Observing behavior and atypically restricted stimulus control

Restricted stimulus control refers to discrimination learning with atypical limitations in the range of controlling stimuli or stimulus features. In the study reported here, 4 normally capable individuals and 10 individuals with intellectual disabilities (ID) performed two-sample delayed matching to sample. Sample-stimulus observing was recorded with an eye-tracking apparatus. High accuracy scores indicated stimulus control by both sample stimuli for the 4 nondisabled participants and 4 participants with ID, and eye tracking data showed reliable observing of all stimuli. Intermediate accuracy scores indicated restricted stimulus control for the remaining 6 participants. Their eye-tracking data showed that errors were related to failures to observe sample stimuli and relatively brief observing durations. Five of these participants were then given interventions designed to improve observing behavior. For 4 participants, the interventions resulted initially in elimination of observing failures, increased observing durations, and increased accuracy. For 2 of these participants, contingencies sufficient to maintain adequate observing were not always sufficient to maintain high accuracy; subsequent procedure modifications restored it, however. For the 5th participant, initial improvements in observing were not accompanied by improved accuracy, an apparent instance of observing without attending; accuracy improved only after an additional intervention that imposed contingencies on observing behavior. Thus, interventions that control observing behavior seem necessary but may not always be sufficient for the remediation of restricted stimulus control.     

Discriminability of frequency of food or stimulus presentations in variable-time schedules

Pigeons responded in a two-alternative forced-choice task in which reinforcement was dependent upon the frequency of events that occurred in an immediately preceding schedule sample. On a given trial the events were either brief food presentations or brief visual and auditory stimulus changes. High levels of stimulus control were obtained by food-presentation schedules only. Discriminative control by frequency or stimulus change was absent. Stimulus control by food frequency was decreased by the imposition of a delay period between the schedule sample and the choice. Moreover, stimulus control by food frequency was related to the ratio of food-presentation schedule pairs when novel schedules were presented in a transfer test.     

Changes in threshold level produced by a signal preceding or following the threshold stimulus

In a previous experiment (Howarth and Treisman, 1958) it was shown that when a warning preceded a stimulus by a fixed interval, the threshold level for the stimulus was higher the longer the fixed interval. A model of this effect was proposed, assuming that the threshold criterion adopted by the subject at any moment was modified by his estimate of the probability of a stimulus at that moment.

Three predictions are derived and tested. It is found that: the standard deviation of the response is constant despite the shifts in threshold induced; the threshold continues to fall as the interval from warning to stimulus is decreased to zero, with the lowest threshold when the warning signal and the stimulus are simultaneous; and that a warning signal following the stimulus by an interval of less than 0 5 to 1 sec. also lowers the threshold.     

Perceived stimulus complexity and food preference development

The importance of perceived complexity, a "collative property" as defined by [Berlyne, D. E. (1967). Arousal and reinforcement. In Nebraska symposium on motivation (pp. 1-110). University of Nebraska Press], to the dynamic development of preference was investigated. Eighty-six female and 82 male subjects rated their liking for and various collative properties of seven very similar orange drinks that differed only in perceived complexity as a result of adding small quantities of other flavours. This was done before and after giving each subject extended experience of one of the drinks, each being used equally often for this purpose. As predicted by the theory of [Dember, W. N., & Earl, R. W. (1957). Analysis of exploratory, manipulatory and curiosity behavior. Psychological Review, 64 (2), 91-96] exposure to a stimulus with a slightly higher complexity than an individual subject's optimally preferred level of perceived complexity, caused an upwards shift in that level, whereas exposure to a less complex stimulus had no such effect. Changes in the appreciation of the drinks predicted by the theory were also observed.     

Stimulus dimensions or stimulus aspects

Summary The questionableness of geometric models of stimulus similarity has led to the development of an alternative approach by Tversky which makes no dimensional or metric assumptions. Rather, stimuli are described as sets of qualitative stimulus aspects and stimulus similarity as a function of common and non-common aspects. According to Restle's model of stimulus similarity, however, the perception of stimuli of a categorial nature can be organized along dimensions because stimulus aspects form dimensions under certain conditions. The present study supports the empirical validity of this assumption. Further it is suggested that contrary to the present opinion quantitative stimulus characteristics are probably not perceived as dimensions in the sense of the geometric models but only dimensions as described in the Restle model.     

The non-occurrence of a stimulus as a signal

Two groups of rats were rewarded for pressing a panel following a varying number of bar presses; a signal following the bar press indicated whether or not a panel press would be rewarded if made before the next bar press. For one group the signal indicating reward was a flash of light, for the other it was the non-occurrence of the flash (in the latter case the flash thus signalled non-reward). The first group learned to withold panel presses except when reward was signalled, but the second group did not. This result is related to the “feature positive effect” of Jenkins and Sainsbury (1970), in which pigeons failed to withold pecks at a negative stimulus display when it was the same as the positive display except for the addition of a distinctive feature.     

Intertrial sources of stimulus control and delayed matching-to-sample performance in humans

Two experiments compared delayed matching-to-sample (DMTS) accuracy under 2 procedures in adults with mental retardation. In the trial-unique procedure, every trial in a session contained different stimuli. Thus, comparison stimuli that were correct on one trial were never incorrect on other trials in that session (or vice versa). In the 2-sample DMTS procedure, the same 2 comparison stimuli were presented on each trial, and their function changed quasi-randomly across trials conditional upon the sample stimulus. Across 2 experiments, 7 of 8 subjects showed the highest overall accuracy under the trial-unique procedure, and no subject showed consistently higher accuracy under the 2-sample procedure. Negative, exponential decay functions fit to logit p values showed that this difference was due largely to the steeper delay-mediated decline in sample control for the 2-sample procedure. Stimulus-control analyses indicated that, under the 2-sample procedure, the selection of the comparison stimulus on Trial N was often controlled by the comparison stimulus selection on Trial N-1 rather than the Trial-N sample stimulus. This source of competing stimulus control is not present in trial-unique procedures. Experiment 2 manipulated intertrial interval duration. There was a small but consistent increase in accuracy as a function of intertrial interval duration under the 2-sample procedure, but not under the trial-unique procedure.     

Acquisition and maintenance of autoshaped key pecking as a function of food stimulus and key stimulus similarity.

The results of a number of recent studies suggest that acquisitions of autoshaped key pecking in pigeons is affected by the similarity of the grain-hopper stimulus and response-key stimulus. In Experiment 1 this hypothesis was tested by training independent groups of pigeons to key peck under six different hopper-stimulus and key-stimulus similarity conditions, and three procedures containing either immediate reinforcement, variable delay of reinforcement, or omission of reinforcement for key pecking. Number of trials to acquisition was found to be related to the similarity variable. Maintained responding was affected by the response-reinforcer contingency. This effect was found both within and between subjects. Under two of the contingencies (automaintenance and omission), maintained responding continued to be affected by the similarity of the hopper stimulus and key stimulus. In Experiment 2 pigeons were given omission training with a hopper light on or off. Both acquisition and maintenance of key pecking were facilitated by the presence of the hopper light. The present findings suggest that much of the responding reported in various automatic shaping and training procedures may reflect the effects of key stimulus/food stimulus similarity.     

Distractors that signal reward attract the eyes

Salient stimuli and stimuli associated with reward have the ability to attract both attention and the eyes. The current study exploited the effects of reward on the well-known global effect in which two objects appear simultaneously in close spatial proximity. Participants always made saccades to a predefined target, while the colour of a nearby distractor signalled the reward available (high/low) for that trial. Unlike previous reward studies, in the current study these distractors never served as targets. We show that participants made fast saccades towards the target. However, saccades landed significantly closer to the high compared to the low reward signalling distractor. This reward effect was already present in the first block and remained stable throughout the experiment. Instead of landing exactly in between the two stimuli (i.e., the classic global effect), the fastest eye movements landed closer towards the reward signalling distractor. Results of a control experiment, in which no distractor-reward contingencies were present, confirmed that the observed effects were driven by reward and not by physical salience. Furthermore, there were trial-by-trial reward priming effects in which saccades landed significantly closer to the high instead of the low reward signalling distractor when the same distractor was presented on two consecutive trials. Together the results imply that a reward signalling stimulus that was never part of the task set has an automatic effect on the oculomotor system.