Coherence showed a peak in V1 that coincided with the V1 power-change peak. Figure 2G illustrates the selective attention conditions with both stimuli presented simultaneously
but only one stimulus behaviorally relevant and therefore selected in any given trial. In the V4 site, attention to either stimulus gave essentially the same activation (Figure 2H), confirming that the site was equally driven by either stimulus. In both V1 sites, attention to their respective driving stimulus led to a slight but highly consistent increase in the frequency of the gamma-band activity (Figures 2I and 2J; p < 0.001 for both V1a and V1b, nonparametric randomization test on peak frequency). This shift was clearly visible also in the raw power spectra (Figures S2G and S2H). Crucially, Figures 2K and 2L demonstrate that the V4 site gamma Rapamycin mouse band synchronized almost exclusively www.selleckchem.com/products/Perifosine.html to the
attended V1 site (p < 0.001 for both V1a and V1b, nonparametric randomization test on gamma-band coherence, see Experimental Procedures for details), despite the fact that both V1 sites were driven equally strongly. The presence of coherence between two sites implies neither zero-phase relationship nor symmetry of mutual influence. To investigate the mutual influences between the example V1 and V4 sites, we determined Granger-causal (GC) influences in the bottom-up and the top-down directions. The GC influence of time series Adenylyl cyclase A onto time series B quantifies the variance in B that is not explained by the past of B but by the past of A (Kamiński et al., 2001; Dhamala et al., 2008). Figures 3B–3E show GC-influence spectra during isolated stimulation with either stimulus 1 (red condition) or stimulus 2 (blue condition). V4 was bottom-up GC influenced in the gamma band selectively by the V1 site that was stimulus driven (Figures 3B and 3C; p < 0.001 for both V1a and V1b, nonparametric randomization test). Similarly, V4 exerted a top-down GC influence in the gamma band selectively to the V1 site that was stimulus
driven (Figures 3D and 3E; p < 0.05 for both V1a and V1b, same test). Figures 3G–3J show GC-influence spectra when both stimuli were presented simultaneously, but only stimulus 1 (red condition) or stimulus 2 (blue condition) were behaviorally relevant. V4 was bottom-up GC influenced in the gamma band almost exclusively by the relevant V1 site (Figures 3G and 3H; p < 0.001 for both V1a and V1b, same test). Similarly, V4 exerted a top-down GC influence in the gamma band primarily to the relevant V1 site (Figures 3I and 3J; p < 0.05, same test). Please note that gray bars below the spectra result from frequency-wise tests followed by multiple comparison correction, while this text reports tests applied directly to the gamma band (see Experimental Procedures for details).