The well-defined RFs indicate that a given electrode was primarily assessing neuronal activity in a small patch of the underlying visual cortex. Figure 1D

shows respective examples for several V4 electrodes (red dots in Figure 1A). In both V1 and V4, the ordered representation of eccentricity and elevation was as predicted by numerous previous studies (Gattass et al., 2005). Figure S1D shows RF outlines from two recording sessions separated by 2 months, illustrating the stability of RF positions and thereby suggesting that the electrodes were in a stable position on the cortex. With these recordings at hand, we engaged the monkeys in the selective visual attention task illustrated in Figure 1E (see Experimental Procedures for details). When the monkey touched a bar and fixated a central dot, two patches of drifting MEK inhibitor grating appeared. The click here two stimuli were always blue and yellow, with the color assigned randomly. After about 1 s, the fixation point assumed the color of one of the stimuli, which was thereby cued as relevant. In each trial, the relevant grating changed curvature at an unpredictable

moment up to 4.5 s after the cue, and the monkey was rewarded for bar releases within a short time window thereafter. Changes in the irrelevant grating were equally probable, but corresponding bar releases were not rewarded. In monkeys K and P, 92% and 94% of bar releases, respectively, were correct reports of changes in the relevant stimulus. In 10% of the trials, only one or the other stimulus was shown in isolation (and its changes had to be

reported) PR 171 to assess stimulus selectivity of the recording sites. For all analyses, we used the period from 0.3 s after cue onset until one of the stimuli changed. Also, for all further analyses, we first calculated local bipolar derivatives, i.e., differences between LFPs from immediately neighboring electrodes. We refer to the bipolar derivatives as “sites.” Bipolar derivation further enhances spatial specificity of the signal and removes the common recording reference, which is important for the analysis of synchronization between sites. Figure 2 shows the results for a single data set including a V4 site activated equally by each of two stimuli and two V1 sites activated exclusively by either one or the other stimulus. Figures 2A–2F illustrate the stimulus selectivity of the different sites during isolated stimulation with stimulus 1 (condition marked red in Figure 2A) or stimulus 2 (condition marked blue in Figure 2A); site “V4” was equally driven by both stimuli (Figure 2B); site “V1a” responded to stimulus 1, but not 2 (Figure 2C); the opposite was the case for site “V1b” (Figure 2D). Figures S2A–S2D show the respective raw power spectra. Figures 2E and 2F demonstrate that V4 showed pronounced interareal synchronization in the 60–80 Hz band selectively with the V1 site that was stimulus driven. In the following, we will refer to the 60–80 Hz band as the gamma band.