Discriminative stimulus control and the effects of concurrent operants.

Discriminative-stimulus-control functions were investigated in a concurrent operant context. Variable-interval reinforcement schedules were arranged for pigeons on two response keys. One key, illuminated with a white vertical line on green background (position irrelevant), was programmed with a given schedule value across groups. For different groups of pigeons, the alternative key, illuminated with green alone, was programmed with twice, the same, or half the reinforcement frequency of the other key. Stimulus-control gradients were collected from both keys as line orientation was varied. On the green-plus-line alternative, flattest gradients were observed when twice the reinforcement frequency was concurrently programmed and the steepest were observed when equal values were concurrently programmed. Also examined were the effects of a programmed changeover delay, important in maintaining the independence of concurrent operants. The changeover delay was found to have relatively minor effects upon stimulus control, despite its typical and marked effects upon steady-state responding.     

Comparison of stimulus control and reinforcement control effects on imitative behavior

An attempt was made to evaluate reinforcement and stimulus control of imitative and non-imitative behavior. The imitative response required the subject to duplicate the experimenter's response by matching blocks that varied in color. The factor designed to evaluate stimulus control was fading, a procedure that systematically varies the differences between the imitative and non-imitative stimuli. The topography and duration of the non-imitative stimuli were faded in. The factors designed to evaluate reinforcement control were differential reinforcement of non-imitative behavior and time out from positive reinforcement. The results showed stimulus control of non-imitation to be more important than reinforcement control, and that reinforcing events were not sufficient to control non-imitation; while the arrangement of stimulus events was sufficient to control non-imitation. These results were related to studies providing evidence for the processes of discrimination and generalization.     

Evidence of interaction between deprivation effects and stimulus control

Discriminative responding in pigeons was studied under multiple variable-interval extinction and variable-ratio extinction schedules, as deprivation was varied. Generally, the greater the accuracy of discrimination that developed during training, the smaller the effect of deprivation upon subsequent performance. This was true both in terms of changes in response rates, and in the relationship between response rates during food reinforcement and extinction. When discrimination was inaccurate, increases in deprivation resulted in disproportionate increases in responding during extinction, as compared to increases during food reinforcement components of the schedule. The results suggest that as stimulus control (accuracy) of responding increases, discriminative performance becomes less and less susceptible to influence by deprivation.     

Reinforcer frequency and restricted stimulus control.

Stimulus control was evaluated in 3 individuals with moderate to severe mental retardation by delayed identity matching-to-sample procedures that presented either one or two discrete forms as sample stimuli on each trial. On pretests, accuracy scores on one-sample trials were uniformly high. On two-sample trials, the correct stimulus (i.e., the one that subsequently appeared in the comparison array) varied unpredictably, and accuracy scores were substantially lower, suggesting that both sample stimuli did not exert stimulus control on every trial. Subjects were then given training sessions with the one-sample task and with a new set of four stimuli. For two of the stimuli, correct matching responses were followed by reinforcers on a variable-ratio schedule that led to a high reinforcer rate. For the other two stimuli, correct responses were followed by reinforcers on a variable-ratio schedule that led to a substantially lower reinforcer rate. Results on two-sample tests that followed showed that (a) on trials in which comparison arrays consisted of one high reinforcer-rate and one low reinforcer-rate stimulus, subjects most often selected the high-rate stimulus; and (b) on trials in which the comparison arrays were either two high reinforcer-rate stimuli or two low reinforcer-rate stimuli and the samples were one high reinforcer- and one low reinforcer-rate stimulus, accuracy was higher on trials with the high-rate comparisons. These results indicate that the frequency of stimulus control by high reinforcer-rate samples was greater than that by low reinforcer-rate samples. Following more training with the one-sample task and reversed reinforcement schedules for all stimuli, the differences in stimulus control frequencies on two-sample tests also reversed. These results demonstrate experimental control by reinforcement contingencies of which of two sample stimuli controlled selections in the two-sample task. The procedures and results may prove to be relevant for understanding restricted stimulus control and stimulus overselectivity.     

The effects of chlorpromazine and imipramine on rate and stimulus control of matching to sample.

Pigeons were trained to perform simultaneous, two-color matching to sample under a multiple fixed-ratio fixed-interval schedule of food presentation. The sequence terminating with a peck on the matching key (a "match") was treated as a unit, analogous to a single key peck in conventional schedules. Except for intermittent reinforcement of matches, no consequent stimulus distinguished matches from mismatches (sequences terminating with pecks on the nonmatching key). The pattern of matches during nondrug sessions resembled that of simpler operants maintained by similar schedules. Matches increased in rate toward the end of both components; mismatch rates increased more slowly. Imipramine increased the rate of mismatches, disrupted schedule patterning, and lowered accuracy in a dose-dependent fashion. Chlorpromazine lowered the overall rate of matches but affected schedule patterns and accuracy less than imipramine. The types of errors during drug sessions were not systematically related to the types of errors that appeared during nondrug sessions. Stimulus control was evaluated for each of the four possible color configurations and was found to be by the entire configuration of colors, not simply by the color of the sample.     

Behavioral control by an imprinted stimulus: long-term effects

Newly hatched ducklings were exposed to imprinting procedures and subsequently trained to peck a key by presenting the imprinting stimulus as the reinforcing (response-contingent) event. Individual ducklings then lived in the apparatus under an arrangement in which each peck produced a 15-sec stimulus presentation. For all ducklings, key-pecks tended to occur in bursts, and as the duckling matured, burst length decreased and the interval between bursts increased. However, even when subjects were 60 days old, some responses still occurred.     

Repeated measurements of reinforcement effects on gradients of stimulus control

Two experiments studied the effects of reinforcement schedules on generalization gradients. In Exp. 1, after pigeons' responding to a vertical line was reinforced, the pigeons were tested with 10 lines differing in orientation. Reconditioning and the redetermination of generalization gradients were repeated from 8 to 11 times with the schedule of reinforcement varied in the reconditioning phase. Stable gradients could not be observed because the successive reconditionings and tests steepened the gradients and reduced responding. Experiment 2 over-came these effects by first training the birds to respond to all of the stimuli. Then, brief periods of reinforced responding to the stimulus correlated with reinforcement alternated with the presentation of the 10 lines in extinction. The development of stimulus control was studied eight times with each bird, twice with each of four schedules of reinforcement. Gradients were similar each time a schedule was imposed; the degree of control by the stimulus correlated with reinforcement varied with particular schedules. Behavioral contrast occurred when periods of reinforcement and extinction alternated and was more durable with fixed-interval, variable-interval, and variable-ratio schedules than with fixed-ratio or differential-reinforcement-of-low-rate schedules.     

Fixed-interval stimulus control

Chlorpromazine was studied for its effects on responding under a second-order schedule in which food was presented following a sequence of 20 one-minute fixed-interval components. A brief visual stimulus occurred at the completion of each fixed interval including the one that terminated with food presentation. Chlorpromazine showed rate-dependent effects in that it increased low rates in the early components of the second-order schedule and, to a lesser extent, decreased high rates in the later components. Chlorpromazine also increased rates in the early quarters within the 1-min fixed-internal components and to a smaller extent decreased rates in the final quarter. The alteration in the patterns of responding within 1-min fixed-interval components terminating in a brief stimulus presentation was substantially less than that which occurred throughout the succession of 1-min fixed-interval components terminating in food presentation, thus suggesting that the presentation of the brief stimulus exerted more control over responding within components than did food presentation over the sequence of components. This result and others suggest that studies using drugs may be useful in elucidating the factors controlling patterns of responding in second-order schedules.     

Subtyping eating disordered patients along drive for thinness and depression

Subtyping individuals who binge eat by “diet-DT” and “depression” has yielded two valid and clinically useful subtypes that predict eating severity, comorbid psychopathology and outcome. The present study aimed to find four subtypes based on these dimensions and test their validity. Besides, it explored the distribution of eating disorder (ED) diagnoses across subtypes given their known heterogeneity, crossover and binge-eating fluctuation.Cluster analysis grouped 1005 consecutively admitted ED adult women into four subtypes, those previously described “DT” (22%), “DT-depressive” (29%), and “mild DT” (25%) and “depressive-moderate DT” (24%). Overall “mild DT” presented lower and “DT-depressive” greater eating and comorbid psychopathology than the rest, whereas “pure DT” and “depressive-moderate DT” presented no differences on bulimic symptoms but in psychopathology (p < .01). Finally, while BN-P patients were mostly and similarly distributed in the “DT” and “DT-depressive” subtypes than in the other, AN were in the new “mild DT” and “depressive-moderate DT” (p < .01). However, BN-NP, BED and EDNOS were similarly represented across subtypes.Results are discussed with regard to 1) the newly emerged subtypes that may explain cases in which DT prevents or does not predict binge eating; 2) the confluence of DT-depression that signaled greater eating and comorbid pathology, particularly self-control problems; 3) ED-DSM-diagnostic criteria.     

Probability effects on stimulus evaluation and response processes.

This study investigated the effects of probability information on response preparation and stimulus evaluation. Eight subjects responded with one hand to the target letter H and with the other to the target letter S. The target letter was surrounded by noise letters that were either the same as or different from the target letter. In 2 conditions, the targets were preceded by a warning stimulus unrelated to the target letter. In 2 other conditions, a warning letter predicted that the same letter or the opposite letter would appear as the imperative stimulus with .80 probability. Correct reaction times were faster and error rates were lower when imperative stimuli confirmed the predictions of the warning stimulus. Probability information affected (a) the preparation of motor responses during the foreperiod, (b) the development of expectancies for a particular target letter, and (c) a process sensitive to the identities of letter stimuli but not to their locations.     

Contextual control of stimulus generalization and stimulus equivalence in hierarchical categorization

The purpose of this study was to determine whether hierarchical categorization would result from a combination of contextually controlled conditional discrimination training, stimulus generalization, and stimulus equivalence. First, differential selection responses to a specific stimulus feature were brought under contextual control. This contextual control was hierarchical in that stimuli at the top of the hierarchy all evoked one response, whereas those at the bottom each evoked different responses. The evocative functions of these stimuli generalized in predictable ways along a dimension of physical similarity. Then, these functions were indirectly acquired by a set of nonsense syllables that were related via transitivity relations to the originally trained stimuli. These nonsense syllables effectively served as names for the different stimulus classes within each level of the hierarchy.