Transfer across CS-US intervals and sensory modalities in classical conditioning of the rabbit

Transcription

1 Animal Learning & Behavir 1984,12 (2), Transfer acrss CS-US intervals and sensry mdalities in classical cnditining f the rabbit E. JAMES KEHOE and PHOEBE E. HOLT University jnew Suth ales, Kensingtn, N. S.., Australia Tw experiments investigated transfer f the rabbit's cnditined nictitating membrane respnse (NMR) frm shrter t lnger CS-US intervals in cnjunctin with a change in CS mdality, fr example, light t tne. In Experiment 1, three experimental grups received initial training with a 400-msec CS-US interval, which prduced substantial CR acquisitin, and three cntrl grups received initial training with a 2,800-msec CS-US interval, which prduced minimal CR acquisitin. Subsequently, the experimental and cntrl grups received training with an 800-,1,800-, r 2,800-msec CS-US trace interval. At the same time, the mdality f the CS was changed frm tne t light (r vice versa). Experiment 2 cntained three grups that received initial expsure t a 400-msec CS-US interval, a 2,800-msec CS-US interval, r just the experimental chambers. Subsequently, all three grups received training with an 800-msec CS-US interval in a different CS mdality. The results f bth experiments revealed substantial psitive transfer acrss CS mdalities frm the 400-msec CS-US interval t the 800-msec CS-US interval. There was als significant transfer t the 1,800-msec but nt the 2,800-msec CS-US interval. The transfer did nt appear immediately n test presentatins f the secnd CS. Rather, the transfer appeared as an enhancement in the rate f CR acquisitin after reinfrced training with the secnd CS had cmmenced. The results are discussed with respect t mechanisms f transfer and facilitatin f trace cnditining. The present experiments examined transfer f training in classical cnditining f the rabbit's nictitating membrane respnse (NMR) under a cmbined manipulatin f CS-US intervais and CS mdality. Many assciative learning paradigms display trace cnditining ver relatively brief intervals. Hwever, the use f a serial cmpund prcedure (CSA-CSB US) substantially facilitates respnse acquisitin t CSA ver relatively lng trace intervals (e.g., Blles, Cllier, Butn, & Marlin, 1978; Kaplan & Hearst, 1982; Kehe, Feyer, & Mses, 1981; Kehe, Gibbs, Garcia, & Grmezan, 1979; Kehe & Mrrw, 1984; Pearce, Nichlas, & Dickinsn, 1981; Rescrla, 1982). Research with serial cmpunds has fcused primarily n the rle f the CSA-CSB relatin, but the substantial trace cnditining f CSA may arise in part frm transfer between the CSB-US and CSA US relatins (Kehe, 1982). Just as acquisitin f an easy discriminatin facilitates subsequent acquisitin This research was supprted by the Schl f Psychlgy, University f New Suth ales, and the Australian Research Grants Scheme (A ). The authrs express their gratitude t Luise Hyman, Luise Mrrw, and Cherie allace-taylr fr their aid in cllecting the data and preparing the manuscript. The authrs als thank R. F. estbrk fr his cmments n the manuscript. Requests fr reprints shuld be sent t E. James Kehe, Schl f Psychlgy, University f New Suth ales, P.O. Bx I, Kensingtn, New Suth ales 2033, Australia. f a hard discriminatin (Lawrence, 1952; Pavlv, 1927, pp ; Seraganian, 1979), experience with ne CS at a shrt, "easy" CS-US interval may facilitate acquisitin t anther CS at a lng, "hard" CS-US interval (Kehe et al., 1981). Recently, estbrk and Hmewd (1982) have fund that txicsis cnditining with ne flavr (e.g., sucrse) at a relatively brief CS-US interval (e.g., 15 min) facilitates the subsequent acquisitin f an aversin t a new flavr (e.g., salt) at a lnger interval (e.g., 3 h). The bundary cnditins that yield transfer acrss CS-US intervals and/r sensry mdalities in classical cnditining f the rabbit are nt well delineated. First, abrupt shifts frm a shrter t a lnger CS-US interval have resulted in dramatic drps in perfrmance, even when the same CS was used thrughut training (Cleman & Grmezan, 1971; Prkasy & Papsdrf, 1965). Hwever, gradual increases in CS-US intervals were fund t sustain high levels f respnding at lng, therwise ineffective CS-US intervals (prkasy & Papsdrf, 1965). Secnd, in examining transfer acrss CS mdalities, Yehle and ard (1969) fund psitive transfer frm a discriminatin between tw pulsed tnes t a discriminatin between tw pulsed light stimuli (and vice versa). Third, in a jint manipulatin f CS-US intervals and CS mdalities, Kehe et al. (1979, Experiment 3) fund n evidence f transfer when they Cpyright 1984 Psychnmic Sciety, Inc. 122

2 TRANSFER AND TRACE CONDITIONING 123 intermixed 4OO-msec light-shck trials (CSB-US) with 1,800-msec tne-shck trials (CSA-US). The curse f acquisitin t the tne CSA in the intermixed training grup appeared identical t that f a grup trained with nly the 1,800-msec CSA-US trace interval. Hwever, Kehe and Mrrw (1984) fund that intermixed training (400-msec CSB-US/2,800 msec CSA-US) prduced higher levels f respnding t CSA in the intermixed training grup than in a crrespnding CSA-US trace cnditining grup. Finally, there is sme evidence f negative transfer acrss sensry mdalities. Specifically, Hinsn and Siegel (1980, Experiment 1) reprted that initial training with ne CS (e.g., tne) at a,600-msec CS-US interval subsequently retarded the rate f CR acquisitin t anther CS (e.g., light) at a 500-msec CS US interval. Als, Scavi (1975) reprted that prir tne-water pairings retarded the rate f CR acquisitin under subsequent light-shck pairings. Investigatins f crss-mdal transfer in ther species and paradigms have yielded equally cmplex results (cf', Church & Meek, in press; Rdgers & Thmas, 1982; Vn right, 1970). EXPERIMENT 1 The present experiment was cnducted t determine whether trace cnditining culd be facilitated by prir training with anther CS in a different sensry mdality at a shrter, mre efficacius CS-US interval. T this end, six grups f animals were used. Three f the grups received initial training with ne CS (e.g., tne) at a 4OO-msec CS-US interval and subsequent training with anther CS (e.g., light) at an 800-, 1,800-, r 2,800-msec CS-US interval. As baselines fr the detectin f transfer effects, three cntrl grups received initial training with a 2,800-msec CS-US interval fllwed by training with an 800-, 1,800-, r 2,800-msec CS-US interval. The baseline cnditin was designed t fulfill tw criteri (1) equal expsure t the handling, apparatus, CSs, and US as experienced by the experimental grups, and (2) minimal excitatry r inhibitry cnditining during Stage 1. The 2,800-msec CS-US interval was chsen as the baseline cnditin, because it clearly meets the first criterin and typically prduces a lw level f excitatry cnditining in the rabbit NMR preparatin (cf. Kehe et al., 1979, 1981). Thus, any differences between the experimental and baseline cnditins wuld prvide a cnservative estimate f psitive transfer. Cnventinally, tw frms f transfer are recgnized, namely "immediate transfer" and "general transfer." In immediate transfer, the alteratin in respnding appears n the initial presentatin f the secnd CS. In general transfer, savings emerge ver the curse f training in the rate r asymptte f re- spnse acquisitin t the secnd CS (Meck & Church, 1982; Seraganian, 1979). Accrdingly, the prcedure f the present experiment was designed t detect bth immediate transfer and general transfer. Methd Subjects. The subjects were 46 naive, female albin rabbits (Oryctlagus cuniculus). On arrival, each rabbit was days ld and weighed apprximately 1.5 kg. All rabbits had free access t fd and water in their hme cages. Apparatus. The apparatus and recrding prcedure fr the nictitating membrane respnse were patterned after thse f Grmezan (1966) as detailed in Kehe et al. (1981). In brief, the subjects were trained individually in eight sund-attenuating, ventilated cnditining chambers. During training, each rabbit was restrained in a Perspex bx, which was held in place within each chamber between metal stays screwed t the flr. A speaker was munted at a 45-deg angle, 8 em anterir t and 16 em abve the subject's head. The speaker prvided bth white nise and an aural es, which was a I,OOO-Hz, 88-dB (SPL) tne superimpsed n an 82-dB ambient nise level prvided by white nise and an exhaust fan. An 8- frsted nen light tube was munted 4 cm abve the speaker. The light tube served as a huselight and was flashed at a rate f 20 Hz in rder t prvide a visual es. The duratin f bth ess was 400 msec, regardless f the es-us interval. The US was a 50-msec, 3-mA, 50-Hz ac shck delivered via stainless steel Autclip wund clips psitined mm apart and 15 mm psterir t the drsal canthus f the right eye. The sequence and timing f stimulus events were cntrlled by an Apple II cmputer equipped with interfaces and sftware develped by Scandrett and Grmezan (1980). Each rabbit's right external eyelids were held pen by N. 3 tailr hks munted n a Velcr strap that fitted abut the head. A muzzle-like headset, fitted abut the snut, supprted a transducer fr mnitring mvements f the nictitating membrane. A small hk was attached t a silk lp sutured in the nictitating membrane f the rabbit's right eye. The hk was cnnected by a thread t ne end f an L-shaped wire lever, which mechanically transmitted the mvement f the nictitating membrane t the transducer. Inside the transducer, mvement f the lever rtated a disk f plarized filter that was interpsed between a lightemitting dide and a pht transistr cvered by a fixed plarized filter. Thus, rtatin f the disk prduced changes in the intensity f the light reaching the transistr thrugh the fixed filter. The signal frm the transistr was amplified and transmitted t an analg/digital cnverter attached t the Apple II cmputer. Prcedure. All rabbits received 1 day f preparatin, 2 days f rest, 1 day f adaptatin, 4 days f Stage 1 training, and 5 days f Stage 2 training. On the preparatin day, hair surrunding the rabbit's right eye was remved, and a small lp f silk (000 Dynex) was sutured int the nictitating membrane. On the adaptatin day, the animals were placed in the cnditining apparatus fr 70 min, but neither a es nr a US was presented. Fllwing the adaptatin day, the animals were assigned t ne f six grups (n =8) accrding t a 2 x 3 manipulatin f the es-us intervals in Stages 1 and 2, respectively. The term "es US interval" refers t the interval between the nset f the es and the nset f the US. In Stage 1, the es-us interval was either 400 r 2,800 msec. In Stage 2, the es-us interval was 800, 1,800, r 2,800 msec. The grups were designated by the significant digits f the es-us intervals in Stages 1 and 2, respectively. Thus, the grups were labeled 4-8, 4-18, 4-28, 28-8, 28-18, and Fr example, Grup 4-8 received a 400-msec es-us interval in Stage 1 and an SOO-msec es-us interval in Stage 2. Half the animals in each grup received tne as the es in Stage 1 and flashing light as the es in Stage 2. The ther half f each grup received the light in Stage 1 and the tne in Stage 2. ith the exceptin f the first day in Stage 2 training, all training days cnsisted f 70 es-

3 124 KEHOE AND HOLT US trials separated by a mean intertrial interval f 60 sec (±20 sec). On the first day in Stage 2 training, Trials I, 2, 3, and 4 were CS-alne trials that prvided a test fr immediate transfer frm Stage 1 t the new stimulus in Stage 2. A cnditined respnse (CR) was defined as any extensin f the nictitating membrane exceeding.s mm which ccurred fllwing the nset f the CS but prir t the nset f the US. T analyze the data, a set f planned rthgnal cntrasts was written (Miller, 1966). The rejectin level was set accrding t a Type I errr rate f.os fr a family fcntrasts. Results Figure 1 shws the mean percentage f CRs in Stage 1 fr each grup pltted as a functin f 35 trial blcks. A fault in the data disk prevented recvery f all the data frm Stage 1. Thus, the data fr Stage 1 are based n ns=4, 8, 3, 4, 8, and 3 fr Grup 4-8,4-18,4-28,28-8,28-18, and 28-28, respectively. The curves in Figure 1 clearly indicate that the 4OO-msec CS-US interval prduced faster and higher levels f CR acquisitin than the 2,800-msec CS-US interval did. Respnding in the grups trained with the 4OO-msec CS-US interval attained terminal levels exceeding 750 CRs, whereas respnding in the grups trained with the 2,SOO-msec CS-US interval rse nly slightly ver training, reaching a mean terminal level. n higher than 240 CRs. Statistical analysis cnfirmed that there was a significant main effect f CS US interval [P(1,24) = 28.23], which interacted with the linear trend ver trials [F(1,24) = 36.96]. Any (!) « ~ (J) c: U) 50 A U 40 ~ C Z 30 U 0 20 a Figure 1. Mean percentage f CRa in Stage 1 f Experiment 1 pltted as a fnnctin f 35-triaI blcks. Grups 4-1, 4-18, and 4-28 received training with a 400-msec CS-US interval, and Grups 28-8,28-18, and received training with a 2,800-msec CS-US interval ther apparent differences between grups failed t attain statistical significance. There was n evidence fimmediate transfer frm Stage 1 t Stage 2. On the first fur trials f Stage 2 training, Grups 4-8, 4-18, and 4-28 shwed mean levels f nly 30, 30, and 00 CRs, respectively. Grups 28-8,28-18, and shwed slightly higher mean levels f 90, 190, and 0 CRs, respectively. If anything, the training with 4OO-msec CS-US interval in ne mdality may have slightly inhibited respnding n the first expsures t the stimulus in the ther mdality. Hwever, the apparent difference between 400- and 2,800-msec CS-US intervals failed t attain statistical significance [F(1,38) = 3.61, P =.06]. Figure 2 shws the mean percentage f CRs in Stage 2 pltted acrss 35-trial blcks. Panel A shws acquisitin curves fr Grups 4-8 and 28-8, Panel B shws the curves fr Grups 4-18 and 28-18, and Panel C shws the curves fr Grups 4-28 and Inspectin f the figure reveals clear evidence f psitive transfer acrss sensry mdalities. Fr each pair f grups, the grup initially trained with the 4()()-msec CS-US interval shwed a higher level f respnding in Stage 2 than that f its cntrl grup initially trained with a 2,800-msec CS-US interval. Psitive transfer was the greatest in the pair f grups trained with the 800-msec CS-US in Stage 2 and diminished in magnitude acrss the 1,800- and 2,800 msec CS-US intervals. The statistical analysis cnfirmed that the level f respnding in Stage 2 was facilitated by prir training with the 4()()-msec CS-US interval relative t the 2,800-msec CS-US interval [P(1,40) = 9.96]. Mrever, there was a significant linear trend acrss Stage 2's CS-US intervals [F(1,40) = 5.82], which interacted with the linear trend ver training trials [F(1,40) = 13.34]. Further inspectin ffigure 2 reveals that the psitive transfer effect was apparent frm the first blck ftraining trials in Stage 2 and cntinued thrughut Stage 2, particularly in the grups trained with the 800- and 1,800-msec CS-US intervals. As a measure f the rate f CR acquisitin, we used the trial numbers f the 1st, 2nd, 3rd, 5th, and th CRs in Stage 2. Figure 3 shws the mean trial fr each f the designated CRs. Since i lb + 1 CR must necessarily ccur at least ne trial later than the i lb CR, there wuld have t be a linear trend acrss the series f designated CRs. T remve this necessary linear trend frm the measure finitial acquisitin, the trial numbers f the 2nd, 3rd, 5th, and th CRs were adjusted dwnward by 1, 2, 4, and 9, respectively. Fr example, if a subject started respnding in Stage 2 by making a series f cnsecutive CRs beginning n Trial 13, then that subject wuld be assigned a value f 13 fr each f the designated CRs. In practice, n subject started respnding with cnsecutive CRs, but 26 fthe 46 subjects did shw at least tw f their

4 C\l A-: B LLJ < 70 '" 4-18 ~ en 60 A E U) rj'~r' U 40 ~ Z LLJ 30 ~'-.F 20 LLJ D. TRANSFER AND TRACE CONDITIONING 125 C 4-28 C Figure 2. Mean percentage f CRs in Stage 2 f Experiment 1 pltted as a functin f 3S-trial blcks. Grups 4-8 and 28-8 were trained with an 800-msec CS-US interval (panel A), Grups 4-18 and with a 1,80-msec CS-US interval (panel B), and Grups 4-28 and with a 2,800-msec CS-US interval (Panel C). initial CRs n cnsecutive trials. The means shwn in Figure 3 reflect this adjustment, and thus, the upward trends indicate that there were actual gaps between trials cntaining CRs. Figure 3 reveals that Grups 4-8, 4-18, and 4-28 generally shwed significantly mre rapid initial CR acquisitin than did Grups 28-8,28-18, and [F(1,40) = 6.47]. Hwever, the differences between the grups were relatively small fr the 1st and 2nd CRs and grew mre prnunced fr the later CRs; this was cnfirmed statistically by a significant interactin between the Stage 1 CS-US interval and the linear trend acrss CRs [F(1,40) =4.41]. The statistical analysis f Stages 1 and 2 included tests t determine whether there were any differences in the cnditinability f the particular tne and light stimuli used in the present experiment. In fact, there were n significant main effects r interactins invlving the tne and light stimuli. Inspectin f the data revealed that differences between respnding t the tne and light were small and incnsistent. EXPERIMENT :2 Figure 3. Mean trial f the lst, 2nd, 3rd, 5th, and th CR in Stage 2 f Experiment 1. The trial number fr each f the designated CRa his been adjusted dwnward t eumiute an upward trend necessitated by the fact that the trial f the n lll + 1 CR must be at least ne trial greater than tbe nibcr. The present experiment was cnducted t replicate the crss-mdal transfer frm the 400- t the 800 msec CS-US intervals. A rest cntrl grup was added in rder t determine whether there was negative r psitive transfer frm the 2,800- t the 800-msec CS US interval (cf. Hinsn & Siegel, 1980). Methd The subjects were 24 female albin rabbits f the same age and weight as used in Experiment 1. The apparatus and prcedures were identical t thse used in Experiment I. The subjects were

5 126 KEHOE AND HOLT assigned t three grups (n=8) designated 4-8, 28-'8, and R-8. Hwever, the death f ne subject reduced Grup R-8 t seven subjects. Grups 4'-8 and 28-8 received training identical t that f their cunterparts in Experiment 1. Grup R-8 received nly restraint and expsure t the chambers fr 70 min/day during Stage 1. During restraint, Grup R-8 was used t bserve spntaneus respnses during intervals crrespnding t thse f the training trials in Grup In Stage 2, Grup R-8 received training with an 800-msec CS-US interval in the same manner as the ther tw grups. Results Figure 4 shws the mean percentage f CRs in Stage 1 as a functin f 35-trial blcks. Examinatin f Figure 4 reveals that Grup 4-8 shwed rapid CR acquisitin t an asymptte near 00 CRs, which was significantly higher than the level f respnding displayed by either Grup 28-8 [F(l,20)= ] r Grup R-8 [F(l,20) = ]. In turn, Grup 28-8 shwed mdest CR acquisitin t an asymptte f 200 CRs, which was significantly greater than the level f spntaneus respnding bserved in Grup R-8 [F(l,20) = 8.03]. The results f Stage 2 cnfirmed thse f Experiment 1; that is, there was n detectable immediate transfer acrss mdalities, but there was substantial general transfer. On the first fur trials f Stage 2, the mean percentage f CRs was 80, 00, and 50 fr Grups 4-8,28-8, and R-8, respectively. Figure 5 shws the mean percentage f CRs in Stage 2 pltted 'P"'" C!' 0 90 SO «I-- en 70 c: 60 rn 50 U I-- 40 Z U 30 Q S 2S-S A R-S Figure 4. Mean percentage f CRs in Stage 1 f Experiment :1 pltted as a functin f 35-trial blcks. Grup 4-8 received training witb a 4OO-msec CS-US inte"ai, Grup 28-8 received training with a 2,800-msec CS-US idte"ai, and Grup R-8 received restraint in tbe cnditining apparatus. C\l 4: I (J) c: III I Z w a ~ / jp----o--o / [ / A R-8 1/ s-: OL.-..._..._L..._...--'L.-..._...--'L Figure 5. Mean percentage f CRs in Stage :1 f Experiment :1 pltted as a functin f 35-trial blcks. AlI grups were trained with an 800-msec CS-US inte"ai. acrss 35-trial blcks. Grup 4-8 shwed rapid acquisitin, particularly in the first blck f trials, whereas Grups 28-8 and R-8 shwed mre gradual acquisitin. All three grups appeared t cnverge at an asymptte arund 850 CRs. The verall level f respnding in Grup 4-8 was higher than the cllective perfrmance f Grups 28-8 and R-8 [F(I,20)=.12]. Althugh Grup 28-8 displayed a slightly higher level f respnding than Grup R-8, the difference was nt significant. Figure 6 shws the mean adjusted trial f the 1st, 2nd, 3rd, 5th, and th CRs fr Grups 4-8, 28-8, and R-8. Examinatin f Figure 6 reveals that Grup 4-8 shwed very rapid CR acquisitin, requiring apprximately 16 trials t attain the first CR and nly a few mre t attain the th CR. Thus, the relatively flat curve fr Grup 4-8 indicates that there was a rapid transitin in perfrmance frm n CRs t virtually 00 CRs. Grups 28-8 and R-8 shwed slwer rates f CR acquisitin. As seen in Experiment 1, the differences between Grup 4-8 and the ther tw grups were smallest fr the 1st CR and grew mre prnunced fr the later CRs. Statistical analysis cnfirmed that there was a significant difference between the linear trend f Grup 4-8 and that f Grups 28-8 and R-8 taken tgether [F(l,20) =6.91]. Althugh Grup 28-8 shwed sme slight savings relative t Grup R-8, the apparent differences failed t attain significance. GENERAL DISCUSSION The present experiments clearly demnstrated psitive transfer acrss a cmbined increase in CS-US interval and change in CS mdality. Specifically,

6 TRANSFER AND TRACE CONDITIONING s: Z 80 ~ u. 60 ~ 50 -e C 30 LU... en 20 ::> J C 0 -c ~ _. 2 3 ~~ / / f / A R-8 I Figure 6. Mean trial f the lst, 2nd, 3rd, 5th, and th CR In Stage 2 f Experiment 2. The trial number fr each f the designated CRs has been adjusted dwnward t eliminate an upward trend necessitated by the fact that the trial f tbe nib+ 1 CR must be at least ne trial greater tban tbe nibcr. after CR acquisitin had ccurred with a CS frm ne sensry mdality at a 4OO-msec CS-US interval, the rate f CR acquisitin t a CS frm anther mdality was facilitated at CS-US intervals f 800 and 1,800 msec but nt 2,800 msec, There was n discernible immediate transfer n the initial presentatins f the secnd CS. Instead, the transfer effect appeared t be an entirely general transfer inasmuch as the rate f CR acquisitin t the new CS was enhanced after reinfrced training had begun. A fundamental interpretative issue cncerns whether the transfer effects represent a frm f stimulus generalizatin r an enhancement in the frmatin f a new assciatin in the secnd stage. A stimulus generalizatin hypthesis wuld cntend that the bserved transfer reflects the ability f the assciatin frmed in Stage 1 t withstand alteratins in the CS US interval and CS mdality s as t raise CR perfrmance in the secnd stage. In fact, there have been demnstratins that generalizatin acrss sensry mdalities can ccur n the basis f tempral patterns shared by the therwise distinctive CSs (Friedes, 1974; Meek & Church, 1982; Seraganian & Ppva, 1976). Hwever, n such transfer appeared in the present results. On the peratinal side, there were minimal similarities in the tempral characteristics f the auditry and visual stimuli. The auditry CS cnsisted f a sharp nset f a pure, cnstant tne, whereas the visual CS cnsisted f the ffset f the huselight fllwed by repeated cycles f nff flashes. Mre imprtantly, there was n evidence f immediate transfer that wuld. be expected had.. 5 Nth CR the subjects shwn stimulus generalizatin alng any cnceivable dimensin f similarity between the auditry and visual CSs. Since the transfer effects were lcalized entirely in the rate f CR acquisitin t the secnd CS, there appears t have been an enhancement in the acquisitin f a new assciatin. It is tempting t infer that the bserved general transfer reflects a glbal change in the efficiency f the rganism's learning system. Hwever, such a spectacular cnclusin des nt even adequately describe the data. In particular, an interpretatin in terms f a vague, but glbal, change in the rganism's learning capacity ffers little basis fr explaining bth the psitive transfer at the shrter CS-US intervals, 800 and 1,800 msec, and its absence at the lngest interval, 2,800 msec. Mrever, there are mre prsaic accunts f transfer which cntend that Stage 1 training neutralizes the backgrund stimuli as a surce f cmpetitin fr the prcessing resurces f the animal (Mackintsh, 1977; Seraganian, 1979; estbrk & Hmewd, 1982). In particular, Mackintsh's (1975) mdel f assciative learning assumes that there is a tradeff between cncurrent stimuli in which the stimulus with the greatest assciative strength n a trial gains a prprtinal increment in its grwth rate parameter while all ther stimuli suffer a prprtinal decrement in their grwth rate parameters. Accrding t this mdel, expsure in Stage 1 t the paired relatin between a salient CS and US generates bth increases in the CS's assciative strength and increases in that CS's grwth rate parameter. Cncmitantly, the cncurrent, but less salient, backgrund stimuli presumably gain assciative strength less slwly and suffer prgressive decreases in grwth rate. In Stage 2, the reductin in the grwth rate fr the backgrund stimuli wuld benefit the new, salient CS by permitting its assciative strength t exceed that f the backgrund stimuli sner than it wuld therwise. Hwever, this benefit wuld appear nly if the new CS began with an appreciable grwth rate. If the new CS had a lw grwth rate, as in the case f a lng CS-US interval, then the neutralizatin f the backgrund stimuli wuld be f n value t the new CS. The present results themselves ffer nly indirect supprt fr the hypthesis that prir training neutralizes the backgrund stimuli. Hwever, the case fr the neutralizatin f backgrund stimuli becmes mre persuasive when viewed in cnjunctin with evidence that backgrund stimuli can acquire smething like excitatry assciative strength when they are "paired" with a US in the absence f an explicit CS. Althugh backgrund stimuli themselves d nt evke vert Cks in rabbit cnditining preparatins, backgrund stimuli d seem t acquire assciative prperties such as the capacity t "blck" subsequent acquisitin t an added explicit CS (Mis & Mre, 1973; Randich & LLrd, 1978). Specif-

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