Does shock modifiability contribute to preference for signaled shock?

The central question asked was whether differential shock modification occurs (posturally induced differences in shock contact time) under signaled and unsignaled conditions using scrambled shock. Shock modifiability was tested with two different shock sources, intensities, and scrambling units by measuring the duration of time subjects were in contact with shock. Subjects were then given a choice between the signaled and unsignaled conditions. Results showed that differential modification of shock contact time did not occur between signaled and unsignaled conditions with any shock source, intensity, or scrambler unit. In addition, subjects preferred the signaled condition. It was concluded that experiments using scrambled shock are not confounded by posturally induced differences in shock contact time.     

Signaled reinforcement effects on fixed-interval performance of the rat

In three experiments using fixed-interval schedules with a 500-msec delay of reinforcement, rats receiving a localized light signal during the delay leverpressed more slowly than rats trained without the signal. In Experiment 1 these groups showed no differences in temporal patterning of responding, but in the remaining two experiments the signaled rats showed better patterning than the unsignaled rats. In Experiments 2 and 3 rats receiving a diffuse tone signal during the delay instead of a light pressed more rather than less rapidly than the unsignaled group. Their patterning was better than that of the unsignaled rats in one of these experiments. Several explanations for both the attenuation and enhancing effects of signaling reward are discussed.     

Can a rat count?

A 2 × 2 factorial experiment was conducted in a licking-suppression situation to test if a rat could count the number of shocks given in a 5-min session under signaled and unsignaled shock conditions. Groups F received three .7-sec grid shocks per session throughout 80 sessions, whereas Groups V received, on any day, one, two, three, four, or five shocks, with a mean of three shocks. The rats’ counting ability was assessed in terms of the post-third-shock acceleration of licking. The results of this measure were compared between Groups F and Groups V on test days in which both groups received three shocks with the identical shock sequence. There was no evidence that rats could count under either signaled or unsignaled shock conditions. The basal rate of licking was less in groups run under the unsignaled shock condition than under the signaled shock condition. The effect of fixed/variable shock frequency upon basal rate of licking was not significant. The results are discussed with reference to the optimal shock density view of Davis and Memmott (1982).     

Concurrent schedules of wheel-running reinforcement: Choice between different durations of opportunity to run in rats

How do animals choose between opportunities to run of different durations? Are longer durations preferred over shorter durations because they permit a greater number of revolutions? Are shorter durations preferred because they engender higher rates of running? Will longer durations be chosen because running is less constrained? The present study reports on three experiments that attempted to address these questions. In the first experiment, five male Wistar rats chose between 10-sec and 50-sec opportunities to run on modified concurrent variable-interval (VI) schedules. Across conditions, the durations associated with the alternatives were reversed. Response, time, and reinforcer proportions did not vary from indifference. In a second experiment, eight female Long-Evans rats chose between opportunities to run of equal (30 sec) and unequal durations (10 sec and 50 sec) on concurrent variable-ratio (VR) schedules. As in Experiment 1, between presentations of equal duration conditions, 10-sec and 50-sec durations were reversed. Results showed that response, time, and reinforcer proportions on an alternative did not vary with reinforcer duration. In a third experiment, using concurrent VR schedules, durations were systematically varied to decrease the shorter duration toward 0 sec. As the shorter duration decreased, response, time, and reinforcer proportions shifted toward the longer duration. In summary, differences in durations of opportunities to run did not affect choice behavior in a manner consistent with the assumption that a longer reinforcer is a larger reinforcer.     

The US-preexposure effect in honeybees

Signaled avoidance was studied in individual honeybees that visited the laboratory regularly to take sucrose solution from a target set on the sill of an open window. During feeding, substrate vibration or airstream was used to signal a brief shock that could be avoided by breaking off contact with the food for a few seconds. Aversive conditioning of the context was measured in terms of return time (the time between successive visits). In Experiment 1, experience with unsignaled shock was found to lengthen return time—which experience with signaled shock did not—and to impair performance in subsequent avoidance training with signaled shock (the US-preexposure effect). In Experiment 2, experience with unsignaled shock given after signaled avoidance training lengthened return time but had no effect on response to the signal in a subsequent extinction test. These results closely resemble the results obtained in analogous experiments with vertebrates.     

How reinforcement context affects temporal production and categorization

The behavioral theory of timing assumes that timing is governed by a pacemaker whose pulses move organisms from one state to the next, and that the speed of the pacemaker covaries with the rate of reinforcement in the experimental context. The goal of the present experiments was to clarify just what constitutes that context. In Experiment 1, pigeons responded on signaled fixed-interval 20-sec and 40-sec schedules of food reinforcement that were presented randomly within sessions (alternating condition) or between sessions (isolated condition). In Experiment 2, pigeons categorized the duration of a short or a long set of intervals in the alternating or the isolated condition. Performance in both experiments was under strong control by the signals, with scalar timing between long and short sets, but no significant differences between the alternating and isolated conditions. The context of reinforcement that determines pacemaker period can thus be specific to a particular timing task and signal.     

A comparison of signaled vs. unsignaled free-operant avoidance in wild and domesticated rats

Three experiments compared the performance of domesticated hooded rats and nondomesticated black rats under signaled and unsignaled free-operant leverpress avoidance. There was no difference between groups in asymptotic shock rates under unsignaled avoidance; however, the black rats avoided much more successfully when the signal was present, while the hooded rats showed little or no improvement. When a longer signal was employed (10 vs. 5 sec), the effects were essentially the same. The black rats generally had higher response rates, and this difference was most pronounced in extinction, where the hooded rats made very few responses. The domesticated rats received a disproportionately large number of shocks early in the session (warm-up) under unsignaled avoidance, but this tendency was much less pronounced for the black rats. However, both groups showed appreciable amounts of warm-up during signalled avoidance. The findings are discussed in terms of differences in levels of activation/arousal between domesticated and nondomesticated animals.     

A concurrent assessment of the positive and negative properties of a signaled shock schedule

A choice and a conditioned suppression procedure were used to assess concurrently the positive and negative properties of stimuli within a signaled shock schedule, Occasional shocks were presented to Ss responding on a variable-interval food schedule. Ss could choose whether shocks occurred alone or whether they were preceded by a 1-min signal. All Ss chose the signaled shock condition over the unsignaled one, even though food reinforced responding in the presence of the signal was suppressed. Rate of responding for food varied across stimulus conditions, with the lowest rate in the presence of the signal and the highest rate in its absence. An intermediate rate occurred under the unsignaled shock schedule. A safety analysis was applied to the data.     

Changeover from unsignaled to signaled avoidance with and without a feedback stimulus in rats

Two experiments are reported in which, by pressing a lever, unsignaled avoidable shocks could be changed to signaled avoidable shocks for periods of 1 min, after which unsignaled avoidance was automatically reinstated. A correlated stimulus identified the signaled schedule. Under these conditions, Ss changed over at a rate sufficient to remain in the signaled schedule more than 90% of the time. Both experiments showed that changeover performance was under the control of the correlated stimulus. Presenting the signaled schedule without the correlated stimulus did not maintain changing over. The second experiment showed that a 1-sec presentation of the correlated stimulus was sufficient to maintain changeover performance. The results were attributed to the conditioned reinforcing properties of the correlated stimulus.     

Preference for food and water in rats as a function of delay of reward

In Experiment I, rats which were both hungry and thirsty were given a choice between a food reward and a water reward. The animals preferred food to water when the reward was delivered immediately, but preferred water to food when a 30-sec delay was imposed in the goalbox before the reward was received. Experiment II replicated the results of the first experiment and showed, in addition, that when the delay was imposed in a separate delay chamber devoid of differential goalbox cues, subjects preferred food to water, similar to the immediate group. The results were discussed in terms of an incentive value process and a competing response hypothesis.     

Shock intensity and warning signal effects on several measures of operant avoidance acquisition

Acquisition of both signaled and unsignaled operant avoidance learning was studied in 64 rats as a function of shock intensity, with three different warning signals used in the signaled procedure. In both signaled and unsignaled avoidance, overall response rate was a progressively increasing function of shock intensity. This was due to both an absolute and relative increase in the frequency of responses at shorter interresponse times with increasing shock intensity. Presence of an effective warning signal in the interval immediately preceding shock increased the probability of an avoidance response in this interval, decreased overall response rate, and reduced shock frequency. A buzzer signal proved most effective, followed by tone and light. However, once a warning signal occurred, the probability of an avoidance response to the signal was virtually independent of shock intensity. Also, an index of avoidance efficiency proved to be inversely related to shock intensity.     

Stimulus duration and conditioned reinforcing value measured by a learning-tests procedure

The relationship between the duration of stimuli and their conditioned reinforcing effect was investigated using a learning-tests procedure. In Experiment 1, stimuli were the same duration on training (stimulus → reward) and test (choice response → stimulus). Ten- and 30-sec stimuli provided effective differential conditioned reinforcement but 3-sec stimuli did not. In Experiment 2, different pigeons had each combination of the 3- and 30-sec stimuli on training and test trials. Evidence of conditioned reinforcement was obtained only for the birds with 30-sec stimuli on both training and test. The results were interpreted as indicating that stimuli become effective conditioned reinforcers on test trials only when their duration exceeds the duration of differential short-term memory cues resulting from a difference in the events that precede them on training and test trials.     

Preference for signaled shock: A test of two hypotheses

In a test of safety signal and preparatory response explanations of the preference for signaled vs. unsignaled shock, three groups of rats were exposed to a different light-tone-shock contingency on each of the two sides of a shuttlebox. One contingency (S/P) provided both a safety signal and a warning stimulus, another (NS/NP) provided neither, and a third (S/NP) provided a safety period but no warning stimulus. Rats preferred either the S/P or the S/NP side of the shuttlebox when the alternate side provided neither safety signal nor warning stimulus. When the safety signal was available on both sides, the side without the warning stimulus was preferred. Results are interpreted as supporting the safety signal hypothesis.     

Measurement of current flow through the rat under signaled and unsignaled grid-shock conditions

The present experiment was run to test the hypothesis that, when shock was signaled, rats would develop effective coping responses so as to reduce the current flow through them. A 1-sec shock was delivered through a grid floor by a fixed impedance ac shock source. The current-flow measure was taken over the last 30 of 90 trials given over 3 days and indexed by “gross skin conductance” or GSC (shock). The rat under the signaled shock condition (n=15) showed higher GSC (shock) than did the rats under the unsignaled shock condition (n=14). Thus, the result contradicted the hypothesis. There was no indication that the rats developed any preparatory response during the 5-sec signal, in terms of either GSC (signal) or posture. The results were discussed with reference to the preparatory-response hypothesis and various other possibilities.     

The psychophysics of remembered duration

Pigeons were trained on a psychophysical choice task to make one response after a 2-sec signal and a different response after a 10-sec signal. Delayed dimensional control was assessed by presenting durations intermediate to the short and long signals and by introducing delays between the signals and choice opportunities. In Experiment 1, choices after intermediate durations were not reinforced; in Experiment 2, one choice was reinforced after the three shortest durations and another was reinforced after the three longest durations. In Experiment 1, the slopes of the psychophysical functions decreased with increases in delays, but the decrease in stimulus control was not unbiased; choice probabilities decreased for longer durations, but did not increase for shorter durations. Experiment 2 revealed the same generalized loss of stimulus control on the temporal dimension, but not the same pattern of bias; temporal control was relinquished equally for shorter and longer durations. These results are evaluated in the context of the subjective shortening model of remembered duration (Spetch & Wilkie, 1983) and Staddon’s theory of timing and remembering (Staddon, 1984).     

Noncontingent and contingent no-choice intervals and concurrent performance

Pigeons responded to changeover-key concurrent variable-interval variable-interval reinforcement schedules while there were intervals during which the changeover key was inoperative (no-choice intervals). In Experiment 1, a multiple schedule on the changeover key signaled choice and no-choice intervals. All subjects showed near-perfect discrimination during initial discrimination training and rapid reacquisition of discrimination following contingency reversals. In Experiment 2, the onset of no-choice intervals was unsignaled and contingent on interchangeover time. The temporal distribution of changeover-key responses conformed to the temporal distribution of choice intervals. The results of both experiments suggest that changeover responding is modifiable as a function of its immediate consequences. The results of Experiment 2, in particular, suggest that time or some correlate of time since the last changeover response can determine subsequent changeover behavior.     

Determinants of pigeons’ choice between certain and probabilistic outcomes

Pigeons chose between 50% and 100% reinforcement on a discrete-trials concurrent-chains procedure with fixed-ratio 1 initial links and fixed-time terminal links. The 100% alternative always provided food after a terminal-link delay, whereas the 50% alternative provided food or blackout equally often after a delay. Additionally, the terminal-link stimuli on the 50% alternative were correlated with the outcomes in signaled, but not in unsignaled, conditions. The effects of intertrial-interval duration and length of the terminal-link delays on choice of the 50% alternative were investigated in four experiments. Preference for the 50% alternative varied with signal condition and duration of the terminal link leading to food, but not with duration of either intertrial interval or the terminal link leading to a blackout. The results are discussed in terms of conditioned-reinforcement effects, Mazur’s hyperbolic-decay model, and delay reduction. This research was supported by a Natural Sciences and Engineering Research Council of Canada research grant awarded to the first author.     

Effect of signaling extra unconditioned stimuli on autoshaping

Acquisition to a target conditioned stimulus (CS) is prevented when extra, unsignaled unconditioned stimuli (USs) are presented with sufficient frequency to remove contingency between target CS and US. Acquisition occurs, however, when the extra USs are signaled by another CS. According to the Rescorla-Wagner theory, signaling reduces contextual conditioning, which otherwise prevents acquisition. Results of Experiment 1 led to the rejection of a rival explanation derived from scalar expectancy theory by showing that acquisition does not occur when only half of the extra USs are signaled. The results of Experiment 2 were, however, contrary to the Rescorla-Wagner theory because they showed equivalent acquisition when the stimulus used to signal the extra USs was also present concurrently with the target CS. Signaling may exert its effect by converting the intertriai interval to CS?.     

The effects of varying the interreinforcement interval on appetitive contextual conditioning

Appetitive contextual conditioning in rats and ringdoves was investigated in six experiments. In Experiment 1, differential contextual training produced greater anticipatory activity in rats in the presence of a context paired with food than it did in rats in the presence of a different context in which food was never presented. Furthermore, the rats showed a preference for the context associated with food when they were given a simultaneous choice test between contexts. In Experiment 2, rats were more active in and preferred a context associated with a variable-time 30-sec (VT30) schedule as opposed to a VT180 schedule. Experiment 3 was a between-subjects replication of the previous experiment. As expected, rats exhibited significantly more anticipatory activity in a context in which food had been presented on a VT30 schedule than they did in a context in which food had been presented on a VT180 schedule. Experiment 4 showed that anticipatory activity was a reflection of context-US associations in ringdoves, and in Experiments 5 and 6, ringdoves also exhibited an inverse relationship between the. amount of anticipatory activity and the length of the interreinforcement interval (IRI). These results reveal a relation between ERI and contextual conditioning opposite from that obtained in studies of aversive conditioning.     

Effect of intertrial interval on variable-interval discrete-trial barpressing

In seven experiments, an effect of the intertriai interval (ITI) duration on barpressing by rats was studied. A stimulus signaled a 15-sec variable-interval trial. The first response after the interval elapsed turned the stimulus off and was rewarded with food. Trials were separated by long (about 300 sec) or short (about 10 sec) ITIs. A within-subjects design established that response rate on trials after long ITIs was lower than that after short ITIs (Experiments 1 and 3–7). The effect was not cumulative (the effect of one and five consecutive short ITIs was the same). Response rate after short and long ITIs was the same when a between-subjects design was used (Experiment 2). Response rate was higher after 160-sec ITIs than after 300-sec ITIs, suggesting that the ITI duration at which all longer ITIs are treated the same (i.e., the upper limit) is greater than 160 sec (Experiment 3). When food, the trial stimulus, a novel stimulus, or a familiar stimulus never paired with food, was presented 10 sec before the next trial during some of the long ITIs, response rate on the next trial was similar to that found after 10-sec ITIs (Experiments 4–6). This similarity suggested that these events could mark the start of the ITI. However, the familiar stimulus did so only when it reliably predicted that the next trial would occur after a short interval. The effect of ITI duration on responding was apparently attributable to response latency. Response latency was greater after long ITIs, but once responding began, it was similar after long and short ITIs (Experiment 7).