Fig. 2.

Predictive stimuli can bias preference when overall reinforcement is equivalent; by decreasing the probability in obtaining the predictive stimulus preference switches towards the alternative. Mean (±SEM) percent choice of the predictive alternative (lever n = 6; light n = 6) when the probability of obtaining the predictive stimulus was (A) 50%, (B) 25%, and (C) 12.5%. (D) Mean (±SEM) percent choice of the predictive alternative during the last four sessions of each training for each phase as a function of decreasing predictive to non-predictive alternative expected values. Lines are best fits from the matching equation for each stimulus condition.