The stimuli were back projected on a screen using a video projector (NEC WT610, 1024 × 768 pixel resolution, 85 Hz) and custom-made software running on an Apple G4 Power PC. The animals viewed the screen at a distance of 57 cm (1 cm = 1° of visual angle). The RDPs were generated by plotting colored dots (white, 13 cd/m2; gray, 1.9 cd/m2; pink, 5.4 cd/m2; green, 0.9 cd/m2; blue, 1.58 cd/m2; red, 0.6 cd/m2; turquoise, 8 cd/m2) on a dark background (black-gray, 0.02 cd/m2) with a density of three dots per degree2 within

a circular stationary virtual aperture. All dots within one RDP moved coherently at a speed of 15°/s and were replotted at the opposite side when Screening Library clinical trial they crossed the border of the aperture. The radius of the aperture was 4°, and its center was 8° away from the fixation spot. The animals started a trial by pressing a button and keeping gaze within a circular window of 2° diameter centered on a small fixation spot (0.06 degree2). After 353 ms, two moving RDPs appeared, one located to the left and the other to the right of the spot. The patterns were composed of white dots on a dark background that moved either up or down relative to the vertical. After a variable interval, from 294

to 646 ms following the RDPs’ onset, the dots in each pattern changed to a different color (e.g., in one pattern to red and in the other to blue). The task for the animals was to select and covertly attend to one RDP (the target) while ignoring the other (the distracter), wait for a brief motion direction change (176 ms duration, 32° intensity click here clockwise from the current direction) in the target, and release the button within 100–650 ms from the change onset. Target direction changes could occur within a time window ranging from 752 to 2940 ms after color-change onset. In order to correctly select the target, the animal had to learn over

several training sessions a color-rank selection rule (gray < pink < green < blue < red < turquoise). Each correctly performed trial was rewarded with a drop of juice. To guarantee that the animal correctly selected the target, on half of the trials, Metalloexopeptidase the distracter pattern located in the opposite visual hemifield changed direction. The monkey had to ignore this distracter change and wait for the target change. Trials in which the monkey responded to the distracter change (false alarms), or did not respond to the target change within the reaction time window (misses), or broke fixation before the target change onset (fixation breaks), were terminated without reward. The different trial types were presented in random sequence. Only correctly performed trials were included in the analysis. Due to limitations in the number of trials that the animals performed during one recording session, we tested only four different colors at a time (instead of six).