Figure 1. Visual category stimuli, task design, and sample trial sequence.

A. Visual features were used as either category-relevant or category-irrelevant features. Category-relevant features defined each category, whereas category-irrelevant features appeared in both sets of exemplars (12 for each category). Across category-feature density conditions (dense or sparse), 3 or 1 category-relevant feature(s) appeared with 2 or 4 category-irrelevant features, respectively. Occurrences of the category-irrelevant features were controlled and counterbalanced. B. Each trial began with the white star in the center of the screen (Trial start). After a single touch, a category cue stimulus appeared on the screen and the rat had to touch it three times (cueing phase) to initiate the choice phase. Touch coordinates on the cue category stimuli were recorded for later analyses. During the choice phase, the category cue stimulus appeared on both left and right sides of the screen and the rat was required to touch one of them for food reward depending on which category cue was given (e.g., left for category A and right for category B, counterbalanced across the rats). A reward was delivered to the food tray on correct trials and a penalty ITI (10 ~ 15 s) was given on error trials (Reward/Penalty phase). Before the rat reached the learning criterion for testing, corrections were given for every error trial in which the trial repeated from the cueing phase until the rat selected the correct choice. After a 3 s ITI, the next trial started.