After center fixation for 400 ms, the monkeys were presented with a central cue that was identical to the search target. The cue stayed on for 200–2500 ms randomly, after which time a search array with 20 stimuli was presented, and the center cue was replaced by the center fixation spot. Monkeys were required to hold fixation at the center of the screen before the search array onset. 3-MA supplier After the onset, monkeys had 4 s to find the target that was the same as
the central cue. No constraints were placed on their search behavior in order to allow them to conduct the search naturally. Monkeys were required to fixate the target stimulus for 700 ms continuously to receive a juice reward. The position of the target on the screen was changed randomly from trial to trial. A memory-guided saccade task was used to determine a cell’s RF and stimulus selectivity. Briefly, the trial started with the monkey fixating a central spot. A peripheral stimulus flashed for 100 ms in one of the stimulus positions used in the search array. After a random period between 500 and 1500 ms, the central spot was extinguished, and the monkey was rewarded for making a saccade to the memorized position of the peripheral stimulus. Before the offset of the fixation learn more spot, monkeys were required to maintain
center fixation. Eleven locations, including nine in the contralateral visual field and two on the vertical middle line, were used, which comprised 11 of the 20 locations used in the search array. Firing rates were compared between the prestimulus period, 200–0 ms before stimulus flash onset, and the poststimulus period, 50–250 ms after the flash onset, using the Wilcoxon rank-sum test, and stimulus locations with significant increased responses (p < 0.05)
were defined to be in the RF. Sites with RFs extending into both hemisfields were rarely found and were excluded from further analyses after a preliminary RF mapping. Multiunit spikes and local field potentials (LFPs) were recorded from the FEF and V4 simultaneously using a Multichannel Acquisition Processor system by Plexon. On a given day, up to four tungsten microelectrodes (FHC) were advanced through the dura in each area. Electrodes within an area were spaced 650 or 900 μm apart. Neural signals were filtered between 250 Hz and 8 PD184352 (CI-1040) kHz and amplified and digitized at 40 kHz to obtain spike data. The location of recordings in both the FEF and V4 was verified with MRI. In both monkeys, we electrically (<50 μA) stimulated in the FEF and elicited eye movements. Eye movements were recorded by an infrared eye tracking system (Eye Link II, SR Research) at a sampling rate of 500 Hz. Recording sites that showed a significant visual response (Wilcoxon rank-sum test, p < 0.05) were included for analysis. The intervals used for this statistical comparison were as described before. Firing rates were calculated with 10 ms nonoverlapping bins.