High frequency oscillations as a correlate of visual perception.

Cortical oscillatory activity in the gamma-band range (>30Hz) is a fundamental mechanism of neural coding that arises during a range of cognitive processes in both animals and humans. Since the first report on high frequency oscillatory synchrony between V1 neurons belonging to the same orientation column (Gray and Singer, 1989, PNAS, 86, 1698-1702), the role of such oscillations in visual perception has been extensively researched. Visual stimuli elicit an early, evoked gamma-band response and a later, induced (neither time nor phase-locked) response. An abundance of experimental evidence now links both evoked and induced high frequency oscillations to a range of visual stimulus properties. On the basis of early studies into gamma-band oscillations in vision, induced high frequency oscillatory activity has been proposed as a putative cortical mechanism of coherent percept formation and object representation while evoked high frequency activity was related to the processing of image features. Recent studies demonstrate that both evoked and induced gamma-band activity are correlated with the speed and accuracy of visual detection and discrimination. Furthermore, induced gamma-band oscillations in the visual cortex are also correlated with fixational eye movement patterns. These direct relations between gamma-band activity and the efficacy of visual perception strongly suggest that cortical high frequency synchronisations constitute a neural mechanism that subserves processes essential for the organised intake and analysis of visual information.