Role of gamma oscillations in processing of natural stimuli

Abstract

Gamma oscillations (~30-80 Hz) are a prominent signature of electrophysiological signals recorded from the brain with proposed roles in various cognitive functions such as attention and visual perception. Although gamma is induced strongly in the visual cortex by visual stimuli such as achromatic gratings, bars and gabors, whether natural stimuli also generate strong gamma is a debated issue. Unlike small achromatic gratings, natural stimuli cover a large visual area, are colorful, and have discontinuities along many dimensions. In this thesis I have studied the effect of these three important aspects of natural stimuli - size, chromaticity and stimulus discontinuities - on gamma oscillations in the local field potential (LFP) recorded from monkey primary visual cortex (area V1), while the monkeys were shown experimentally designed visual stimuli. In the first part, we show that large grating stimuli induce a second gamma oscillation (~25-45 Hz, termed as slow gamma) in addition to the traditionally known gamma oscillation (~45-70 Hz, fast gamma) in the LFP with distinct tuning to stimulus size, orientation and contrast. Fast gamma have a shorter latency, stronger spike-entrainment and are coherent over shorter distances than the slow gamma, suggesting that the two gamma oscillations may be involved in processing across different spatial ranges. In the second part, we show that colored stimuli generate gamma oscillations of extremely high magnitude in V1 LFP, far exceeding the gamma generated by optimally tuned achromatic gratings. Further, they are the strongest for reddish (long-wavelength) hues, depend critically on the purity (saturation) of the hue and are highly correlated with only the positive L-M cone contrast generated by the color stimuli, suggesting that gamma could be a marker of specific mechanisms underlying this computation in V1. In the concluding part, we show that gamma oscillations are very sensitive to stimulus discontinuities, reducing drastically in the presence of small discontinuities in grating orientation, spatial phase, contrast or an annular cut. Our results suggest that gamma may behave like a resonant phenomenon that depends critically on the excitation-inhibition balance in the neuronal network. These results may give useful insights into mechanisms of gamma oscillations, their role in natural vision and chromatic and achromatic visual processing in the primate primary visual cortex.