Fig 6.

Psychometric functions obtained with the Free Operant Psychophysical Procedure—pecks to one key, say L, are reinforced (according to one or more VIs) only during the first half of the trial, and pecks to the other key, R, are reinforced (also according to one or more VIs) only during the second half of the trial. In the top panels, when the overall reinforcement rate favored the L key (first one of the two VI schedules), the psychometric function shifted to the right; when it favored the R key (last one of the two VI schedules), it shifted to the left. The middle panels show that when the overall reinforcement rates differ, the psychometric function shifts only if the local reinforcement rates differ around the middle of the trial; the bottom right panels show that when the overall reinforcement rates are equal, the functions shift provided the local reinforcement rates differ in the middle of the trial. The data are from Bizo & White (1995a) (top left panel) and Machado & Guilhardi (2000) (remaining panels). The curves show the fit of the LeT model.