Such a precession would result in fully inconsistent gamma phase relations and a complete absence of gamma coherence. Gamma coherence would actually be destroyed by any of the observed frequency differences: a 6 Hz frequency difference would lead to complete precession and loss of coherence six times per second, and a 2 Hz difference would lead to complete precession and loss of coherence two times per second. PI3 kinase pathway An absence of coherence would be inconsistent with CTC. However, we found clear V1-V4 gamma coherence. The presence of gamma coherence demonstrates that gamma phases did not freely precess against each other, but rather that gamma rhythms had a consistent phase relation. Thereby, the

observation of coherence rules out the abovementioned simple interpretation of the slight frequency differences. Rather, the synopsis of our findings suggests one of the following scenarios or a combination of them: (1) the frequencies of the synchronized rhythms at the V4 site and the relevant V1 site are always identical on a moment-by-moment basis, yet the common frequency fluctuates and the local circuits resonate at different frequencies, giving rise to different peak frequencies in the time-averaged power spectra; (2) our ECoG recordings in V4 reflect a mixture of (at least) two gamma rhythms in V4, one entrained by the attended V1 gamma and a second at a slightly lower

frequency; and (3) in the third scenario, the different gamma frequencies play mechanistic roles in bringing about the selective interareal synchronization. There is one crucial additional ingredient to this scenario, namely a theta-rhythmic gamma-phase reset across mTOR inhibitor V1 and V4, which we have described previously

(Bosman et al., 2009). After the reset, the attended V1 gamma and the unattended V1 gamma partly precess relative to the slightly slower V4 gamma. The attended V1 gamma is of higher frequency than the unattended V1 gamma and therefore precesses faster. Correspondingly, in each gamma cycle, the attended V1 input enters V4 before the unattended V1 input. The earlier entry together with feedforward inhibition makes the attended V1 input entrain V4 at the expense of the SB-3CT unattended V1 input (Fries et al., 2007; Vinck et al., 2010). The selective entrainment of V4 by the attended V1 gamma rhythm further enhances the gain of the attended V1 input and reduces the gain of the unattended V1 input (Fries, 2005; Börgers and Kopell, 2008). The theta-rhythmic reset of interareal gamma-band synchronization is supported by our data (Figures 8 and S4). It probably corresponds to a reset of attentional selection and, under natural viewing conditions, might subserve the theta-rhythmic sampling of multiple objects in a scene, either overtly (Otero-Millan et al., 2008) or covertly (Fries, 2009; Landau and Fries, 2012). Importantly, the third scenario, with partial precession, also leads to selective coherence, as observed here.