Figure 1. The sequence of the stimuli in the conditional discrimination task.

Two kinds of trials were used in the task: paired trials, and CS-alone trials. In a paired trial, a chamber light stimulus is delivered before the CS. The duration of the interval between the end of the light stimulus and the CS onset is 3 s to 5 s, averaging 4 s. In a CS-alone trial, the CS alone is delivered without reinforcement.