334-347 stimulus interaction and between-trials proactive interference in monkeys

7/30/2019 334-347 Stimulus Interaction and Between-trials Proactive Interference in Monkeys.

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J o u r n a l o f E x p e r i m en t a l P s y c h o l o g y :A n i m a l B eh av i o r Processes1981, V o l . 7, No. 4 , 334-347C opy r i g h t 1981 by the A m e r i c a n P s y ch o l o g i c a l A s s o c i a t io n , I n c .0097-7403/81/0704-0334S00.75

S t imu l u s I n t e r a c t i o n a n d B e tw e e n - T r i a l s P r o a c t i v eI n t e r f e r e n c e in M o n k e y sT h o m a s J . R e y n o l dsR o c k e f e l l e r U n iv e r s i t y Douglas L. MedinUniversity of Illinois a t U r b a n a - C h a m p a i g n

M o s t t h e o r e t i c a l a c c o u n t s o f p r o a c t i v e i n t e r f e r e n c e i n d e l a y e d -m a t c h i n g - t o - s a m -p le p a r a d i g m s f o c u s o n processes l i nk ed to t i m e . Tw o e x p e r i m e n t s q u e s t i o n e dt h i s e x c lu s iv e fo c u s o n t e m p o r a l f a c t s. B e t w e e n - t ri a l s p r o a c t iv e i n t e r f e r e n c e w a ss tudied in a s i t u a t i o n in w h i c h th e s i m i l a r i t y o f consecu t ive t r i a l s w a s v a r i e da l o n g t h e d i m e n s i o n s o f c o lo r , f o r m , a n d p o s i t i o n . A l l o f t h e s e f ac t o r s a s wel l a sth e s i m i l a r i t y o f s a m p l e a n d t e s t c o n t e x t s c o n t r ibu t e d to m e m o r y p e rf o r m a n c e .A m a t h e m a t i c a l m o d e l b a s e d o n t h e a s s u m p t i o n s t h a t s i m i l a r i t y s t r o n g l y i n f l u -e n c e s m e m o r y - b a s e d j u d g m e n t s a n d t h a t o v e r a l l s i m i l a r i t y is d e t e r m i n e d b y t h em u l t i p l i c a t i v e i n t e r a c t i o n o f c o m p o n e n t d im e n s io n s g a v e a n e x c e l l e n t q u a l i t a t i v ea n d q u a n t i t a t iv e a c c o u n t o f t h e da t a in b o t h e x p e r i m e n t s . These r e s u l t s s u p p o r ta b r o a d e r v iew o f f a c t o r s d e t e r m i n i n g p r o a c t iv e i n t e r f e r e n c e a n d a r e i n c o n s i s t e n tw i t h th e ide a t h a t t h e s e m u l t i p l e f a c t o r s c a n b e t r e a t e d a s i n d e p e n d e n t .

T h e t h e o r e ti c a l a n a l y si s o f m e m o r y p r o -cesses in a n i m a l s ha s advanced s t ead i ly inr ecen t yea r s (D'Amato, 1973; Medin, Rob-erts, & Davis, 1976; Spear, 1973, 1976,1978; Wa gn er , 1976). On e u l t i m a t e a d v a n -tage of broad theories is that processes thata re i n fe r r ed on the bas i s o f empi r i ca l ly de-n n e d r e l a t i o n s c a n b e compared a n d testeda c r o s s e x p e r i m e n t a l p a r a d i g m s a n d subjec tpopu l a t i ons . S ince th i s gene ra l i ty i s r eachedin o n l y t h e a d v a n c e d stages o f t h e o r y d e -v e l o p m e n t , c u r r e n t t h e o r i e s o f a n i m a l m e m -ory s t i l l tend t o be b o u n d to the specific ex -p e r i m e n t a l p r o c e d u r e s a n d q u e s t i o n s f r o mwhich they were deve loped. The r ecen t p rog-ress tha t h a s b e e n m a d e in a n a l y z i n g p a r -t i c u l a r p a r a d i g m s , h o w e v e r , s u g g e s t s t h a t i tm a y b e t i m e l y to e v a l u a t e th e e x t e n t tow h i c h concep t s a r i s ing i n o n e s u b a r e a o fm e m o r y r e s e a r c h m i g h t b e f r u i t f u l l y app l i edto o the r a rea s . In the p resen t a r t i c l e , we an -a ly ze p r o a c t i v e i n t e r f e r e n c e in a de layed-

Th i s r e s e a r c h w a s s u p p o r t e d b y U . S . Pu b l i c H e a l t hServ ice G r a n t M H 3 24 8 9 a n d N a t i o n a l S c ie n c e F o u n -d a t i o n G r a n t B N S 7 9 - 2 2 67 8 , Th e f i r st a u t h o r w a s a l s os u p p o r t e d by U . S . Pu b l i c H e a l t h S e r v i c e Po s t d o c t o r a lF e l l o w s h i p M N 0 7 5 5 1 . C a r o l y n M e r v i s a n d M a r k A l -tom prov ided h e l p f u l comments o n earlier d r a f t s o f t h em a n u s c r i p t .R e q u e s t s fo r r e p r i n t s s h o u l d be s e n t to D o u g l a s L .M e d i n , P s y ch o l o g y D e p a r t m e n t , U n i v e r s i t y o f I l l i n o i s ,603 E. D a n i e l , C h a m p a i g n , I l l i n o i s 6 1 8 2 0 .

m a t c h i n g - t o - s a m p l e p a r a d i g m i n r e l a t i o n t oo u r o w n i d e a s c o n c e r n i n g t h e i n t e r a c t i o n o fc o m p o n e n t c u e d i m e n s i o n s i n d i s c r i m i n a t i o nl e a r n i n g (e.g., Medin, 1975, 1976).T w o p r o m i n e n t t h e o r i e s o f a n i m a l m e m -ory , the independent trace strength and com-p e t i t i o n m o d e l (R o b e r t s & G r a n t , 1 9 7 4 ,1976) an d t h e t e m p o r a l d i s c r i m i n a t i o n h y -pothes i s (D'Amato, 1973), a re l a r g e l y b a s edo n data collected w i t h t h e d e l ay e d - m a t c h -i n g - t o - s am p l e (DMTS) p a r a d i g m . I n t h ebasic task , a s a m p l e s t i m u l u s is presen ted ,a n d a f t e r a d e l a y i n t e r v a l , a test i s givenb e tw e e n t h e i d e n t ic a l s a m p l e a n d a n e ws t i m u l u s , with choice o f t h e s a m p l e b e i n g

correct a n d r e w a r d e d . C o m m o n l y u s e d v a r i -a t i o n s o n t h e basic t a s k i n c l u d e a r e q u i r e dresponse to the s a m p l e , w h i c h m a y o r m a yn o t b e r e w a r d e d , f ix e d i n t e r t r i a l i n t e r v a l v e r -su s subjec t- ini t ia ted t r ia ls , a n d n o n r e w a r dv e r s u s t im e o u t f o l l o w i n g i n c o r r e c t r e -sponses. (See Medin e t al. , 1976, fo r a rev iewo f DMTS p r o c e d u r e s .) T y p i c a l l y , a n i m a l sa r e g iven ex tens ive p rac t i ce wi th a s m a l ln u m b e r o f s a m p l e a n d c o m p a r i s o n s t i m u l i .B o t h t h e o r ie s p r o v i d e a n a c c o u n t o f p r o a c -t i v e i n t e r f e r e n c e t h a t f o c u s e s o n t e m p o r a lf ac to r s a s determinants o f fo rge t t i ng .Th e m a j o r a s s u m p t i o n s o f trace s t r e n g t ht h e o r y a r e ( a ) that c h o i c e p r o b a b i l i t y is di-r ec t ly r e l a t ed to m e m o r y t r a c e s t r e n g t h , (b )33 4


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PROACT IVE I N T E R F E R E N C E 335that strength f o r a stimulus is accumulatedas a negatively accelerated function of ex-posure time and decays as a negatively ac-celerated f u n c t i o n o f time since presentation,and (c) that memory traces for d i f f e r e n ts t i m u l i grow a n d decay independently(Grant, 1975; Grant & Roberts, 1973; Rob-erts & Grant, 1974, 1976). In this model,forgetting (as well as remembering) isstrictlya f u n c t i o n o f temporal parameters. Proactiveinterference occurs because incorrect choices t i m u l i acquire strength f r o m earlier trialsw h e n they appeared as the sample and werecorrect. That is to say, when a small numbero f stimuli a r e used, th e incorrect stimuluso n one trial might have been correct on ani m m e d i a t e l y preceding trial.In a similar way, the temporal discrimi-n a t i o n hypothesis (D'Amato, 1973) empha-sizes th e role o f time as the major factor inDMTS performance. Since each stimulusappears f r e q u e n t l y as both the sample andthe incorrect test alternative, the task be-comes o n e o f determining w h i c h stimulushas appeared most recently. Thus, the sam-ple f r o m the current trial recedes into a seto f sample events organized in time, a n d suc-cessfu l DMTS performance requires nots i m p l y memory retrieval b u t also accuraterelative recency judgments.B o t h theories have been quite successfulin predicting many of the DMTS results inthe literature, including delay, sample du-ration, sample set size and sample interfer-ence ef fec ts (see Reynolds & Medin, 1979,fo r a recent review). Worsham (1975)s howed that the choice accuracy ofcapuchinm o n k e y s could be predicted qualitatively o nthe basis of the relation between successivetrial stimuli. When the sample on trial hadbeen the incorrect test stimulus on Trialn 1 ("hard sequence"), performance wa ss ign i f i can t ly worse than w h e n the Trial s a m p l e h a d n o t appeared f o r t h e previousthree or f o u r trials ("easy sequence"). Sim-i l a r l y , Grant (1975) observed that pigeonsw e r e less accurate w h e n the correct and in-correct stimuli reversed roles between trialsthan in a control condition in which only th esecond trial was presented. Further, strongproactive interference w a s f o u n d when th ecorrect choice on Trial had been incorrect

o n Trial n - 1, but no interference was ex-hibited if a newstimulus was correct onTrialn. These results can be readily handled byeither of the two theories s o f a r considered.

There are other results, however, whichsuggest that a n exclusive emphasis o n time-linked processes may be ignoring importantdeterminants o f matching performance.Medin (1976) tested monkeys in a DMTSparadigm in which f o r m w a s t h e relevantdimension and color was the irrelevant di-m e n s i o n a n d constant w i t h i n , a n d variablebetween, trials. The stimuli were two circlesa n d t w o triangles, o n e o f each painted blacka n d the other white, and thus choice testsw e r e always between a circle and a triangleo f the same color. On consecutive trials, ei-ther the same or the different form was cor-rect in either the same or the d i f f e r e n t colora n d occupying either the same or the dif-f e r e n t position. The factorial combination o fthese three conditions yielded eight relationswhich were presented equally often on trials.The results indicated clearly that interfer-ence magnitude depended on overall simi-larity between trials. Although f o rm was theo n l y relevant dimension, both color and po-sition contributed to performance. When thesame f o r m , color, and position were correcto n consecutive trials, the monkeys averaged.7 7 correct. A change in either the color orthe position of the correct object betweentrials reduced accuracy by approximately.10, and performance was at chance whenth e correct f o r m w a s changed. Finally, i f thecorrect and incorrect stimuli reversed rolesbetween trials, accuracy fel l below chanceto .32 correct.These results are consistent with the ideathat stimulus similarity i s an important fac-tor in between-trials proactive interference.In the past, it has often been convenient toignore stimulus similarity in treatments o fDMTS, since by design, discriminabilitym a y not have been an important factor. Sim-ilarity does become critical, however, wheni n f o r m a t i o n about individual stimulus com-ponents must be used in the evaluation ofa current choice test. Before elaborating thispoint, the idea of stimulus similarity needsto be developed in some detail.W h i le similarity ef fec ts have been largely


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33 6 T H O M A S J . R E Y N O L D S A N D D O U G LA S L . M E D I Ni g n o r e d in t h e o r e t i c a l t r e a t m e n t o f DMTS,s t i m u l u s s imi l a r i ty and genera l iza t ion havealways been central issues in the discrimi-n a t i o n l e a r n i n g l it e r a tu r e . It is n a t u r a l , th e n ,to look to theo r ies o f di sc r imina t ion l ea rn in gas a gu ide to how s t imu lus s im i l a r i ty effectss h o u l d be represented in DMTS. In the re-m a i n d e r o f this article, we develop a n d adaptMedin 's (1975, 1976) context model of d is-c r i m i n a t i o n l e a r n i n g to the between- t r ia lsp roac t ive in te r fe rence DMTS parad igms . Inthe process , we emphas ize how proact ive in-terference is critically dependent on the sim-i l a r i ty of the test cues a n d con tex t to thesample and in te r fe r ing even ts . I t i s a rguedtha t t emp ora l f ac to rs com pr i se jus t on e o fm a n y dimens ions a long which s i m i l a r i t ym u s t be represented an d just one o f the fac-tors m o d i f y i n g p e r f o r m a n c e .The con tex t model as sumes tha t when as t imu lus is presented, i n f o r m a t i o n concern-in g the s t im u lu s and i t s con tex t i s s to red to -gether in m e m o r y . Fo r re t r i eva l to occur ,bo th the cue an d it s assoc ia ted con tex t mus tac t iva te the cue-con tex t no de in me m ory s i -mul taneous ly . Expl ic i t predictions are de-r ived f rom t h e a s s u m p t i o n s t h a t ( a ) p r o ba-bi l i ty of re t r ieval of a cue and i ts context isreduced by c h an g e s in e i t h e r c o m p o n e n t ,with dif ferences a long each c u e an d con tex tdimension represented by a similari ty p a -r am e t e r be t we en 0 an d 1. A p a r a m e t e r v a lu eof 1 represents ident i ty along a d i m e n s i o n ,an d a v a l u e o f 0 indicates total lack o f gen-era l iza t ion fo r tha t d imens ion , (b) The s im-ilari ty parameters fo r a l l dimensions a recom bined by an in tera ct ive, specif ical ly mul-tiplicative, ru le y ie ld ing a s ingle s imilar i tymeasure governing t ransfer a n d i n te r fe r -ence.A l t h o u g h the context model developed byMedin (1975 , 1976) uses the l an gua ge o fret r iev al p rocesses, th e cen t ra l idea tha t cuesan d context are represented in m e m o r y asa n i n teg ra ted even t , with s imi l a r i ty o f indi-v i d u a l c o m p o n e n t s c o m bi n i n g b y a n i n te r -act ive ru le , does not depend on the ret r ievalcons t ruct . A recogn i tion process m odel couldbe developed which embodies th e s am e in -teract ive assumptions in terms of select ionf rom ac t ive sho r t - t e rm memory rep resen ta-t ions based o n s imilar i ty . The c u r r e n t ex -p e r i m e n t s a re directed a t the distinction be -

tween i ndependen t and in te rac t ive s imi la r i t yru l e s and no t a t the specific m e m o r y p r o c es smodel that embodies these assumptions .In app ly ing the con tex t model to the be-tween- t r i a l s parad igm, it is as s u m e d t h a t o na choice test, p e r f o r m a n c e m a y b e con t ro l l edby t h e m e m o r y re p r e se n t a t io n o f t h e c u r r e n tsample, the immediate ly preceding choicet es t , o r bo th . The p robabi l i ty o f con t ro l i scom puted as a d irec t f u n c t i o n of the phys ica la n d t em po ra l s im i l a r i ty o f the t es t to the twopr io r even ts , as de te rmined by the mu l t ip l i -cative rule. This analys is suggests a n in ter-p l a y o f f ac t o r s d e t e r m i n i n g the re l a t ive in -f luence of the s am p l e and the preced ing tes trep resen ta t ions : Whi le s ample has the ad-van tage o f be ing c lose r in t ime to the t es t ,the preceding test m a y have g rea te r phys ica ls imi l a r i ty to the cu r ren t t es t s e t t ing .Wh e r e as th e c o n t e x t m o d e l a s s u m e s t h a tindiv idual d imensions interact in produc ingthe i r effects, the re a re o t h e r d i s c ri m i n a t i o nl e a r n i n g t h e o r i e s t h a t a s s u m e t h a t c o m p o -n e n t d i m e n s i o n s a r e i n d e p e n d e n t an d ad d i -t ive (e.g., Spence, 1936; a n d m o d e l s re-viewed by Flagg & Medin, 1973). In orderto d e t e r m i n e which view m o r e n e a r l y de-scribes the data, s ep a r a te m a t h e m a t ic a l fo r -m u l a t i o n s of the i n te rac t ive a n d i n d e p e n -dence as sum pt ions are f i t to the e x p e r i m e n t a ldata. These experiments a lso provide somee s ti m a t e o f t h e m ag n i t u d e t h a t f ac t o r s o t h e rt h a n t i m e h a v e in i n f l u e n c i n g p e r f o r m a n c e .

E x p e r i m e n t 1In the f i rs t e x p e ri m e n t , m o n k e y s with ex -tens ive DMTS exper ience were t es ted witha between- t r ia ls proact ive in terference (PI)p rocedure s imi l a r to tha t used by Medin(1976) . The roles o f co lo r a n d f o r m werereversed in order to exten d the gen era l i ty ofth e Medin ( 1976 ) results . Th e basic designis shown in Figure 1. Condit ions are de-scribed in t e r m s o f re l a t ions be tween con-secut ive t r ia ls . In the f igure , the preced ingtrial consisted o f a black square p resen ted

as the s am p l e s t i m u l u s a n d t h e n a p p e a r i n go n th e left for the choice test. F o r an y pairof t r ia ls , the correct co lor may be the sameo r dif ferent , th e f o r m m ay e i t h e r r e m a i n t h es a m e o r shift between t r ia ls , and the positiono f the r e war d e d s t i m u l u s m ay be t h e s a m e


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PROACT IVE I N T E R F E R E N C E 337or different. Thus an SSS trial is an exactrepetition of the preceding trial; SDD refersto a trial in which the same color is correctb u t the f o r m has been changed and the po-si t ion of the rewarded object has shifted be-tween trials. Note also that sample presen-tations as well as correct test responses arerewarded.B e f o r e presenting the quantitative models,i t m a y be h e l p f u l to develop some qualitativepredictions. A ll models would expect con-dit ion SSS to yield better performance thanc o n d i t i o n DSD. In each case the test con-d i t i o n s are the same on consecutive trials,b u t f o r D S D t h e reward conditions havebeen reversed and for SSS the reward con-ditions have not been reversed. The inde-pendence and interactive assumptions makecontrasting predictions concerning the rel-ative diff icul ty of other trial types. Considerconditions SSS and SDS. According to thecontext theory, the change in f o r m asso-ciated with condition SDS should decreasethe likelihood that the representation asso-ciated with the preceding trial will influencep e r f o r m a n c e . Since responding on the basis

o f information from the preceding trialshou ld facilitate performance, condition SDSis predicted to produce lower performancethan condition SSS, which is not associatedw ith a change in f o r m . B y t h e same logic,w h e n a preceding trial might interfere withp e r f o r m a n c e , a change in f o r m betweentrials should reduce this interference. Thuscondition DDD is predicted to yield betterp e r f o r m a n c e than condition DSD.The independent cue model does not sharethese predictions. If we compare conditionsSSS and SDS, both should be helped bypositive transfer f r o m the preceding testa l o n g the relevant color dimension. In ad-dition, SSS also matches the prior stimulialong the irrelevant (form) dimension, andt h u s a constant amount o f generalization isadded to both test objects in this condition.Whether this added generalization facili-tates, impairs, o r h a s n o e f fec t o n perfor-mance depends on the particular choice ruleemployed. If i t is assumed that th e i n f o r -mation underlying choice performance fol-lows Weber's law, than adding a constantto both test alternatives makes differences

SAMPLETEST T R I A L N-l

|w| [wjSSS SSD

[w) +(B) (B) (w;

S D D S D ST R I A L N

DD DWj+

'W j+ DOS

[WJ+ DSS DSD

Figure 1 . Rel a t i ons between consecut ive t r ia l s i n Expe r imen ts 1 and 2 . ( In Tr ia l s SSS, SSD, etc, S =sa me a nd D = different; each se t of letters indicates color, f o r m , a n d posi t ion, in that order. B = black;W = white ) .


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338 T H O M A S J. R E Y N O L D S A N D D O U GL A S L . M E D I Nh a r d e r to detect, a nd th e i n d e p e n d e n t , a d-ditive model would predict condition SDSto be eas ier than con di tion SSS. Me din ,Reyno lds , and Park inson ( 1980) reviewedal te rna t ive cho ice ru les a n d conc luded tha tn o n e h a s been p roposed tha t s imu l t aneous lypredicts condition SSS to be better t h a n SDSand cond i t ion ODD to be bet te r than DSD.Th us , th e additive a n d multiplicative modelsmake d is t inc t ly d i f fe ren t qua l i t a t ive p red ic-t ions . These qual i ta t ive predict ions a s wella s t h e m o r e f o r m a l q u an t i t a t i v e p r e d i c t i o n sare t ested in the fo l lowing exper im en ts .

MethodSubjects. Th e subjec ts were th ree 5- 8 yr-old j u n g l e -b o r n f e m a l e r h e s u s m o n k e y s (Macaco mulatto) wi t hex t ens i ve d i s c r im i n a t i o n a n d DMTS t r a in ing (see M edin ,1980; M e d i n e t al . , 1980, fo r d e t a i l s ) . Th e y w e r e m a i n -t a i n e d o n a 12:12 h r l i g h t / d a r k c y c l e i n i t i a t e d a t 0700h o u r s a n d w e r e f ed l a b o r a t o r y c h o w d a i l y a f t e r t e s t i n g .Apparatus. Th e mo n k e y s w e r e t e s te d in a d a r k e n e droom w i th ex te rna l sounds masked by white noise. Twoc o m p a r a b l e W i s c o n s i n G e n e r a l Test A p p a r a t u s e s( W G T A s ) , o n e p a i n t e d g r a y a n d t h e o t h e r b l a c k , w e r eused, wi th a given monkey a lways tested o n the s a m ee q u i p m e n t . Th e g r a y W G T A w a s l i g h t e d by two 15-Wf l u o r e s c e n t b u l b s ; th e b l a c k o n e w a s i l l u m i n a t e d b y f o u r60-W incandescent bulbs. The three food wells o n t h ef o r m b o a r d s were spaced 15 cm cen te r to cen te r . Thes t i m u l i were two c i rc le s ( r adiu s = 3 .5 cm) an d twos q u a r e s ( 6 X 6 c m) c u t f r o m .64-cm-thick p l y w o o d , o n eo f e a c h p a i n t e d b l a c k and the o t h e r w h i t e .Procedure. The m on keys were tes ted twice da i ly , 49t r i a l s / s e s s i o n , 5 d a y s / w k fo r a t o t a l of 24 sessions. Eacht r i a l be g a n w i t h th e p r e s e n t a t i o n o f a s a m p l e s ti m u l u sove r the c e n t e r food wel l , w h i c h th e m o n k e y di s p l a c e dfo r a r a i s i n r e w a r d . Th e n , a f t e r a d e l a y of 6, 12, or 24sec, a choice w a s given between th e s a m p l e s t i m u l u s a n da n a l t e r n a t i v e choice s t i mu lu s . The two cho ice s t imu l idif fered in co lo r but had the s a m e f o r m a n d were p re -s e n t e d o v e r th e side food wel ls . Choice of the s a m p l ewa s rewarded with a r a i s in . A nonc o r r ec t i on procedurew a s used, and the i n t e r t r i a l i n t e r v a l was a c o n s t a n t15 sec.Th e m a i n v a r i a b l e o f i n t e r e st w a s t h e be t w e e n - t r ia l sr e l a t i o n s a s o u t l i n e d in F i g u r e 1. For any tw o c o n s e c u t i v et r i a l s , e i t h e r th e s a me c o lo r o r t h e d i f f e r e n t co lo r w a sc o r r e c t , th e fo rm (c i r c le o r s q u a r e ) o f t h e s a m p l e a n dchoice s t i m u l i e i t h e r r e ma i n e d th e s a m e o r shi f t ed , a n dth e p o s i t i o n o f t h e correct tes t s t i m u l u s w a s e i the r th es a m e o r d i f f e r e n t . Th es e v a r i a t i o n s p r o d u c e a t o t a l o fe i g h t m a i n betw een -tr ial s re la t ion s (SSS, SSD, SDD ,SDS, O D D , DOS, DSS, and DSD, where S and D referto s a m e a n d di f fe ren t , r e spect ive ly , a n d t o c o lo r , f o r m ,a n d p o s i t i o n , in t h a t o r d e r ) . C o lo r w a s t h e r e l e v a n t di -m e n s i o n on a l l t r i a l s , and fo rm wa s cons tan t and i r r e l -e v a n t w i t h i n a t r i a l a n d v a r i a b l e be t w e en t r i a l s . E a c ho f the 24 c o m b i n a t i o n s o f c o n d i t i o n s a n d d e l a y w a s gi ventwice in a session . Since th e ma j o r f o c u s w a s o n h o w t h e

r e l a t i o n be t w e en t r i a l s a f f e c t e d p e r f o r m a n c e , a "set-up"t r i a l , chosen a t r a n d o m f rom th e 12 s t i m u l u s - d e l a y c o n -dit ions , was given as the i n i t i a l t r ial of each session.Th e t r i a l c o n d i t io n s w e r e r u n i n o n e o f s ix q u a s i - r a n -d o m o r d e r s w h i c h w e r e c o n s t r u c t e d s u c h t h a t e a c h c o m-b i n a t i o n o f c o n d i t i o n a n d d e l a y w a s preceded by eacho t h e r t r i a l s e q u e n c e a bo u t e q u a l ly of t en i n s ix sessions.E a c h mo n k e y w a s given a d i f f e r e n t sequence o f se t -upt r i a l s , so tha t no two subjec ts were eve r tested with p re -c i se ly th e s a m e c o m bi n a t i o n o f s t i mu l i a n d t r i a l s .Results

The m o n k e y s av e r age d 91% correct on these t -up t r i a l s which began each day . Th emean p ropor t ion co r rec t fo r each be tween-t r ia l condi t ion a s a f u n c t i o n o f delay is pre-sen ted in Table 1 (each subjec t con t r ibu ted48 observat ions t o each po in t ) . At a l l delayin te rva l s , condi t ion SSS was bet te r thanSDS and condi t ion DDD was better thanDSD, resu l ts cons is tent with th e c o n t e x tmodel ' s in te rac t ive as sumpt ions and incon-s i s ten t wi th the independen t genera l iza t ionm o d e l . O v e r a l l , p e r f o r m an c e r an g e d f r o m92% correct to 42% correct, which indicatesr a t h e r d r am a t i c e f f e c t s o f t r i a l cond i t ions .The data were also very stable across sub-jects. Fo r each monkey , cond i t ion SSS wasbet te r than S D S a nd condi t ion D D D w a sbet te r than DSD.Er ro r da ta were ana lyzed by a De lay XTrial Type X Prac t ice (six session blocks)with in-subjects an a l y s i s o f var i ances . Therewas a s ignif icant main effect o f delay, F(2,4) = 14.1, M S e = 56.3, p< .05, wit h sl ightlym o r e e r r o r s o c c u r r i n g a t the 24-sec than a tthe 6- and 12-sec delays. There wa s also amain e f fec t o f t r ia l type, F(7, 14) = 26.5,M S e = 102.2, p < .01, with th e f o u r s a m eco lo r cond i t ion s be ing equ a l ly good and wi thcondition DSD being worse tha n DSS whichwa s in t u r n w o r se th a n al l the o ther cond i -t ions (Ne w m an -Ke u l s , ps < .01, with im -provemen t ev iden t be tween the second andth i rd s ix -sess ion b locks (Newman-Keu ls ,p < .01). N o n e of the in teract ions wa s sig-ni f ican t , a n d , fo r p u r p o s e s o f theo re t ica lanalysis , data were collapsed over practicea n d delay.Theoretical Analysis

Context model. A s s u m p t i o n s : In ap p l y -in g the con tex t model to the between-trials


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P R O A C T I V E I N T E R F E R E N C E 339Table 1Mean Proportion Correct fo r Each Trial Condition as a Function of Delay in Experiment I

Col o rSSSSDDDD

Tr i a l condi t ion :Form

SSDDDDSS

Posi t ionSDDSDSSD

6.9 2.90.87.90.87.88.75.56

Delay (in sec)12

.9 2.90.88.89.76.76.61.46

24.90.80.8 1.72.69.67.58.42

Note. The N = 3. S = s a m e ; D = dif fe ren t .

i n t e r f e r e n c e paradigm, i t was assumed tha tthe represen ta t ion of the s a mp le f rom thecu r r e n t t r i a l a nd th e r e p re s e n t a t i on o f t h eimmedia te ly p receding choice te s t in f lu-enced pe r fo rm an ce on the bas is o f the ir s im -i lar i ty to the cur ren t te s t cues a n d contex t .Highe r o rde r e f fec ts a r i s ing f rom other s t i l le a r l ie r samples a n d choice tests were a s-s u m e d to be negl igible and were not repre-sented in the model .The p robab i l i ty o f con t ro l by the sampleo r t h e preceding tes t w a s computed direct lyf rom the ov e ra ll s im i l a r i t y of the presentedtest cues a n d contex t to the p re v ious s a mp lean d test events , according to a mul t ip l ica t iveru le . S imi la r i ty a long each d imens ion wa srepresented by a pa ramete r be tween 0 a n d1 (c = s imi la r i ty of black to white; / = s im-i l a r i ty o f circle to squa re ; p = s imi la r i ty o fr i g h t to lef t food well position s; / = s imilar i tyo f t ime of re t r ieva l to t ime of s to rage ; x =s imi l a r i ty o f context between a s ing le sam ples t i m u l u s presented over the center food wellan d two choice s t imuli presented over theside food wells) . Probabi l i ty o f con t ro l ,P(CON), w a s th e n s imp ly e q ua l to the prod-uct o f the re levan t sim i la r i ty pa ram ete rs(mult ip l ica t ive rule) . In other words,P(CON) was reduced by any changes ine i ther cues or context between events . Timeis s imply treated as ac t ing l ike any o the rs t i m u l u s d ime ns ion .Refe rence to Figure 1 m a y make theseassumptions clearer. The probability thatthe tes t ing se t t ing would f avo r con t ro l by thepreceding test i s jus t t for condit ions SSSan d DSD, s ince t ime is the only dimension

o f dif fe rence be tween the two cons e cu t iv etes t t r ia ls . Fo r cond i t i ons S SD an d DSS, thecor responding p robab i l i ty w o u l d be t-p,s ince posi t ion is an a d d i t i ona l d ime ns ion o fdif ference. B y t h e s a me r a t i ona l e , th e p rob -abilities w o u ld be t -/for condi t ions SDS a n dOD D a n d t-f-p fo r cond i t i ons S DD an dDOS.N o w consider th e p r o b a b i l it y t h a t th e testse t t ing will f avo r c o n t r o l by the i n f o r m a t i o nassociated with th e s a mp le p re s e n t a t i on .This is the p r o d u c t of the p a r a m e t e r fo r t e m -p o ra l s imi l a r i t y (t) and the p a r a m e t e r fo rr e p r e s e n t i n g th e s imi la r i ty of the s ing le sam -p le con te x t to the test setting (x). Th e v a l u eo f the t e m p o r a l p a r a m e t e r s h o u l d be h ig h e rfo r th e s a m p l e a n d t e s t than fo r two con-secutive test t r i a l s , s ince the sam ple and te sta re close in t ime . In pract ice , the v a l u e fo rth e sample represen ta t ion is a l w a y s a p r o d -u c t ( x - t ) , a n d i n t h e p re s e n t e x p e r ime n t sthese p a r a m e t e r s c a n n o t be est imated sep-a ra te ly .Th e cont ro l p robab i l i t i e s fo r the s a mp lea n d the p receding te s t a re su m m ar ized inTable 2. The subscripts associa ted with th et e mp ora l s im i l a r i t y p a r a m e te r s r e cog n izethe dif fe rence in t ime be tween th e s a mp lea n d c u r r e n t test and the p receding te s t andc u r r e n t test. I t is a ssum ed tha t the va lue o ft decreases with the r e t e n t i o n i n t e r v a l bu tt h a t o th e r p a r a me te r s a r e t i m e - i n d e p e n d e n t .Table 2 a lso embodies o u r a s s u m p t i o n sconcerning the probabi l i ty o f m a k i n g a cor-rect response , given tha t e i the r the samplee v e n t or the p receding test t r i a l p rov ides thebasis o f p e r f o r m a n c e . A g a i n r e f e r e n c e to


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340 T H O M A S J . R E Y N O L D S A N D D O U G L A S L . M E D I NTable 2Context Model Control and Choice Probabilities for the Eight Trial Conditions

Col o r

SSSSDDDD

Tr i a l co n di t io n :Form

SSDDDDSS

Posi t ion

SDDSDSSD

P(CON)s ample :A

x-t,

x-t,

X-tt

x-t,x-t,

P(rn Choicetest:B S a m p l e

1/b 1+C1

p-tb 1+c1/ P - f b T + 7i/'b i + ci/b 1+C1/P-'b T + Tip - f b 1+C1

/b 1+C

probabi l i t iesPr i o rtest

1\+cpc+ p

Pc+ p1

\+cpcp\+cpcc+ pc~ c + pcp1+cp

Note. S = same; D = d i f f e r e n t .Figure 1 i s he lp fu l . I t i s as sum ed tha t thep robabi l i ty o f a correct response is e q u a l tothe rat io of s imilar i ty of the correct objectto the con tro l l ing even t div ided by t h a t v a l u ep l u s the s im i l a r i ty o f the inco r rec t objec t a ndth e c o n t r o l l in g e v e n t . On t r i a l s in wh i c h th es a m p l e e v e n t c o n t r o l s p e r f o r m an c e , fo r ex-ample, the correct object d iffers f rom thes a m p l e in t i m e (t) and pos i t i on 1 (p), and thei n c o r r e c t object differs f rom the s am p l e inposi t ion (p), t ime (t), a n d color (c). Theprobabi l i ty o f a co r rec t r esponse i s then p-t / ( p - t + c - p - t ) o r 1/(1 + c) .Th e s am e r e as o n i n g ap p l i e s to t r i a l s o nw h i c h the p reced ing tes t t r i a l con t ro l s pe r -f o r m a n c e . On SSS t r ia ls the correct objectdiffe rs f rom the correct object of the pre-ced ing t r i a l in t ime , whi le th e i nco r rec t o b-ject differs in t ime , co lo r , a n d pos i t ion . Th eprobabi l i ty o f a correct response is t/(t + t-c-p) o r 1/(1 + c-p), as is indicated in T ab l e2. N o t e t h a t in th is case p e r f o r m a n c e is ac-t u a l l y pred ic ted to be g rea te r whe n the p re-ceding tes t contro ls response, s ince 1/(1 +cp ) i s g rea te r th an 1 / (1 + c).The ov era l l p ro ba bi l i ty o f a co r rec t r e -s p o n s e i s c o m p u t e d a s t h e p r o bab i l i t y t h a tth e s a m p l e e v e n t co n t r o ls p e r f o r m a n c e m u l -

t ipl ied by the p robabi l i ty o f be ing co r rec t ,given t h a t t h e s am p l e c o n t r o l s p e r f o r m an c e ,p lu s th e p r o bab i l i t y t h a t th e preceding tes tc o n t r o l s p e r f o r m an c e t i m e s t h e p r o bab i l i t yo f being co r rec t, g iven tha t the p reced ingt e s t c o n t r o l s p e r f o r m an c e , p l u s t h e p r o ba-bil i ty t h a t n e i t h e r c o n t r o l s p e r f o r m an c e an dth e animal guesses correct ly .Combining information. Gi v e n t h a t th ep r o bab i l i ty o f c o n t r o l by t h e c u r r e n t s am p l ea n d th e p r o bab i l i t y o f c o n t r o l by the p re-ceding test a re independen t , f o u r logical pos-sibilities exist: (a ) sample on ly con t ro l s pe r -f o r m a n c e [ A ( l B)] ; (b ) prior tes t o n l yc o n t r o l s p e r f o r m a n c e [ B ( l - A ) ] ; (c) bo t hs am p l e a n d pr io r test c o n t r o l p e r f o r m a n c e( AB ) ; an d ( d ) n e i t h e r c o n t r o l s p e r f o r m an c e[(1 - A ) ( l B )] . In cases a and b, it wasa s s u m e d tha t cho ices were based en t i re ly o nth e s ing le con t ro l l ing i n f o r m a t i o n . W h e nn e i t h e r e v e n t wa s i nvo lved , i t was a s s u m e dt h a t th e m o n k e y g u e s s e d (wi th p r o b a b i l i t ycorrect = .5). F i n a l l y , for the cases in which

1 This v a l u e o f p would not be expected to be the s a m eas the v a l u e o f p associa ted with shif ts in posit ion be-tween t r i a l s . On t r i a l s in which th e sa m p l e c o n t r o l s per -f o r m a n c e , however , p cancels out o f the pred ic t ion e q u a -t i o n s so we need not be c onc e rned w i t h it s specif ic value.


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P R O A C T I V E I N T E R F E R E N C E 341both the sample a n d test i n f o r m a t i o n we r ei n f l u e n t i a l , two poss ibi l i t ies were inves t i-gated. Submodel P I as s u m e d t h a t th e m o n -ke y wa s e q u a l l y l ikely t o use e i ther even t a st h e bas is o f r e s p o n d i n g . S u bm o d e l N o P Ias s u m e d tha t cho ices were based o n t h e c u r -r e n t s am p l e w h e n e v e r it c o u l d c o n t r o l p e r -f o r m a n c e a n d t h a t i n t e r f e r e n c e f rom th ep r i o r t r i a l o c c u r r e d o n l y w h e n th e s a m p l ew a s n o t i n f l uen t i a l .These two submodels have f ive indepen-d e n t p a r am e t e r s . Th r e e a re re lated to p h y s -ical s imi l a r i ty (c, f, p), o n e i s associated withthe t empora l s imi l a r i ty of the co r rec t a n dpreceding tes t t r ia l (/b ) , and the o t h e r is thec o m bi n e d t e m p o r a l a n d con tex tua l s imi l a r -ity o f th e c u r r e n t te st an d t h e c u r r e n t s am p l e( x - f a ) . Both models embody the in te rac t ives imi l a r i ty as sumpt ions , b u t t h e N o P I m o d e lassumes that the prior test trial does not alterth e p r o bab i l it y t h a t th e c u r r e n t s am p l e wi llbe used as the basisof p e r f o r m an c e . We nextdevelop predict ions fo r an i n d e p e n d e n t g e n -

eralization model and then eva lua te the abil-it y of the models to p red ic t the da ta .Independent generalization model. Inorder to e v a l u a t e th e i n t e r ac t i v e a ss u m p t io n so f the con tex t model , an a l t e rna t ive modeltha t hypo thes izes tha t genera l iza t ion f romc o m p o n e n t d i m e n s i o n s is addi t ive a n d i n -d e p e n d e n t wa s a l so fo rm u la te d . Spec if ica l ly,it w a s as s u m e d t h a t th e cho ice s t imu l i ha dsome in i t i a l base s t reng th ( j) tha t cou ld bein c r em e n t e d b y a n a m o u n t r w h e n a s t im u l u sap p e a r e d a s a s am p l e a n d t h a t th e r e war df rom th e preced ing tr i a l added s t ren g th tothe choice s t imul i in propor t ion to thei r s im-i lar i ty (i n color [c], f o r m [/], o r posi t ion[p]) to the correct object on the precedingt r i a l . On a n S SS t r i a l , fo r e x a m p l e , the cor-rect s t imu lus wou ld have a s t reng th equa lto i ( i n i t i a l v a l u e ) + r ( f rom s am p l e r e -w a r d ) + (c + f + p [f rom the precedingtest]), an d t h e i n c o r r e c t s t im u l u s wo u l d h av ejus t strength /'+/(from the preceding test).On a DSD t r i a l , the co r rec t objec t wou ldT a b l e 3Predictions of the Independent Generalization Model

Tr i a l condi t ion :Color Form

S S

S S

S D

S D

D DD D

D SD S

PositionS

DDS

DS

S

D

Predic ted pr o po r t i o nco r re c t(i+r+c+f+p)

(i+r+c+f+p) + (/+/)(i+r+c+f)

(i+r+c+f) + (i+f+p)(i+r+c)

(i+r+c) + (i+p)(i+r+c+p)(i+r+c+p) + (0

(t+r)(i+r) + (i+c+p)(i+r+p)

(i+r+p) + (i+c)(i+r+f+p)

(i+r+f+p) + (1+f+c)(i+r+f)(i+r+f) + (i+f+c+p)

Note. The symbols a re ; ( i n i t i a l s t r e n g t h ) , r (s t reng th f rom s ample reward) , a n d c,f, and p (s t rength f rom sh a r i ngsa me color, fo rm, or posi t ion , respectively , as were rewarded on the imm edia tely preceding test tr ial). S = same;D = di f fe ren t .


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342 T H O M A S J . R E Y N O L D S A N D D O U G L A S L . M E D I Nh av e s t r e ng th i + r + f, a n d t h e incor rec tob jec t wou ld hav e s treng th i + c +/+/.Choice probabi l i t ies fo r th is mode l werecomp u te d a s t h e r a t i o o f a ccumu la t e ds t reng th fo r the test s t im ul i ; fo r exam ple , thepredicted proport ion correct on SSS t r ia lsw o u l d b e (/ + / + c + f + p ) / [ ( i + r +c + f + p) + (i + f)]. The predict ions fort h e v a r i ous t r i a l t y p e s a r e s umma r iz e d i nTable 3. The choice rule of the independentgene ra l iza t ion mode l l e ads to the predictiondiscussed ear l ie r that SD S t r ia ls will be bet-ter t h a n SSS trials. Looking at the corre-spond ing predic tion equa t ions , one m ay no tethat condit ion SSS differs f rom SD S o n l yin th e factor/which will t end to move pe r-f o r m a n c e in condi t ion SSS closer to chance .Data fits. Each of the mode ls unde r con-sideration has f ive p a ra me te r s t o be esti-ma te d . Fo r this purpose , a grid search pro-ce d u re w a s run on a PDF 11/10 comp u te rfo r th e independence mode l a n d each of thein te rac t ive submode ls in o rde r to f ind pa-rameter values that minimized the me a nsquared dev ia t ion (D 2) o f predicted and ob-served propor t ions . The best f i t t ing p a r a m -

eters are presented in Table 4, a long withD 2, mean abso lu te dev ia t ion (\B\), chi-s q ua re fo r goodness of f i t , and r2. For thein te rac t ive mode l f i ts , the q u a n t i t y x-t&,while theore t ica l ly the p rod uc t of two c o m -p one n t s , cou ld n o t be m e a n i n g f u l l y sepa-r a t e d . Th us a s ing l e p a r a m e te r v a l u e w a sest imated fo r the product .In addi t ion to incor rec t ly p redic t ing tha tcondi t ion SDS w o u ld be easier than SSS,th e q ua n t i t a t i v e f i t fo r the i nd e p e nd e ncemode l wa s q u i t e p oo r , with a m e a n a b s o l u tedeviation of .07 and r2 equa l to only .63. Theidea t h a t t h e com p on e n t d im e ns ions e x e r t a nind e p e nd e n t i n f luence o n P I does n o t seemto ca p tu re th e m a i n t r e n ds in th e d a t a .Th e in te rac t ive mode l tha t a ssumes tha tth e cu r r e n t s a mp le a l one con t ro l s p e r fo r -mance in the case in which bo th the sampleand the p rev ious te s t po ten t ia l ly sha re con-trol ( the N o P I s ub mod e l ) f a r e s s ome w h a tbet te r , but the r2 v a l u e is stil l o n l y . 88 andth e ave rage abso lu te dev ia t ion o f predictedand observed values is .04.The ve rs ion of the con te x t mod e l i nco r -p o ra t i ng p roa c t iv e in t e r f e r e n ce p rod uce d ex -

Table 4Best Fitting Parameters for Independence and Interactive Models

Exper i -m e n t Best fitting p a r a m e t e r s a n d indices of f i tIndependence model

12 CloseFa r

N o P I12 CloseFa r

PI12 CloseFar

/.16.25.67

x-t..53.44.42.77.99.73

r.9 5.86.87

( b

.91

.62.97

.78

.94.99

c.34.32.65

c.00.01.32.08.25.41

/.09.15.00

In te ract ive/

.1 9

.45.52

.20

.44.48

P.08.16.90

modelsP

.53

.35.1 4

.59

.60.16

D 2.007.003.005

D2

.002

.002.002

.000

.000.001

\D\.07.04.06

\D \.04.04.04.01.01.02

X20)123.9

31.950.5

X 2 ( 3 )106.628.325.6

2.81.25.8

r 2.63.76.8 2

r2

.8 8

.84.9 1

.99

.99.98Note. In E x p e r i m e n t 1 , N = 3; in E x p e r i m e n t 2, each g r o u p N = 2. PI = proac t ive i n t e r f e r e n c e .


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P R O A C T I V E I N T E R F E R E N C E 343cellent fits to the data. The average absolutedeviation of predicted and observed valueswas less than .01, and the model accountedfo r 99% of the variance. Although the fittingprocedures were designed to minimizesquared deviations rather than chi-square,the actual chi-square value was nowherenear statistical significance. Observed valuesfo r the eight main trial types, along withpredicted values for the PI submodel, ares h o w n in Table 5.Discussion

The results of the first experiment sup-ported both the qualitative and quantitativepredictions of the interactive models, whilebeing clearly inconsistent with the assump-tion that color, f o r m , and position were ad-ditive and independent in their contribu-tions. The data fit for the No PI interactivesubmodel indicated further that interferencew a s important even w h e n the current samplei n f o r m a t i o n exerted some control over per-formance.

The best account of the data was providedb y the interactive model that assumes thatPI occurs to the extent that the precedingtrial event i n f l u e n c e s performance. An im-portant implication of this model is thatproactive interference is not solely time-de-pendent but is also a f u n c t i o n of the simi-larity of the test and potentially interferingsituations. According to the parameter es-

timates of the PI submodel, the probabilitythat the preceding test trial was i n f l u e n t i a lranged from .09 on SDD and DDS trials to.78 on SSS and DSD trials. The fact thatp e r f o r m a n c e ranged f r o m 48% to 92% cor-rect in the various conditions reinforces theconclusion that PI exerts large i n f l u e n c e onperformance, an influence nicely accountedfo r b y t h e submodel.To test the generality of these models andto f u r t h e r explore their implications, we rana f o l l o w - u p experiment in which w e a t -tempted to experimentally manipulate therelative likelihood that the representationsassociated wi t h the current sample and thepreceding test w o u l d control performanceonthe subsequent choice test. This was done inExperiment 2 by altering the contextual sim-ilarity of the sample and the test presenta-tions.

Experiment 2Relations between the sample and the testsetting were varied by changing the sepa-

ration of the f o o d wells and the color of thef o r m b o a r d . These changes were designed toeither increase or decrease the distinctive-ness of the sample and the test presentations.In the Close condition, the food wells weremoved close together, and the entire f o r m -board waspainted a neutral tan. For the Farcondition, the f o o d wells were widely sepa-

Ta b le 5Mean Proportion Correct fo r Each Trial ConditionE x p e r i m e n t 2

T r ia l condi t ion :C o l o r

SSSSDDDDS e t - u pN

FormSSDDDDSSt r i a l

Pos i t ionSDDSDSSD

E x p e r i m e n t 1O b ta in e d

.92.86.8 5.8 4.7 7.77.65.48.913

Predic ted,9 2.86.8 3.8 5.76.79.64.48

CloseO b ta in e d

.82.78.79.81.65.75.65.48.772

Pred ic ted.83.77.79.81.66.73.65.48

Fa rO b ta in e d

.83.60.64.74.48.71.66.27.732

Pred ic ted.85.62.64.75.48.66.66.30

Note. Pred ic ted v a l u e s a re der ived f rom the proac t ive i n t e r f e r e n c e vers ion of the c o n t e x t m o d e l . S = same; Dd i f f e r e n t .


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344 T H O M A S J . R E Y N O L D S A N D D O U G L A S L . M E D I Nr a t e d , a n d t h e b a c k g r o u n d s u r r o u n d i n g th ec e n t e r ( s a m p l e ) food wel l w a s p a i n t e d t a n ,the b a c k g r o u n d o f o n e choice food wel l w a sr e d , a n d t h a t of t he o t h e r w a s g r e e n .Th e p r i m a r y p u r p o s e fo r E x p e r i m e n t 2wa s to investigate how the model's p a r a m -e t e r v a l u e s c h a n g e d w i t h e x p l ic it c h a n g e s inth e s t i m u l u s s it u a t i o n . F ir s t o f a l l , th e v a l u eo f p shou ld be sensitive to the distance be-tween choice food wel ls , wi th th e Close con-di t ion associa ted with grea te r pos i t i on s im-i l a r i t y t h a n t h e Fa r c o n d i t i o n . I t m i g h t a l s ob e expec ted tha t co lo r a n d f o r m d i f f e r e n c e sw o u l d be more sa l i en t (have smal l e r s imi -l a r i t y v a l u e s ) w h e n th e s t i m u l i a r e closelyj u x t a p o s e d t h a n w h e n t h e y a r e widely sep-arated. A th i rd prediction is that the relativel ike l ihood of the r e p r e s e n t a t i o n s of the p r e -ced ing test a n d c u r r e n t s a m p l e b e i n g i n f l u -e n t i a l w il l va ry with food we l l sepa ra t i on ,wi th th e c u r r e n t s a m p l e e x e rt in g m o r e co n -t r o l i n t he C l o s e c o n d i t i o n t h a n i n t he Fa rc o n d i t i o n .W h e n a l l o f these f ac to r s a re c o m b i n e d ,one can m a k e a reasonably c lear predic t ionc o n c e r n i n g t he o v e r a l l diff icul ty of the Closea n d F a r c o n d i t i o n s . H a v i n g th e food wellsclose together should increase the l ikel ihoodt h a t th e s a m p l e p r e se n t a t io n will c o n t r o l p e r -f o r m a n c e , an d i f the co lo r d i f fe rence is m o r edist inc t ive w h e n the s t imul i a re c lose , thent h a t to o will f a c i l it a t e p e r f o r m a n c e . O v e r a ll ,t h e n , the C lose con di ti on sho u ld y ie ld be t t e rp e r f o r m a n c e than the Far condi t i on , pa r t i c -u l a r l y o n those t r i a l types in w h i c h c o n t r o lby the preceding test trial would lower per-f o r m a n c e subs t an t i a l ly (e.g., di f fe ren t co lo ra n d di f fe ren t posi t ion t r ia l s) .Method

Subjects and apparatus. The t h r ee m onk ey s fromE x p e r i m e n t 1 wer e t es ted a g a i n o n t h e s a m e W G T A s ,bu t n ew f o r m b o a r d s were used . For the Close c o n d i t i o n ,th e t h r e e food wells were sp a c ed 7.7 cm c e n t e r to c e n t e r ,a n d th e e n t i r e b o a r d w a s p a i n t e d t a n . F o r t h e F a r c o n -d i t i o n , th e food wel ls w e r e spaced 20.3 cm c e n t e r toc e n t e r , a n d o n e t h i rd of the b o a r d w a s p a i n t e d r ed , o n et h i rd g r e e n , a n d t h e m i d d l e t h i r d t a n . Tw o b o a r d s w e rec o n s t r u c t e d , o n e wi t h red on the l ef t a n d g re e n o n t h er i g h t , th e o t h e r w ith r e d a n d g r e e n r e v e r s e d .Procedure. E x p er i m en t 2 w a s r u n with th e s a m eo v e r a l l p r o c e d u r e a s E x p e r i m e n t 1. Br ie f l y , the 24 c o m -b i n a t i o n s o f e x p e r i m e n t a l c o n d i t i o n s a n d d e l a y w e r ep r e s e n t e d t w i c e /sess i on p l u s a "set-up" t r i a l fo r a t o t a lo f 49 t r i a l s / sess i on . The m o n k e y s w e r e run two sess ions/

d a y , 5 d a y s / w k , fo r 24 d a y s . The Close a n d F a r c o n -d i t i o n s w e re a l t e r n a t e d b e t w e e n sess ions a c c o rd i ng toa n A B B A s e q u en c e , and the two t r i c o l o r e d f o rm b o a rd swere used on a l t e rn a te Far sessions. Once aga in , no twosubjec ts saw the s a m e s e q u e n c e o f s t i m u l i a n d t r i a l s ,s ince e a c h h a d a u n i q u e se t -up t r i a l o r d e r . Th e i n t e r t r i a li n t e r v a l was a c o n s t a n t 15 sec.Results

P e r f o r m a n c e on the C lose and Far se t -upt r i a l s ave raged 77% cor rec t and 73% cor rec t,respectively. The slightly l o w e r p e r f o r m a n c eo n these t r ia l s m a y re f lect d i s r u p t i o n a s s o -c ia t ed w i t h t h e n e w f o r m b o a r d s . T h e m e a nproportion correct for each t r ia l condi t ionis f r o m tw o m o n k e y s , s in c e th e th i rd devel-oped a s t rong pos i t i on b i as i n the Far con-d i t i on which pe r s i s t ed th rough 20 sessions.As in E x p e r i m e n t 1, b e t w e e n - t r i a l s c o n d i-t i ons heav i ly i n f l u e n c e d p e r f o r m a n c e , a n da g a i n SSS w as the best an d DS D t h e w o r s tc o n d i t i o n . As pred ic ted , the C lose condi t i onyielded better p e r f o r m a n c e t h a n the Far con-d i t i on , a n d th i s d i f fe rence w a s g r e a t e r w h e nrewarded co lo r o r pos i t i on shi f ted be tweentrials.

The re la t ive diff icul ty o f di f fe ren t t r i a ltypes was aga in ve ry s t ab le ac ross sub jec t s .For both the Close and Far condi t ions , eachm o n k e y p e r f o r m e d b e tt e r o n S S S t r ia l s t h a no n SDS t r ia l s a n d bet ter o n D D D t r ia l s t h a no n D SD tr ia ls , a s expected by the c o n t e x tm ode l . Tab le 6 presen t s th e m e a n p r o p o r t i o ncorrect by trial condit ion and delay for theClose and Far condi t i ons .P r e l i m i n a r y a n a l y s e s o f six-session blocksindicated that there were nos ign i f ican t p r ac -t ice ef fec ts (F < 1), a n d t h e d a t a w e r e t h e r e -f o r e co l lapsed ac ross days . The o ve ra l l ana l -ysis o f v a r i a n c e i n d i c a t e d t h a t th e m a i neffec ts o f d e l a y , F(2, 2) = 54.47, MSe =1.48, p < .05, and t r ia l types, F(l, 7) =16.86, MS; = 33.75, p < .01, as wel l as thei n t e r ac t ions o f C lose ve r sus Far condi t i onwith trial types, F(l, 7) = 4.27, MSe = 13.69,p < .05, and of t r ia l types wi th delays, F ( 14,14) = 3.21, MS, = 7.79, p < .05, were sig-ni f ican t .Theoretical Analysis

The d a t a fi t t ing pro cedures fo l l owed thoseused in E x p e r i m e n t 1. P a r a m e t e r s w e r e es-t ima ted sepa ra t e ly fo r the C lose and Far


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PROACTIVE INTERFERENCE 345T a b le 6Mean Proportion Correct for Each Trial Condition for the Close and Far Conditions and as aFunction of Delay

Trial condition:Color

SsSsDDDD

FormSSDDDDSS

Pos i t ionSDDSDSSD

6se c.87.7 3.77.79.76.85.75.49

CloseD e la y

12se c.8 5.8 5.84.89.61.7 3.58.53

24se c.74.77.76.75.57.68.62.4 2

6se c.84.54.64.71.55.80.64.31

Fa rD e l a y

12sec.84.68.69.8 1.45.68.71.23

24sec.79.57.60.71.44.65.65.27

Note. S = s a m e ; D = d i f f e r e n t .

conditions, wi th performance collapsed overdelay intervals.2 A grid search was con-ducted to minimize mean squared deviationsfo r each of the models under consideration.

The independent generalization model didn o t fit the results accurately. Condition SSSwas better than SDS, contrary to its predic-tion, and the average absolute deviation was.04 in the Close condition and .06 in the Farcondition. The No PI version of the inter-active model yielded an average absolutedeviation of .04 in both conditions, but thePI submodel was much more accurate in itspredictions. The PI version of the contextmodel had an average absolute deviation of.01 for the Close condition and .02 for theFar condition, and it accounted for nearlyall the variance. These estimates are detailedin Table 4, and the predicted and observedvalues for the model are given in Table 5.Not only does the PI interactive model fitthe data accurately, but also the associatedparameter estimated changed in the ex-pected manner. The similarity parameter forposition is much higher in the Close condi-tion than in the Far condition. Similarityparameters for color and form, in contrast,are higher in the Far condition. Finally, es-timates for x-tz and r b indicate that com-pared with the Far condition, the Close con-dition increased the likelihood that therepresentation associated with the currentsample would control choices and decreasedthe probability that the representation as-

sociated with the preceding test trial wouldexert a n i n f l u e n c e .

General DiscussionThe major empirical results, and their as-

sociated theoretical implications, were rea-sonably clear-cut. In each experiment,changes in color, changes in f o r m , a n dchanges in position all a f f ec t ed between-trials proactive interference. Color was therelevant dimension, pos i t i on was variableand irrelevant, and f o r m was constant w i t h i ntrials and irrelevant, yet each of these factorscontributed to performance. In the secondstudy, separation and distinctiveness of thefood wells also produced d i f f e r e n c e s inproactive interference. It seems clear, there-f o r e , that theories of animal memory thatf o c u s exclusively o n temporal information intheir accounts of forgetting are severely lim-ited. The present results imply that changesin a variety of attributes interact to deter-m i n e memory performance.The data of these experiments have beenrelated to a theory that assumes that overallsimilarity is the product o f similarities o f

2 Idea l ly , w e w ould l ike to address th e mo d e l s di rec t lyto delay effects . A l t h o u g h the effect of de lay was sta-t i s t i ca l ly r e l i a b l e , i t was no t o f su f f ic ien t m a g n i t u d e t oa l l o w us to d i s t i n g u i s h a m o n g d i f f e r e n t t r e a t m e n t s o ff o rg e t t i n g . The m a i n c o n c l u s i o n s w o u l d n o t be c h a n g e dif s e p a r a t e f i t s o f the mode ls were conducted a t e achd e l a y i n t e r v a l .


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34 6 THOMAS J . REYNOLDS A ND DOUGLAS L . M E D I Ni nd iv idua l c o m p o n e n t s a n d t h a t p e r f o r m a n c eis con t ro l l ed by pr io r even ts , p rov ided tha tthe test cues and context are similar to theirm e m o r y r e p r e s e n t a t i o n s . A m a t h e m a t i c a lm o d e l i nco rpo ra t ing th i s pe rspec t ive , the PIvers ion of the con tex t model , gave a n excel-l e n t ac c o u n t of the observed data in bo the x p e r i m e n t s . Accord ing to th i s model , over -a ll s i m i l a r i t y is d e t e r m i n e d by a m u l t i p l i -c a t i v e r a t h e r t h an an ad d i t i v e c o m bi n a t i o no f componen t d imens ions . Th is imp l ies tha ta s a l i en t d i f fe ren ce a lon g a s ing le d im ens ionbe tween th e test s i tua t ion a n d some ear l i e reven t c an be suff ic ient to preven t the rep-r e s e n t a t i o n o f tha t ea r l i e r even t f rom in f lu-e n c i n g test behavior despi te the presence o fs imi l a r i ty a l o n g m an y o t h e r d i m e n s i o n s .Even though co lo r was the re levan t d imen-sion a n d fo rm wa s i r r e l e v an t a n d c o n s t a n twith in t r ia ls , a sa l i en t change in f o r m be -tween trials could el iminate a n y between-t r ia ls pro act ive inf lu en ces ( i.e ., i f the f o r ms imi l a r i ty parameter,/, were zero) . A m o d e la s s u m i n g t h a t c o m p o n e n t di m e n s i o n s a r e i n -dependen t and addit ive in their inf luence, bywa y o f c o n t r a s t , wa s n e i t h e r q u a l i ta t i v e l y n o rq u a n t i t a t i v e ly suppor ted .A s discussed ear l ier , th e process modelchosen to specify the m e c h a n i s m o f c o n t r o lby th e m e m o r y o f a p r i o r e v e n t is qui te sep-a r a t e f rom th e i n t e r ac t i v e s i m i l a r i t y a s-sumpt ions . A r e t r i e va l model c an be devel-oped by a s s u m i n g t h a t r e t r ie v a l is a f u n c t i o no f s i m i l a r i t y a n d t h a t the c o n t r o l o f p e r f o r -m a n c e b y pr io r even ts depends so le ly o nmemor ies that h av e been activated. Alter-n a t i v e l y , a r e c o g n i t i o n s h o r t - t e r m m e m o r y(STM) mo del cou ld be der ived by as su m ingt h a t c o n t r o l o f p e r f o r m a n c e is d e t e r m i n e dby select ion f rom ac t ive STM r e p r e s e n t a -t i ons , where select ion i tse l f depends o n s im-i l a r i t y . These poss ibi l i t ies , of course, are notm u t u a l l y exclusive.The vers ion of the con tex t model g iv ingthe bes t accou n t o f the da ta as sum ed tha tw h e n bo t h th e s am p l e e v e n t a n d t h e preced-in g test contro l choice behav io r , an imals a rea s l ikely t o u se the preceding tes t a s t h e y a ret o u se the s a m p l e as the bas i s fo r r e s p o n d i n g .T h e s u bm o d e l t h a t a s s u m e d t h a t t h e p r e -c e d i n g t e s t c o n t r o l s p e r f o r m an c e o n l y w h e nth e s am p l e d o es n o t e x e r t a n y i n f l u e n c e o nth e tes t did not f i t the d a t a n e a r l y a s well .

This d i f fe rence suppor t s the c l a im tha tp r o ac t i v e i n t e r f e r e n c e in t h i s p a r ad i g m in -volves a direct compet i t ion o f m e m o r y tracesr a ther than a guess ing b ias in the absenceo f r e le v a n t i n f o r m a t i o n .In th e presen t exper imen ts , th e s a m p l ep resen ta t ions were a lways rewa rded , a p r ac -tice t h a t i s n o t u n i f o r m i n D M T S exper i -m e n t s . A l t h o u g h o n e c o u l d a r g u e o n s o m eg r o u n d s t h a t r e war d i n g a s am p l e p r e s e n t a -t ion shou ld a l low i t t o compete more effec-t ively with the p r io r t es t t r i a l , i t may havehad the oppos i te e f fec t . The fac t tha t bo thsample presentations and tests were re-war d e d m a y w o r k to b l u r th e d is t inc t ion be -tween them . I t is qui te po ss ible tha t i f sam plep r e s e n t a t i o n s we r e n o t r e war d e d , be t we e n -t r i a l s p roac t ive in te r fe rence wou ld be re -duced cons iderably .In app ly ing the con tex t m odel to the p res -en t exper imen ts , w e as sum ed tha t de lay in -t e r v a l s c h an g e t h e p a r am e t e r f o r t e m p o r a ls imi l a r i ty whi le l eav ing th e o t h e r s i m i l a r i t ypara m ete rs in t ac t . A n a l t e rn a t iv e poss ib il i tythat deserves serious cons idera t ion is thats t i m u l u s an d c o n t e x t a t t r ibu t e s m ay be c o m eless distinctiv e with t i m e , in w h i c h case the i rs imi l a r i ty pa ram ete rs w ou ld be expec ted toi n c r e as e . Un f o r t u n a t e l y , n e i t h e r o f t h e e x -per imen ts p roduced de lay e f fec t s o f a s u f -f ic i en t m a g n i t u d e t o a l lo w th e e v a l u a t i o n o fthese a l t e r n a t ive possibilities.The p resen t resu l t s concern ing p roac t ivei n t e r f e r e n c e an d i ts r e l a t io n t o s i m i l a r i t y a r econs i s ten t wi th o ther work i n o u r l a b o r a t o r ye x am i n i n g retroactive in terference in DMTS(Medin et al. , 1980) . S t imu l i in te rpo la tedbe tween a s am p l e p r e s e n t a t i o n an d t h e testp roduced n o e f f e c t , i n t e r f e r e n c e , o r fac i l i -t a t i o n , d e p e n d i n g o n t h e s i m i l a r i t y of thes am p l e an d t h e i n t e r p o l a t e d s t i m u l u s an dwh e t h e r the two s t im u l i were as soc ia ted withs a m e o r d i f f e r e n t o u tc o m e s . Those data sup-p o r t bo t h t h e g e n e r a l n o t i o n t h a t a s t im u l u se v e n t accesses s i m i l a r s t o r e d i n f o r m a t i o na n d th e specific idea th a t overa l l s im i l a r i tyis governed by a mult ip l icat ive combinat ionr u l e . Those resu l ts a s well as the resu l t s o fth e p r e s e n t e x p e r i m e n t s a r e c o n s i s t e n t withth e propos i t ion tha t di f fe ren t s t im u l u s e v e n t sc a n b e t r e a t e d a s i n d e p e n d e n t o n l y fo r thecase in wh i c h s t i m u l i a re v e r y d i s s im i l a r .S ince m os t de la yed-m atch ing s tud ies use


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P R O A C T I V E I N T E R F E R E N C E 347s m a l l sets o f a l t e r n a t i v e sample stimuli, therei nev i t ab ly a r e n u m e r o u s o p p o r t u n i t i e s fo rearlier s t i m u l u s episodes to be accessed bylater s t i m u l u s events. Nor can one escapec o n s i d e r a t i o n o f c u i n g effects by u s i n g dif-fe ren t s t imu l i o n every trial. E x p e r i m e n t 2demonstrated that the s i m i l a r i t y of the s a m -p l e c o n t e x t to the test context i n f l u e n c e d thelikelihood that th e sample presentation wouldcontrol p e r f o r m a n c e o n a s u b s e q u e n t choicetest. For certain p u r p o s e s one can m i n i m i z ec u i n g effects, but i t seems clear that sucheffects a re f a i r l y g e n e r a l a n d merit closers c r u t i n y .

Taken as a whole , the present studiesmake a strong case against the practice oftreating proactive in te r fe rence e f fects inDMTS solely in terms o f processes l inkedto time. S i m i l a r i t y along both r e l e v a n t andi r r e l e v a n t s t i m u l u s d i m e n s i o n s a s well a scontextual s imi la r i ty contributes i m p o r -t a n t l y to m e m o r y p e r f o r m a n c e . The test set-t i n g i s n o t some n e u t r a l context in whichr e s p o n s e tendencies to the correct and in-correct choice objects c o m p e te r a th e r , thetest setting is more appropriately conceivedo f as a potent cue d e t e r m i n i n g access to s im-i l a r p r i o r e v e n t s .

R e f e r e n c e sD ' A m a t o , M. R. D e l a y e d m a t c h i n g a n d s h o r t - t e r mm e m o r y i n m o n k e y s . In G. H. B o w e r (Ed.) , The psy-chology of learning an d motivation ( V o l . 7). NewYo r k : A cade m ic Press, 1973.Flagg, S. F. , & M e d i n , D. L. C o n s t a n t i r r e l e v a n t c u e sa n d s t i m u l u s g e n e r a l i z a t i o n in m o n k e y s . Journal of

Comparative and Physiological Psychology, 1973,85 . 339-345.G r a n t , D. S. P r o a c t i v e i n t e r f e r e n c e in pigeon sho r t - te rmm e m o r y . Journal of Experimental Psychology: An-imal Behavior Processes, 1975, / , 207-220.G r a n t , D . S., & R o be r t s , W . A . Trace i n t e r a c t i o n inp ig e on s h o r t - t e r m m e m o r y . Journal of ExperimentalPsychology, 1973, 101 , 2 1 - 2 9 .M e d i n , D. L. A t h e o r y o f c o n t e x t in d i s c r i m i n a t i o n

l e a r n i n g . In G. H. Bower (Ed.) , The psychology oflearning and motivation ( V o l . 9) . New Yo r k : A c a -d e mi c Press, 1975.M e d i n , D . L . A n i m a l mo d e ls a n d m e mo r y mo d e l s. I nD. L. M e d i n , W . A . Rober ts , & R. T. D a v i s (Eds.),Processes of animal memory. Hil lsda le , N. J . : E r l -b a u m , 1976.Medin , D. L. P r o a c t i v e i n t e r f e r e n c e in mo n k e y s : D e l a ya n d i n t e r s a m p l e i n t e r v a l e f fe c ts a r e n o n c o m p a r a b l e .Animal Learning & Behavior, 1980, 8, 553-560.M e d i n , D . L., R e y n o ld s , T. J., & Pa r k i n s o n , J . K . St im-u l u s s i m i l a r i t y a n d r e t r o a c t i v e i n t e r f e r e n c e a n d f a c il -i t a t i o n i n m o n k e y s h o r t -t e r m m e m o r y . Journal o fExperimental Psychology: Animal Behavior Pro-cesses, 1980, 6, 1 1 2 - 1 2 5 .M e d i n , D. L., R o be r t s , W . A ., & D a v i s , R. T. (Eds.) ,Processes of animal memory. H i l l s d a l e , N . J.: Er l -b a u m , 1976.Reyno lds , T . J ., & M e d i n , D. L. S t r e n g t h v s. t e m p o r a l -o r d e r i n f o r m a t i o n in d e l a y e d - ma t c h i n g - t o - s a mp le p e r -f o r m a n c e by m o n k e y s . Animal Learning & Behavior,1979, 7, 294-300.Rober t s , W. A. , & G r a n t , D. S. S h o r t - t e r m m e m o r y inthe p igeon w ith p r e s e n t a t i o n t i m e p r e c i s e ly c o n t r o l l e d .Learning and Motivation, 1974, 5, 393-408.Rober ts , W . A ., & G r a n t , D. S. Studies o f s h o r t - t e r mm e mo r y i n t h e p i g eo n u s i n g t h e d e l a y e d - ma t c h i n g - t o -s a mp le p r o c e d u r e . In D. L. M e d i n , W . A . Rober ts ,& R. T. D a v i s (Eds .) , Processes of animal memory.H i l l s d a l e , N . J . : E r lba u m, 1 9 7 6 .Spear, N . E . R e t r i e v a l o f m e m o r y in a n i m a l s . Psycho-logical Review, 1 9 7 3 , 50 , 163-194 .Spear, N . E . R e t r i e v a l o f me mo r i e s . In W. K. Estes(Ed.), Handbook of learning an d cognitive processes(Vol . 4 ). Hil lsda le , N.J . : E r l b a u m , 1976.S p e a r , N . E . The processing of memories: Forgettingand retention. H i l l s d a l e , N . J . : E r lba u m , 1 9 78 .Spence , K. W. The n a t u r e o f d i s c ri m i n a t i o n l e a r n i n g ina n i m a l s . Psychological Review, 1936, 43, 427-449.W a g n e r , A . R . P r i mi n g i n S TM : A n i n f o r m a t i o n p r o -c e ss in g m e c h a n i s m f o r s e l f -g e n e r a t e d o r r e t r i e v a l - g e n -era ted depression in p e r f o r m a n c e . In T. J . Tighe &R. N. L e a t o n (Eds.) , ffabituation: Perspectives fromchild development, animal behavior, and neurophys-iology. H i l l s d a l e , N . J . : E r lba u m, 1 9 7 6 .W o r s h a m , R. W. T e m p o r a l d i s c r im i n a t i o n f a c to r s i n thede layed m a t c h i n g - t o - s a m p l e t ask in m o n k e y s . AnimalLearning & Behavior, 1975, 3, 93-97.

Rece ived Ju ly 1 4, 1980Revision received M ay 4 , 1981

334-347 stimulus interaction and between-trials proactive interference in monkeys

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