. It is possible that outputduration more accurately reflects limits on the memorytrace, but that pronunciation duration is measuring an

independent rehearsal mechanismone in which pronuncia-tion speed controls the likelihood that subvocal rehearsals oflist fragments occur during encoding. The cyclic rehearsalnotion, of course, has been an essential component of somevery impressive treatments of recall by Estes and colleagues(Lee & Estes, 1977, 1981). A genuine integration of cyclicrehearsal and forgetting during output would require exten-

sive data relating rehearsal rates and strategies to p ropertiesof subsequent recall. At this point, we do not know, forexample, whether cyclic rehearsal rates are more similar torecall output or to reading-pronunciation times. In the mode lbelow, we restrict ourselves to consideration of output timeas an index of effective retention intervals. This, however,reflects a desire for simplicity more than a strong claimagainst rehearsal mechanisms.

Finally, it is logically possible that the ability of bothoutput times and pronunciation times to account for thedifferences in accuracy of recall is epiphenomenaltheresult of both measures being related to a third, differentfunctional mechanism. One example of such an explanationis embodied in the feature-coding model of Neath andNairne (1995). They accounted for word-length effects onimmediate span by introducing the notion of segments inword storage. Words that take longer to say are coded inmore segments, and each segment introduces increasedprobabilities of memory loss. In their model, the segmentalstorage is causal, and pronunciation duration is related to thenumber of storage segments by a linear regression. Theiranalysis might be extended by postulating a second relationbetween segmental storage and recall output times. In thefeature model, segmental storage is a surrogate for time.Segmental storage increases the probability of forgetting,and is by hypothesis directly related to pronunciation time,and hence is strongly related to output time. In a similar

approach, G. D. A. Brown and Hulme (1995) proposed thatword-length effects can be accounted for by opposingprocesses of trace decay and redintegration.

Other Phenomena in Span

Articulatory suppression in the output-time model An-other argument given in support of Baddeley's (1986)construct of the articulatory rehearsal loop is the damagingeffect of articulatory suppression on short-term recall (Bad-deley & Lewis, 1984). However, articulatory suppression ismost effective w hen it extends throughout the recall interval(Baddeley & Lewis, 1984; Baddetey, Lewis, & Vallar,

1984). Under the articulatory rehearsal loop model, onemight expect that articulatory suppression would be mostdamaging if it interfered with rehearsal during stimulusinput and the retention phases of the span task. Under theoutput-time model, articulatory suppression during encod-ing may interfere with articulatory-phonemic coding of thestimuli at input, but articulatory suppression during recall isespecially important because it interferes with output and isalso likely to increase interference during the output period(Baddeley, 1986).

Nontemp oral effects on span. There is a host of phenom-ena that we do not expect to account for by differences in


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330 DOSHER AND MA

recall output times. These include essentially all phenomenathat rest on differences between materials in encoding orinterference factors. Examples of these effects includelexicality effects (Hulme et al., 1991, 1995; Roodenrys,Hulme, & Brown, 1993) and phonemic-confusability seteffects (e.g., Schweickert et al., 1990). In a lexicality effect,words are recalled better than nonwords when the sets are

controlled for pronunciation duration; we expect that wordsare encoded more efficiently and possibly suffer less interfer-ence from subsequent items than nonwords. In the phonemic-confusibality effect, it has been shown that confusable lettersets (by d,g,p .. .) exhibit reduced spans relative to discrim-inable letter sets (a,/, m . . . ) even though they are spoken atapproximately the same rate; we expect that the confusableletters suffer more interference from subsequent items thanthe discriminable letters. Output-time differences betweenmaterials that differ strongly on encoding or item-interfer-ence properties can provide only a partial explanation fordifferences in span between these materials. For a modelstructure that accomodates both encoding and interference

differences between materials, see the Serial-position datasection below.

A Forgetting Model of Span

Correct list reproduction. In this section, we consider asimplified descriptive model of serial recall that accounts forthe probability of completely correct list reproduction. Thissimple structural model is not meant to apply at the moredetailed process level of some existing quantitative modelsof memory (Drewnowski, 1980; Lee & Estes, 1977; Lewan-dowsky & Murdock, 1989; Neath & Nairne, 1995; Shiflrin& Cook, 1978), but rather is meant to explore the adequacy

of an output-time model in accounting for our data.The descriptive model makes several assumptions: The avail-

ability of the information supporting perfect reproduction of thememory list is represented by a strength value that varies fromtrial to trial. The strength of initial encoding is reduced byforgetting over output times, resulting in a diminished strengthvalue by the time output is complete. Hie distribution of strengthvalues for lists of a certain length and output duration is assumedto be Gaussian with mean u and standard deviation o\ Theprobability of correct recall is related to a threshold on strength: Ifinformation is of sufficiently high strengthexceeds a thresholdTthen perfect recall results. If not, then a recall error occurs.The use of a threshold strength to determine correct recall issimilar to mechanisms in the recall model of Lewandowsky andMurdock (1989).

In our descriptive model, the form of forgetting over timetakes on the exponential form (cf. Murdock, 1982; Norman& Wickelgren, 1969; Wickelgren & Norman, 1966; Wickel-gren, 1970). 6 The forgetting equation is

where

Table 4Forgetting-Model Parameters and Goodness of Fit

Mode andpredictor variable

KeypressOutput timeOutput durationPronunciation duration

SpokenOutput timePronunciation duration

\

8.668.51

8.55

9.019.66

.082

.088

.275

.122

.367

T

5.115.11

4.79

5.404.28

r 2

.988

.993

.958

.985

.955

Range

.942-.925

.966-.975

.878-.915

.914-.977

.905-.971

Note. Values are reported for fits to the average data. The rangesListed for r 2 ar e for individual participant fits. The parameter valueslisted are for a model in which P c 1 [T, p(t), a] , where T is arecall threshold on a strength distribution with mean u(t) = \ e "P ' attime t (or for the corresponding list length), and standard deviationCT = 1. \ = encoded strength for the entire list prior to forgetting;3 = the forgetting rate; P c = proportion correct;


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Serial-position data. An extensio n of this forgettingmodel considers the output times and error rates for eachserial position. The serial-position data are considered onlybriefly, averaged across subjects. Statistical details areomitted for brevity. The purpose here is simply to illustratehow a similar model form can be extended to account forserial-position data. In the serial-position m odel, we use thetotal retention delay, including the tim e following the item instudy and the time prior to output of each item in recall, as apredictor of accuracy on that item:

P k, L, M ) is the probability of correct recall for position k of listlength L for materials set M. The a(k)s are encoded strengths ateach input position, and tfk, L, M) is me retention time for that

position, list length, and materials set Hie a(k)s and fl areconstant over materials because these materials were matched

Figures 10A-10C show the average serial-position datafor keypress recall trials of the current experiment for eachof the materials. Only keypress recall data are shownbecause only the keypress mode measured item-by-item

recall times. Error rates are higher for wo rds than for lettersand are higher for letters than for digits especially at the endsof lists. This is where cumulative output times differ mostacross the materials. The solid lines show the predictions ofthe output-forgetting model. Figures 10IM0F graph theobserved versus the predicted values of the percentagecorrect for the three materials sets, in each output position.Although there are some systematic deviations between themodel and the data for the longest list length, nonetheless thefit is quite good (R2 = .93, r2 = .94).

2 4 6 8

Serial Position

0 2 4 6 8 10

Serial Position

0.0-

2 4 6 8 10

Serial Position

0.0-

0.0 0.4 0.8Predicted Percent Errors

1.0-

0.8-e| 0 - 6 -

|o.4Q .

0.2-

o o

tt rs //

/


o o


Figure 10. Serial-position data and mode l fits for the three materials sets. Percentage correct isshown as a function of list position for each list length for keypress recall for average data. A: Datafor the digit set; B: data for the letter set; C: data for the word set. The symbols are the data, and thelines represent a m odel fit based on the m easured rec all time at each position of each list length. Eachsymbol corresponds to data from a list of a particular length, from four to nine items. D: Therelationship between predicted and observed percentage of errors for the digit set; E: this relationshipfor the letter set; F: this relationship for the word set. The m odel fits quite w ell.


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332 DOSHER AND MA

In this model, the a(k)s and p are the same for the threematerials sets, and the time elapsed during study is the sam efor the three materials sets, so predictions about the patternof differences in accuracy over materials result from differ-ences in the time required to output items. The fact that themodel fits the serial-position data quite well lends strongsupport to the importance of delays du ring output. Items areforgotten if they cannot be produced before they decay. Thisanalysis provides very specific support for the claim ofimportance of output delays by Cowan et al. (1992) and byAvons et al. (1994). A more detailed analysis and a modelthat includes both item and time loss are reported in a relatedarticle in which study rate is varied to partially decoupleitem interference and time losses (Dosher, 1994).

Conclusions

1. In explaining the temporal limits on immediate m emoryspan for different materials, an explanation based on tempo-ral limits on recall output provides an alternative to an

explanation based on the temporal limits of an articulatoryrehearsal loop. In our data, measured output times areconsistently somewhat better than pronunciation times inequalizing span performance for matched materials. Al-though it is premature to eliminate rehearsal explanations oflimits on span, output-time contributions to the limits onspan deserve serious consideration and further study.

2. Output time alone is not a complete predictor ofperformance. Other factors, even such apparently mild onesas output mode, modulate the amount of loss in a givenoutput time. Manipulations such as irrelevant speech orarticulatory suppression are expected to m odulate the impactof output time (as well as initial coding) even m ore strongly.As we began by stating, time-related loss does not reflectjust passive decay; it reflects generalized activity-basedinterference from m ental processing.

3. There are two contributions to forgetting in STM:item-related interference and nonspecific time-related loss.Our materials sets were matched for phonem ic discriminabil-ity and also for recognition and, hence, by hypothesis, foritem-related interference. Differential performance reflectednonspecific time-related loss during the time of recalloutput. Time-related losses do not preclude item-relatedinterference. Other nonmatched materials sets, such aswords and nonwords or discriminable and confusable letters,would produce differential interference that might easilydominate the limits imposed on output time.

4. A completely general descriptive model integrates bothitem-related interference and nonspecific time-related forget-ting (Dosher, 1994). We proposed a simplified time-relatedforgetting mo del. This model accounts very well for the dataof our experiments both at the level of completely correct listrecall and at the level of serial-position data.

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Appendix

Full Data Graphs for Each of the Individual Participants

Figures A1-A4 show the percentage of correct recall as afunction of list length for (A) spoken reca ll and (B) keypress rec all,the percentage of correct recall as a function of output time for (C)spoken recall and (D) keypress recall, and the percentage of correct

recall as a function of pronunciation duration for (E) spoken recalland (F) keypress recall. For all 4 participants, recall output timesequate performance slightly better than pronunciation durationdoes, although pronunciation also does quite well.

Spoken Recall

4 6 8 10List Length

B Keypress Recall1.0

0.24 6 8 10

List Length

A Spoken Recall1.0

0.0-I4 6 8 10

List Length

B Keypress Recall

i 830.6-I0.4io.2

0.0

\ " \

4 6 8 10List Length

2 4 6 8Spoken Output Time (s)

2 4 6 8 10Keypress Output Time (s)

0.40.2

Q-0.0


2 4 6 8 10Keypress O utput Time (s)

0.5 2.0 3.5Pronunciation Duration (s)

0.5 2.0 3.5Pronuncia tion Duration (s) Pronuncia tion Duration (s) Pronunciation Duration (s)

Figure AL Percentage of correct recall data for Participant JW. Figure A2. Percen tage of correct recall data for Participant SL.


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FORGETTING MODEL OF SHORT TERM MEMORY 335

Spoken Recall B Keypress Recall

f c 0 2

o. 0.04 6 8

List Length4 6 8 10

List Length

A Spoken Recall1.0 i

0.0 i4 6 8

List Length10

B Keypress Recall1.0

4 6 8List Length





Pronunciation Duration (s)

0.5Pronunciation Duration (s) Pronunciation Duration (s) Pronunciation Duration (s)

Figure A3 . Percen tage of correct recall data for Participant LM . Figure A4. Percentage of correct recall data for Participant UJ.

Received Ju ly 5,1996Revis ion rece ived Apr i l 2 8 ,1 997

Accep ted May 4 ,1997

Documents

Output Loss or Rehearsal Loop - Output-time Versus Pronunciation-time Limits in Immediate Recall for Forgetting-matched Materials