Two different definitions of behavioral contrast have been used for multiple schedules. One, interschedule, definition identifies contrast as changes in the rates of responding which occur when subjects move from one multiple schedule to another. The other, intraschedule, definition emphasizes changes in the rates of responding which occur relative to a baseline rate of responding. The baseline is the rate of responding emitted during a multiple schedule that supplies equal rates of reinforcement in the two components. The distinction between these two definitions is important for empirical and theoretical reasons. For example, theoretical confusion has arisen when the interschedule definition has been used to test and reject theories which implicitly define contrast by the intraschedule definition. min extinction (ext), mult VI 2-min VI 2-min, mult VI 2-min VI 30-sec, and mult VI 2-min VI 2-min. Positive contrast would occur if the rate of responding emitted during the constant, VI 2-min, component increased when subjects moved from the mult VI 2-min VI 2-min to the mult VI 2-min ext schedule, or from the mult VI 2-min VI 30-sec schedule to the final mult VI 2-min VI 2-min schedule. Both would be positive contrast because responding in the constant component increased when the rate of reinforcement obtained from the variable component decreased (either from VI 2-min to ext or from VI 30-sec to VI 2-min). Negative contrast would occur if the rate of responding emitted during the VI 2-min component decreased when subjects moved from the mult VI 2-min ext to the mult VI 2-min VI 2-min schedule or from the mult VI 2-min VI 2-min to the mult VI 2-min VI 30-sec schedule. Both decreases would be negative contrast because constant-component responding decreased when the variable rate of reinforcement increased (from ext to VI 2-min or from VI 2-min to VI 30-sec).The second, intraschedule definition (e.g., McSweeney, 1978), assesses positive and negative contrast by comparing the rates of responding emitted during component schedules which supply unequal rates of reinforcement to baseline rates of responding emitted when the components supply equal rates. The equal baseline rates of reinforcement are both iden-457 1979, 32,[457][458][459][460][461] NUMBER 3 (NOVEMBER)
Five rats pressed levers for food reinforces delivered by several concurrent variable-interval variable-interval schedules. The rate of reinforcement available for responding on one component schedule was held constant at 60 reinforcers per hour. The rate of reinforcement available for responding on the other schedule varied from 30 to 240 reinforcers per hour. The behavior of the rats resembled the behavior of pigeons pecking keys for food reinforcers. The ratio of the overall rates of responding emitted under, and the ratio of the time spent responding under, the two components of each concurrent schedule were approximately equal to the ratio of the overall rates of reinforcement obtained from the components. The overall rate of responding emitted under, and the time spent responding under, the variable component schedule varied directly with the overall rate of reinforcement from that schedule. The overall rate of responding emitted under, and the time spent responding under, the constant component schedule varied inversely with the overall rate of reinforcement obtained from the variable component. The local rates of responding emitted under, and the local rates of reinforcement obtained from, the two components did not differ consistently across subjects. But they were not exactly equal either.
A series of studies have reported that responding is faster when letter pairs to be matched are projected to two hemispheres rather than one. Four experiments described here tested this bilateral field advantage and identified factors that influence its extent. Subjects were shown letter pairs drawn from the ensemble "AaBb", and classified the letter pairs as "Match" if the letters had the same name (regardless of case) and "No Match" if they did not. In the first two experiments the letter pairs were presented unilaterally (both letters in one visual field), bilaterally (one letter in each visual field), or centrally (both letters on the vertical midline, above and below fixation), in order to investigate how the bilateral field advantage is influenced by screen location. The third experiment added a bilateral-diagonal position (to check for artefacts related to horizontal scanning strategies), and the fourth experiment added distractor digits (to equate initial processing demands in the bilateral and unilateral conditions). Results indicate that the bilateral field advantage is a robust phenomenon, although several manipulations reduced its magnitude. Implications of these findings for models of hemispheric collaboration and interhemispheric processing are discussed.
Twenty-five normal subjects made "same-different" responses to dot patterns presented in the LVF, RVF or bilaterally. Task difficulty was manipulated in each condition by varying the number of dots in the two patterns presented from two to four to six. The pairs of patterns always had the same number of dots on a given trial. Response latency and accuracy worsened as the number of dots increased for all three presentation conditions and for both "same" and "different" judgements. Overall, responding was faster and the number of errors lower on Bilateral presentations. For response latencies to identical patterns of dots, the size of the bilateral advantage increased relative to RVF responding as task difficulty increased but did not change significantly relative to LVF responding. When the two patterns were not identical the size of the advantage did not change as task difficulty increased. "Same" judgements were faster but less accurate than "different" judgements. A model of hemispheric interactions is proposed to account for the findings.
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