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Note on the invariance of response latency in shuttlebox avoidance learning
LEARNING
AND
MOTIVATION
7, 108- 116 (1976)
Note on the Invariance of Response Latency in Shuttlebox Avoidance Learning ROBERT C.BOLLES,SEWARD A. MOOT, AND KELLY NELSON University
of Washington
Rats trained to avoid shock by running in a shuttlebox showed an orderly increase over trials in the probability of avoidance but no corresponding change over trials in the latency of the avoidance response. Both responding in the presence of shock (escapes) and responding prior to shock onset (avoidances) appeared to be stationary in time. Latencies were found to vary with different experimental conditions but in every case to be invariant over trials.
The course of avoidance learning in the shuttlebox is almost invariably assessed by the proportion of avoidance responses that occur on successive blocks of trials; rarely are latency data reported. The response-probability measure typically increases in a manner which is fairly orderly for individual animals and quite orderly for groups of animals. But there are too few data on response latency to draw any conclusion about how this measure changes during the course of training. The present studies were undertaken simply to provide such data. EXPERIMENT
I
Method Subjects. The subjects were 24 experimentally naive male rats of Long-Evans descent, approximately 100 days old. Apparatus. The shuttlebox was 76 cm long, 20 cm wide and 18 cm high, with no door or hurdle. It was placed in a sound-attenuating chamber and lighted by a 15-W bulb directly above the shuttlebox. A ventilation fan produced a background noise level of 64 db, and the buzzer cue for shock increased this level to approximately 76 db. The floor of the shuttlebox was constructed of l-cm stainless steel bars spaced 2.5 cm center to center. A Grason-Stadler shock generator provided scrambled shock of nominal 1.O-mA intensity. Responses were defined automatically by micro-switches attached to the floor which operated when the rat moved 10 cm past the midpoint. Response latencies were recorded to the nearest 0.1 sec. Supported by research grant GB-20801 from National Science Foundation. Requests for reprints should be addressed to R. C. Bolles, Department of Psychology, University of Washington, Seattle, Washington 98105. 108 Copyright 0 1976 by Academic Press. Inc. All rights of reproduction in any form reserved.
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Procedure. The animals were randomly assigned to three groups, and trained in a single session of 50 trials with a variable intertrial interval (mean = 90 set). The Avoidance group was trained under conventional conditions: shock was preceded by a IO-set buzzer S + l. A response occurring in this interval terminated S+ and avoided the shock. If an animal failed to respond during the IO-set interstimulus interval, the shock came on but could be escaped (and the S + terminated) by a response. The Signaled Escape group was run under the same conditions except that all shocks were unavoidable, so that all trials were necessarily escape trials. Responses occurring in the presence of the S + alone had no programmed consequences. The third group, the Unsignaled Escape group, was trained under the same conditions except that there was no S + . Thus, this group was trained under conventional escape conditions. Results Consider first the behavior of the Avoidance group. Figure 1 shows an orderly increase in the number of avoidance responses in successive blocks of trials. Because three of the eight animals learned the response quite quickly and performed at a high level, the mean is higher than the median, but both measures of average avoidance responding were consistent with results typically reported for rats in a shuttlebox: the group was avoiding shock about 50% of the time after 50 training trials, and their performance did not seem to have reached an asymptote. The mean latency (actually the time required to run about 10 cm past the midline of the box) averaged over both escape and avoidance trials showed a similar gradual decline over trial blocks. But note that this overall decrease in latency must reflect in part the increasing frequency of avoidance responses which are necessarily quicker than escape responses. 1 Here we follow Church’s (1971) suggestion to call the shock cue or warning stimulus an .S+ rather than a CS.
IO-TRIAL
BLOCKS
FIG. 1. Percentage of avoidance responses of rats trained under avoidance conditions.
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The results of analyzing latencies of avoidance and escape responses separately are given in Fig. 2. Also included in Fig. 2 are response latencies for the signaled and unsignaled escape groups. Each data point in the figure is the median, over subjects, of the median response latency over a block of five trials. Note that the data are presented in smaller blocks than in Fig. 1 to better reveal any trends in the early trials. Latencies are taken relative to the start of the trial, i.e., 10 set before scheduled shock onset, to make the results comparable for the different groups. These results tell a very different story: in the Avoidance group the latency of the avoidance response provided no evidence of learning. The course of avoidance learning as measured by the probability of avoidance was clearly not reflected in the response latency measure. There was a suggestive, but not reliable (t = 1.4) decrease across animals in latency between the first and second avoidance response. Nor was there any apparent learning of the escape response in any of the groups. The escape response appeared to have so much strength initially that little further strengthening was possible. But perhaps there was escape learning and it is obscured by the way the data are presented in Fig. 2. One possibility is that the unit of time in the figure is too long. Thus, it might be argued that the rat learns to escape in the first trial or two, and this learning is obscured by taking medians over five-trial blocks. This was not the case. For the different groups, mean performance on the first escape trial was from .3 set to .9 set (for the Unsignaled group)faster than the respective median speed over the first block of trials. Individual r-tests showed that the median latency over trials 2-5 was significantly longer than on trial 1 for each group. In other words, escape latencies were actually getting longer over the first few trials. This
5-TRIAL
BLOCKS
FIG. 2. Median latencies in seconds, by 5-trial blocks for the three groups on escape trials, and for the avoidance-trained animals on avoidance trials. Note the expansion of the ordinate, and note that there were no avoidances in the first trial block.
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anomaly suggests that some decremental factor, perhaps a tendency to freeze, comes into play in the first few trials to interfere slightly with what is initially an extremely strong response. So it is evidently not the case that learning occurred too quickly to be detected by our analysis. Nor was it the case that a few subjects in a group showed learning to escape; the group means tell the same story as group medians, and inspection of individual records revealed no clear instance of gradually declining latencies. Another possibility is that our time scale was too short to demonstrate learning. Perhaps some escape learning would have become evident with continued training. Some suggestion of gradual learning was shown by the Unsignaled Escape group. First, note that in this group the latency averaged about .5 set longer than in the Signaled Escape group. This difference was significant on several trial blocks. Perhaps the IO-set S+ permits the rat to make some postural adjustment or orienting response so that it can respond more rapidly when shock comes on. Alternatively, perhaps the animals in the Unsignaled Escape group are subject to more fear, or engage in more freezing, or find the whole situation more aversive than the animals in the other groups (Badia & Culbertson, 1972). Whatever the reason for this performance decrement in the Unsignaled Escape group, it appeared quickly, and then disappeared over the course of 50 trials. An analysis of the linear trend over trial blocks showed it to be highly reliable (F( 1,62) = 9.7,~ < .Ol); the linear trends for the other groups were not significant. Even an ad hoc analysis of the most suggestive portion of the data, over trial blocks 2 to 4, failed to reveal significant trends for the other groups. The gradual improvement in the Unsignaled Escape group may be attributed to learning, but much of this learning simply ameliorated decremental factors arising early in training, and all of it is clearly secondary to the rat getting from one end of the apparatus to the other. There is every reason to believe that there would be further secondary learning, i.e., acquisition of postures and orientations, with continued training in all groups, but such learning would merely refine what is already a very strong response. Such improvements in motor skill do not constitute the stuff of which escape learning is made, or avoidance learning either. The basic fact is that the animals responded in 11.6 set (median = 11.1 set) on the first escape trial, and there was therefore little opportunity for faster responding. EXPERIMENT
2
The results of Experiment 1 indicate that by a response-latency criterion there is little or no learning either of escaping shock or of avoiding it in the shuttlebox. While the probability of running in the presence of the S+ alone increases in a continuous manner, the response seems to remain essentially stationary in time. When the shock comes on, the rat always runs, and this response too seems to remain essentially stationary in time. The purpose of
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Experiment 2 was to see if comparable results would be found with somewhat different subjects. Previous work in our laboratory had indicated that female Wistar rats perform much better in the shuttlebox than male Long-Evans. Would the former show a different pattern of response latencies? Method Subjects. The subjects were eight male rats of Long-Evans descent (the same stock as the animals in Experiment l), and eight female rats of Wistar descent. All were experimentally naive and approximately 100 days old. Apparatus and procedure. The apparatus was the same as that used in Experiment 1. Both groups were given 100 trials of avoidance training with the same parameters as in Experiment 1. Results
The male Long-Evans showed poor learning, reaching an overall mean level of avoidance responding of only 1% (median = 7 compared with 24% in Experiment 1). Two subjects performed reasonably well, but four gave three or fewer avoidances in the 100 trials. By contrast, the group of female Wistars performed very well, reaching 66% avoidance (median = 6%) overall, and six of the eight subjects achieved 20 or more consecutive avoidances. In both cases the learning curves were comparable in shape to that shown in Fig. 1; the two groups reached their highest level of performance in the fourth 20-trial block. Thus, by a response-probability measure, both groups showed the gradual type of approach to their respective asymptotic performance levels that is typically reported for the rat in the shuttlebox. The response-latency data, calculated as a median over animals of the median for each animal’s avoidance and escape latencies over 20-&l blocks, are shown in Fig. 3. Here, as in Experiment 1, there is no indication of a systematic decrease in response latency. There appears to be no learning of either the escape response or the avoidance response in either group. A closer look at the first few trials indicates no fast learning in either group. The means of the first escape latency were 10.8 and 10.9 set for the two groups, as against final trial-block latencies of 10.6 and 10.9 set (measured from S + onset). Individual t-tests indicated that on several trial blocks the female Wistars were reliably faster than the male Long-Evans on either escape trials or avoidance trials. But the main finding is that while the two groups gave quite different overall avoidance performances, they were alike in demonstrating no decrease in either avoidance latencies or escape latencies. Nor was there any such evidence in the records of any individual animal.
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BLOCKS
FIG. 3. Median response latency (set) on avoidance and escape trials of female Wistar (FW) and male Long-Evans (ML) rats.
EXPERIMENT
3
Another case of no change in response latency of rats in a shuttlebox was reported by Maier, Albin, and Testa (1973). These investigators found very little avoidance in the first 30 training trials, but they also found no systematic change in the mean latency of the escape response, which hovered around 2 set in their various groups. Maier, et al. interpreted running in the shuttlebox as an unlearned or unconditioned reaction to shock, and they discovered that they could not get “learned helplessness” in rats using such a response. But they discovered that they could obtain the learned-helplessness effect by modifying the response requirement so that the rats had to run one way and then back. Maier, er al. found that this double response was initially much slower than the usual shuttlebox response, and that it was, apparently, slowly acquired over 30 training trials. The purpose of Experiment 3 was to examine further the double-run response. We were most concerned with whether avoidance learning could be obtained using this novel response requirement, and, if it could, whether such learning would be reflected in the response-latency measure. Method Subjects. The subjects were six naive female rats of Wistar descent, approximately 100 days old. Apparatus and procedure. All conditions were the same as in Experiment 2 except that the interstimulus interval was increased to 20 set
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to provide more opportunity for avoidance responses to occur. Both avoidance and escape responses were defined in terms of adouble run, i.e., the rat had to run one way and then back before a response was counted. It was possible to run one way in the presence of the S+ alone and then terminate shock quickly by running back; however, this occurred only rarely. Results By the conventional response probability criterion, the double run was learned as an avoidance response; there were 6% avoidances on the first 20-trial block and 5% avoidances on the fifth 20-&l block. But the data on response latency, summarized in Fig. 4, were the same as before, i.e., they showed no evidence of learning. Again, it was not the case that plotting medians over trial blocks obscured some learning occurring in the first few trials. The medians on trial 1, over trials l-5, and over trials l-20 were all within .2 set (the basic time unit) of 21.0 sec. GENERAL
DISCUSSION
The results of Experiment 1 indicate that under certain conditions, particularly when shock is unsignaled, there may be subtle changes in behavior over trials, but these changes seem best attributed to “secondary” learning, such as the acquisition of postural and orientational behaviors, which play only a minor role in getting the rat efficiently from one side of the shuttlebox to the other. In general, our results are like those of Maieret al. (1973) in showing that under normal conditions, there is little or no change over trials in the latency of escape responding in a shuttlebox. There are also some differences in results. Maier et al. reported latencies of about 2 to 5 set following shock onset; under comparable conditions our escape latencies were generally less than 1 sec. Another difference between our results and ESCAPE l
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FIG. 4. Median response latency (set) of animals required to
NII
forth and back.
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those of Maieret al. is that when they imposed the double-run requirement on their animals, they found slow escape responding, with a mean of about 13 set from shock onset on the first trial block and a progressive decrease in latency to a mean of about 7 set on the last trial block. Although the reliability of this decrease was not reported, it is strongly suggestive of escape learning. By contrast, our animals always performed much more #quickly on double-run escape trials, in approximately 1 set, and evidenced no decrease in latency (Fig. 4). Our shorter latencies would seem to support, even better than their own data, Maier et al.‘s interpretation that shock elicits running in the shuttlebox. It is difficult to give any other account of behavior that is executed in 1 set on the first trial. We suggest that the stimulus properties of our particular apparatus, together with our use of other specific parameters, made shock an especially strong elicitor of unconditioned running, and that with behavior of such great initial strength learning either cannot occur or cannot be manifest. This fixed or unlearned quality of the escape response may be surprising, but it presents no major problem of interpretation. The emergence of avoidance behavior, i.e., responses occurring quickly enough to prevent shock, poses a more interesting interpretive puzzle. We find that response probability, the probability that the running response will occur during a fixed time interval, increases steadily over a number of trials, but we find that when running occurs it always occurs at the same time in the interval. Within an S-R framework, the learning of a response is generally attributed either to the loss of competing behaviors or to the conditioning of the learned response to stimuli which precede those that evoke it initially. Either mechanism can increase the probability of the response occurring in a given stimulus situation. But both mechanisms should also make the response occur progressively sooner. Thus, if the acquisition of running in the shuttlebox is to be attributed to the loss over trials of competing behaviors, say, freezing, initially elicited by S +, then how are we to account for the anomaly that freezing is gradually lost only at a particular time during the S +? That is, how can freezing be gradually lost at a particular point in the interval (so that running becomes gradually more probable) while being maintained at other points in the interval (so that running does not occur at other times)? And if the acquisition of running is to be attributed to the gradually strengthened connection between S + and running, then why does running not occur sooner and sooner after the onset of S+? We are so accustomed to gradual increases in response speeds in appetitive situations that we take them for granted, and our S-R machinery handles them easily, even in cases where different response measures are not well correlated. But here the machinery fails us; neither a competing response nor an associative strength interpretation looks very convincing. Nor is it obvious how one could invoke a combination of these mechanisms to explain our results. Another consideration is that in the avoidance situation there is no
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differential reinforcement for faster responding. When the avoidance response first appears it is already fast enough to prevent shock, even allowing for considerable variability about the mean, and there is no reason for the speed of the response to change. The differential consequences of running or not running make the probability of running increase, but there are no differential consequences of running a little faster or a little slower so speed does not change. This functional argument has merit, but it provides no mechanism for learning. More specifically, it ignores the anomalies encountered by the conventionalS -R mechanism that werejust discussed. It could be argued that the S + itself has little direct associative control over running; running is really controlled by the fear that is conditioned to the S+ in the first training trials. If this were the case then the latency of avoidance running would be governed by how long it takes conditioned fear to build up to the appropriate level, and not by the associative strength of the running response per se. Indeed, to make the argument fit the present data it would have to be assumed that running was an unconditioned response to a particular level or state of fear, or that it became maximally conditioned to some level of fear during the first escape trials. Otherwise there should have been some change in latency under some condition somewhere in the course of avoidance learning. This argument, then, bypasses the anomalies encountered by the S-R learning mechanisms. It asserts, in effect, that there is no associative stimulus-response learning here, and that the gradual increase in the probability of the avoidance response reflects only the gradually increasing probability that the response will be appropriately motivated. This conclusion is also suggested by a cognitive view of avoidance learning (e.g., Bolles, 1975). We may think of the rat as already having a strong tendency to run in the shuttlebox once it perceives that its end of the box is dangerous and/or that the other end of the box is safe. We may think of this perception as initially unlikely, and as later somewhat uncertain but gradually becoming more probable. Thus, the perception of danger and safety may or may not occur on a given trial, but when it occurs, it occurs quickly and releases a fixed, prepotent running response. REFERENCES Badia, P. & Culbertson, S. The relative aversiveness of signaled vs unsignaled escapable and inescapable shock. Journal of the Experimental Analysis of Behavior, 1972, 17, 463-471. Bolles, R. C. Theory of motivation, 2nd ed. New York: Harper and Row, 1975. Church, R. M. Aversive behavior. In J. W. Kling and L. A. Riggs (Eds.), Experimental psychology. New York: Holt, Rinehart & Winston, 1971. Maier, S. F., Albin, R. W., & Testa, T. J. Failure to learn to escape in rats previously exposed to inescapable shock depends on nature of escape response. Journal of Comparative and Physiological Psychology. 1973. 85, 581-592. Received May 10, 1974 Revised March 1, 1975
AND
MOTIVATION
7, 108- 116 (1976)
Note on the Invariance of Response Latency in Shuttlebox Avoidance Learning ROBERT C.BOLLES,SEWARD A. MOOT, AND KELLY NELSON University
of Washington
Rats trained to avoid shock by running in a shuttlebox showed an orderly increase over trials in the probability of avoidance but no corresponding change over trials in the latency of the avoidance response. Both responding in the presence of shock (escapes) and responding prior to shock onset (avoidances) appeared to be stationary in time. Latencies were found to vary with different experimental conditions but in every case to be invariant over trials.
The course of avoidance learning in the shuttlebox is almost invariably assessed by the proportion of avoidance responses that occur on successive blocks of trials; rarely are latency data reported. The response-probability measure typically increases in a manner which is fairly orderly for individual animals and quite orderly for groups of animals. But there are too few data on response latency to draw any conclusion about how this measure changes during the course of training. The present studies were undertaken simply to provide such data. EXPERIMENT
I
Method Subjects. The subjects were 24 experimentally naive male rats of Long-Evans descent, approximately 100 days old. Apparatus. The shuttlebox was 76 cm long, 20 cm wide and 18 cm high, with no door or hurdle. It was placed in a sound-attenuating chamber and lighted by a 15-W bulb directly above the shuttlebox. A ventilation fan produced a background noise level of 64 db, and the buzzer cue for shock increased this level to approximately 76 db. The floor of the shuttlebox was constructed of l-cm stainless steel bars spaced 2.5 cm center to center. A Grason-Stadler shock generator provided scrambled shock of nominal 1.O-mA intensity. Responses were defined automatically by micro-switches attached to the floor which operated when the rat moved 10 cm past the midpoint. Response latencies were recorded to the nearest 0.1 sec. Supported by research grant GB-20801 from National Science Foundation. Requests for reprints should be addressed to R. C. Bolles, Department of Psychology, University of Washington, Seattle, Washington 98105. 108 Copyright 0 1976 by Academic Press. Inc. All rights of reproduction in any form reserved.
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Procedure. The animals were randomly assigned to three groups, and trained in a single session of 50 trials with a variable intertrial interval (mean = 90 set). The Avoidance group was trained under conventional conditions: shock was preceded by a IO-set buzzer S + l. A response occurring in this interval terminated S+ and avoided the shock. If an animal failed to respond during the IO-set interstimulus interval, the shock came on but could be escaped (and the S + terminated) by a response. The Signaled Escape group was run under the same conditions except that all shocks were unavoidable, so that all trials were necessarily escape trials. Responses occurring in the presence of the S + alone had no programmed consequences. The third group, the Unsignaled Escape group, was trained under the same conditions except that there was no S + . Thus, this group was trained under conventional escape conditions. Results Consider first the behavior of the Avoidance group. Figure 1 shows an orderly increase in the number of avoidance responses in successive blocks of trials. Because three of the eight animals learned the response quite quickly and performed at a high level, the mean is higher than the median, but both measures of average avoidance responding were consistent with results typically reported for rats in a shuttlebox: the group was avoiding shock about 50% of the time after 50 training trials, and their performance did not seem to have reached an asymptote. The mean latency (actually the time required to run about 10 cm past the midline of the box) averaged over both escape and avoidance trials showed a similar gradual decline over trial blocks. But note that this overall decrease in latency must reflect in part the increasing frequency of avoidance responses which are necessarily quicker than escape responses. 1 Here we follow Church’s (1971) suggestion to call the shock cue or warning stimulus an .S+ rather than a CS.
IO-TRIAL
BLOCKS
FIG. 1. Percentage of avoidance responses of rats trained under avoidance conditions.
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The results of analyzing latencies of avoidance and escape responses separately are given in Fig. 2. Also included in Fig. 2 are response latencies for the signaled and unsignaled escape groups. Each data point in the figure is the median, over subjects, of the median response latency over a block of five trials. Note that the data are presented in smaller blocks than in Fig. 1 to better reveal any trends in the early trials. Latencies are taken relative to the start of the trial, i.e., 10 set before scheduled shock onset, to make the results comparable for the different groups. These results tell a very different story: in the Avoidance group the latency of the avoidance response provided no evidence of learning. The course of avoidance learning as measured by the probability of avoidance was clearly not reflected in the response latency measure. There was a suggestive, but not reliable (t = 1.4) decrease across animals in latency between the first and second avoidance response. Nor was there any apparent learning of the escape response in any of the groups. The escape response appeared to have so much strength initially that little further strengthening was possible. But perhaps there was escape learning and it is obscured by the way the data are presented in Fig. 2. One possibility is that the unit of time in the figure is too long. Thus, it might be argued that the rat learns to escape in the first trial or two, and this learning is obscured by taking medians over five-trial blocks. This was not the case. For the different groups, mean performance on the first escape trial was from .3 set to .9 set (for the Unsignaled group)faster than the respective median speed over the first block of trials. Individual r-tests showed that the median latency over trials 2-5 was significantly longer than on trial 1 for each group. In other words, escape latencies were actually getting longer over the first few trials. This
5-TRIAL
BLOCKS
FIG. 2. Median latencies in seconds, by 5-trial blocks for the three groups on escape trials, and for the avoidance-trained animals on avoidance trials. Note the expansion of the ordinate, and note that there were no avoidances in the first trial block.
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anomaly suggests that some decremental factor, perhaps a tendency to freeze, comes into play in the first few trials to interfere slightly with what is initially an extremely strong response. So it is evidently not the case that learning occurred too quickly to be detected by our analysis. Nor was it the case that a few subjects in a group showed learning to escape; the group means tell the same story as group medians, and inspection of individual records revealed no clear instance of gradually declining latencies. Another possibility is that our time scale was too short to demonstrate learning. Perhaps some escape learning would have become evident with continued training. Some suggestion of gradual learning was shown by the Unsignaled Escape group. First, note that in this group the latency averaged about .5 set longer than in the Signaled Escape group. This difference was significant on several trial blocks. Perhaps the IO-set S+ permits the rat to make some postural adjustment or orienting response so that it can respond more rapidly when shock comes on. Alternatively, perhaps the animals in the Unsignaled Escape group are subject to more fear, or engage in more freezing, or find the whole situation more aversive than the animals in the other groups (Badia & Culbertson, 1972). Whatever the reason for this performance decrement in the Unsignaled Escape group, it appeared quickly, and then disappeared over the course of 50 trials. An analysis of the linear trend over trial blocks showed it to be highly reliable (F( 1,62) = 9.7,~ < .Ol); the linear trends for the other groups were not significant. Even an ad hoc analysis of the most suggestive portion of the data, over trial blocks 2 to 4, failed to reveal significant trends for the other groups. The gradual improvement in the Unsignaled Escape group may be attributed to learning, but much of this learning simply ameliorated decremental factors arising early in training, and all of it is clearly secondary to the rat getting from one end of the apparatus to the other. There is every reason to believe that there would be further secondary learning, i.e., acquisition of postures and orientations, with continued training in all groups, but such learning would merely refine what is already a very strong response. Such improvements in motor skill do not constitute the stuff of which escape learning is made, or avoidance learning either. The basic fact is that the animals responded in 11.6 set (median = 11.1 set) on the first escape trial, and there was therefore little opportunity for faster responding. EXPERIMENT
2
The results of Experiment 1 indicate that by a response-latency criterion there is little or no learning either of escaping shock or of avoiding it in the shuttlebox. While the probability of running in the presence of the S+ alone increases in a continuous manner, the response seems to remain essentially stationary in time. When the shock comes on, the rat always runs, and this response too seems to remain essentially stationary in time. The purpose of
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Experiment 2 was to see if comparable results would be found with somewhat different subjects. Previous work in our laboratory had indicated that female Wistar rats perform much better in the shuttlebox than male Long-Evans. Would the former show a different pattern of response latencies? Method Subjects. The subjects were eight male rats of Long-Evans descent (the same stock as the animals in Experiment l), and eight female rats of Wistar descent. All were experimentally naive and approximately 100 days old. Apparatus and procedure. The apparatus was the same as that used in Experiment 1. Both groups were given 100 trials of avoidance training with the same parameters as in Experiment 1. Results
The male Long-Evans showed poor learning, reaching an overall mean level of avoidance responding of only 1% (median = 7 compared with 24% in Experiment 1). Two subjects performed reasonably well, but four gave three or fewer avoidances in the 100 trials. By contrast, the group of female Wistars performed very well, reaching 66% avoidance (median = 6%) overall, and six of the eight subjects achieved 20 or more consecutive avoidances. In both cases the learning curves were comparable in shape to that shown in Fig. 1; the two groups reached their highest level of performance in the fourth 20-trial block. Thus, by a response-probability measure, both groups showed the gradual type of approach to their respective asymptotic performance levels that is typically reported for the rat in the shuttlebox. The response-latency data, calculated as a median over animals of the median for each animal’s avoidance and escape latencies over 20-&l blocks, are shown in Fig. 3. Here, as in Experiment 1, there is no indication of a systematic decrease in response latency. There appears to be no learning of either the escape response or the avoidance response in either group. A closer look at the first few trials indicates no fast learning in either group. The means of the first escape latency were 10.8 and 10.9 set for the two groups, as against final trial-block latencies of 10.6 and 10.9 set (measured from S + onset). Individual t-tests indicated that on several trial blocks the female Wistars were reliably faster than the male Long-Evans on either escape trials or avoidance trials. But the main finding is that while the two groups gave quite different overall avoidance performances, they were alike in demonstrating no decrease in either avoidance latencies or escape latencies. Nor was there any such evidence in the records of any individual animal.
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BLOCKS
FIG. 3. Median response latency (set) on avoidance and escape trials of female Wistar (FW) and male Long-Evans (ML) rats.
EXPERIMENT
3
Another case of no change in response latency of rats in a shuttlebox was reported by Maier, Albin, and Testa (1973). These investigators found very little avoidance in the first 30 training trials, but they also found no systematic change in the mean latency of the escape response, which hovered around 2 set in their various groups. Maier, et al. interpreted running in the shuttlebox as an unlearned or unconditioned reaction to shock, and they discovered that they could not get “learned helplessness” in rats using such a response. But they discovered that they could obtain the learned-helplessness effect by modifying the response requirement so that the rats had to run one way and then back. Maier, er al. found that this double response was initially much slower than the usual shuttlebox response, and that it was, apparently, slowly acquired over 30 training trials. The purpose of Experiment 3 was to examine further the double-run response. We were most concerned with whether avoidance learning could be obtained using this novel response requirement, and, if it could, whether such learning would be reflected in the response-latency measure. Method Subjects. The subjects were six naive female rats of Wistar descent, approximately 100 days old. Apparatus and procedure. All conditions were the same as in Experiment 2 except that the interstimulus interval was increased to 20 set
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to provide more opportunity for avoidance responses to occur. Both avoidance and escape responses were defined in terms of adouble run, i.e., the rat had to run one way and then back before a response was counted. It was possible to run one way in the presence of the S+ alone and then terminate shock quickly by running back; however, this occurred only rarely. Results By the conventional response probability criterion, the double run was learned as an avoidance response; there were 6% avoidances on the first 20-trial block and 5% avoidances on the fifth 20-&l block. But the data on response latency, summarized in Fig. 4, were the same as before, i.e., they showed no evidence of learning. Again, it was not the case that plotting medians over trial blocks obscured some learning occurring in the first few trials. The medians on trial 1, over trials l-5, and over trials l-20 were all within .2 set (the basic time unit) of 21.0 sec. GENERAL
DISCUSSION
The results of Experiment 1 indicate that under certain conditions, particularly when shock is unsignaled, there may be subtle changes in behavior over trials, but these changes seem best attributed to “secondary” learning, such as the acquisition of postural and orientational behaviors, which play only a minor role in getting the rat efficiently from one side of the shuttlebox to the other. In general, our results are like those of Maieret al. (1973) in showing that under normal conditions, there is little or no change over trials in the latency of escape responding in a shuttlebox. There are also some differences in results. Maier et al. reported latencies of about 2 to 5 set following shock onset; under comparable conditions our escape latencies were generally less than 1 sec. Another difference between our results and ESCAPE l
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FIG. 4. Median response latency (set) of animals required to
NII
forth and back.
LATENCY
OF SHUTTLEBOX
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those of Maieret al. is that when they imposed the double-run requirement on their animals, they found slow escape responding, with a mean of about 13 set from shock onset on the first trial block and a progressive decrease in latency to a mean of about 7 set on the last trial block. Although the reliability of this decrease was not reported, it is strongly suggestive of escape learning. By contrast, our animals always performed much more #quickly on double-run escape trials, in approximately 1 set, and evidenced no decrease in latency (Fig. 4). Our shorter latencies would seem to support, even better than their own data, Maier et al.‘s interpretation that shock elicits running in the shuttlebox. It is difficult to give any other account of behavior that is executed in 1 set on the first trial. We suggest that the stimulus properties of our particular apparatus, together with our use of other specific parameters, made shock an especially strong elicitor of unconditioned running, and that with behavior of such great initial strength learning either cannot occur or cannot be manifest. This fixed or unlearned quality of the escape response may be surprising, but it presents no major problem of interpretation. The emergence of avoidance behavior, i.e., responses occurring quickly enough to prevent shock, poses a more interesting interpretive puzzle. We find that response probability, the probability that the running response will occur during a fixed time interval, increases steadily over a number of trials, but we find that when running occurs it always occurs at the same time in the interval. Within an S-R framework, the learning of a response is generally attributed either to the loss of competing behaviors or to the conditioning of the learned response to stimuli which precede those that evoke it initially. Either mechanism can increase the probability of the response occurring in a given stimulus situation. But both mechanisms should also make the response occur progressively sooner. Thus, if the acquisition of running in the shuttlebox is to be attributed to the loss over trials of competing behaviors, say, freezing, initially elicited by S +, then how are we to account for the anomaly that freezing is gradually lost only at a particular time during the S +? That is, how can freezing be gradually lost at a particular point in the interval (so that running becomes gradually more probable) while being maintained at other points in the interval (so that running does not occur at other times)? And if the acquisition of running is to be attributed to the gradually strengthened connection between S + and running, then why does running not occur sooner and sooner after the onset of S+? We are so accustomed to gradual increases in response speeds in appetitive situations that we take them for granted, and our S-R machinery handles them easily, even in cases where different response measures are not well correlated. But here the machinery fails us; neither a competing response nor an associative strength interpretation looks very convincing. Nor is it obvious how one could invoke a combination of these mechanisms to explain our results. Another consideration is that in the avoidance situation there is no
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differential reinforcement for faster responding. When the avoidance response first appears it is already fast enough to prevent shock, even allowing for considerable variability about the mean, and there is no reason for the speed of the response to change. The differential consequences of running or not running make the probability of running increase, but there are no differential consequences of running a little faster or a little slower so speed does not change. This functional argument has merit, but it provides no mechanism for learning. More specifically, it ignores the anomalies encountered by the conventionalS -R mechanism that werejust discussed. It could be argued that the S + itself has little direct associative control over running; running is really controlled by the fear that is conditioned to the S+ in the first training trials. If this were the case then the latency of avoidance running would be governed by how long it takes conditioned fear to build up to the appropriate level, and not by the associative strength of the running response per se. Indeed, to make the argument fit the present data it would have to be assumed that running was an unconditioned response to a particular level or state of fear, or that it became maximally conditioned to some level of fear during the first escape trials. Otherwise there should have been some change in latency under some condition somewhere in the course of avoidance learning. This argument, then, bypasses the anomalies encountered by the S-R learning mechanisms. It asserts, in effect, that there is no associative stimulus-response learning here, and that the gradual increase in the probability of the avoidance response reflects only the gradually increasing probability that the response will be appropriately motivated. This conclusion is also suggested by a cognitive view of avoidance learning (e.g., Bolles, 1975). We may think of the rat as already having a strong tendency to run in the shuttlebox once it perceives that its end of the box is dangerous and/or that the other end of the box is safe. We may think of this perception as initially unlikely, and as later somewhat uncertain but gradually becoming more probable. Thus, the perception of danger and safety may or may not occur on a given trial, but when it occurs, it occurs quickly and releases a fixed, prepotent running response. REFERENCES Badia, P. & Culbertson, S. The relative aversiveness of signaled vs unsignaled escapable and inescapable shock. Journal of the Experimental Analysis of Behavior, 1972, 17, 463-471. Bolles, R. C. Theory of motivation, 2nd ed. New York: Harper and Row, 1975. Church, R. M. Aversive behavior. In J. W. Kling and L. A. Riggs (Eds.), Experimental psychology. New York: Holt, Rinehart & Winston, 1971. Maier, S. F., Albin, R. W., & Testa, T. J. Failure to learn to escape in rats previously exposed to inescapable shock depends on nature of escape response. Journal of Comparative and Physiological Psychology. 1973. 85, 581-592. Received May 10, 1974 Revised March 1, 1975