Topography of spatially directed conditioned responding: effects of context and trial duration

Domesticated quail (Coturnix japonica) received a discrete conditioned stimulus (CS) at one end of the experimental chamber paired with the opportunity to copulate with a female quail (the unconditioned stimulus) in a goal box located 112 cm away. Approach to the CS (sign tracking) and approach to the goal area (goal tracking) were measured. The duration of exposure to the experimental context (C) was varied in Experiment 1, and the duration of the conditioning trials (T) was varied in Experiment 2 for independent groups, creating C/T ratios of 1.0, 1.5, 4.5, 45, and 180. Contrary to previous reports of a direct relation between the C/T ratio and conditioned responding, in the present experiments, a shift in the topography and stimulus control of conditioned behavior occurred. Low C/T ratios (1.0-4.5) produced goal tracking controlled by contextual cues, whereas high C/T ratios (45 and 180) produced sign tracking controlled by the discrete CS.     

The effects of aversive conditioned stimuli on the timing of unsignalled avoidance responding in rats

Rats received unsignalled avoidance training in a shuttlebox. After acquisition, classical conditioning sessions were administered. The Excitatory Group received signals paired with shock, the Inhibitory Group received signals explicitly unpaired with shock, while the Random Control Group received signals and shocks randomly in time. When subsequently superimposed on the avoidance baseline, the excitatory signal increased response rate, the inhibitory signal decreased rate and the random signal had no effect compared to signal-only control data. Unsignalled avoidance generates temporal gradients of responding; the major purpose of this study was to determine how these rate changes were reflected in the baseline temporal behavior. Very clear and simple summation rules emerged: In the presence of the excitatory signal, rats acted as if they were ahead of their actual location in the response-shock interval regardless of where the signal occurred; the inhibitory signal had the opposite effect while the random signal did not change the temporal behavior relative to signal-only control data. These results were interpreted within a motivational framework.     

Naloxone decreases responding for conditioned reinforcement in rats

That opioids can mediate unconditioned reinforcement is well established, but there is little evidence indicating whether they modify conditioned reinforcement. Here, a tone which initially served as a discriminative stimulus for the availability of water reinforcement was established as a conditioned stimulus. When later given a choice between pressing a lever producing the tone (but not water) or one which produced no effect, rats chose the tone-producing lever 66% of the time. Naloxone (3.0 mg/kg) reduced overall responding and completely eliminated the preference for the tone-producing lever. Results are briefly discussed in terms of the importance of understanding mechanisms serving conditioned reinforcement.     

Reduction of shock duration as negative reinforcement in free-operant avoidance

Rats were trained on a free-operant procedure in which shock duration was controlled by responses within a limited range of interresponse times. Shocks of 1.6-mA intensity occurred randomly with average density of 10 shocks per minute. As long as interresponse times were 15 seconds or less, any shocks received were at the briefer of two durations (.3 second). Whenever interresponse times exceeded 15 seconds, any shocks received were at the longer duration (1.0 second). For six of eight animals, avoidance responding developed quickly and reached levels of better than 90%. Four yoked animals stopped responding within the first few sessions. Shock duration reduction without change in shock probability or intensity was sufficient for the acquisition and maintenance of avoidance responding.     

Temporal control of conditioned responding in goldfish

The peak procedure was used to characterize response timing during acquisition and maintenance of conditioned responding in goldfish. Subjects received light-shock pairings with a 5- or 15-s interstimulus interval. On interspersed peak trials, the conditioned stimulus light was presented for 45 s and no shock was delivered. Peaks in the conditioned response, general activity, occurred at about the time of the expected unconditioned stimulus, and variability in the activity distribution was scalar. Modeling of the changes in the activity distributions over sessions revealed that the temporal features of the conditioned response changed very little during acquisition. The data suggest that times are learned early in training, and, contrary to I. P. Pavlov's (1927/1960) concept of "inhibition of delay," that timing is learning when to respond rather than learning when not to respond.     

The effect of rate of delivery of response-independent shocks upon avoidance responding

Two hooded rats were trained to bar-press to avoid electric shock on a continuous avoidance schedule with response-shock and shock-shock intervals equal. The rate of delivery of response-independent shocks superimposed on this schedule was varied. The response-independent shocks led to generally higher response rates but, with responses during shock omitted, the rates decreased as the response-independent shock rate was increased. The actual shock rate received by the subjects was linearly related to the maximum potential shock rate. There was an increasing, negatively accelerated function between percentage avoidance and response rate, but there was no consistent relation between the number of shocks avoided and response rate. Response rate decreased as the potential shock rate increased, but responding was maintained even when as few as 15% of the shocks could be avoided.     

Positive conditioned suppression: effects of CS duration

During a brief conditioned stimulus (15 or 30 sec) that terminated with the response-independent delivery of banana pellets, operant responding reinforced by other food pellets according to a variable-interval schedule of reinforcement was suppressed in the squirrel monkey. Conditioned stimuli of longer duration (1, 2, and 3 min) did not reliably affect the rate of operant performance. Brief conditioned stimuli generated homogeneous response patterns of nearly complete suppression. Increasing the CS duration did not enhance responding, as previously reported, but led to alternate bursting and pausing, which suggested a loss of control by the conditioned stimulus. The results suggest that the magnitude of "positive" or "negative" conditioned suppression reflects the strength of the classical conditioning process.     

Probability matching: encouraging optimal responding in humans.

Two hundred undergraduate students participated in a repeated-trials binary choice procedure in which choice of one outcome was correct on 75% of trials. Subjects received 192 trials and were divided into five conditions: (1) control; (2) subjects were given the actual probabilities; (3) subjects were told if they did well they could leave early; (4) competition condition; (5) midway through the task subjects were asked to recommend a strategy for another subject. Half of the subjects in each group were told that the best they could do was to be correct on 75% of the trials. This manipulation permitted assessment of the hypothesis that subjects in probability-matching tasks are seeking a strategy that will be correct on 100% of the trials. The results partially confirmed this hypothesis. In addition, two of the variables improved performance significantly (giving probabilities and asking subjects to recommend a strategy). However, while subjects in all groups improved significantly over trials, optimal choice did not occur in this task.     

Effect of adrenocorticotropic hormone on conditioned avoidance in rats interpreted as state-dependent learning.

Rats were given one training trial that was followed 2 days later by one test trial in a "step-out" passive avoidance task. Each rat was injected with either adrenocorticotropic hormone (ACTH) or placebo before training and before testing. Four groups of rats were used, representing the 4 possible training-testing injection combinations: placebo-placebo, placebo-ACTH, ACTH-placebo, and ACTH-ACTH. ACTH given in testing increased avoidance for subjects that had received ACTH in training and decreased avoidance for those that had received placebo in training.     

Experimental control of superstitious responding in humans

Superstitions were demonstrated with human subjects when presses on one button were reinforced on a VI 30-sec schedule while presses on a second were never reinforced. Superstitious responding, on the second button, was often maintained because presses on that button were frequently followed by reinforcement for a subsequent press on the first button. The introduction of a changeover delay (COD), which separated in time presses on the second button and subsequent reinforced presses on the first button, reduced or eliminated the superstitious responding of these subjects. Some complex superstitions were also demonstrated with other subjects for which the COD was in effect from the beginning of the session.