Overall and local contrast in multiple schedules: Effects of stimulus similarity and discrimination performance

Abstract

An experiment with pigeons related overall and local behavioral contrast to similarity between stimuli signaling multiple-schedule components. Similarity was defined both physically and by discrimination performance. Initial and final baseline conditions used two equal random-interval schedules. During two intervening test periods, the schedule accompanying one component was changed to extinction. In the first test, components alternated strictly; in the second test, random component sequences were used. Signaling wavelength stimuli were separated by 1.5, 2, or 14 nm. Overall positive contrast occurred reliably, but its amount depended neither on wavelength difference nor on discrimination performance. Local positive contrast was less frequently observed when signaling stimuli were physically dissimilar; however, the effect was most closely related to actual discrimination performance. The relationship between discrimination and local contrast was nonmonotonic, indicating maximum local contrast at intermediate discriminations.Extensive research and theory have addressed behavioral contrast in multiple schedules (see review by Williams, 1983). In efforts to understand this phenomenon, researchers have distinguished between overall and local forms. Overall positive contrast is defined as an increase in response rate during a constant schedule component, along with a reduction in reinforcement rate during alternative components. Local contrast has been assessed in two ways, one that considers between-component sequences and one that considers within-eomponent patterning. Between-eomponent local contrast is positive if overall response rate during a given component is relatively high when the immediately preceding component had been associated with a relatively low reinforcement rate. Within-eomponent local contrast is positive when response rate is elevated during the initial portion of a relatively rich component, yielding a decreasing within-eomponent pattern. Negative overall and local contrast have similar forms, but in opposite directions.Although overall and local contrast are conceptually related, the two forms may not always coexist. For example, local contrast usually diminishes with exposure to unequal schedules (e.g., Nevin & Shettleworth, 1966), but overall contrast does not. In his review, Williams (1983) pointed to the transitory nature of local contrast as a major feature that distinguishes local from overall contrast. In this and subsequent papers (Williams, 1988; This research was supported by Grant BNS 80-25515 from the National Science Foundation. I thank StephenR. Bloughfor his help with the regressionanalysesand Donald S. Blough for his suggestions concerningthe manuscript. The author's address is Department of Psycho1-ogy, Brown University, Providence, RI 02912. Williams & Wixted, 1986), Williams reasoned that withincomponent local contrast accounts for overall contrast during early exposure to unequal schedules. However, as the local form dissipates, the overall form persists because of an...

Effects of Expectations for Different Reward Magnitudes on Neuronal Activity in Primate Striatum

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Absence of Anticipatory Contrast in Rats Trained on Multiple Schedules

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Incentive Relativity and the Specificity of Reward Expectations in Honey Bees

“…The magnitude of incentive contrast effects in these studies (i.e., the responses of subjects that experience a reward shift relative to the responses of subjects that experience the secondary reward continually, but are otherwise treated identically) reveals that reward expectations are under direct stimulus control. In simultaneous incentive contrast experiments, where two stimuli with different schedules of reinforcement are presented alternately, the behavioral contrast that results from the transition of reinforcement schedules is more pronounced, for example, when the reinforced stimuli share many common elements (Bloomfield, 1972;Blough, 1988;Bower, 1961;Chechile & Fowler, 1973). In addition, static contextual cues-the apparatus and other background cues-may contribute to contrast effects induced by a reduction of reward (Dachowski & Brazier, 1991;Daniel, Wood, Pellegrini, Norris, & Papini, 2008;Flaherty, 1982).…”