Memry & Cgnitin 1974, Vl. 2, N.3, 467-471 The influence f ne memry retrieval n a subsequent. 1* memry retrieva GEOFFREY R. LOFTUS and ELIZABETH F. LOFTUS University f Washingtn, Seattle, Washingtn 98195 Ss prduced an instance f a categry and fllwing zer r tw intervening items prduced a secnd instance f the same categry. The secnd instance was prduced mre quickly than the initial instance. This finding, in cnjunctin with ther data reprted in the paper, indicate that the reductin in latency fr the secnd instance is due mstly t a reductin in the rate with which the categry is searched. In an experiment by Freedman and Lftus (1971), Ss were shwn a nun categry plus a restricting letter r adjective and were asked t name an instance f the categry which began with the letter r which was characterized by the adjective. Reactin time t prduce the respnse was measured. The data were discussed in terms f a mdel that pstulated a hierarchical memry cmpsed f nun categries (e.g., animals) with subsets (e.g., birds, dgs) and supersets (e.g., living things) f each categry. Retrieval frm this hierarchical structure was assumed t cnsist f at least tw majr steps: (1) entering the apprpriate categry and (2) searching the categry fr an apprpriate member. The times t execute Step 1 and Step 2 are hereafter dented t 1 and t 2, respectively. The duratin f t 1 was estimated t be abut.25 sec by the fllwing reasning. Ss saw stimuli presented with the categry either first (e.g., fruit-p) r secnd (e.g., Pvfruit] and with at least a 71-sec interval between the nun and restrictr. Reactin times were measured frm the presentatin f the secnd member f the pair. When the categry came secnd, the ttal retrieval prcess began nly after its presentatin and included bth t 1 and t 2, accrding t the mdel. When the categry came first, hwever, t 1 culd be cmpleted befre the restrictr was shwn. Fr example, given the stimulus fruit-p, the S culd enter the categry "fruits" during the interval. Since measured reactin time begins when "P" is presented, measured reactin time excludes t 1 in this case. The decrease in reactin time when the categry is shwn first vs secnd can therefre be equated with t 1, which is excluded ill the frmer case and included in the latter. Mre recently, Lftus (1973) asked Ss t prduce a member f a categry and a shrt time later asked them t prduce a different member f that categry. This was accmplished by shwing a categry-letter pair (e.g., fruit-p), which asked the S fr an apprpriate instance, *Requests fr reprints may be sent t either Lftus, Department f Psychlgy, University f Washingtn, Seattle. Washingtn 98195. The research was supprted by a Natinal Institute f Mental Health grant t E. Lftus and a Natinal Science Fundatin grant t G. Lftus. Appreciatin is expressed t Thmas O. Nelsn fr his cmments n the manuscript. then, fllwing zer, ne, r tw intervening items, shwing the same categry paired with a different letter (e.g., fruit-a), which asked fr a different instance. Interest centered arund the questin f whether the speed f retrieving the secnd instance f a categry was affected by the retrieval f the first instance andr the lag between the tw retrievals. The results indicated that respnse latency fr the secnd instance was shrter than respnse latency fr the first instance and increased mntnically with the number f intervening items. Fr example, a S's baseline time t name a fruit beginning with the letter "P" was 1.52 sec. Hwever, it tk him 1.22 sec t prduce the same respnse if he had named a different fruit n the previus trial and 1.29 sec t prduce the respnse if he had named a different fruit tw trials back. The results f the Lftus (1973) study thus indicate that the prcess f retrieving infrmatin frm a categry facilitates a subsequent retrieval frm that categry. Hwever, in this experiment the S was presented with the categry name and restricting letter simultaneusly; retrieval time thus included bth t 1 and t 2. Cnsequently, the facilitatin effect culd have invlved a reductin in t 1 r t 2 r bth. The present experiment is designed t distinguish amng these three pssibilities. In sme cnditins f the present experiment, an interval was inserted between the categry name and the letter and the stimuli were presented either in the rder categry-letter r in the rder letter-categry [as in the Freedman & Lftus (1971) study]. As nted abve, this prcedure allws an estimatin f t 1. Additinally in the present experiment, the S was required t name an instance f a categry and shrtly thereafter was asked t name a secnd instance f the categry [as in the Lftus (1973) study]. This design is sufficient t determine the lcus f the reductin in reactin time t name a secnd categry instance. Figure 1 shws three pssible patterns f results. Suppse first that nly categry entry time, t 1, is reduced when a secnd categry instance is prduced. In this case, the results shwn in Fig. la shuld btain: the 467
468 LOFTUS AND LOFTUS a RT cat.g'1 leu., paired with tw letters), 80 filler stimuli were used. The filler stimuli als cnsisted f a categry plus a letter. Sme f the filler categries were used nly nce; thers appeared twice with tw different letters. Thus, each S saw 240 unique stimuli (80 critical categries, each paired with tw letters, plus 80 filler stimuli). b RT ~ c RT ~ 0 2 initial Fig. 1. Three pssible patterns f results fr the relatinship between time and the number f intervening items (lag) between tw appearances f a critical categry. letter-categry cnditins (which include t 1 ) shuld depend n the prir retrieval, whereas the categry-letter cnditins (which exclude t 1 ) shuld nt. Cnversely, suppse that nly categry search time, t 2, is reduced when the secnd categry instance is prduced. Such a situatin wuld lead t the results shwn in Fig. lb. Bth the categry-letter and the letter-categry cnditins include t 2, s they shuld be affected equally by the initial retrieval. The final pssibility is that bth t 1 and t 2 are reduced. This situatin wuld predict the results shwn in Fig. lc. Here, the categry-letter cnditin (which includes t 2 but nt t 1 ) shuld be affected by the initial. retrieval, but the letter-categry cnditin (which includes bth t 1 and t 2 ) shuld be affected t a greater degree. METHOD Subjects Eighteen Ss frm the New Schl fr Scial Research received $5 fr their participatin in tw J-h sessins, which ccurred n 2 cnsecutive days. N S had previusly participated in a memry experiment. Materials Each stimulus was printed in blck letters n a 5 x 8 in. index card. A stimulus always cnsisted f a categry name plus a letter (e.g., fruit-p). Eighty critical categry names were selected frm the Battig and Mntague (1969) and Shapir and Palerm (1970) categry nrms. Each f the categry names was paired with tw different letters. If "dminance" is defined as the frequency with which a wrd is given as an exemplar f a categry, then ne f the tw categry-letter stimuli will be referred t as mre dminant than the ther. In additin t the 160 critical stimuli (80 categries each Design There were three within-s factrs: rder (categry-letter vs letter-categry), interval (simultaneus presentatin f the stimuli vs 2.5-sec interval between the categry name and the letter), and lag (Lag 0, Lag 2, and initial presentatin). These factrs were cmbined factrially, thereby giving a 2 (rders) by 2 (intervals) by 3 (lags) by 18 (Ss) design. Each S received a different permutatin f the 240 items with the fllwing restrictins: (1) The initial presentatin f a critical categry-letter pair was fllwed after zer r tw intervening filler items (i.e., at Lag 0 r at Lag 2) by the presentatin f the same categry paired with a different letter. Each S received 40 stimuli presented at Lag 0 and 40 at Lag 2. (2) On half f the trials, Ss saw the stimulus crrespnding t the high dminant instance befre seeing the stimulus crrespnding t the lw dminant instance. Fr the remaining trials, the reverse arrangement held. A given categry was presented in the rder dminant-nndrninant fr half the Ss and in the reverse rder fr the remaining half f the Ss, Prcedure Each S was tld that he wuld see items cnsisting f categries and letters and that he was t respnd with a wrd in the categry that began with the given letter. He Was given examples and tld t respnd as quickly as pssible, but t avid errrs. The S sat in frnt f a screen with a windw cvered by half-silvered glass. An index card cntaining the stimulus was placed in a dark enclsure behind the mirrr and was presented by illuminating the enclsure. A micrphne was placed in frnt f the S, and he respnded by speaking int it. A trial cnsisted f the fllwing: (a) a card with the item printed in large type was placed in the darkened enclsure; (b) the E said "ready" and pressed a buttn which illuminated the first member f the stimulus pair; (c) either simultaneusly r after a 2.5-sec interval, the secnd member f the pair was autmatically illuminated and an electric timer started; (d) the S's verbal respnse activated a vice key that stpped the timer and terminated the trial. A warm-up perid f 20 trials preceded the experimental trials each day. RESULTS Only crrect respnses (96%) t the critical stimuli were included in the fllwing analyses. Median latencies were btained fr each S's respnses in each f the 12 cnditins. Fr each cnditin, mean latencies were then btained by averaging the medians frm individual Ss; these means are pltted in Figs. 2 and 3. Figure 2 shws the results when the 2.S-sec interval was inserted between the categry and the letter. In bth the letter-categry and categry-letter cnditins, a secnd instance f a categry is prduced faster than the first' instance; furthermre, a secnd instance is prduced faster at Laga than at Lag 2. Figure 3 indicates that the same pattern f results btains when letter and nun are presented simultaneusly. A 2 (rders) by 2 (intervals) by 3 (lags) analysis f variance was dne n the latency data. Significant effects were fund fr lag [F(2,34) = 6.57, P <.05],
THE INFLUENCE OF ONE MEMORY RETRIEVAL 469 categry-letter rder [F(1,17) = 14.71, P <.01], "and interval [F(1,17) = 33.52, p<.ol]. Nne f the tw-way r three-way interactinswas significant (F < I fr all cases). DISCUSSION Dependence f Memry Retrievals A number f studies have indicated that the time t retrieve infrmatin frm a semantic categry is decreased if that categry has been accessed a shrt time previusly. Cllins and Quillian (1970), fr example, have shwn that the time required t answer such questins as "Is a canary a bird?" is decreased by as much as 600 msec if infrmatin abut canaries has been accessed n the previus trial. Using a smewhat different paradigm, Meyer and Schvaneveldt (Meyer & Schvaneveldt, 1971; Meyer, Schvaneveldt, & Ruddy, 1972; Schvaneveldt & Meyer, 1973; Meyer, 1973) have shwn the same thing. In these experiments, Ss were required t classify letter strings as wrds r nnwrds. The general finding was that the reactin time t classify a. letter string as a wrd is faster if the S has just classified a semantically similar wrd as ppsed t a semantically dissimilar wrd. Thus, fr example, the time it takes t classify "butter" as a wrd is faster if "butter" is preceded by "bread" than if it is preceded by "nurse." Tw general classes f mdels have been prpsed t handle such results. A lcatin shifting mdel (Meyer & Schvaneveldt, 1971) assumes that when a S has finished prcessing a member f a particular categry and must then shift t begin prcessing a secnd categry, the shift time is dependent upn the semantic distance between the tw categries. An activatin mdel, n the ther hand, assumes that when items in a categry are prcessed, ther items are "excited" r "activated" t the extent that they are semantically similar t the infrmatin being prcessed. Tw further assumptins are made: first (Warren, 1970) that activatin decays 2.5 sec. interval.: 1.60 1.90 1.80 RT 1.10 1.50 (J 2 letter. Categry Fig. 2. Mean reactin time in secnds as a functin f the number f intervening items (lag) between tw appearancesf a critical categry. Items were presented with a 2.S-sec interval between the categry and the letter. Letter Categry Letter simult. 2.20 2.10 RT 2.00 1.90 1.80 0 2 initial Categry Categry. Lette, Fig. 3. Mean reactin time in secnds as a functin f the number f intervening items (lag) between tw appearancesf a critical categry. The categry and letter were presented simultaneusly. away ver time and secnd that activated items are mre readily accessible than nnactivated items. The results f the present experiment tgether with the data f Meyer et al (1972) and Lftus (1973) discnfirm the lcatin shifting mdel and supprt the activatin mdel. All f these experiments invlve the fllwing srts f cmparisns. Let T represent target infrmatin whse time t be prcessed is the dependent variable f interest. Let R represent infrmatin which is semantically related t T, and finally let Uland U 2 represent infrmatin which is semantically unrelated t T. Nw cnsider three cnditins: Cnditin a: Prcess U1 ; Prcess U2 ; Prcess T. Cnditin b: Prcess R;Prcess U 2 ; Prcess T. Cnditin c: Prcess U1 ; Prcess R; Prcess T. The data shw that T is prcessed fastest in Cnditin c, next fastest in Cnditin b, and slwest in Cnditin a. Bth the lcatin shifting mdel and the activatin mdel crrectly predict that reactin time in Cnditin c wuld be faster than reactin time in Cnditins a and b. Hwever, the predictins f the tw mdels differ with regard t the relatinship between Cnditins a and b. A lcatin shifting mdel incrrectly predicts that reactin time wuld be the same fr Cnditins a and b, since in bth cases the S is shifting frm the unrelated categry, U 2 t T. An activatin mdel, n the ther hand, crrectly predicts the btained pattern f results. This is because in Cnditin b, T is assumed t have been activated by R, and this activatin has nt decayed by the time T is prcessed. In Cnditin a, n the ther hand, T is nt assumed t have been activated at all; therefre, time t prcess T wuld be lnger. Prcessing Stages In the utset f this reprt, it was nted that the semantic retrieval mdel prpsed by Freedman and
470 LOFTUS AND LOFTUS Table 1 Time Estimates (in Secnds) fr Memry Retrieval Stages as a Functin f Three Lag Cnditins Lag Cnditin Retrieval Stage. Lag 0 Lag 2 Initial t, Categry entry time 0.20 0.22 0.27 t 2 +k Categry search timeplusbaseline 1.47 1.65 1.69 t 3 Eye mvement time 0.14 0.14 0.13 t, Extraencding time 0.21 0.16 0.22 Lftus (1971) pstulates tw majr prcessing stages: entering a categry (which takes time td and searching the categry (which takes time t 2 ). Anther stage, taking time k, is a baseline stage, invlving respnse executin, etc. Unfrtunately, these stages are nt sufficient t handle the data frm the present experiment. T see why this is s, cnsider the reactin times t initially access a categry. These reactin times fall int a 2 by 2 design with rder (categry-letter vs letter-categry) and interval (2.5 sec vs simultaneus) as factrs. Accrding t the Freedman-Lftus mdel, the prcessing times invlved in initial access shuld be as fllws: Cnditin 1, categry-letter; interval: RT1 = t 2 Cnditin 2, letter-categry; interval: RT 2 =t 1 +t 2 Cnditin 3, categry-letter; simultaneus: RT3 =t l + t 2 Cnditin 4, letter-categry; simultaneus: RT 4 =t l + t 2 Thus reactin times fr Cnditins 2-4 shuld be equal t each ther and shuld differ (by t 1 ) frm the reactin time t Cnditin 1. Hwever, the data indicate that all fur reactin times differ frm ne anther, thereby necessitating the pstulatin f additinal prcessing stages. First, in Cnditin 4, the predispsitin t encde the categry befre the letter may cnflict with nrmal left-t-right reading habits. Thus, an additinal eye fixatin culd smetimes ccur in Cnditin 4 relative t the ther three cnditins. We shall label the time fr this additinal eye fixatin t 3 Secndly, when categry and letter are presented simultaneusly (Cnditins 3 and 4), reactin time must include the time t encde bth stimuli. With a 2.5-sec interval, n the ther hand (Cnditins 1 and 2), reactin time includes the time t encde nly ne f the tw stimuli. Let the extra encding time required in Cnditins 3 and 4 be designated by t4. We are nw in a psitin t include the tw new stages in the fur initial reactin times. (1a) Categry-letter; interval: RT 1 = t 2 =1.69 sec (1b) Letter-categry; interval: RT2 = t 1 + t 2 +k :: 1.96 sec (l c) Categry-letter; simultaneus: RT 3 = t 1 + t 2 + t4 +k:: 2.18 sec (ld) Letter-categry; simultaneus: RT 4 = t 1 + t 2 + t 3 + t 4 > 2.31 sec By apprpriate manipulatins f Eqs. la-4a, we find that t 1 = 0.27 sec (RT 2 - RTd; (t 2 ) = 1.69 sec (RTd; t3 :: 0.13 sec (RT4 - RT 3 ) ; and t4 = 0.22 sec (RT 3 - RT 2 ). The estimate f0.27 sec fr t 1 (categry entry time) cincides well with previus estimates btained by Freedman and Lftus (1971) and Lftus and Freedman (1972). The estimate f 0.22 sec fr t4 (encding time) is far greater than ne wuld expect if "encding" meant nly the prcess f pattern-recgnizing the visual stimulus (cf. Sperling, 1963, wh estimated 10 msec per item fr the pattern-recgnitin prcess). Thus the btained estimate f 0.22 sec must include a great deal mre prcessing, althugh it is impssible in the present experiment t determine what such encding might cnsist f. Finally, since an eye fixatin usually lasts n the rder f 200-300 msec, the estimate f 0.13 sec fr t 3 (extra fixatin time) is smewhat less than ne wuld expect. A pssible reasn fr this discrepancy is that additinal eye fixatins may nt be made n all f the Cnditin 4 trials. The ntin f an extra eye fixatin smetimes ccurring in Cnditin 4 is, fcurse, easily testable. One mre parenthetical remark shuld be made. As nted abve, the interactin f interval time and categry-letter rder was nt significant. If the null hypthesis f n interactin is accepted, then inspectin f Eqs. la-4a indicates that t 1 = t3. (This can be seen either by the fact that RT 3 - RT 1 :: RT 4 - RT 2 r by the fact that RT 2 - RT 1 = RT4 - RT 3, bth fwhich are true under the null hypthesis.) Hwever, since nthing in the present experiment necessarily warrants acceptance f the null hypthesis, the equality f t 1 and t 3 shuld nt be taken very seriusly. What Stage Des Activatin Affect? Using the lgic utlined abve, it is pssible t btain estimates f t 1, (t 2 ), t3, and 4 fr secnd categry presentatins at Lags 0 and 2. These estimates, alng with the estimates given abve fr initial presentatin, are shwn in Table 1. The statistical analyses f the data indicate that the nly parameter which reliably changes ver lag cnditin is t 2, If we make the reasnable assumptin that k remains cnstant ver lag cnditins, then t 2, the categry search time, cnstitutes the lcus f the activatin effect. This finding agrees with the cnclusin f Meyer (1973, p. 30), wh nted that "The semantic distance between categries... may affect the search rate fr the secnd categry." The invariance f encding time (t 4) ver lag cnditin is smewhat at dds with the finding fmeyer et al (1972, Experiment 3) that encding time appears t be shrtened by prir prcessing f semantically similar infrmatin. The reasn fr this discrepancy is nt entirely clear. A pssible explanatin may lie in the fact that the prcessing delay between the tw categries was much shrter in the Meyer et al experiment than in the present experiment, and the activatin decay functin fr encding time may be
THE INFLUENCE OF ONE MEMORY RETRIEVAL 471 different frm the analgus decay functin fr search rate. REFERENCES Battig, W. F., & Mntague, W. E. Categry nrms fr verbal items in 56 categries: A replicatin and extensin f the Cnnecticut categry nrms, Jurnal f Experimental Psychlgy Mngraph. 1969, 80(3, Pt. 2). Cllins, A. M., & Quillian, M. R. Facilitating retrieval frm semantic memry: The effect f repeating Part f an inference. In A. F. Sanders (Ed.), Attentin and perfrmance III. Amsterdam: Nrth-Hlland, 1970. Freedman, J. L & Lftus, E. F. Retrieval f wrds frm lng-term memry. Jurnal f Verbal Learning & Verbal Behavir. 1971, 10, 107-115. Lftus. E. F. Activatin f semantic memry. American Jurnal f Psychlgy, 1974, in press. Lftus, E. F., & Freedman. J. L. Effect f categry-name frequency n the speed f naming an instance f the categry. Jurnal f Verbal Learning & Verbal Behavir, 1972, 11. 343-347. Meyer, D. E. Crrelated peratins in searching stred semantic categries. Jurnal f Experimental Psychlgy 1973 99 124-133. ' " Meyer, D. E., & Schvaneveldt, R. W. Facilitatin in recgnizing pairs f wrds: Evidence f a dependence between retrieval peratins. Jurnal f Experimental Psychlgy 1971 90 227-234. ' " Meyer, D. E., Schvaneveldt, R. W., & Ruddy, M. G. Activatin f lexical memry. Paper presented at the meeting f the Psvchnmic Sciety, St. Luis, Nvember 1972. Schvaneveldt, R. W., & Meyer, D. E. Retrieval and cmparisn prces~es in semantic memry. In S. Krnblum (Ed.), Attentin and perfrmance IV, New Yrk: Academic Press 1973. ' Shapir, S. I., & Palerm, D. S. Cnceptual rganizatin and class membership: Nnnative data fr representatives f 100 categries. Psychnmic Mngraph Supplements, 1970, 3(11, Whle N. 43). Sperling, G. A mdel fr visual memry tasks. Human Factrs, 1963,5,19-31. Warren, R. E. Stimulus encding and memry. Unpublished dctral dissertatin, University f Oregn, 1970. (Received fr publicatin September 17, 1973; revisin accepted December 6,1973.)