Figure 1.

Procedure used in Roper and Zentall (1999). Pigeons chose between two alternatives. Choice of one alternative (e.g., left) was followed by either a stimulus (e.g., red) 50% of the time that was always followed by reinforcement or a different stimulus (e.g., green) 50% of the time that was never followed by reinforcement. Choice of the other alternative (i.e., right) was followed by either of two stimuli (blue or yellow) both of which were followed by reinforcement 50% of the time. Spatial location and colors were counterbalanced.