Figure 2.

Procedure used in Gipson et al. (2009). Pigeons chose between two alternatives. Choice of one alternative (e.g., left) was followed by either a stimulus (e.g., red) that was always followed by reinforcement on half of the trials or a different stimulus (e.g., green) that was never followed by reinforcement on the remaining trials. 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 75% of the time. Spatial location and colors were counterbalanced.