A total of 313 neurons were recorded in the PFC (99 in monkey CC and 214 in monkey ISA). The average firing rate of neurons recorded in PFC was 7.4 Hz (interquartile range of firing rate 3-MA order was 1.7 to 10.1 Hz). Only local field potentials from electrodes with at least one isolated unit were used for all of our analyses, ensuring the electrode was in the appropriate cell layer. Animal eye position was monitored using an infrared eye-tracking system (Eyelink, SR Research), which sampled the eye position at 240 Hz. Behavioral control was handled by Cortex (http://www.cortex.salk.edu). Animal procedures

followed all guidelines set by the Massachusetts Institute of Technology Committee on Animal Care and the National selleck kinase inhibitor Institutes of Health. Code used in the analysis was custom written in MATLAB (MathWorks) or R (R Foundation for Statistical Computing). The task began with the presentation of a fixation spot

at the center of the screen. The monkeys were required to acquire and maintain fixation within three degrees of this spot until making a behavioral response. Immediately after fixation was acquired, both the rule cue and response targets appeared and remained on screen for the duration of the trial. The rule cue was a colored border around the display indicating the feature of the stimulus the monkey needed to discriminate on the current trial. The animals were trained to perform two different rules: color and orientation. Each rule was associated with two different cues in order to distinguish rule-related activity from cue-related activity (see Figure S1A for example neurons encoding the rule and not the individual cues). After the presentation of the rule cue, the animals were required to maintain fixation for a “preparatory” time period before the onset of the stimulus. The duration of the preparatory period was randomized for each monkey (227–496 ms for monkey CC, 86–367 ms for monkey ISA; different ranges were the result of iteratively lowering the preparatory period during training

while equalizing performance between animals). At the end of the preparatory period, a test stimulus, oriented either vertically or horizontally and colored either red or blue, appeared at the center of the screen. The test stimulus consisted of small shapes (colored and aligned appropriately). The identity Thalidomide of these small items changed from session to session, ensuring the animals generalized the rules. After the onset of the stimulus, the monkeys were free to make their response: a single saccade to either the left or right target. The correct saccade direction depended on both the stimulus identity and the current rule in effect (Figure 1A). For the color rule, a red stimulus required a saccade to the right, and a blue stimulus a saccade to the left. For the orientation rule, a horizontal stimulus required a saccade to the right, and a vertical stimulus a saccade to the left.