Figure 2.

Behavioral task. a, Experiment 1. The task began with central fixation after which the first stimulus appeared for 800 ms. After a 500 ms delay, a second stimulus appeared. When the second stimulus was identical to the first stimulus (AA trial), the monkey had to release the lever. When the two stimuli were different, the monkey had to continue pressing the lever and wait for another 500 ms until a third stimulus appeared. The third stimulus was always identical to the second one; thus, the monkey had to release the lever. When the second stimulus was different from the first one, it was selected either from a group other than that of the first stimulus (Easy condition) or from the same group (Difficult condition). b, Experiment 2. Instead of the Easy condition used in experiment 1, the Grouping condition was used. When the second stimulus belonged to the same group as the first stimulus, the monkey had to release the lever (AA′ trial). When the two stimuli belonged to different groups (ABB′ trial), the monkey had to continue pressing the lever and wait for another 500 ms until a third stimulus appeared. The third stimulus always came from the same group as the second stimulus; thus, the subject had to release the lever. The condition alternated between blocks composed of ∼100 trials.