Schematic diagram of the behavioral tasks and the monkey brain. a. Delayed match-to-sample task. The monkey was required to locate the salient stimulus in the cue frame and release a lever when a matching stimulus appeared. b. Schematic diagram of the monkey brain and areas where recordings were performed. Abbreviations: AS, arcuate sulcus; IPS, intraparietal sulcus; PS, principal sulcus; STS, superior temporal sulcus. c. Reaction-time version of the task. The monkey was required to release the lever immediately if a target was present and continue to hold if it was absent.

Behavioral response time as a function of stimulus set size. The reaction time version of the task was used. a. Average of 30 sessions is plotted for one monkey. b. Average of 13 sessions is plotted for the second monkey using the dense stimulus array. Error bars represent standard deviations across individual trials.

Population firing rate. a–c. Average discharge rates from dlPFC (N=278), LIP (N=187), and area 7a (N=71) respectively. Peri stimulus time histograms are plotted with mean discharge rate elicited during presentation of target in the receptive field (RF) and distractor in the receptive field conditions over fixation (F), cue (C), and delay (D) period. Shaded area represents standard error of the mean discharge rate. The dotted vertical lines illustrate the time of cue onset and offset. Gray arrows in each panel indicate the population visual response latency; black arrows, the population time of neuronal target discrimination. Insets are schematic illustrations of stimulus condition in or out of the receptive field; each neuron’s receptive field was located at a different position.

Receiver operating characteristic (ROC) analysis and mutual information analysis. a. Area under the ROC curve is plotted as a function of time. Solid vertical line represents the time of cue onset. Dotted line indicates the average mid-point between the peak and baseline of recorded neurons irrespective of area. Red, cyan, and blue curves represent dlPFC (N=278), LIP (N=187), and area 7a (N=71) respectively. b. Cumulative distribution of neurons with significant area under the ROC curve. Each vertical arrow indicates the time point when 50% of neurons of an area reached significance (dlPFC: N=170, LIP: N=114, 7a: N=31). c. Proportion of neurons with significant mutual information as a function of time. Total sample size, dlPFC: N=793, LIP: N=406, 7a: N=485. d. Cumulative distribution of neurons that represented significant mutual information (dlPFC: N=137, LIP: N=83, 7a: N=20). Vertical arrow indicates the time point when 50% of neurons in an area reached statistical significance.

Relationship between discriminability and time of target discrimination. Area under the ROC curve over the entire stimulus presentation period (discriminability value) is plotted as a function of the time of target discrimination for each neuron for a, dlPFC b, LIP. Solid lines represent regression of area under the ROC curve on time of target discrimination. Arrows indicate the top quartile of ROC values obtained after pooling the values of both areas together; vertical dotted lines represent the mean time of target discrimination in each area based on neurons with ROC values above the arrow.

Population responses during the reaction-time task. a,b. Mean discharge rate of target in the receptive field (RF) and distractor in the receptive field during fixation (F), cue (C), and delay (D) periods are plotted (dlPFC: N=42, LIP: N=36 neurons from one monkey). The arrows indicate the visual response latency (gray arrow) and the time of neuronal target discrimination (black arrow). c,d. Average discharge rate synchronized to lever release; only neurons recorded simultaneously are included here (dlPFC: N=17, LIP: N=9). Vertical red line and gray zone represents mean and 1 standard deviation around the time of stimulus presentation, relative to the response time.

Population responses in the difficult-discrimination task. a. Responses of target in the receptive field (red lines) and distractor in the receptive field (blue lines) are plotted for the three brain areas at four levels of difficulty (dlPFC: N=58, LIP: N=55, area 7a: N=25). The arrows indicate the response onset latency (gray) and the time of neuronal target discrimination (black arrow). Data from salient stimuli of both colors have been pooled together (only one color is shown in the insets). b. Average difference in responses between a salient stimulus and a distractor in the difficult-discrimination task. Vertical lines represent time of target discrimination in dlPFC (red line) and LIP (cyan line).

ROC and mutual information analysis for responses in the difficult-discrimination task. a–c. Average ROC values obtained from three levels of difficulty, respectively. Vertical lines illustrate the time of cue onset. d–f. Cumulative distribution of neurons with significant area under the ROC curve. g–i. Cumulative distribution of neurons that represented significant mutual information. Each vertical arrow indicates the time point when 50% of neurons in an area reached significance.