No abstract

Blocking was investigated in a free-operant procedure by presenting a response-contingent signal prior to reinforcer delivery. At issue was the way in which blocking effects previously reported with this procedure are related to conditioned reinforcement effects. also previously found with similar procedures. Signal presentation decreased response rate when delay of reinforcement was 0 or .5 sec, but the signal increased response rate when the delay of reinforcement was increased to 3 sec. Thus, which effect (blocking or conditioned reinforcement) occurred depended critically on the response-reinforcer interval.

We find that additivity of trichromatic color matches holds, even in parafoveal vision, for most observers if rods are prevented from seeing the stimuli. Some heterozygous women are exceptions. The failure of the additivity law for these women implies that their eyes contain more than three types of cone with different spectral sensitivities.

Two experiments are reported that challenge the interpretation of previous results with the signal-key procedure, in which the discriminative stimuli are located on a response key different from the key associated with the operant response requirement. Experiment 1 replicated the procedure of Keller (1974), and found that contrast effects on the operant key occurred reliably for only one of four subjects. High rates to the signal key initially occurred for only one subject, but modifications of the procedure produced substantial rates to the signal key for all subjects. In all cases, however, signal-key behavior was greatly reduced by the addition of a changeover delay which prevented reinforcement within 2 seconds of the last peck to the signal key, suggesting that signal-key pecking was maintained primarily by adventitious reinforcement. Experiment 2 modified the signal-key procedure by using three response keys, so that the discriminative stimuli on the signal key controlled different responses during all phases of training. With this modification, reliable contrast effects on the operant key occurred for all subjects, suggesting that the failure to find contrast in previous studies has been due to the confounding of changes in the discrimination requirements with changes in relative rate of reinforcement. The results challenge the additivity theory of contrast, and suggest that "elicited" behavior plays a minor role, if any, in the determination of contrast effects in multiple schedules.Key words: signal-key procedure, additivity theory, multiple schedules, behavioral contrast, key peck, pigeons A major influence on theories of behavioral contrast was the development of the "signalkey" procedure by Keller (1974). With this procedure the response requirements for both components of a multiple schedule are associated with one response key that has a constant stimulus, whereas the discriminative stimuli for the two components are located on a second response key, which has no response requirement. Its rationale was derived from Catania's (1971) notion of "topographical tagging", on the assumption that behavior controlled by the response contingency will occur on the "operant" key, whereas that elicited by the discriminative properties of the stimulus will occur to the signal key.Two aspects of the results with the signal-key procedure have been of major interest. First, contrast effects generally do not occur when "operant pecks" are considered in isolation. Second, the change from nondifferential reinforcement to differential reinforcement (e.g., Requests for reprints should be sent to Ben A. Williams,