The influence of 'switching' on the psychometric function in the free-operant psychophysical procedure

Fifteen rats were trained under the 'free-operant psychophysical procedure' , using a sucrose reinforcer. The training sessions consisted of 50-s trial s in which reinforcers were available on a variable-interval 30-s schedule; in the first 25 s of each trial, reinforcers were only available for respo nses on lever A, whereas in the last 25 s they were available only for resp onses on lever B. Data were collected in probe trials (4 per session) in wh ich no reinforcers were delivered, during the last 10 sessions of each phas e of the experiment. In phase 1 (70 sessions), repetitive switching between the levers was prevented by withdrawal of lever A after the first response on lever B in each trial. In phase 2 (40 sessions), this constraint on swi tching was removed. In phase 3 (40 sessions), it was reinstated. In all 3 p hases, the response rates on lever A declined and the response rates on lev er B increased as a function of time from the trial onset. The response rat e on lever B, expressed as a percentage of the overall response rate, confo rmed to a two-parameter logistic function. Removal of the constraint on swi tching did not alter the indifference point (the time corresponding to 50% responding on lever B), but did reduce the slope of the function, this bein g reflected in an increase in the Weber fraction. The changes were reversed when the constraint on switching was reinstated. The results show that con straining switching altered the slope of the psychometric function; thus ca ution is needed in interpreting psychometric functions obtained with the fr ee-operant psychophysical procedure in terms of theoretical models of timin g processes. (C) 1998 Elsevier Science B.V. All rights reserved.