Coding Position in Short-term Memory - Semantic Scholar

IN TERNATIO NAL JOU RNA L OF PSYCH OLOGY, 1999, 34 (5/6), 403± 409

Coding Position in Short-term Mem ory Richard N.A. Henson University C ollege, London , UK

M any formal models of short-term memory have recently been proposed (e.g. Anderson & M atessa, 1997; Brown, Preece, & Hulme, in press; Burgess & Hitch, 1992; H enson, 1998; Lee & Estes, 1981; Lewandowsky & M urdock, 1989; M urdock, 1995; N airne, N eath, Serra, & Byun, 1997; N eath, 1993; Page & Norris, 1998). A n important issue addressed by these models is the problem of serial order: that is, how people store and retrieve a novel sequence of items in the appropriate order. A popula r solutio n to this problem is to assu me that each item is coded for its positio n within a sequence. The present article discusses three differen t means of codin g position . It is argued that the pattern of errors people make when they misrecall a sequence supports the hypothesis that positio n is coded relative to the start and the end of a sequence. Other evidence, however, suggests that positio nal codin g is also sensitive to temporal factors. A new model is described that reconciles these two strands of evidence. Plusieurs modeÁ les formels de me moire aÁ court terme ont e te re cem ment propose s (Anderson & M atessa, 1997; Brown, Preece, & Hulme, sous presse; Burgess & Hitch, 1992; Henson, 1998; Lee & Estes, 1981; Lewandowsky & M urdock, 1989; M urdock, 1995; Nairne, N eath, Serra, & Byun, 1997; Neath, 1993; Page & Norris, 1998). Une question importante aborde e par ces modeÁ les est le probleÁ me de l’ ordre se riel, c’ est-aÁ -dire co mment les gens em magasinent et reÂcupeÁ rent une nouvelle se quence d’ items dans l’ ordre approprieÂ. Une solutio n populaire aÁ ce probleÁ me est de supposer que chaque item est code selon sa positio n da ns une seÂquence. Cet article discute de trois facË ons diffe rentes de coder la positio n. Il est propose que le patron d’ erreurs, lorsqu’ une se quence n’ est pas rappele e correctement, appuie l’ hypotheÁ se d’ un codage de la positio n en rapport avec le de but et la ® n d’ une se quence. D’ autres donn e es suggeÁ rent que le codage de la positio n est aussi sensible aÁ des facteurs temporels. U n nouveau modeÁ le est de crit qui incorpo re ces deux ensembles de re sultats.

M odels of serial order in short-ter m me mory can be classi® ed as chaining, ordinal, o r pos itional models (H enson, 1998) . C haining models, such as TO DA M (L ewandowsky & M urdock, 1989 ; M urdo ck, 1995) , store order via associatio ns between successive ite ms. Th e order of items can be reconstru cted by chaining along th ese associations, such that each ite m beco mes the cue for retrieval of its successor. H owever, tho ugh a long-standing a nd recurring idea (e.g. Ebbinghaus, 1964 ; Jordan, 1986) , there is in fact little evidence to suppo rt chaining mo dels of short-ter m me mory (H enson , 1996 ; H enson, No rris, Page, & Ba ddeley, 1996) . Ordinal models, such as the Primacy M od el (Page & No rris, in press) store order via the relative strengths of item represen tatio ns in me mory. These models escape many of the criticisms of chaining models. H owever, becau se order is stored relatio nally, ordinal models cannot accoun t fo r position al errors in serial reca ll (see following). These errors demand some approxi mate coding of ite m positions in a sequence, as assu med in position al mo dels, such as the List M emory M odel (A nderson & M atessa, 1997) , the A rticulatory L oop M odel (B urgess & Hitch, 1992 , 19 98), the O SCA R M odel (B rown et al., in press), the Start-E nd M odel (H enson, 1 998) , th e Perturbation M o del (L ee & E stes,

19 81), and th e Positional D istinctiven ess M o del (N airne et al., 1997 ; Neath, 1993).

THREE CODINGS OF POSITION T he position s of ite ms within a sequence can be de® ned in te mpo ral, absolu te, or relative ter ms. A cod ing of te mpo ral p osition assu mes th at each item is associated with its ti me o f o ccurren ce (Y nte ma & Trask, 1963) , perhaps relative to the start (B rown et al., in press) or end (G lenb erg & S wanson, 1986 ; N eath , 1993 ) of a sequence. In the OS CA R model of B row n et al. (in press), for exa mp le, items are associated w ith the states of te mporal oscillato rs of different frequencies (e.g. the hour a nd minute hands of a clock, F ig. 1a). By resetting the oscillators (rew inding the clock), the order o f items can be recalled. A coding of absolute p osition assu mes that items are associated w ith their ordinal position (® rst, second , third, etc.), rega rdless o f th eir time of occurrence (A nderson & M atessa, 1997; B urgess & H itch, 199 2 ). In the connectionist model of Bu rgess and H itch, for examp le, items are associated w ith a context signal, implemented as a w indow of activity that moves across an array of no des

Request s for reprints shoul d be addressed to D r R. Henson , Institut e of Cognitive N euroscie nce, Univers ity College London , 17 Queen Square, London WC 1N 3AR, U K (Tel: 1 44 171 833 7483; E- mail: rhenson @ ® l.ion.ucl.ac.uk). This work was support ed by a BBSRC grant awarde d to Neil Burges s and G raham Hitch.

q

1999 Internationa l U nion of Psychological Science

404

HE NSO N

fro m left to right (Fig. 1b). In one version of th is model (Burgess & H itch, 1992) , the con text sign al is event-d riven, in that the w indow only moves when a new ite m is presented , regardless of the delay between successive items. In oth er words, ite ms are associated w ith th eir absolute positio n fro m th e start of the list. A coding of relative po sition assu mes that ite ms are coded with respect to both the start and the end of a sequ ence (H enson, in press; H ough ton, 1990) . T he StartE nd M odel (SE M ) of H enson (19 98) for exa mp le, assumes a start marker, which is strongest at the start of a sequence and decreases in strength toward s the end , and an end marker, w hich is weakest at the start of a sequ ence and increa ses in streng th towards the end (Fig. 1c). T he relative strengths of the start an d end markers therefo re p rovide an approximate two-dimensional code for each po sition w ithin the sequence. A ssociating items w ith these marker stren gths codes their position relative to the end as well as the start of a sequ ence.

A te mporal coding of position is sensitive to p resentation rate, such that items fu rther apart in time are associated w ith mo re distinctive positional codes (cf. a line of telegraph poles receding into th e distan ce, N eath & C row der, 19 90). An absolute coding o f po sition, h owever, is insensitive to presentation rate, in that the code for the second ite m in a sequence presented rapidly is identical to the code for th e second item in a sequence presented slow ly. An absolute cod ing of p osition (relative to the start of a sequence) is also insensitive to th e length o f a sequ ence, in that th e code fo r th e th ird ite m in a sequence of three is identical to th e code for th e third ite m in a sequ ence of ® ve. A relative coding of position , h owever, is sensitive to sequence leng th, in that the code for the third ite m in a sequ ence of three is different fro m the code for the third ite m in a sequence of ® ve: The fo r mer item is cod ed at the end o f the sequence, w hereas the latter ite m is coded in the mid dle of the sequence.

EVIDENCE FROM ERROR PATTERNS IN SERIAL RECALL

(a )

1

2

3

4

P o sition

(b ) 1 2 3

Positional models of seria l order derive support fro m two main typ es of error. Th e ® rst occurs when a sequ ence is grou ped into subsequences by, for examp le, inserting a pause every third ite m (e.g. Ryan 196 9 ; Wickelgren, 1967) . T houg h th e total number of errors is reduced by grou ping, one cla ss o f error actually increases (L ee & E stes, 1 981 ; N airne, 1991) . Th ese interp ositions (H enson, 1996 ) are transpositions betw een group s that maintain their position w ithin grou ps, such as the swapping of middle-g roup items illu strated in F ig. 2a. Th e relatively high incidence of isolated interpositio ns (H enson, 1996 ) indicates th at items are so mehow coded for their positio n w ithin grou ps. T he second type of position al error occurs between recall of successive sequences. In a typical serial recall

(a ) Interp osition s

4 Po sitio n

(c)

R

S

M

H

Q

J

Y

E

(b ) P rotru sio n s

1

2

3

R

M

H

Q

J

Y

V

M

F

L

J

Z

4

P o sitio n FIG. 1. Illustrations of positional coding in (a) the temporal ter ms of B row n et al. (in press), (b) the absolute ter ms of Bu rgess and H itch (1992), and (c) the relative ter ms of H enson (1998).

FIG. 2. Positional errors in seria l recall: (a) in terpositions between groups, and (b) protrusions betw een trials.

CO DIN G POSITIO N IN STM

exp eriment, participa nts atte mpt multiple trials of p resentation an d recall. D eta iled a nalyses of the errors in such experiments reveal that erroneous ite ms are more likely than chance to o ccur at the same po sition in the previous trial (C onrad, 1 960 ; Estes, 1991) . H enson (1996 ) called such errors protrusions (Fig. 2b). Such proactive interference of position al info r mation indicates that ite ms are so mehow coded for their p osition w ith in trials. B oth interpo sitions and protrusion s are exa mples o f a general tendency for substitutions betw een sequences to maintain their position w ith in a sequence. H ow ever, most previou s de monstrations of such position al errors have used sequences of equal leng th presented at a constant rate, for which th e de® nitio ns of te mporal, absolute, and relative positio n are confounded. On ly recently have these errors been used to investigate the nature of positional codes in short-ter m memory.

405

(a )

R M H

Q

J

Y

(b )

R

M H

Q

J

Y F

(c) Temporal vs. Absolute Position To test w hether position within g roups is coded in tempo ral or absolute ter ms, N g (1996 ) exa mined th e pattern of interpositions between grou ps p resented at different rates. In on e condition, the middle gro up in three groups of three items w as presented twice as slowly as the ® rst and last group. C onsidering only the ® rst two grou ps for simplicity (F ig. 3a), the question was w hether the th ird item in the ® rst group was more likely to transpose w ith the third ite m of the second gro up (a s predicted by an absolute cod ing of position fro m the start), or w ith the second item of the second group (as predicted by a temporal coding of po sition fro m the start, given that these ite ms o ccurred at the sa me time relative to the start of a grou p). T he for mer errors proved mo re co m mo n, favouring an absolute coding of position. T his result held w ith both visual and auditory presentation o f the stimuli (N g & M aybery, 1999). O ne p roblem w ith N g’s exp eriments is th at pa rticipan ts may have rehearsed the ite ms at rates that differed fro m th e objective presentatio n rates. Indeed, it is possible that they rehearsed the groups at the sa me rate (e.g. their max imu m rate o f subvocal articulation), in which case the predictions of a te mporal coding of po sition no long er differ fro m those of an absolute coding. O ne solution to th is problem is to repeat the experiments w ith concurrent articulato ry suppression , which shou ld minimise any rehearsal (B addeley, 198 6 ). T he results of such an exp eriment are currently being a nalysed (N g & M aybery, 1999) .

Absolute vs. Relative Position A bsolute and relative codings of po sition cannot be distinguished by N g’s experiments. To test whether position w ithin group s is cod ed in absolute or relative ter ms, H enson (in press) exa mined th e pattern of interpositions between group s o f different sizes. In o ne condition

R

M

Y F T em p o ra l

H

Q J

L Z K G X A b so lute

R ela tive

FIG. 3. Positional errors respecting tempora l, absolute or relative position w ith (a) groups presented at different rates, (b) groups of different size, and (c) trials of different length. (Note: tempora l and absolute position are de® ned relative to the start of a sequence in these exa mples.)

(F ig. 3 b), a group of three items w as fo llowed by a group of four. T he question was whether th e third item in the ® rst group was more likely to tra nspo se w ith the th ird item of the second g roup (as predicted by an absolute coding of p osition fro m the start), or w ith the fou rth item of the second group (as predicted by a relative coding o f position, given th at these ite ms occurred at the end of a group). Th e latter errors proved more co mmon, favourin g a relative coding o f position. In a second exp eriment, H enson exa mined the pattern of p rotru sions between trials of different length (F ig. 3c). A relative coding o f po sition w as aga in supported by the ® n ding that items at the end of one report were more likely to have occurred at the end of the prev iou s rep ort tha n at the same ab solute position in the previo us repo rt. T hese results are d if® cult to explain in ter ms of different rehearsal rates, because there is no obviou s reason fo r participants to rehearse long er sequences faster than shorter ones, such that the total rehearsal time for the different sequence len gths is equated. F urther more, though th e ® nding of end-to-end errors is not incompatible w ith a te mporal or absolute coding o f po sition de® ned relative to the end (rather than start) of a sequence, fu rther ana lysis also revealed that start-to-start

406

HE NSO N

errors were more co m mo n than errors between items at the same tempo ral or absolute p osition relative to the ends o f sequences. T hese data therefo re suggest that position is coded relative to b oth the start and the end of a sequ ence. O ne mig ht ob ject that there is so me p roperty un ique to the ® n al item in a sequence that explains why end-toend sub stitu tions were so co m mon. A more general test of a relative coding of position needs to exa mine the pattern of errors between th e middle p ositions of sequ ences. For example, a general coding of relative po sition predicts that the second item in a gro up of three is most likely to sub stitu te w ith the third ite m in a group of ® ve. Indeed , a more comprehensive test of te mporal, absolute, and relative position migh t involve sequences like those depicted in F ig. 4a, in w hich a sequence of three ite ms is followed by a sequence of ® ve items presented three times as fast. C onsider the secon d item in the ® rst sequence: A te mporal cod ing of po sition relative to the sta rt of a sequence p red icts that this ite m is mo st likely to substitu te for the fou rth ite m of the second sequ ence. A lternatively, a temporal coding of position

(a )

R

M

Q V

J

T em p ora l

F

H

A b so lu te

M

R ela t ive

H R eca ll C u e

R

M T im e

THE START-END M ODEL O f th e three p ositiona l models outlined in F ig. 1, the results given favour the SEM (H enson, 1998) . H owever, the data also raise question s regarding the nature of the end marker assumed by this model. First, th ere is the question of how the end marker grows in strength toward s th e end of a sequence, when that end h as not yet occurred in tim e. O ne possibility is that the strength of the end marker correspo nds to the d egree of expectation for the end of the sequ ence (H enson, 1998) . T his is a plausible assu mption w hen the sequence lengths are known in advance (such as the grou ps in E xperiment 1 of H enson, in press). H owever, th e length of the sequ ences used in H enson’s Exp eriment 2 varied fro m trial to trial in an unp redictable man ner. In other wo rds, participan ts did no t k now the length of a seq uence in advance, making a n expectancy interpretatio n of the end marker stren gth less plausible. Ano ther possible solutio n is describ ed next.

L

(b )

R

relative to the end of a sequence predicts that this item is most likely to substitute for the seco nd item of the second sequence. A n absolute coding o f positio n relative to the sta rt of a sequence p redicts that this item is most likely to substitute for the seco nd item of the seco nd sequence. A lternatively, an absolute cod ing of p osition relative to the end o f a seq uence predicts that this item is most likely to substitute for the fourth item of the second sequence. O nly a relative coding of position predicts that the second ite m in the ® rst sequ ence is most likely to sub stitute for the third ite m of the second sequence.

H R eca ll C u e

FIG. 4. Predictio ns for future experiments: (a) co mbined test of temporal, absolu te and relative positional coding of nonter minal items (note: temporal and absolute position are de® ned relative to the start of a sequence in these exa mples), and (b) test for the effects of temporal spacing on serial recall performance.

OSCILLATOR-CODING OF RELATIVE POSITION A model d eveloped by H enson and B urgess (1997 ) de monstrates how a relative positional coding can be implemented w ith tem poral oscillators. T his model assu mes that people possess a number of internal oscillators w ith a rang e of different frequencies. H owever, rather th an co mbining th e states o f these o scillators to fo rm a single timing signal, as in OS CA R (B row n et al., in press), H enson and Burgess assu med th at each oscillator comp etes separately to best represent a sequence. T he ``best o scillators’ ’ are th ose with a half-period clo sest to the tempo ral duration o f the sequence (see Fig. 5a). T hus sequences of d ifferen t te mp oral durations are represented by d ifferent oscillators. Importan tly, however, the positional codes associated w ith items are de® ned by the p hase of the o scillators at the p oint in time w hen each ite m was p resented. T he use o f ph ase info r mation auto matically de® n es position relative to both the start and the end of a sequence: The phase of an oscillator that is associated with an item at the end of a short sequence, for exa mple, w ill be identical to the phase of a slow er oscillato r that is associated with an

CO DIN G POSITIO N IN STM

(a ) R M H Q J Y

fa st o scilla to r

R M H Q J Y

best o scilla to r

R

M

H

Q J Y

slo w o scilla t or

(b )

R

M

H

_

Q

J

Y

_

407

In order to detect the end of a sequence, H enson a nd B urgess assumed additional input fro m a rhyth mic tra cker. T his tracker ¯ ags the end o f a sequence whenever an ite m fails to occur in time w ith th e beat de® ned by the previous items. T hus the p ause after th e third ite m in F ig. 5b w ill sign al the end of the ® rst group. (Surprisingly, the precise tem poral characteristics that d eter mine grouping in short-ter m memory do not appear to have been stu died .) For such gro uped sequ ences, oscillators co mpete to represent b oth the p osition of each ite m w ithin a group, and the positio n o f each grou p w ithin the sequence. Stru ctured seq uences are therefore rep resented by o scillators of several d ifferent frequ encies; fast on es coding item po sitions in grou ps, slower on es coding group positions in groups of groups, etc. In o ther wo rds, the mo del is extendible to hierarchies of positio nal codes (L ee & E stes, 198 1 ; though see H enson & Bu rgess, 1997 , fo r the proble ms w ith synchronizing o scillators at each level of a hierarchy).

EVIDENCE FROM TEMPORAL PRESENTATION SCHEDULES

(c) R

M

R M

H

H

Q

Q

J

J Y

Y

FIG. 5. Illustrations of (a) competition betw een oscillators to best represent a sequence (Henson & Burgess, 1997), (b) rhythmic parsing of grouped sequences, and (c) oscillator representations of in creasing and decreasing schedules of N eath and C rowder (1996).

item at the end of a long sequence. Position al codes are therefore independent of both the number of items in a sequ ence and the rate of presentatio n of tho se items. M ore p recisely, the model assumes that items are associated w ith th e phase of all oscillato rs as they are presented. When the seq uence ends (a s de® n ed later), the oscillators w ith a frequency that mean s they are clo sest to co mpleting a half-p eriod (i.e. closest to their state at the start of the sequ ence) are selected to represent the sequ ence. An i mp ortant advantage o f th is model over the SE M (H enson, 199 8) therefo re is th at it does not need to kn ow the leng th or du ration of a sequence in advan ce. B ecause multiple oscillato rs co mp ete du ring p resentation, many possible ``parsings’ ’ of a sequence are maintained in parallel, and o nly w hen a sequence ends is the correct ``parsing’ ’ selected. A t recall, oscillators of any freq uency can be cho sen in order to recall the ite ms serially (depending on th e desired speed of recall). T he strength w ith which ite ms are cued at each time point du ring recall is deter mined by the similarity in the phases of the recall oscillators and th e o scillators that won the co mpetition to represent the sequ ence. (For a more detailed mathem atical for malization of this model, a nd de monstrations that it can reproduce app ropriate similarity gradien ts, see Henson & B urgess, 199 7.)

A n alternative test o f position al coding involves co mparing serial recall perfo rma nce as a fun ction of d ifferent tempo ral p resentation schedules. S everal stud ies have show n that measures of the recency effect are sensitive to th e ratio of inter-item interval to retention interval (e.g. B jork & W hitten , 1974 ; G lenberg & Swa nson , 1986 ; N eath & C rowder, 1990) , consisten t w ith th e distinctiveness mod els of N eath (199 3) an d N airn e et al. (1997) . H owever, these studies have used either free recall or recognition, for which the use o f position al infor mation is n ot strictly necessary. M oreover, the recency effect is no t n ecessarily th e best ind ex of positional coding in short-ter m memory. The regression slop e over the last few serial positio ns, for exa mple, (N airne et al., 199 7) is likely to depend on several factors, including overall perfor mance level. This is pa rticularly relevant to serial recall, where other factors affecting recency includ e, for exa mp le, the suppression of prev ious respon ses (see H enson, 199 8). Th ese studies do not th erefore provide direct evidence for temp oral effects on positiona l coding in short-ter m me mory for serial order. O ne clear prediction of a te mporal cod ing of po sition is that, a ll else being equal, ite ms more widely separated in time w ill be coded more distinctively. T his sug gests that slower presentation rates sho uld p roduce superior serial recall perfor mance. H owever, o ne problem with this prediction is that rehearsal rates are again likely to differ fro m o bjective presentation rates. W hen rehearsal was prevented by articulato ry suppression, B addeley a nd L ew is (1984 ) fo und th at serial recall was worse w ith slow presentation rates, con trary to what might b e expected fro m a te mp oral coding of positio n. (N eath & C row der, 19 96, showed that slower presentation rates did aid p erfo r mance when mean p resentation rates were as fast as ® ve items per second , but the danger with such rapid

408

HE NSO N

presentation rates is that they do not allow such effective encod ing of items.) A second problem with varying presentatio n rates is that other factors that affect serial recall may also be a fun ction of time. For exa mple, phonological info r mation is often assumed to b e rap idly forgotten fro m short-ter m memory (Tehan & Hu mphreys, 199 5). T he greater lag betw een p resentation and recall of each ite m that is entailed by the slow er presentatio n rates of B addeley and L ewis (1984 ) might impair seria l recall despite more d istinctive p ositional codes. W hat is req uired is a co mparison of serial recall perfor mance w hen the temporal spacing between items is varied but th e mean time betw een presentation and recall o f ite ms is kept constant. In F ig. 4b, for example, te mporal d istinctiveness theory predicts superior perfor mance fo r the slower presentation rate. Assu ming a linear relationship betw een lag a nd the recall probab ility of each item (an d that rehearsal is prevented by articulato ry suppression ), the o scillator model of H enson and B urgess (1997) , h owever, predicts no difference between the two presentation schedules, because th e positional cod es are equivalent in bo th cases. A study by N eath and Crowder (19 96) suggests that the timing o f presentation can affect position al cod ing in other situations. T hey co mpared serial recall under two presentation sched ules: an increasing schedule in which the interite m interval increased w ith serial position , a nd a decreasing schedule in which the interitem interval decreased w ith serial positio n (F ig. 5c). Perfor mance w ith th e increasing schedule w as superior to that w ith the decreasing schedule. B ecause the total presentation time and mean lag between presentation and recall was equal in both schedules, th is d ifference is dif® cult to attribute to encod ing or rehearsal differences, or to d ecay of item infor matio n. T he temporal distinctiveness explanation offered by N eath an d C rowder w as that participants adopted a forward perspective (fro m the start of the sequences), fro m w hich it is more bene® cial for later items to be w idely spaced in time th an it is for early items (cf. the telegraph pole analog y). Th is sensitiv ity to increasing and decreasing presentation schedu les is n ot necessarily problematic for th e oscillator mod el of Henson and B urgess (1997) , however. A ssu ming that the rhyth mic tracker does not identify any rhythm o r grouping in eith er schedule, sequences w ill b e coded by the same o scillators in both cases (Fig. 5c). Inter-ite m intervals that increase toward s the end of a sequ ence w ill therefore be associated w ith increasing p hase d ifferences, whereas inter-ite m intervals that decrease toward s the end of a sequence will be associated w ith decreasin g phase differences. L arger phase differences b etween successive positions result in more distin ctive coding of th ose p ositions, just a s in temporal distinctiveness th eory. T hus the mod el can app eal to the sa me explanation of a forw ard perspective that was p roposed by Neath and C rowder (though one might also consider other consequences of changing the

distinctiveness of successive positio ns, such as the effects of reca lling early ite ms on the probability o f recalling subsequ ent ite ms; cumulative effects that are cha racteristic of serial recall, H enson , 1996) . T he oscillator mod el of Henson and Bu rgess (1997 ) only models the coding o f p osition: It is not a co mplete process model of serial recall. A s mentioned earlier, the probab ility of recalling each ite m correctly during serial recall depends on other fa ctors such as response co mpetition , response suppression, available ite m infor matio n, and the retention interval. Th e co mpetition between items for recall at each position (n or mally via a variant of th e L uce cho ice rule) is assumed by almost every model of serial recall. T his co mp etitio n explains why it is the coding of position relative to the coding of surrou nding p ositions th at is impo rtant in deter mining the probab ility of correct recall of ite ms. Respo nse suppression, the p rocess th at reduces the probability of an ite m being recalled more than once, is also assu med by nearly every mod el of serial recall. T his w ill affect the oscillator model’s predictions fo r overall perfor mance level an d serial position curves. F inally, the model n eeds to ad dress the issue of retention interval before it can be co mpared w ith the temporal distinctiveness th eory of N eath an d C row der (1990) . Indeed, recent evidence fro m free recall (N airne et al., 1997 ) suggests that the absolute duratio n of a retentio n interval exerts an effect despite a constant ratio of inter-item interval to retention interval, w hich is not predicted by te mporal d istinctiveness th eory. O ne possibility is that th e forgettin g during a retention interval re¯ ects the decay of the associations betw een items and the oscillators representin g the sequence (cf. B urgess & H itch, 19 98). In this case, the effects o f the retentio n interval are d ifferent from those of the inter-ite m interval: T hat is, there are effects of ti me on short-ter m memory for serial o rder that are distinct fro m its effects on p ositional cod ing.

CONCLUSIO N A review of recent data suggests that models of shortter m me mory for serial order must include so me for m of positional infor mation associated w ith the ite ms in a sequ ence, and th at this info r mation shou ld b e de® ned relative to bo th the sta rt and th e end of that sequence. A n ew model was outlined that codes po sitions in such relative ter ms via the phases of oscillators. T his model not only overco mes the p roblems associated with predicting the end of a sequence, but also allows so me in¯ uence of temporal factors on p ositional coding, particu larly when no rhythmic grouping is identi® ed w ith in a sequ ence. The challenge for future develop ments of this model is to specify more precisely how the timing of items effects their grou ping a nd to address the issue of retention interval in order to co mpare the mo del to theories o f te mp oral or positional distinctiven ess.

CO DIN G POSITIO N IN STM

REFERENCES A nderson, J.R ., & M atessa, M . (1997). A production system theory of serial memory. Psychological Review, 104, 728± 748. Baddeley, A.D. (1986). Working me mory. O xford: Clarendon Press. Baddeley, A .D., & Lewis, V.J. (1984). W hen does rapid presentatio n enhance digit span? Bulletin of th e Psychonomic Society, 22, 403± 405. Bjork, R., & Whitten , W.B. (1974). Recency-sensitive retrieval process in long-te rm free recall. Cognitive Psycholo gy, 6, 173± 189. Brown, G.D.A ., Preece, T., & Hulme, C. (in press). O scillatorbased memory for serial order. Psychological Review. Burgess, N., & H itch, G. (1992). Toward a network model of the articulatory loop. Journal of M emory and Language, 31, 429± 460. Burgess, N., & Hitch, G. (1998). M emory for serial order: A network model of the phonological loop and its timin g. M anuscript sub mitted for publica tion . Conrad, R . (1 960). Serial order intrusions in immediate memory. British Journal of Psychology, 51, 45± 48. Ebbingha us, H. (1964). M emory: A contribution to experimental psychology. New York: Dover. Estes, W.K. (1991). On types of item codin g and sources of recall in short-ter m memory. In W.E. Hockley & S. Lewandowsky (Eds.), Relating theory and data: In honor of Bennet B. M urdock (pp. 175± 194). H illsdale, NJ: Lawrence Erlbaum A ssociates Inc. Glenberg, A.M ., & Swanson, N.G. (1986). A te mporal distinc tiveness theory of recency and modality effects. Journal of Experimental Psycholo gy: Learning, M emory and Cognition, 12, 3± 15. Henson, R.N.A . (1996). Short-term me mory for serial order. Unpublis hed doctoral dissertation , U niversity of Cambridge, UK . Henson, R.N.A . (1998). Short-term memory for serial order: The Start-End M odel of serial recall. Cognitive Psychology, 36, 73± 137. Henson, R .N.A. (in press). Position al info rmation in shortterm memory: Relativ e or absolute? M emory and Cognition. Henson, R .N.A ., & Burgess, N. (1997). Representations of serial order. In J.A. Bullinar ia, D.W. Glasspool, & G. H oughto n (Eds.), 4th N eural Computation and Psychology Workshop (pp. 283± 300). London : Springer. Henson, R .N.A., N orris, D.G., Page, M .P.A., & Baddeley, A .D. (1996). U nchaine d memory: Error patterns rule out chainin g models of im mediate serial recall. The Q uarterly Journal of Experimental Psychology, 49A, 80± 115. Houghto n, G. (1990). The problem of serial order: A neural

409

network model of sequence learning and recall. In R . D ale, C. M ellish, & M . Zock (Eds.), Current research in natural language generation (pp. 287± 319). London : A cademic Press. Jordan, M .I. (1986). Serial order: A parallel distributed approach (ICS Report 8604). San Diego, CA: University of California, Institute for Cog nitiv e Science. Lee, C.L., & Estes, W.K. (1981). Item and order info rmation in short-term memory: evidence for multilevel perturbation processes. Journal of Experimental Psychology: Human Learning and M emory, 7, 149± 169. Lewandowsky, S., & M urdo ck, B.B., Jr. (1989). M emory for serial order. Psychological Review, 96, 25± 57. M urdo ck, B.B., Jr. (1995). Developing TO DAM : Three models for serial order info rmation. M emory and Cognition, 23, 631± 645. Nairne, J.S. (1991). Positiona l uncertainty in long-term memory. M emory and Cognition, 19, 332± 340. N airne, J.S., Neath, I., Serra, M ., & Byun, E. (1997). Positiona l distinctiveness and the ratio rule in free recall. Journal of M emo ry and Language, 37, 155± 166. N eath, I. (1993). D istinctiveness and serial positio n effects in recognition . M emory and Cognition, 21, 689± 698. N eath, I., & Crowder, R .G. (1 990). Schedu les of presentatio n and te mporal distinctiveness in hu man memory. Journal of Experimental Psychology: Learning, M emory and Cognition, 16, 316± 327. N eath, I., & Crowder, R .G. (1996). Distinctiveness and very short short-term serial positio n effects. M emory, 4, 225± 242. N g, L.H.H . (1996). Are time-dependent oscillators responsible for te mporal grouping effects in short-term me mory. U npublished Honors thesis, U niversity of Western Australia. Ng, L.H.H., & M aybery, M . (1999). Temporal grouping effects in short-term memory: An evaluation of time-dependent models. M anuscript submitted. Page, M .P.A ., & N orris, D.G. (1998). The primacy model: A new model of immediate serial recall. Psychological Review, 105, 761± 781. Ryan, J. (1969). Groupin g and short-term memory: Different means and patterns of grouping. The Q uarterly Journal of Experimental Psychology, 21, 137± 147. Tehan, G., & H u mphreys, M .S. (1995). Transient phon emic codes and im munity to proactive interference. M emory and Cognition, 23, 181± 191. W ickelgren, W.A . (1967). Rehearsal grouping and hierarchical organization of serial positio n cues in short-term memory. Quarterly Journal of Experimental Psychology, 19, 97± 102. Y ntema, D.B., & Trask, F.P. (1963). Recall as a search process. Journal of Verbal Learning and Verbal Behavior, 2, 65± 74.