Abstract
Gamma-band (25-90Hz) activity in the local field potentials (LFPs), EEG and MEG is present throughout the cerebral cortex and has been linked to many cognitive functions and mental disorders. It has been hypothesized that gamma activity, as a clock or stop watch signal, plays important roles in perception, attention and memory, which apparently require consciousness, by synchronizing signals in different cortical regions. However, gamma activity can also be found in the brains of anesthetized animals or even in brain slices, which most people would agree lack consciousness. To understand the role of gamma-band activity, we looked for differences in gamma-band activity generated in different cortical states. We characterized the frequency of the gamma peak and the durations of oscillatory epochs using time-frequency analysis of the LFP recorded in the primary visual cortex (V1) of awake and anesthetized macaques. We found that while the frequency at the peak of gamma activity in awake (~60Hz) was significantly higher than in anesthetized (~40Hz) monkeys, gamma-band bursts were not significantly different in awake and anesthetized – in both states the bursts resembled filtered broad-band noise: brief in time and variable in frequency. Such randomness of gamma dynamics can be explained by a cortical model: a recurrent network driven by Poisson noise. The model accounts for the upwards shift of peak gamma frequency in the awake cortex by a change of network parameters. We conclude that gamma activity in V1 is too random to act as a ‘clock’ or ‘stop watch’ in the perceptual or cognitive functions that have been proposed. Instead gamma band activity may be an epiphenomenon and its function needs to be re-evaluated.
Meeting abstract presented at VSS 2012