Discriminative properties of briefly presented stimuli

In Experiment I, pigeons' responses produced food according to a fixed-interval schedule while responses on the key also produced brief stimuli according to a variable-interval schedule. Each brief stimulus reset the fixed interval. Thus, a brief stimulus occurred irregularly but a fixed minimum time separated the occurrence of food from a brief stimulus. Pauses followed brief stimuli and were followed by an accelerated response rate until another brief stimulus or food occurred. In Experiment II, four control procedures were examined. (1) Brief-stimulus presentations were omitted, producing a loss of response patterning. (2) A second-order schedule was studied with fixed-interval components. This schedule produced patterning following brief stimuli similar in kind and degree to that found in Experiment I. (3) A conjoint schedule was arranged in which food was no longer separated from the stimulus by a fixed time; pauses following the stimulus no longer resulted. (4) A brief food reinforcer replaced the brief visual stimulus, resulting in a constant response rate with no pausing following the brief food stimulus. The results suggest that the brief-stimulus effects were due to discriminative functions produced by the fixed temporal relation separating the stimulus from food.     

Discriminative properties of shock-correlated reinforcement in an operant context

A two-choice discrete operant procedure was devised for the study of shock-correlated reinforcement effects in rats. In the presence of one auditory stimulus, responding on one response lever was reinforced with food; with another auditory stimulus, responding on a second lever was reinforced. It was found that discrimination performance of one group, relative to appropriate control groups, was facilitated when electric shock was correlated with reinforcement on one lever and not on the other. Further, relative discrimination levels were found to be higher on the lever correlated with the shock than on the alternate lever. The significance of the results for operant within-S studies and for a mediational theory of shock-correlated reinforcement was discussed.     

Analysis of the pavlovian properties of signals for punishment

Rats learned to suppress lever pressing in the presence of a compound stimulus that signalled either response-independent shock (conditioned suppression) or response-dependent shock (punishment). Suppression to one element of this compound was blocked if the other element had previously signalled the same contingency as that holding in the presence of the compound. Two experiments, however, failed to find significant evidence of blocking if the pretrained element had signalled conditioned suppression and the compound signalled a punishment contingency. On the other hand, if one stimulus signalled punishment of one response it was able to block the acquisition of control by the second stimulus when the compound signalled punishment of a different response. Finally, although animals showed no suppression to one element of a compound signalling conditioned suppression if the other had previously signalled punishment, this apparent blocking effect concealed that what they learned was that the added element signalled the cancellation of the instrumental contingency. These results provide little support for the view that stimuli signalling punishment contingencies suppress responding in whole or in part by virtue of their Pavlovian relation to shock.     

Discriminative and reinforcing properties of two types of food pellets

In Experiment I some discriminative functions of food pellets were studied by developing a multiple schedule of reinforcement (mult FR 30 FI 3) in which the delivery of a standard laboratory food pellet as a reinforcer set the occasion for reinforcement on every 30th response (FR 30), and the delivery of a sucrose food pellet as a reinforcer set the occasion for reinforcement after a 3-min interval (FI 3). Discriminative stimulus control by the type of pellet was also demonstrated by reversing the operant discrimination and having the standard pellet control the FI 3 and the sucrose pellet control the FR 30. In Experiment II a mult FR 30 FR 30 with two bars was developed; a standard food pellet was followed by an FR 30 on Bar 1 and extinction (ext) on Bar 2, while a sucrose pellet was followed by an FR 30 on Bar 2 and ext on Bar 1. A control rat was placed, for comparison, on a mixed (mix) FR 30 FR 30 schedule with two bars, but neither bar correlated with the type of food pellet. In Experiments I and II the similarity between pellet controlled multiple schedules and multiple primed schedules was discussed, as was the comparability of transitions and effectiveness of control between pellet controlled multiple schedules and multiple schedules providing continuous exteroceptive stimuli.     

Interactions between the discriminative and aversive properties of punishment

Punishment acquires a discriminative property when it is selectively paired with either reinforcement or extinction. At the milder punishment intensities, the discriminative control exerted by punishment is similar to the discriminative control exerted by a response-produced neutral (nonaversive) stimulus. However, the effect of the aversive property is apparent as the intensity of the punishment is increased. The aversive property of the punishment acts to enhance the discriminative control when the punishment is selectively applied during extinction periods, and to attenuate the discriminative control when the punishment is selectively applied during reinforcement periods. One major difference was found between the control exerted by the punishment and the response-produced neutral simulus: Responding greatly increased after the S^Δ punishment but not after the S^Δ neutral stimulus; this increase in responding was independent of the punishment intensities studied.     

Discriminative properties of morphine that modulate associations between tastes and lithium chloride

Wistar rats learned to withhold consumption of a target solution when morphine preceded presentation of the target solution and lithium chloride (LiCl) and to consume the same target solution when saline preceded the presentation of the solution. After this serial feature discrimination training, morphine did not block the formation of a Pavlovian association between saccharin and LiCl but did suppress consumption of familiar tap water. After Pavlovian conditioning, morphine blocked the formation of an association between saccharin and LiCl but did not suppress consumption of a familiar tap water solution. The roles of morphine and saline can be interchanged. It appears that the morphine discriminative stimulus is calling up a representation of neither the conditioned stimulus nor the unconditional stimulus alone, but rather a modified representation of some aspect of their association.     

Discriminative stimulus properties of drugs: a brief history and a selective review

A brief history of the background of drug discrimination research is given, focusing on the concepts of "drug dissociation" or state-dependent learning; an alternative explanation to the state dependency concept is to view the drug as a discriminative stimulus. In drug discrimination learning both a qualitative and a quantitative dimension is identified as governing the discrimination, examples of which are provided in the text. Examples of how drug discrimination techniques may assist in future research are outlined.     

Fixed-ratio punishment

Responses were maintained by a variable-interval schedule of food reinforcement. At the same time, punishment was delivered following every nth response (fixed-ratio punishment). The introduction of fixed-ratio punishment produced an initial phase during which the emission of responses was positively accelerated between punishments. Eventually, the degree of positive acceleration was reduced and a uniform but reduced rate of responding emerged. Large changes in the over-all level of responding were produced by the intensity of punishment, the value of the punishment ratio, and the level of food deprivation. The uniformity of response rate between punishments was invariant in spite of these changes in over-all rate and contrary to some plausible a priori theoretical considerations. Fixed-ratio punishment also produced phenomena previously observed under continuous punishment: warm-up effect and a compensatory increase. This type of intermittent punishment produced less rapid and less complete suppression than did continuous punishment.     

Fixed-interval punishment

The average rate of bar-pressing maintained by a variable-interval schedule of milk reinforcement in 33 rats was found to be a decreasing function of intensity of concurrent punishment and, over a wide range of shock intensities, was inversely related to punishment frequency. Cumulative records were, however, negatively accelerated during 30-min punishment sessions with complete suppression occurring earlier and earlier (after fewer and fewer shocks) as intensity increased. In addition, acceleration was often observed between successive fixed-interval shock presentations and, at low and moderate intensities, bursts of responding occurred after each shock. The time to recover between punishment sessions (post-punishment recovery) was an increasing monotonic function of punishment intensity.     

Conditioned punishment

Responses of pigeons were maintained by a VI schedule of food reinforcement. Conditioned punishment was programmed by having these responses concurrently produce an originally neutral stimulus. The effectiveness of this response-contingent stimulus was maintained by infrequent and prearranged stimulus-shock pairings delivered independently of responses. This conditioned punishment procedure reduced the overall response rate as long as the procedure was in effect. The extent and durability of the reduction was a function of the intensity of the shock that was paired with the stimulus. Analysis of the reduction in the overall response rate revealed: (1) a reduction of responses occurring in the absence of the response-contingent stimulus, which was designated as a “punishing” effect, and (2) a reduction of responses during the response-contingent stimulus, which was designated as a “suppressive” effect.     

Multiple punishment schedule

Concurrent schedules of punishment and fixed-ratio reinforcement were studied in six white Carneaux pigeons. Two stimuli were alternated on the translucent key which served as the response mechanism. The pigeons were exposed to continuous punishment in one stimulus and first response punishment in the other. It was found that continuous punishment suppressed the fixed-ratio performance more than did punishment of the first response. Typically, a ratio run of responses was completed in spite of the onset of continuous punishment. Responding was then reduced for the remainder of the continuous punishment stimulus, except for those times when responding was initiated near the end of the stimulus. The results indicate that a multiple schedule can be used to compare different schedules of punishment in the same organism.     

Discriminative conditioning of prism adaptation

Two experiments were used to demonstrate that adaptation to ll-deg prism displacement can be conditioned to the stimuli associated with the goggles in which the prisms are housed. In Experiment 1 it was found that repeated alternation between a series of target-pointing responses while wearing prism goggles and a series of responses without prism goggles led to larger adaptive shift when S was tested with nondisplacing goggles than when tested without goggles. The results of Experiment 2 indicated that the adaptation revealed in the first experiment was primarily proprioceptive, rather than visual. Surprisingly, most Ss reported greater difficulty during the exposure period in overcoming the negative aftereffect than they did the prism-induced error.     

Summation of punishment suppression

In two experiments, eight rats were trained to lever press with food on a variable-interval schedule. Bar pressing produced shock on a variable-interval schedule in the presence of two independently presented stimuli, a light and a tone. Two rats in each experiment received alternative presentations of the light and the tone and were consequently always in the presence of a stimulus that signalled variable-interval punishment. The other two rats in each experiment were treated similarly except that they received periods in which neither light nor tone was present. During these periods, bar pressing was not punished. The two stimuli that signalled punishment were then presented simultaneously to evaluate the effect of stimulus compounding on response suppression. The subjects trained without punishment-free periods did not show summation to the compound stimulus; the subjects trained with punishment-free periods showed summation of suppression. The major difference between the two experiments was the longer mean interval of variable-interval punishment used in the second experiment. This manipulation made the summation effect more resistant to extinction and thus increased its magnitude.