Cross-modal transfer as a function of initial training level in classical conditioning with the rabbit

Transcription

1 Animal Learning & Behavior 1987, 15 (1), Cross-modal transfer as a function of initial training level in classical conditioning with the rabbit BERNARD G. SCHREURS and E. JAMES KEHOE University ofnew South Wales, Kensington, New South Wales, Australia Initial reinforced training with a conditioned stimulus (CSA) from one sensory modality subsequently facilitates the rate of acquisition of the rabbit's nictitating membrane response to a second conditioned stimulus (CSB)from a different sensory modality. In Experiment 1, the level of transfer was a direct function of the number of CSA-US pairings (0, 15, 30, 60, and 120). In Experiment 2, cross-modal transfer appeared to be maximal after initial training was conducted to a performance criterion as small as two CRa to CSA. The results are discussed with respect to theories of transfer, particularly a layered network model of conditioning that suggests that CR acquisition to each CS depends on two sequentially organized associations, one unique to the CS and one that is common to all associations involving the target response system. Kehoe and Holt (1984) recently examined transfer of training in classical conditioning of the rabbit's nictitating membrane response (NMR). They first conducted training in which a conditioned stimulus (CSA) in one sensory modality (e.g., tone) was paired with an electricalpulse unconditioned stimulus (US). After conditioned responses (CRs) were established to CSA, transfer training was begun with a new stimulus (CSB) in another sensory modality (e.g., light). At the beginning of transfer training, CSB was presented four times to determine whether there was immediate transfer from CSA to CSB. In fact, immediate transfer was negligible; the mean level ofresponding on the four test presentations was less than 10% CRs, which failed to differ from that of control groups. Although immediate transfer was not detected, general transfer was evidenced by extremely rapid CR acquisition during CSB-US pairings, as compared with that of control groups, which received initial exposure either to the experimental chambers or to the CSA and US separated by a long CS-US interval. One of the boundary conditions yielding transfer of training across sensory modalities that remains to be delineated is the level of initial training. There are some rabbit NMR data to suggest indirectly that general transfer may occur with only minimal initial training. Kehoe, Morrow, and Holt (1984) found that general transfer was undiminished after an extinction procedure had virtually eradicated responding to the initial CS. Further afield, Scavio, Ross, and McLeod (1983) have found that as few as 15 CS-US pairings substantially enhanced subsequent CR acquisition to the same CS even after an extinction procedure involving 480 CS-alone presentations. In addition, Ross and Scavio (1983) reported that, although in- This research was supported by Grant A from the Australian Research Grants Committee. Correspondence should be sent to E. J. Kehoe, School of Psychology, University of New South Wales, Kensington, N.S.W. 2033, Australia. sufficient to produce CR acquisition, as few as 15 CS US pairings at one CS-US interval influenced the rate of CR acquisition when subjects were shifted to another CS US interval. Accordingly, the primary aim ofthe present experiments was to explicitly manipulate the initial level of training with CSA in order to reveal the minimum amount of training that would produce general transfer. Another aim ofthe present experiments was to further examine the possibility that the facilitation of CR acquisition to CSB represents stimulus generalization along some unspecified dimension of similarity between CSB US training and prior CSA-US training. In previous experiments, stimulus generalization has been defined in terms of immediate transfer as assessed during a small number of presentations of CSB immediately prior to CSB-US training (Holt & Kehoe, 1985; Kehoe & Holt, 1984; Kehoe et al., 1984). In addition to the previous assessment procedure for immediate transfer, the present experiments included test presentations ofcsb during initial CSA-US training and, conversely, test presentations of CSA during the course of CSB-US transfer training. As a test for subthreshold generalization, Experiment 1 assessed whether CSB could modify an unconditioned response (DR) and, more importantly, whether there were changes in any effect ofcsb on the UR over the course ofcsa-us training. Specifically, CSB-US and US-alone trials were administered at the beginning and end ofcsa US training (cf. Wagner, Thomas, & Norton, 1967; Sears, Baker, & Frey, 1979). A third aim ofthe present experiments was to evaluate a network model ofconditioning that accounts for general transfer (Kehoe, 1985, 1986). In brief, the model (illustrated in Figure 1) contains three "sensory" elements, one each for CSA (A), CSB (B), and the US. The outputs of elements A and B project to an intermediate element (X), and the output from X projects to a "response" element (R), which in tum gives rise to the observed behavior 47 Copyright 1987 Psychonomic Society, Inc.

2 48 SCHREURS AND KEHOE en Cl -l W u.. w > i= a. w U wa: CR/UR Figure 1. A neural network model ofconditioning with "sensory" elementsa, B, and US, an intermediate adaptive element X, and an adaptive "response" element R. (CR/UR). Both ofthe non-sensory elements (X, K) receive an input from the US element. The A-X, B-X, and X-R connections are subject to changes governed by a linear operator process (Sutton & Barto, 1981). The firing of both X and R is an all-or-none event that is governed by a random threshold variable. Thus, for example, X fires in response to an input from A only if the A-Xconnection weight exceeds the threshold value on that trial. Likewise, R fires only if the X-R connection exceeds the threshold value on that trial. Inputs from the US are assumed to always trigger both X and R. According to the proposed network model, CR acquisition to CSA and subsequent general transfer to CSB proceed in the following fashion. At the beginning of training, only outputs from the US to X and R are effective. Initially, the CSA input is unable to trigger the intermediate element X, but each CSA input renders its connection with X eligible for modification for a briefperiod following CSA onset (Sutton & Barto, 1981). Ifa US input triggers X during that eligibility period, the A-X connection weight will receive an increment. As the A-Xconnection strengthens over successive CSA-US pairings, A will begin to trigger X and thus render the X-R connection eligible for increments by the US input to R. CRs to CSA will begin to occur only when the A-X and X-R connections become strong enough so that A triggers X and then X triggers R. In subsequent transfer training with CSB, immediate transfer would not appear, because the B-Xconnection is ineffective. However, once CSB-US pairings begin and the B-X connection starts to strengthen, the earliestfirings ofxbyb would be immediately translated into CRs via the previously established X-R connection, thus yielding a facilitation of CR acquisition to CSB. In summary, CRs to CSA rely upon the successive strengthening of the A-X and X-R connections, but CRs to CSB require only the establishment of the B-X connection, which then capitalizes on the existing X-R connection. In addition to accounting for general transfer, the proposed network model generates two predictions regarding the effects ofmanipulating the level ofinitial training with CSA. First, the model predicts that the magnitude of general transfer should be proportional to the number ofinitial CSA-US pairings, because general transfer rests upon the strength ofthe X-R connection, which, in tum, requires prior strengthening of the A-Xconnection. Thus, a few CSA-US pairings would strengthen only the A-X connection and thus fail to lead to general transfer. A larger number of CSA-US pairings would be needed to strengthen the A-Xconnection sufficiently to produce any strengthening of the crucial X-R connection. Because both CRs to CSA and general transfer presumably depend upon strengthening of the X-R connection, general transfer to CSB would not be expected until there had been enough CSA-US pairings to produce some CRs to CSA. Second, using the same logic, the model makes the novel prediction that if initial training is truncated after a few CSA US pairings, partial strengthening ofthe A-Xconnection could be revealed subsequently if CSA were tested during CSB-US training. Specifically, after CSB-US pairings strengthen the B-Xconnection and begin to progressively strengthen the X-R connection, presentations of CSA would begin to evoke CRs via the previously established A-X connection and the newly established X-R connection. More specifically, the emergence of CRs to CSA would be expected to parallel the emergence of CRs to CSB. EXPERIMENT 1 The present experiment examined general transfer as a function of the number of CSA-US pairings. Method Subjects. The subjects were 40 naive female albino rabbits (Oryctolagus cuniculus), each days old andweighing approximately 1.5 kg. The animals had free access to food and water in their home cages. Apparatus. The apparatus and recording procedure for the NMR were patterned after those ofgormezano (1966; Kehoe, Feyer, & Moses, 1981). The CSs were (1) a looo-hz, 85-dB (SPL) tone superimposed on an ambient noise level of 81 db, and (2) a 20-Hz

3 CROSS-MODAL TRANSFER 49 flashing ofthe neon tube that served as the houselight. The US was a 50-msec, 3-mA, 5o-Hz ac electrical pulse delivered via stainless steel Autoclip wound clips positioned 10 rom apart and 15 rom posterior to the dorsal canthus of the right eye. The sequence and timing ofstimulus events were controlled by an Apple ITcomputer equipped with interfaces and software developed by Scandrett and Gormezano (1980). To monitor movements ofthe nictitating membrane, a small tinned copper wire hook was attached to a silk loop sutured in the nictitating membrane of the rabbit's right eye. The other end of the hook contained a loop that fitted over the end of an L-shaped piano wire lever, which operated a photoelectric transducer. The signal from the transducer was amplified and transmitted to an analog/digital converter installed in the computer (Scandrett & Gormezano, 1980). No straps were used to restrain the external eyelids. Procedure. All rabbits received I day of preparation, 2 days of recovery, 1 day of adaptation, 2 days of Stage 1 training, and 4 days ofstage 2 training. On the preparation day, hair surrounding the rabbit's right eye was removed, a small loop of silk (000 Dynex) was sutured into the nictitating membrane, and the animals were returned to their home cages for 2 days of recovery. On the adaptation day, the animals were placed in theconditioning apparatus for 70 min, but neither a CS nor a US was presented. Following adaptation, theanimalswere assigned randomly to one offive groups (n=8). The groups varied in the number of CSA-US pairings presented during Stage 1 and were designated PO, PIS, P30, P60, and P120 according to the number of CSA-US pairings received. Half of the animals in each group received the tone as CSA and the light as CSB; the other half received the light as CSA and the tone as CSB. All subjects were restrained in the conditioning chambers for the same period of time on each day of Stage 1 (70 min). At the beginning of Day 1 and at the end of Day 2, all subjects received a series of test trials for immediate transfer to CSB; the test trials consisted of four CSB-alone trials, four CSB-US trials, and four US-alone trials. On the test trials, the intensity of the US was.5 ma, a value selected to be just above threshold for elicitation ofthe DR. Group PO received no other stimulus presentations during Stage I. Groups PIS, P30, and P60 received 15, 30, and 60 CSA-US pairings, respectively, on Day 2. Group PI20 received 60 CSA-US pairings on both Day I and Day 2. During Stage I, the CS-US interval was 300 msec, and trials were presented at a mean intertrial interval (1TI) of 60 sec (range sec). With the exception ofthe first day ofstage 2, each day ofstage 2 contained 60 CSB-US pairings, 3 CSB-alone test trials, and 3 CSAalone test trials. On the first day of Stage 2, Trials I, 2, 3, and 4 were CSB-alone test trials that provided a further assessment of immediate transfer from CSA to CSB. Throughout Stage 2, the duration of each CS was 600 msec, and the CSB-US interval was 600 msec, a value that produces a moderate rate ofcr acquisition and avoids both ceiling and floor effects in detecting transfer (cf. Kehoe & Holt, 1984). The mean m was 60 sec. A CR was defined as any extension ofthe nictitating membrane exceeding.5 rom and occurring after the onset ofthe CS but prior to its termination. Unless otherwise noted, planned contrasts were used to analyze the data, and the rejection level was set according to a Type I error rate of.05. Results and Discussion The level of CR acquisition to CSA in Stage 1 was a direct function of the number ofcsa-us pairings. Specifically, the mean terminal levels of responding at the end of CSA-US pairings were 0%,0%,5%,49%, and 89% CRs for Groups PO, PI5, P30, P60, and PI20, respectively. For Group PO, which was not exposed to CSA-US pairings, the terminal level of responding represents the base rate of spontaneous responses during periods corresponding to CSA in the other groups. There was no discernible immediate transfer from CSA to CSB. On CSB-alone test trials, no subject displayed any CRs to esbeither before or after CSA-US training. To assess any subthreshold CR-evoking capacity by CSB, we examined the magnitude ofa UR to a weak US (.5 rna) following CSB. Specifically, we computed the ratio of the UR magnitude on CSB-US trials to the UR magnitude on US-alone trials. There was no significant change in this ratio from the beginning of Day 1 (M=.98) to the end of Day 2 (M= 1.06) (F < 1). Furthermore, a comparison of the ratios for Group PO versus the pretrained groups failed to reveal any differences in the UR magnitude ratios (F < 1). The panels of Figure 2 depict the percentage of CRs on CSB-US trials (Panel a) and CSA test trials (Panel b) during Stage 2 training. In Panel a, the set ofdata points designated as "IT" indicates the level of immediate transfer as assessed by the level of responding on the first four presentations of CSB alone. The data revealed little, if any, immediate transfer from Stage 1 to Stage 2. In particular, Groups PO, PI5, and P30 all showed 0% CRs to CSB. One subject in Group P60 responded to every CSBalone trial (M= 13%), 1 subject in Group PI20 produced one response to CSB, and another subject in Group PI20 produced two responses to CSB (M=I2%). Anyapparent differences among groups in levels of responding to CSB failed to reach statistical significance [F(1,35) = 2.68, p >.10]. Further inspection of Panel a reveals strong evidence of general transfer, its magnitude being a direct function ofthe number ofprevious CSA-US pairings. In particular, the levels of responding during the first 30 trials of CSB-US training were 3%, 1%, 6%, 30%, and 42% CRs for Groups PO, PI5, P30, P60, and PI20, respectively. Statistical analysis corroborated the effect of initial training with a significant linear trend across groups [F(1,35) = 6.27], which interacted with a linear trend across 3Q-trial blocks [F(1,35) = 9.88]. Inspection of Panel b of Figure 2 reveals that, during the course of CSB-US pairings, responding on CSA test trials in Group POremained at base-rate levels (M = 0% CRs), thus providing another instance in which there was no detectable immediate transfer from a CS in one modality to a CS in another modality. In contrast, Groups PI5 and P30 showed progressive increases in responding on CSA test trials from base-rate levels less than 6 % CRs to asymptotic levels of 34 % and 62 % CRs, respectively. Finally, responding to CSA in Groups P60 and PI20, which entered Stage 2 training displaying considerable responding to CSA, showed marginal increases over the course of Stage 2, reaching levels of 85% and 90% CRs, respectively. Statistical comparisons confirmed that there was a significant direct effect ofthe number ofprior CSA-US pairings on responding to CSA during Stage 2, which was reflected by a linear trend across groups [F(I,35) = 51.89]. In addition, the observed increase in responding to CSA across days of CSB-US training was

4 50 SCHREURS AND KEHOE (a) CSB-US PAIRINGS CSA TEST TRIALS lr) 0:: o ~ UJ 40 o 0:: o 15 UJ A IT TRIAL BLOCKS TEST TRIAL BLOCKS Figure 2. Perceot CRs 00 CSB-US trials (Panel a) and CSA test trials (Panel b) during Stage 2 training in Experiment 1. corroborated by a significant linear trend across test-trial blocks [F(I,35) = 16.44]. The apparent interaction of groups X test-trial blocks failed to reach the declared level of significance [F(I,35) = 3.75, p <.06]. The pattern of responding to both CSB and CSA strongly supports predictions from the proposed network model ofconditioning. (1) According to the model, both the emergence of CRs to CSA in Stage 1 and the early appearance of CRs to CSB in Stage 2 rely upon the initial establishmentofthe X-R connection following establishment of the A-X connection. In agreement with the model, a considerableand corresponding number ofcsa US trials were needed to produce both the emergence of CRs to CSA and the occurrence of general transfer to CSB. Inspection of the terminal levels of responding to CSA in Stage 1 reveals that only Groups P60 and P120 showed an appreciable number ofcrs to CSA. Likewise, only Groups P60 and P120 showed obvious facilitation in the acquisition of CRs to CSB. (2) According to the network model, a small number of initial CSA-US pairings should strengthen the A-X connection without strengthening the X-R connection to any appreciable degree. However, as the X-R connection is strengthened during the subsequent CSB-US training, CSA should progressively become able to elicit CRs via the previously established A-Xconnection and newly establishedx-r connection. Consistent with this prediction, Groups Pl5 and P30, which showed virtually no CRs during their CSA US pairings, displayed progressive increases in responding to CSA during Stage 2 that paralleled the course of CR acquisition to CSB. Moreover, responding to CSA in both Groups P15 and P30 appeared to reach asymptotic levels that were considerably less than 100% CRs. These low asymptotic levels further support the proposed model, because they can be construed as reflecting the limit of CR evocation that would be imposed if the initial CSA US pairings only partially strengthened the A-Xconnection. Finally, where the A-X connection had either received no previous strengthening (Group PO) or virtually complete strengthening (Groups P60 and PI20), there was little evidence of change in responding to CSA during CSB-US training. EXPERIMENT 2 Experiment 2 examined general transfer as a function of a performance criterion during CSA-US training, namely the number of CRs evoked by CSA before training was terminated. At an empirical level, the use of a fixed performance criterion represents a conventional alternative to a fixed amount of training for manipulating the degree ofinitial learning (cf. Ellis, 1965, pp ). Method Subjects. The subjects were 28 naive female rabbits of the same age and weight as those used in Experiment 1. Apparatus and Procedure. Except where indicated, the apparatus and procedure were identical to those used in Experiment 1. All

5 CROSS-MODAL TRANSFER 51 rabbits received 1 day of preparation, 1 day of adaptation, 1 day ofstage 1 training, and 4 days ofstage 2 training. The tone served as CSA and the flashing light served as CSB. Four groups designated C2, C5, CIO, and C20 (n=6) were presented CSA-US pairings until the animals achieved a criterion of 2, 5, 10, or 20 total CRs, respectively. The required number of CRs did not have to beconsecutive in occurrence. Upon reaching the designated criterion, each subject was immediately removed from the conditioning chamber and returned to its home cage. All subjects achieved criterion within 180 CSA.-US pairings. A further 4 subjects were assigned to Group CO, which did not receive any CSA-US pairings during Stage 1. Instead, subjects in Group CO were restrained in their chambers for 180 min, the maximum time allowed for CSA US pairings in the other groups. Finally, Stage 2 training for all subjects was identical to that of Experiment 1. That is, each day consisted of 60 CSB-US pairings, 3 CSB-alone test trials, and 3 CSA-alone test trials. Immediate transfer was assessed on the first day ofstage 2 by CSB-alone presentations on Trials 1, 2, 3, and 4. Results and Discussion An examination ofstage 1 training indicated that once CRs began to occur, subsequent CRs followed in relatively rapid succession. Accordingly, the number oftrials required to achieve the different criteria differed only slightly. Specifically, thecriteriaof2, 5,10, and20crs were achieved in 62, 103,80, and 94 CSA-US pairings, respectively. Any apparent linear or quadratic trend in the group means failed to reach statistical significance (Fs < 1). Despite the relatively uniform mean number of trials to criterion, there was considerable betweensubjects variability in the number of trials required to reach criterion (range: 20 to 180 trials). In Stage 2 training, immediate transfer from CSA to CSB was less than 10% CRs across all five groups (F < 1). Once CSB-US pairings began, general transfer appeared to be in evidence for all groups that had received prior CSA-US training. In the first 3D-trialblock, the mean response level shown by Group CO was only 1% CRs; for Groups C2, C5, C10, andc20, they were 24%, 34%, 14%, and 38% CRs, respectively. A statistical comparison of Group CO versus Groups C2, C5, CIO, and C20 approached, but failed to reach, the declared level ofsignificance [F(1,23) = 3.69, p <.06]. However, a comparison between the extreme groups, COversus C20, did indicate a significant general transfer effect [F(1,23) = 4.96, p <.05]. In summary, the general transfer effects were not as reliable as those that have been seen when more extensive initial training has been given (Kehoe & Holt, 1984; Kehoe et al., 1984); nevertheless, initial training to even a minimal performance criterion appeared to yield nearly as great an effect as have the more extensive training regimes. Likewise, initial training to a minimal performance criterion also yielded high levels of responding on CSA test trials during Stage 2. Across tests for CSA administered during CSB-US training, Group CO showed only 2% CRs, whereas Groups C2, C5, CI0, and C20 showed 68%,68%,61 %, and 88% CRs, respectively. Statistical comparisons confirmed a significant difference between Group CO versus the other groups [F(1,23) = 25.81]. Among Groups C2, C5, CI0, and C20, any apparent trends failed to reach significance. GENERAL DISCUSSION The results of Experiments 1 and 2 provide a coherent picture regarding the amount of initial CSA-US training needed to facilitate CR acquisition during subsequent CSB-US training. Forthe conditioning parameters prevailing in the present experiments, somewhere between 30 and 60 CSA-US pairings appears to have been the minimum number that yielded discernible general transfer. In Experiment 1, Groups P15 and P30 displayed little, if any, facilitation of CR acquisition to CSB, whereas Groups P60 and P120 showed considerable facilitation of CR acquisition to CSB. A similar pattern appears to be true for Experiment 2 when its results are examined in terms of the number of initial CSA-US pairings. In Experiment 2, a meanof 62 CSA-US pairings were required for subjects to reach even the lowest criterion (2 CRs). In fact, only 1 subject reached its designated criterion (5 CRs) in fewer than 30 CSA-US pairings. Subsequently, there was almost a uniform rate ofcr acquisition to CSB among the groups that had received prior CSA-US training. Across Groups C2, C5, CIO, and C20 in Experiment 2, the mean percentage ofcrs in the first block of CSB-US training was 28% CRs, whereas it was a mean of 1% CRs in the rest control, Group CO. The magnitude of this difference agrees with that seen when Group P60 (M = 30% CRs) and Group PO (M = 3% CRs) in Experiment 1 are compared. Because the precise number ofcsa-us pairings needed to produce general transfer depends undoubtedly on other conditioning parameters, such as the CSA-US interval, a more general rule for determining the amount ofinitial training can be induced from the correspondence between the emergence of CRs to CSA and facilitation ofcr acquisition to CSB. In Experiment 1, only the groups that displayed an appreciable likelihood of CRs during CSA US training, namely Groups P60 and P120, showed facilitation of CR acquisition to CSB. By the same token, in Experiment 2, the CSA-US training to a criterion ofjust two ers appeared sufficient to facilitate CR acquisition to CSB. With regard to a theory ofgeneral transfer, the present results provide detailed support for the following key features of the proposed network model. (1) According to the model, the first few CSA-US pairings only strengthen the A-Xconnection, because the X-R connection does not become eligible for strengthening until an A input starts to trigger X in advance of a US input. Nevertheless, the A-Xconnection can presumably be detected by CSA test trials during subsequent CSB-US training as it strengthens the shared X-R connection, thus permitting CSA inputs to evoke a CR via the previously established A-Xconnection and the newly established X-R connection. In Experiment 1, this hypothesis was confirmed in Groups P15 and

6 52 SCHREURS AND KEHOE P30, in which CRs to CSA emerged only during CSB US training. (2) According to the model, a relatively large number ofinitial CSA-US pairings enables the successive strengthening ofthe A-Xand X-R connections, thus leading to both the appearance ofcrs to CSA and subsequent facilitation of CR acquisition to CSB. In agreement with the model, the emergence of CRs to CSA appeared to predict the subsequent facilitation of CR acquisition to CSB. With regard to the results of Experiment 2, it is perhaps surprising that even the occurrence of two CRs to CSA predicts facilitation of CR acquisition to CSB. However, on each trial, both the likelihood ofcsa's triggering X and the likelihood of X's triggering R depend on a variable threshold. Thus, according to the model, a CR represents the product of two probabilistic events. Ordinarily, both the A-Xand X-R connections would have to be moderately well established for both connections to exceed their respective thresholds on any given trial. Therefore, even the minimal CR criterion in Experiment 2 ensured that a moderate X-R connection had been established prior to CSB-US training. In addition to explaining general transfer and the effects ofinitial CSA-US training, the proposed layered network model can also explain the data that prompted the present study, namely the ability of general transfer to survive extinction ofthe CR to CSA (Kehoe et al., 1984). Accordingto computer simulations of the model (Kehoe, 1985, 1986), general transfer survives because the X-R connection remains largely intact during extinction. During the extinction procedure, the A-Xconnection declines at a steady rate. As the A-Xconnection weakens and X's frequency offiring declines, the X-R connection is eligible for modification less and less often. In this way, the X-R connection is largely protected from extinction and thus remains intact. With the X-R connection still in place, the introduction of CSB-US training is still able to take advantage of the X-R connection in yielding CRs as the B-X connection begins to strengthen. To the knowledge ofthe authors, the proposed layerednetwork model represents the first rigorous account of general transfer applicable to classical conditioning and related associative learning procedures. Nevertheless, there have been a large number of previous attempts to identify the sources of general transfer. The proposed sources have included (1) adaptation to the apparatus, (2) development of a "US representation" (Rescorla & Heth, 1975), (3) neutralization ofbackground stimuli that might otherwise compete with the CS for the subject's processing resources (Mackintosh, 1977; Seraganian, 1979; Westbrook & Homewood, 1982), and (4) stimulus generalization from the initial CS to the new CS based on shared temporal or stimulus patterns (Friedes, 1974; Meck & Church, 1982; Seraganian & Popova, 1976), and (5) superordinate learning in which the animal acquires not only specific associations but also a sensitivity to the structural relations among the stimuli, responses, and reinforcers (Holt & Kehoe, 1985; Levine, 1959; Rodgers & Thomas, 1982). Among the proposed sources of general transfer, previous investigations indicate little, if any, contribution from adaptation, the US representation, and neutralization of background stimuli. In controlling for these sources, previous demonstrations ofgeneral transfer with the rabbit NMR preparation, as well as other preparations, have included control conditions with the same exposure to the apparatus, the initial CS, and the US as the transfer condition (Holt & Kehoe, 1985; Kehoe & Holt, 1984; Kehoe et al., 1984; Thomas, Miller, & Svinicki, 1971; Westbrook & Homewood, 1982). In all cases, the rate of response acquisition by these control groups in transfer testing fell well below that of groups with a history of initial stimulus-reinforcerpairings. When a naive rest control group was also available, the rate of response acquisition in the control groups exposed to the CS and US differed little from that of the rest control group (Kehoe & Holt, 1984; Westbrook & Homewood, 1982). In addition to the controls used in transfer studies, there are converging lines of evidence to show that presentation of the US in the rabbit NMR preparation does not contribute to the general transfer effect through the development of a "US representation," sensitization, or pseudoconditioning. Explicitly unpaired or truly random presentations ofa US and CS have never yielded evidence of even transitory sensitization or pseudoconditioning in the rabbit NMR preparation (Gormezano, Kehoe, & Marshall, 1983; Ross & Scavio, 1983). In fact, US-alone presentations generally retard CR acquisition during subsequent CS-US pairings. As few as 50 US-alone presentations can produce some retardation (Mis & Moore, 1973, Experiment 1). With respect to stimulus generalization as a source of general transfer, immediate transferacross stimulus modalities can occur when the original and test stimuli share the same temporal pattern (Meck & Church, 1982; Seraganian & Popova, 1976). Nevertheless, overt stimulus generalization does not appear to be a necessary precursor to general transfer. The aggregate of cross-modal transfer data collected with the rabbit NMR preparation has failed to reveal any statistical evidence for immediate transfer that occurs in conjunction with the highly reliable general transfer effect. In the transfer experiments conducted so far, immediate transfer has been routinely assessed on the first four presentations of CSB in transfer training. Thus, any individual animal could show 0, 1, 2, 3, or 4 responses to those presentations ofcsb. Including the subjects from Groups P120 and POof Experiment 1, 126 experimental subjects hadreceived extensive prior training with a CSA at a relatively short CS-US interval, and 117 control subjects had received either no prior training or prior exposure to a long CS-US interval that failed to produce initial CR acquisition. Among the experimental subjects, 79%, 13%, 3%, 2%, and 2% showed 0, 1,2, 3, and 4 responses, respectively. Similarly, among the control subjects, 85%,9%, 3%, 0%, and 1% showed 0, 1,2,3, and 4 responses, respectively. Any apparent difference between the two distributions

7 CROSS-MODAL TRANSFER 53 failed to attain statistical significance [~(4) = 4.52, P >.30]. In the present experiments, more extensive attempts to detect immediatetransfer from CSA to CSB also failed to yield any evidence of stimulus generalization. In particular, testing ofcsb immediately before and immediately after CSA-US pairings yielded no responding. Likewise, in the naive rest control groups (PO and CO), repeated tests of CSA during CSB-US pairings failed to reveal any immediate transfer. Finally, attempts to deteet subthreshold immediate transfer in the pretrained groups by examining the ability ofcsb to modify the UR magnitude yielded negative results. Having eliminated other possible contributors to general transfer, there remains the possibility that some superordinate learning process occurs during CSA-US training. For example, Thomas (1970) has explained general transfer between discrimination learning problems by contending that the animal acquires not only a specific discrimination during initial training, but also a "general attentiveness" to stimulus differences. Thomas's hypothesis is a special case of a more general hypothesis that, during initial training, the animal acquires not only specific associations but also a sensitivity to similar structural relations among stimuli, responses, and/or reinforcers (Behar & LeBedda, 1974; Holt & Kehoe, 1985; Levine, 1959; Rodgers & Thomas, 1982). In agreement with this hypothesis, Holt and Kehoe (1985) found that contiguous CSA-US relations in initial training uniquely facilitated subsequent acquisition based on contiguous CSB US relations even when embedded in a discrimination task. Likewise, initial discrimination training along a visual dimension facilitated subsequent discrimination training along an auditory dimension. 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