5° × 1.5° which was flashed for 100 ms in one of six positions arranged on a circle with radius equal to the eccentricity that elicited the maximal response in the RF mapping task. Monkeys were required to maintain fixation of the central spot. After a delay of 750 ms, the fixation spot was turned Sirolimus order off and the monkeys had to saccade to the memorized position of

the peripheral stimulus and maintain their gaze at the peripheral location within a 3° × 3° window for 200 ms in order to be rewarded with juice. Monkeys were required to hold a bar to initiate the trial and subsequently fixate a central spot (0.4° × 0.4°) on the screen. Successful fixation within a 3° × 3° window for 1,500 ms was followed by the appearance of three isoluminant, sinusoidal, drifting gratings (2° diameter, drifting rate 1 cycle/s), one red, one blue, and one green, positioned at the same distance from the center of the screen (usually within 4°–8°) and distributed radially around Obeticholic Acid datasheet the fixation point at 120° intervals. Following a variable period of time (0–1,000 ms), the fixation spot was replaced by a small square cue whose color indicated the stimulus to be attended. The monkeys had to shift their attention to the target stimulus (while maintaining fixation of the central cue) and wait for the target to change color. The

color change could happen any time between 250 and 3,000 ms after the cue onset. In one-third of the trials, one distracter changed color before the target, in one-third both distracters changed color before the second target (with a minimum delay of

400 ms), and in one-third only the target changed color. The animals were required to ignore any color changes of the distracter stimuli and respond only to the target color change by releasing the bar within 600 ms. Successful completion of the trial was rewarded with a drop of juice. If the monkeys released the bar prematurely, did not respond to the target color change within the specified time, or broke fixation, the trial was aborted. We manipulated task difficulty by making the color changes subtle so that the monkeys needed to attend to the target in order to detect the change and respond correctly. We decreased the magnitude of color change to the point that the monkeys performed between 80% and 85% to ensure that they did not rely on a bottom-up, stimulus-driven approach but they rather used the cue to attend to the target. We used a Multichannel Acquisition Processor system (Plexon) to record spikes and local field potentials (LFPs) from FEF and V4 simultaneously using up to four tungsten microelectrodes in each area. The recording procedure has been described in detail before (Gregoriou et al., 2009a) and is briefly outlined in the Supplemental Information. Briefly, spike data were obtained after filtering between 250 Hz and 8 kHz, amplifying and digitizing the signal at 40 kHz.