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Abstract:
The phase of spikes in the gamma cycle has previously been shown to be modulated by stimulus orientation in macaque V1 (Vinck et al. 2010, Womelsdorf et al. 2012). These studies suggested that such stimulus-dependent phase shifts selectively facilitate the impact of neurons that fire earlier in the gamma cycle, on their targets. However, such phase coding schemes implicitly depend on the generation of a consistent gamma oscillation frequency across stimulus conditions. To test this, we examined whether the stimulus orientation and the eye of presentation affected the peak gamma frequency in V1. Two macaque monkeys were trained to passively fixate on a central spot, while one of two orthogonally oriented gratings was presented monocularly through a mirror stereoscope for a period of one second. In different trials either the eye of presentation or the orientation, were changed. Local field potential (LFP) signals recorded from 168 sites across multiple sessions were found to exhibit significant coherence with concurrently recorded single-unit spikes in the gamma frequency range. The power spectral density of each of those LFPs was fit as the sum of a power function and a gaussian function, and the center of the gaussian was taken as the peak gamma frequency. We found that, across sites the peak frequency varied between 30 Hz and 45 Hz in both monkeys. Moreover, within each site there was a significant shift in the peak frequency both with orientation (median shift ~1.99Hz) as well as the eye of presentation (median shift ~0.89Hz). There was no systematic relationship between the direction of shift and stimulus preference. Given that the orientation-dependent phase shifts reported earlier were of a very small magnitude (only a few degrees), it follows that such frequency changes could be detrimental for phase-shift coding schemes that rely on entrainment of multiple cortical areas to a single ‘clock-like' signal. Alternatively, neural mechanisms implementing phase computations could be relatively invariant to frequency changes by using more intricate signal properties like instantaneous phase.
