Manipulation of CS-US conditional probability and of the US-US trace interval on conditioning to the CS and to a background stimulus in a CER situation

LEARNING

AND

MOTIVATION

18,

371-391 (1987)

Manipulation of CS-US Conditional Probability and of the CS-US Trace Interval on Conditioning to the CS and to a Background Stimulus in a CER Situation J. TANNER, J.N.P. Oxford

RAWLINS, University.

AND J. H. MELLANBY England

Two experiments examined the relationship between conditioning to the CS and background using a novel CER paradigm, in which a long background stimulus played the role of more conventional contextual cues. Experiment I manipulated the probability of US occurrence given the CS (p(US/CS)). Conditioning to the background was not a monotonically decreasing function of p(US/CS) at all shock intensities, and conditioning to the CS was remarkably insensitive to the value of p(US/CS) when assessed off the baseline. Experiment 2 manipulated the trace interval between the CS and US. Although conditioning to the CS decreased as the trace interval increased, conditioning to the background was dependent upon whether it served as the interstimulus interval (ISI; interval between the CS and US) or intertrial interval (ITI; interval between CS-US pairs) stimulus. Conditioning to the IS1 background decreased as the trace interval increased, but conditioning to the IT1 background at first increased, but then decreased as the trace interval was further increased. These results are discussed with respect to the adequacy of contemporary models of conditioning. o 1987 Academic

Press, Inc.

It is well established that simply pairing a conditioned stimulus (CS) with an unconditioned stimulus (US) is insufficient to ensure the formation of a CS-US association. Rescorla (1968) demonstrated that for successful conditioning the probability of US given the CS (p(US/CS)) must be greater than the probability of US given no CS (p(US/cS)). He found that the extent of conditioning to the CS was a function of the discrepancy between p(US/CS) and p(US/cS), and that when p(US/CS) = p(US/cS) there was no conditioning to the CS. Odling-Smee (1975) also manipulated CS-US conditional probability, but considered conditioning to background This work was supported by the United Kingdom Medical Research Council and constituted part of the requirement for the D.Phil. at Oxford University. Requests for reprints should be sent to Dr. J. N. P. Rawlins, Department of Experimental Psychology, Oxford University, South Parks Road, Oxford OX1 3UD, England. 371 0023-%%I87 $3.00 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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stimuli. He reasoned that since CSs are always presented in a compound with background stimuli, the situation considered by Rescorla is one in which the probability of US given CS plus background (p(US/CS + X)) is varied relative to the probability of US given background only @US/X)). Rescorla and Wagner’s (1972) theory of Pavlovian conditioning assumes that all stimuli present in a conditioning situation compete for the available associative strength. Therefore, as Odling-Smee points out, the RescorlaWagner theory predicts that increases in conditioning to the CS, resulting from a greater discrepancy between p(US/CS + x) and p(US/X), should be accompanied by decreased conditioning to the background and vice versa. That is, there should be a reciprocal relationship between conditioning to the CS and to the background. Odling-Smee’s (1975) results supported this prediction in that the extent to which animals avoided a black compartment in which they had been shocked was found to be inversely related to p(US/CS + X) when p(US) was fixed. At the extremes, therefore, when all shocks were signaled by the CS (p(US/CS + X) = l), animals still preferred the black compartment, but when all shocks were unpaired with CS presentations (p(US/CS + X) = 0), they showed marked avoidance of the black compartment and preferred a white alternative. Taken together Rescorla’s (1968) and Odling-Smee’s (1975) results offer support to the Rescorla-Wagner theory. However, a reciprocal relationship between CS and background might also be expected from Mackintosh’s (1975) attentional theory and from the Pearce-Hall model of Pavlovian conditioning (Pearce & Hall, 1980). Experiment 1 in the present study also considered the effect of varying p(US/CS + X), but in a situation that allowed simultaneous assessment of conditioning to the CS and background. This was achieved by substituting a long background stimulus for more conventional background cues (or for Odling-Smee’s black compartment) and by programming CS and US presentations to occur during the background, and only during this stimulus. If the long background stimulus functions like Odling-Smee’s black compartment, then one would similarly expect to observe increased conditioning to the CS and decreased conditioning to the background, as p(US/CS + X) is increased, with p(US) fixed. Since such a reciprocal relationship between conditioning to the CS and background might be predicted by a variety of theories of Pavlovian conditioning, the primary intention was not to evaluate those competing theories. Rather it was to develop a paradigm that allows a sensitive assessment of conditioning to both CS and background, which might then be profitably employed in other situations, and to consider the distribution of conditioning between CS and background with variations in p(US/CS + x) as a function of shock intensity. Using the same paradigm, Experiment 2 in the present study also assessed the distribution of conditioning between CS and background

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when the CS-US trace interval was varied. Although the temporal gradient for trace conditioning is highly dependent upon the response system being monitored, it is typically true that the greater the trace interval the weaker is the conditioning to the CS (cf. Mackintosh, 1983, pp. 202210). The influence of temporal contiguity has not been adequately incorporated into current models of Pavlovian conditioning, but Marlin (1981), using Odling-Smee’s black-white compartment arrangement, found a reciprocal relationship between conditioning to the CS and background as the trace interval was manipulated. She found, for example, that when shock occurred immediately after CS offset there was marked conditioning to the CS and little avoidance of the black compartment, but that with a CS-US trace interval of 30 s there was little conditioning to the CS and almost complete avoidance of the initially preferred black compartment. The similarity between this and the situation in which CS-US conditional probabilities are manipulated suggests that overshadowing of the CS by the background, increasing as either p(US/CS + X) is reduced or the CS-US trace interval is increased, might be the common denominator in these two situations (Dickinson, 1980, pp. 61-70). Whatever the theoretical interpretation, however, if, in the present study, the background stimulus functions like the black compartment, one might also anticipate a reciprocal relationship between the CS and background as the trace interval is manipulated. EXPERIMENT

1

Experiment 1 manipulated the probability of shock given the CS (p(US/CS)), with the probability of shock @(US)) fixed. Five groups of animals received different conditional probabilities of shock given the CS (p(US/CS) = 0, 0.25, 0.50, 0.75 and l), and conditioning to the background and CS was examined as a function of shock intensity (0.15, 0.20, 0.25, and 0.30 mA). METHOD Subjects The subjects were 40 adult male Sprague-Dawley rats obtained from the Oxfordshire Laboratory Animal Colony, initially weighing 296-417 g. They were caged in fours under a 12-h light/dark cycle (lights on 0800-2000 h). Food deprivation was gradually introduced in stages, commencing 18 days before the start of behavioral testing. The regime throughout testing consisted of l-h access to Dixon’s laboratory food pellets between 1330-1430 h. Water was freely available. Testing occurred daily between 1000 and 1300 h.

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RAWLINS.

AND MELLANBY

Apparatus

Training was conducted in four identical Campden Instruments CI 410 Skinner boxes, controlled by a NOVA-4 minicomputer programmed in ACT-N (Millenson, 1975). Each 24 x 22 x 20-cm box was enclosed in a sound-attenuating chamber, which provided masking noise from an extractor fan. A single fixed lever was located to the left of the magazine and background illumination was provided by a 2.8-W bulb located in the center, above the magazine. Reinforcement consisted of 3-s access to a 10% sucrose solution (w/v). Two stimuli were used. The flashing light stimulus was produced by a 2.8-W overhead light flashing at a frequency of 2 Hz, and the 20-Hz click train was produced by a Campden Instruments CI 258 click generator. The clicks were delivered through loudspeakers mounted in the ceilings and produced a 12-dB increase in noise level above background, which was 70 dB. The boxes were equipped with grid floors through which scrambled footshock could be delivered from Campden Instruments CI 521C shock generators and CI 521s scramblers. Procedure Days l-10: Handling. The rats were handled 10 min per cage per day. Days 11-14: Shaping: On the first day of shaping the magazine flap

was secured open, free reinforcers were delivered every 30 s, and every leverpress was reinforced. During the next 3 days the magazine flap was left to move freely, the density of free reinforcers was gradually reduced, and leverpresses continued to be reinforced. Session lengths were 15 min. Days 15-20: Random interval (RI) pretraining. When all rats were making at least 100 responses per session, they were placed on progressively increasing RI schedules. There were 2 days of each of RI 8 s, RI 16 s, and RI 32 s. Session lengths for RI 8 s and RI 16 s were 15 min; session lengths for RI 32 s and all subsequent sessions were 30 min. Days 21-64: RI 64-s stabilization. The rats were run for a further 44 days on a RI 64-s schedule in order to stabilize baseline response rates. At the end of this phase, animals were allocated to one of 5 experimental groups (n = 8), matched for baseline response rates. Days 65-109: Shock. Within the 30-min session there occurred two 5min intrusion periods, during which a background stimulus was turned on. For half the animals in each group this stimulus was the flashing light, and for the other half it was the clicker. The timing of these intrusion periods varied randomly across squads and days, with the constraint that they were separated by at least 6 min. A total of 8 x 15-s CS presentations and 8 x 0.5-s shocks occurred during the two intrusion periods. For those animals receiving the flashing light stimulus as back-

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ground, the CS was the clicker, and for those animals receiving the clicker background, the CS was the flashing light. A minimum of 3, and a maximum of 5, CS presentations and shocks occurred during each intrusion period. The five experimental groups differed in the probability of shock given the CS (p(US/CS)), and this was achieved by varying the number of CS-shock pairs. Group 0 received no CS-shock pairs (CSUS), 8 unsignaled shocks (US) and 8 unreinforced CS presentations (CS); Group 0.25 received 2 x CS-US, 6 x US, and 6 x CS; Group 0.50 received 4 x CS-US, 4 x US, and 4 x CS; Group 0.75 received 6 x CS-US, 2 x US, and 2 x CS; and Group 1 received 8 x CS-US. Within an intrusion period CS-US, US, and CS events were programmed randomly and independently, with the constraint that events were separated by at least 16 s. For the first 10 sessions shocks were of 0.15 mA intensity, for the next 10 sessions they were 0.30 mA, for the next 10 sessions they were 0.20 mA, and for the final 15 sessions they were 0.25 mA. (The final phase was longer because a fault developed in one of the boxes.) Days 110-114:

RI 64-s.

There were 5 further days of RI 64 s in order to allow recovery of baseline response rates. Day 115: Probe. The probe trial consisted of a 2-min unreinforced presentation of the CS, the timing of which varied randomly across squads. Data Analysis

and Collection

Total number of responses during the session were collected. These were expressed as rates per minute and square root transformed to achieve a data distribution appropriate for parametric analysis of variance. For the purposes of allocation to groups, the means of Days 60-64 were considered. Shock. Three measures were collected. Responses during the 5 min preceding each intrusion period were used to assess changes in baseline rates; responses during the 15 set preceding each CS were used to assess conditioning to the background; and responses during the CS were used to assess conditioning to the CS. Responses during baseline, background, and CS were square root transformed and analyzed separately. To examine the conditioning of these stimuli as a function of shock intensity, consideration was restricted to the means of Days 6-10 for each shock schedule, since during this time changes in response rates were small and unsystematic. During the final phase, when shocks were 0.25 mA, for those animals affected by the box fault the means of Days 11-15 were considered instead. This problem did not affect the outcome, RI 64-s stabilization.

376

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RAWLINS,

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MELLANBY

however, since analyses restricted only to those animals not affected produced analogous results. Probe. Responses during the 2-min CS presentation and baseline responses during the 2-min pre-CS period were collected and square root transformed. The results were assessed by analyses of variance using the GENSTAT library implemented on an ICL 2988 computer. The basic design was a 5 x 2 with factors of Group (0, 0.25,0.50,0.75, 1) and Stimulus (flashing light, clicker). Since the five groups formed an ordered sequence of increasing p(US/CS), the analyses also included a polynomial regression on Group. For the shock phase there was an additional factor of Shock (0.15, 0.20, 0.25, 0.30) and a polynomial regression on Shock. Results RI 64-s Stabilization

The five experimental groups were well matched for baseline response rates, and no effects were significant. For Days 60-64 group mean square root response rates per minute varied between 9.23 and 9.35. Shock

The effect of Stimulus (flashing light/clicker) was not significant, and neither did it interact significantly with either Group or Shock. In the analyses reported below, therefore, it is not considered. Figure 1 shows square root response rates during baseline, background, and CS during Days 6-10 for each shock intensity. It is evident that changes in baseline response rates were small, but systematic. As shock intensity was increased, the response rates of Groups 0 and 0.25 became somewhat depressed relative to other groups. This produced a significant linear effect of Shock (Lin. Shock: F(1, 90) = 53.98, p < .OOl) and a significant Lin. Shock x Lin. Group interaction (F(1, 90) = 10.21, p < .Ol). Changes in response rates during background were more dramatic. Across all shock intensities, it is clear that as p(US/CS) was increased, responding during the background also increased. This produced a significant overall effect of Group (F(4, 30) = 3.54, p < .025) and significant Lin. Group (F(4, 30) = 12.83, p < .Ol). Across all groups, it is evident that shock intensity also had a powerful effect: as shock intensity was increased, response rates during background decreased. This produced a highly significant effect of Lin. Shock (F(1, 90) = 257.10, p < .OOl). The quadratic effect of Shock was also significant (Quad. Shock: F( 1, 90) = 10.30, p < .Ol), however, and this arose because the difference between 0.15 mA and 0.20 mA was greater than the differences between 0.20 and 0.25 mA or between 0.25 and 0.30 mA. One final feature concerning conditioning to the background is that there was a progressive tendency for Group 0.25 to respond less during the background than Group 0, as

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shock intensity was increased. When shock intensity was 0.30 mA, therefore, this dip in responding offset the more general tendency for background response rates to increase as p(US/CS) was increased. This produced a significant Lin. Shock x Quad. Group interaction (F(1, 90) = 12.87, p < .OOl). In general, response rates during the CS showed an opposite trend to background response rates as p (US/CS) was manipulated. Overall, CS response rates decreased as p(US/CS) increased, and this produced a significant effect of Lin. Group (F(1, 30) = 7.91, p < .Ol). The quadratic effect of Group was also significant (Quad. Group: F( 1, 30) = 5.05, p < .05), however, and this arose because the difference between Groups 0 and 0.25 was greater than the differences between other adjacent groups. This effect was particularly evident with shock intensities of 0.25 MA and 0.30 mA, when it is clear that Group 0 responded appreciably more

378

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RAWLINS.

AND MELLANBY

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FIG. 2. Probe trial data for Experiment 1. Upper half: group mean square root response rates during baseline and CS on the probe trial and, for comparison, square root response rates on the final day of shock treatment (Day 15, 0.25 mA). Lower half: conditioning to the CS expressed as a conventional suppression ratio relative to baseline on the probe trial, and relative to baseline and background on Day 15.

during the CS than other groups, which did not differ from one another. As with responding during the background, shock intensity exerted a powerful influence on CS rates, producing decreased responding for all groups as shock intensity was increased (Lin. Shock: F(1, 90) = 246.91, p < .OOl; Quad. Shock: F(1, 90) = 23.95, p < .OOl). Probe The effect of Stimulus was not significant, and neither did it interact significantly with group. In the analyses reported below, therefore, it is not considered. The upper half of Fig. 2 shows square root response rates during baseline and CS on the probe trial and, for comparison, square root

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379

response rates on the final day of shock treatment (Day 15, 0.25 mA). It is clear that giving the five additional RI 64-s sessions, before the probe trial, was successful in abolishing baseline response rate differences between groups. As regards CS response rates, it is clear from inspection of Fig. 2 that Group 0 responded appreciably more than other groups, which did not differ from one another. This produced significant effects of Lin. Group (F(l) 30) = 13.40, p < .OOl) and Quad. Group (F(l) 30) = 6.23, p < .025). It is evident, therefore, that off-the-baseline assessment of conditioning to the CS produced a pattern analogous to that seen on the baseline, when one was actually measuring response rate during the CS plus background. That this is so is illustrated in the lower half of Fig. 2, which shows conditioning to the CS expressed as a conventional suppression ratio (Annau & Kamin, 1961) relative to baseline on the probe trial, and relative to baseline and background on Day 1.5. If, on the baseline, measurement of response rate during the CS represents conditioning to the CS plus conditioning to the background, then expressing CS rate as a suppression ratio relative to background gives an indication of the expected conditioning to CS only. It is clear from Fig. 2 that the expected conditioning to the CS. derived in this way, predicts a much more gradual increase in conditioning as p(US/CS) is increased than was actually observed. Rather, conditioning to the CS observed on the probe trial is more accurately predicted, from the on-the-baseline data, by expressing CS rate as a suppression ratio relative to baseline. Discussion As would be predicted from Rescorla’s (1968) and Odling-Smee’s (1975) results, therefore, a reciprocal relationship between the conditioning to the CS and background was generally observed. As p(US/CS) was increased, conditioning to the CS increased and conditioning to the background decreased. In the present experiment, however, decreased conditioning to the background, as p(US/CS) was increased, was not monotonic at all shock intensities. When shock intensity was 0.30 mA, the group for whom p(US/CS) = 0.25 showed more conditioning to the background than the p(US/CS) = 0 group. Since Odling-Smee (1975) did not examine CS-US conditional probabilities of less than 0.5 it is impossible to make a comparison. He did, however, manipulate shock intensity and found, as in the present experiment, that increases in shock intensity produced increased conditioning to the background for all groups. Closer examination of conditioning to the CS in the present experiment reveals a picture very different from that reported. by Rescorla (1968). CS rates on the probe trial in the animals for whom p(US/CS) = 0.25 were just as suppressed by the CS as those in animals for whom p(US/CS) = 1. Rescorla, by contrast, found conditioning to be a function of the discrepancy between p(US/CS) and p(US/CS). In the present experiment,

380

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RAWLINS,

AND

MELLANBY

for Groups 0, 0.25, 0.50, 0.75, and 1 these discrepancies were -0.25, - .0625, 0.375, 0.687, and 1, respectively; clearly sufficiently different to expect differences on the basis of Rescorla’s findings. The reason for this discrepancy is not clear, but one obvious procedural difference is the realization of conditional probabilities. In the present experiment CS-US conditional probabilities were mainpulated by varying the number of CS-US pairs; if the CS predicted shock on a given trial, that shock always occurred immediately after CS offset. In Rescorla’s experiments, however, manipulation of CS-US conditional probabilities varied the probability of shock occurring during the CS, but the timing of those shocks was randomly programmed. It is suggested that if a CS, in addition to providing probabilistic information as to the occurrence of shock, also provides precise temporal information about when any US can occur, increased conditioning to that CS may result. If correct, this suggests that temporal information should be reflected in the calculation of conditional probabilities. In the present experiment this would have the effect of reducing the differences between groups, since the temporal information provided by the CS was the same for all groups. Another procedural difference concerns the extent of training. In the present experiment animals experienced 80 CSs and 80 USs during each of the fn-st three phases, and 120 of each in the fourth phase: 360 of each in all. It seems likely that learning would have been close to asymptotic by the time the probe measures were taken. The analyses of the results supported this view that behavior was stable over the last 5 days of each phase, as there were no statistically significant findings including Days as a factor. Thus, the animals had very substantial experience of the CS on the baseline, in the presence of the background stimulus, before subsequent off-the-baseline assessment. It is possible that because of this experience animals were unable to adjust their CS rates to take account of the influence of the background, which on probe trials was not presented. A final possibility is that the apparent ineffectiveness of manipulating p(US/CS) resulted from a floor effect. However, the combined mean CS rate per minute for Groups 0.25, 0.50, 0.75, and 1 on the probe trial was 14.22. One would imagine that this level of responding should have allowed some scope for differentiation between the groups. Although the reason for the discrepancy between Rescorla’s (1968) results and the present findings has not been established, it is clearly an important issue. Rescorla’s paper was influential in the development of theories that questioned the sufficiency of temporal contiguity, but that emphasized instead the relative predictiveness of all stimuli present. This continues to be a dominant theme. In the present experiment, however, conditioning to the CS appeared to be invariant to changes in the relative predictiveness of the background. The present analysis suggests that

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some clarification of this issue might be forthcoming if the temporal information provided by the CS and the length of training are both systematically examined. Of course, temporal factors may also play a different role in conditioning. It is well established that the temporal relation between the CS and the US may have quite different values for optimal conditioning to be seen in different systems (e.g., Vandercar & Schneiderman, 1967). If our selections of stimulus durations for the CS and background were less than optimal, then this might conceivably account for the otherwise anomalous conditioning to baseline seen in all groups (even though all USs were delivered during the background stimulus), and the increased conditioning to background seen in Group 1 at higher shock levels. Associative learning theories do not, at present, attempt to incorporate the degree to which deviation from an optimal CS-US interval modifies learning. The most obvious way to include this factor would be by adjusting the learning rate constant, but this would not change asymptotic values for associative strength in the Rescorla-Wagner (1972) model, and it should be recalled that our animals had received extensive training, and their response rates were apparently stable, judging by our analyses of variance. Furthermore, such a move would still not enable this model to account for the nonmonotonic changes seen in conditioning to background as p(US/CS) increases from 0.0 to 1.0. One other possibility is that some of the presently observed, but unexpected, effects are due to higher order conditioning (Rizley & Rescorla, 1972), which would tend to oppose the influence of first-order conditioning, and would thus tend to obscure the differences between contemporary models based on first-order relations alone. This would be disadvantageous only if first-order effects are by far the most influential. In the analysis of one-trial blocking (Dickson, Nicholas, & Mackintosh, 1983) and of the serial potentiation effect (Pearce, Nicholas, & Dickinson, 1981), however, second-order effects seem to exert a powerful influence, and this suggests that the interaction between first- and second-order conditioning may be an important issue. If so, the next generation of theories (cf. Daly & Daly, 1982) may not be differentiated on the mechanisms assumed to underlie first-order conditioning (US versus CS effectiveness), but on the assumptions concerning the respective influences of first- and secondorder conditioning, and their interaction. EXPERIMENT 2 Experiment 2 examined the influence of CS-US contiguity on the distribution of conditioning between CS and background. It was run concurrently with Experiment 1 and employed the same paradigm. Five groups of animals experienced different trace intervals between CS offset and shock onset (0,3,6, 12, and 24 s), and conditioning to the background

382

TANNER,

and CS was examined and 0.30 mA).

RAWLINS,

AND

MELLANBY

as a function of shock intensity

(0.15, 0.20, 0.25,

Method Subjects

The subjects were 40 adult male Sprague-Dawley rats obtained from the Oxfordshire Laboratory Animal Colony, initially weighing 309-415 g. They were caged in fours under a 12-h light/dark cycle (lights on 0800-2000 h). Food deprivation was introduced as in Experiment 1, and consisted of l-h access to Dixon’s laboratory food pellets between 1630 and 1730 h. Water was freely available. Testing occurred daily between 1300 and 1600 h. Apparatus

Training was conducted in eight identical Campden Instruments CI 410 Skinner boxes of the sort described in Experiment 1. The same two stimuli were used. Procedure

The schedule of the experiment was exactly as described in Experiment 1. The only aspect that differed was the nature of the shock phase. Days 6.5-109: Shock. Two 5-min intrusion periods occurred during the 30-min session, randomly timed, but with the constraint that they were separated by at least 6 min. A total of eight CS-shock pairs occurred during the two intrusion periods, with a minimum of three and a maximum of five occurring in each. The timing of these CS-shock pairs varied randomly across both squads and days, with the constraint that they were separated by at least 16 s. As in Experiment 1, CSs were 15 s and shocks 0.5 s. The five experimental groups differed in the length of the trace interval between CS offset and shock onset. For Groups 0, 3, 6. 12, and 24 the trace intervals were 0, 3, 6, 12, and 24 s, respectively. As in Experiment 1, there were 10 sessions of 0.15mA, 10 of 0.30-mA, 10 of 0.20-mA, and 15 sessions of 0.25-mA shock intensity. Data Analysis

and Collection

Shock. Four measures were collected. Responses during the 5 min preceding each intrusion period, responses during the 15 s preceding each CS, responses during the CS, and responses during the trace interval. These were square root transformed and analyzed separatedly. To assess conditioning of these stimuli as a function of shock intensity, we again considered the means of Days 6-10, or the means of Days 1 l15 for those animals affected by the box fault in the 0.25-mA phase. As in Experiment 1, the results were assessed by analyses of variance

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using the GENSTAT library. The design was a 5 x 2 with factors of Group (0, 3, 6, 12, 24) and Stimulus (flashing light, clicker). The analyses again included a polynomial regression on Group, with the interval values considered on a logarithmic scale. For the shock phase there was an additional factor of Shock and a polynomial regression on Shock levels. As in Experiment 1, the factor of Stimulus was not significant, and it is not considered in the analyses reported below. Results RI 64-s Stabilization

The five experimental groups were well matched for baseline response rates and no effects were significant. For Days 60-64 group mean square root response rates per minute varied between 7.07 and 7.31. Shock

Figure 3 shows square root response rates during baseline, background, CS, and trace during Days 6-10 for each shock intensity. It is evident that changes in baseline response rates were small, and that there were no systematic differences between groups. Shock intensity exerted an effect, however, producing decreased responding with increased shock intensity (Shock: F(3,90) = 5.80, p < .Ol; Lin. Shock: F(l,90) = 10.01; Quad. Shock: F(1, 90) = 6.43, p < .025). As regards response rates during background, it is clear that the background played two roles in this experiment. It was the stimulus present during the intertrial interval (between CS-US pairs, background on Fig. 3), but it was also present during the interstimulus interval (between CS offset and shock onset, trace on Fig. 3). When serving as the intertrial stimulus, it is clear that responding during the background was not a monotonic function of the trace interval. Animals experiencing a trace interval of 6 s responded less during the background than either animals experiencing a zero or 24-s trace. This produced a significant quadratic effect of Group (Quad. Group: F(1, 30) = 12.49, p < .Ol). This was true at all shock intensities, but it is clear that Shock itself exerted a powerful influence, producing decreased responding for all groups with increased shock intensity (Lin. Shock: F(1, 90) = 144.53, p < .OOl). As shock intensity was increased, there was also a tendency for the 12- and 24-s trace groups to show a greater reduction in background response rates than the zero-trace group. The magnitude of this effect was insufficient to produce a significant Shock x Group interaction, however (Lin. Shock x Lin. Group: F(1, 90) = 2.46, p > .05). When serving as the interstimulus stimulus, by contrast, responding during the background was a monotonic function of the trace interval. Background response rates increased as the trace interval increased, producing a highly significant linear effect of Group (Lin. Group: F(1,

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30) = 152.58, p < .OOl). As regards the influence of shock intensity, it is clear that the expected decrease in response rates as shock intensity was increased was partially obscured by a decrease in responding as a function of increased experience. This produced highly significant linear and quadratic effects of Shock (Lin. Shock: F(1, 90) = 23.55, p < .OOl; Quad. Shock: F(1, 90) = 50.76, p < .OOl). It meant, for example, that background response rates during the trace interval were lower when shock intensity was 0.25 mA than when it was 0.30 mA. The confounding of experience with the task makes the relationship between shock intensity and response rate difficult to interpret, but it is clear that over time animals increasingly differentiated between the background when it played the role of intertrial stimulus, and when it functioned as the interstimulus stimulus. Response rates during the CS were as one might expect, with increased responding as the trace interval increased. The rate of increase was greater between Groups 6 and 12 s than between other adjacent groups, however, and this resulted in both the linear and quadratic effects of

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FIG. 4. Probe trial data for Experiment 2. Upper half: group mean square root response rates during baseline and CS on the probe trial and, for comparison, square root response rates on the final day of shock treatment (Day 15, 0.25 mA). Lower half: conditioning to the CS expressed as a conventional suppression ratio relative to baseline on the probe trial, and relative to baseline and background on Day 15. Trace intervals are plotted on a logarithmic scale.

Group being significant (Lin. Group: F(1, 30) = 35.52, Group: F(1, 30) = 10.46, p < .Ol). Shock intensity powerful influence, with CS rates decreasing for all intensity was increased (Lin. Shock: F(1, 90) = 204.32, Shock: F(1, 90) = 10.18, p < .Ol).

p < .OOl; Quad. again exerted a groups as shock p < .OOl; Quad.

Probe

The upper half of Fig. 4 shows square root response rates during baseline and CS on the probe trial and, for comparison, square root response rates on the final day of shock treatment (Day 15, 0.25 mA). It is evident that on the probe trial baseline response rate differences between groups were minimal (Groups: F(4, 30) = 1.04, p > .05), but

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that CS rates increased as the trace interval increased (Lin. Group: F( 1, 30) = 19.37, p < .OOl). The lower half of Fig. 4 shows CS rates expressed as a suppression ratio relative to baseline on the probe trial, and as a suppression ratio relative to baseline and background on Day 15. Since on Day 15 differences between groups in background response rates were relatively small, expressing CS rate relative to baseline or background makes little difference. There is the suggestion that animals responded to the CS on the probe trial in a way similar to the CS plus background on the baseline, however, in that the CS rates of Groups 3 and 6 s did not differ on the probe trial. These groups also did not differ on the baseline, but if conditioning to the CS and to the background are assumed to be additive, one might have expected a difference. Discussion In the present experiment, therefore, we did not observe the anticipated reciprocal relationship between conditioning to the background and CS as the trace interval was manipulated. Conditioning to the CS decreased as the trace interval increased, but it was clear that the background played two roles, and that these became increasingly differentiated as experience with the task grew. When the background served as the intertrial stimulus, as the trace interval increased conditioning at first increased, but after 6 s it then decreased. When the background served as the interstimulus stimulus, however, conditioning paralleled that of the CS, decreasing as the trace interval increased. The present results would thus seem to be at variance with those reported by Marlin (1981). She examined trace intervals of 0, 10, and 30 s, and found that avoidance of the compartment in which animals were shocked increased linearly as the trace interval increased. There are too many procedural differences to allow a comparison-length of the CS, shock intensity, extent of training, nature of the background, method of assessing conditioning to background-but the differences would not seem to be a function of extent of training per se. In the present experiment during the first 3 days of the shock phase, when shock intensity was 0.15 mA, all groups were similarly and noticeably suppressed to the background. Finally, the CS for long trace groups did not appear to become established as a conditioned inhibitor, as has been reported by Hinson and Siegel (1980). Although this suggestion was not critically evaluated, in that the CS was not presented in compound with a known excitatory stimulus off the baseline (cf. Rescorla, 19691, it is nevertheless clear, on the baseline, that response rates during the CS never exceeded those during the background. Since the background acquired noticeable excitatory properties when shock intensity was 0.30 mA, if the CS for long trace groups had acquired inhibitory properties, one might have expected some

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enhancement of responding during the CS. It is certainly clear that the performance of the long trace groups did not resemble that of the group for whom the CS and shock were explicity unpaired in Experiment 1, who, by contrast, showed marked elevation of responding during the CS at all shock intensities. These findings again raise the issue of whether the influence of temporal contiguity has been adequately incorporated into modern conditioning theories. The study of temporal contiguity seems to have suffered from the more vigorous interest in the influence of relative predictiveness as a determinant of conditioning, but it appears that the models so generated are not readily applicable to simple trace conditioning situations like the present one. The present results are, however, consistent with the framework for trace conditioning recently proposed by Kaplan and Hearst (1982). Kaplan and Hearst suggest that although initially there may be excitatory conditioning to the general context, if definite cues distinguish the inter-trial interval (ITI) from the interstimulus interval (IX), “local” contexts become established, and these acquire excitatory and inhibitory properties of their own, the nature and extent of these properties being dependent upon such things as relative salience and duration, and relative probability of US occurrence. In Kaplan and Hearst’s (1982) experiments, these cues were often provided by “filling” the IS1 with another stimulus or by similarly filling the ITI. However, they also found relative duration of the IT1 and IS1 to be an effective discriminative cue, observing enhanced discrimination as the IT1 was increased relative to the ISI. In simple trace conditioning, as in the present experiment, this cue is available, and might serve to establish an excitatory IS1 background together with a nonexcitatory or inhibitory IT1 background. This would clearly be consistent with the presently observed differential responding during the background, when it served as either the interstimulus or intertrial stimulus. Moreover, on the basis of Kaplan and Hearst’s results one might expect this discrimination to be more difficult for the long trace groups in the present experiment, since for them the mean IT1 more closely resembled the ISI: indeed, in Group 24, there could be occasional ITIs that were shorter than the ISIS. Although for all groups discrimination of the background as interstimulus or intertrial stimulus improved over time, for long trace groups this discrimination was only clearly evident toward the end of training, suggesting that the discrimination was indeed more difficult in Groups 12 and 24. These two groups were, however, similar to one another. Kaplan and Hearst’s (1982) framework might also be used to explore the more specific effects presently observed. If the discrimination between IS1 and ITI background is accepted, it might be expected that conditioning both to the CS and to the IS1 background decreases as the trace interval

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increases. As the trace interval is increased, the CS is placed in a progressively less favorable temporal relationship with the US and the potential for the CS to be overshadowed by the ISI background might also be increased. Decreased conditioning to the IS1 background as the trace interval is increased might also stem from a less favorable temporal relationship, in the sense that as the duration increases, the animal’s ability to predict accurately the precise timing of the US is progressively reduced. The final feature of the present results to be discussed is the conditioning to the background when it served as the IT1 stimulus. On the basis of Kaplan and Hearst’s analysis one might expect very little conditioning to the IT1 background at all trace intervals. The IT1 background is temporally removed from the US, and it is also likely to be overshadowed by better predictors of the US, namely the CS and the IS1 background. This analysis, however, assumed perfect discrimination between the IT1 and IS1 background, and it has been suggested that this discrimination is poorer for long trace groups. Poorer discrimination between the IT1 and IS1 might serve to increase the excitatory strength of the IT1 background by generalization. One might speculate, therefore, that as the trace interval is decreased, the excitatory strength of the IS1 background increases, but generalization between the IS1 and IT1 is low; and for long trace groups IS1 background is lower, but generalization is high. All that is required is for an intermediate level of excitatory strength to the IS1 background and an intermediate level of generalization between the IS1 and ITI, to result in more conditioning to the IT1 background than either high/low or low/high levels of excitatory strength and generalization, respectively. In principle, therefore, the framework suggested by Kaplan and Hearst (1982) provides a suitable one for accommodating the present results. The identification of the two roles of the background as IS1 and IT1 stimulus is not only consistent with the general features of the present data, but may also be explored to explain the specific effects. The framework also suggests directions that future research might follow. As regards the present experiment, it would seem profitable to explore further the determinants and consequences of generalization between IS1 and IT1 background. More generally, the framework suggests drawing parallels between simple trace conditioning and the serial potentiation effect (cf. Pearce et af., 1981); a situation in which the CS-US trace interval is filled not by the background, but by another explicit CS. GENERAL DISCUSSION

In the present study we manipulated two variables that have been identified as important determinants of conditioning: the probability of the US following the CS (Experiment 1) and the trace interval separating

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the CS and US (Experiment 2). On the basis of Odling-Smee’s (1975) and Marlin’s (1981) results we antici(pated that as p(US/CS) decreased or the trace interval increased, conditioning to the CS would decrease and conditioning to the background increase. This was not observed, however. In Experiment 1, conditioning to the background was not a monotonically decreasing function of p(US/CS) at all shock intensities, and conditioning to the CS seemed invariant to the value of p(US/CS) in the 0.25 to 1 range, when assessed off the baseline. In Experiment 2, although conditioning to the CS decreased as the trace interval increased, it was clear that the background played more than one role. When it served as the ISI background, conditioning decreased as the trace interval increased, but when it served as the IT1 background, conditioning at first increased, but then decreased as the trace interval was further increased. The different outcomes of Experiments 1 and 2 suggest that simple overshadowing of the CS by the background, increasing as either p(US/CS) is reduced or the trace interval is increased, is insufficient to explain the observed results. Rather, it is clear that the trace conditioning procedure resembles a discrimination problem, and that the different roles of the background, as IS1 and ITI, must also be taken into account. We suggest that one way to make progress would be to incorporate higher order conditioning into current models of associative learning (cf. Daly & Daly, 1982). Furthermore, it seems clear that temporal contiguity still needs to be properly incorporated into formal models of conditioning, and the analysis of trace conditioning as a ‘discrimination problem has not yet been addressed by these theories. Finally, we should consider the paradigm used in these experiments, in which a long background stimulus plays the role of more conventional contextual cues. Much of the recent interest with contextual conditioning has focused on the informational aspects that contextual cues provide, rather than on their unique properties. If this continues to be a dominant theme, the present paradigm may be a useful alternative to Odling-Smee’s black-white compartment arrangement. If the context is provided by a conventional stimulus of long duration, it need not be present throughout the session, and this allows one to assess simultaneously conditioning to the context and to the CS on the baseline. It also, of course, allows one to assess conditioning to the context and CS using the same measure. The present manipulation of shock intensity indicates, moreover, that this assessment may be a very sensitive one. Finally, if CS-US conditioning is embedded in a background stimulus, changes in baseline response rates are very small. This may be useful if off-the-baseline assessment of conditioning is required, when baseline response rate differences pose unwanted interpretative problems.

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