Fig. 1.

Stimuli used in the final phase of the size discrimination (top). Labels on the circles indicate actual diameters, in pixels, used in the experiment, but were not shown to the monkeys. At each level of difficulty (bottom), there were two absolute distracter sizes. The easiest distracters (i.e., difficulty level 1) were 24 pixels larger or smaller in diameter than the target, and the hardest (i.e., difficulty level 5) identical to the target