A RE‐EXAMINATION OF LOCAL CONTRAST IN MULTIPLE SCHEDULES 1

Self Cite

Pigeons were trained on multiple schedules with component stimuli of different degrees of similarity. In Experiment 1, a two-component schedule was used in which the two stimuli were either two line orientations or a line orientation versus a diffuse color. Reinforcement rate was varied in one component to determine the effects of stimulus similarity on different aspects of behavioral contrast. Contrast in terms of average response rates (molar contrast) was larger with less similar stimuli. Local contrast effects at the beginning of the component were larger for more similar stimuli, but these effects were more variable and did not attain statistical significance. Independent of the level of molar contrast, the local pattern of schedule interaction differed for the two levels of similarity: with more similar stimuli, the maximum degree of interaction occurred at the beginning of the components and then decreased; with less similar stimuli, the degree of interaction increased throughout the components and was at its maximum near their end. In Experiment 2, the same three stimuli were used while reinforcement rate in the middle component of a three-component sequence was varied; this isolated the effects of the preceding schedule from those of the following schedule. Contrast effects were generally greater in the target component preceding the variable schedule, and these were enhanced by less similar stimuli. Contrast in the target component following the variable schedule was manifested primarily in terms of the behavior at the beginning of the component, and these effects were inconsistently related to stimulus similarity. The functional separation of the effects of stimulus similarity on the different locations of contrast suggest that "anticipatory contrast" and "local contrast" depend upon different mechanisms, thus excluding any account of contrast solely in terms of relative rate of reinforcement.An important distinction in the study of contrast effects in free-operant schedules is that between local versus molar contrast. Molar contrast refers to the average change in response rate as a function of the change in reinforcement rate in the alternative component(s); local contrast refers to the larger than average rate changes in the early portions of the target component (immediately following the transition from the alternative schedule).

Section: Resultsmentioning

confidence: 99%

The effects of stimulus similarity on different types of behavioral contrast

Williams

1988

Self Cite

“…Figure 1 shows that the size and pattern of the contrast effect across the constant component changed systematically with continueddiscriminationtraining, so that the maximum contrast effect occurred toward the end of the component prior to the change in reinforcement rate at the point of component transition. Williams (1974) and Buck, Rothstein,and Williams (1975) referred to this finding as the followingschedule effect, but subsequently the terminology was changed to anticipatory contrast in order to emphasize its similarity to anticipatory contrast in consummatory licking procedures (see Flaherty, 1996). The issues posed are what role does anticipatory contrast play in behavioral contrast generally and what are its determinants?…”

Section: Separable Components Of Behavioral Contrast Local Versus Molmentioning

confidence: 99%

Behavioral contrast redux

Williams

2002

Self Cite

Contrastis a ubiquitousconceptin experimental psychology. Most well known are contrast effects with respect to sensory systems, exemplified by color and brightness contrast. In other areas of psychology, the concept of adaptation level has wide applicability. In all of these domains, the effect of a given stimulus value seemingly involves a comparison process such that the psychological impact of a stimulus is due not solely to its absolute value, but also to its value relative to that of other stimuli presented either in the same context or in the past. Although the empirical phenomenon of contrast is pervasive, whether it is regarded as a basic psychological process or, instead, is to be explained in terms of more fundamental mechanisms varies widely across different areas of psychology. REINFORCEMENT CONTRAST:A BRIEF HISTORYThe concept of contrast has played a major role in reinforcement theory, both in generating voluminous research and in posing theoretical challenges. There are two major classes of contrast, with relatively separate experimental literatures. Successivecontrast is said to occur when the level of behavior maintained by a given reward outcome varies with the nature of the reward outcome previouslypresented. The most well known example was reported by Crespi (1942). The running speeds in a straight alley of rats maintained on a constant level of reward amount were compared with those of other rats for which the same amount of reward was presented either after experience with a smaller reward or after experience with a larger reward. Subjects shifted from a smaller amount ran faster than subjects with the constant amount throughout, which Crespi labeled an elation effect, whereas rats shifted from a larger amount ran slower than those with the constant amount throughout, which was labeled a depression effect. As a result of these and similar findings, Hullian learning theory modified the concept of incentive(the K parameter) to become a performance variable, which ultimately made it difficult to distinguish the Hullian account from other competing accounts explicitly cognitive in nature. Successive contrast effects have continued to be investigated widely even with the decline in influence of Hullian learning theory, often in the context of physiological manipulations to assess their underlying basis. Because Flaherty (1996) has provided a comprehensive review of this literature, no further account will be provided here.The second type of contrast procedure is when the behavior produced by a constant reward payoff is affected by the reward conditionsthat are alternately presented for the same response under different stimulus conditions. This procedure is sometimes labeled simultaneouscontrast, a misnomer because the different reward conditions are not simultaneously present. However, the different conditions do occur in close temporal proximity, whereas in the successive contrast procedure the different reward conditions are often experienced with significant temporal separations. Although sim...

“…Overall contrast, according to Williams's analysis, depends on the value of the following component, which the subject learns to anticipate. This explanation rests on evidence that the direction of steadystate contrast depends on the schedule during the following, rather than the preceding, component; for example, rates in a constant component are higher when the following schedule is lean than when it is rich (Buck et al, 1975;Williams, 1979Williams, , 1981Williams, , 1982Williams & Wixted, 1986). Since following-schedule control is acquired with training, it appears and persists as local contrast diminishes.…”

Section: Discussionmentioning

confidence: 99%

“…However, as the local form dissipates, the overall form persists because of an anticipatory effect that emerges as exposure continues. This effect reflects control by the schedule during the following component: Within-eomponent rates follow a rising pattern when the following schedule is to be relatively lean; they drop when the following schedule is to be relatively rich (see also Buck, Rothstein, & Williams, 1975). Applied to positive contrast, then, this analysis would attribute overall contrast initially to local response rate elevations at rich component onsets.…”

mentioning

confidence: 98%

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

Blough

1988

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...