During the fixation condition (Figure 4C, right panels), both units showed lower firing rates; however, u26 still showed differences between responses to targets and distracters. Thus, this unit selected the target even during fixation where both RDPs were irrelevant. On the other hand, the second unit (u79) shows a constant low firing rate for both targets and distracters during the entire fixation period. These two units represent extreme cases in our fixation data set. The average neuron showed some response

after the color change, mainly to targets, and no response to distracters. A common finding in most units was a progressive buildup of responses after the onset of the two white RDPs during the main task relative to fixation. In order to examine the trend across the recorded neural population, we normalized in each unit responses PARP inhibitor to targets and distracters corresponding to the different distances to the mean response during a 300 ms time window prior to the color-change onset during main task trials. We aligned all units to their preferred target location, and pooled responses across cells to obtain

normalized population responses (Figure 4D). In agreement with the single-cell data, the population responses showed a pattern intermediate between the two example neurons. During the main task (Figure 4D, left panel), responses to all stimuli gradually increased following the onset of the two selleck kinase inhibitor white RDPs (see Temporal Dynamics of the Response Modulation). During the interval of 100–400 ms after the color-change onset, responses to targets increased by similar amounts (p = 0.83, one-way ANOVA), whereas responses to distracters were differentially suppressed as a function of ordinal distance (p = 0.043, one-way ANOVA). The results were similar in both animals

(see Figures S3A and S3B for population responses corresponding to Ra and Se). During fixation there was no response buildup after the onset of the two white RDPs, but only a slight response increase to targets after the heptaminol color change (Figure 4D, right panel). In this condition we did not observe differences in response as a function of distance for any of the stimuli (p = 0.062 for targets and p = 0.696 for distracters, one-way ANOVAs). In order to characterize the dynamic of response changes during the tasks across the population of neurons, we computed for each unit and distance a modulation index (MI) between the responses to each stimulus (target and distracter), and the average response across the 300 ms preceding the onset of the two white RDPs (baseline; see Experimental Procedures). During the task condition, MIs corresponding to both stimuli and the three distances departed from zero (horizontal dashed line) toward more positive values at the onset of the color change (Figure 5A).