Figure 1.

Classification task. (A) Subjects classified stimuli one by one without feedback or prior information regarding class structure. A blank gray screen preceded each trial, initiated by key-press. After 1 s, a small fixation cross appeared, followed (200 ms later) by a stimulus. At 500 ms after stimulus onset, the screen was blanked again. Subjects had 1.5 s from stimulus onset to relate it to a class. Late responses were discarded and signaled to the subjects. (B) Examples of stimuli used for classification. (C) Statistical distributions of the stimuli, reflecting the frequency of their presentation. Each subject participated in one of the three possible distribution conditions during all of the sessions.