Lowet, E., Roberts, M. J., Bosman, C. A., Fries, P., & De Weerd, P. (2015)
European Journal of Neuroscience, 2015, 43(10): 1286-1296.
Neuronal gamma-band synchronization (25-80 Hz) in visual cortex appears sustained and stable during prolonged visual stimulation when investigated with conventional averages across trials. However, recent studies in macaque visual cortex have used single-trial analyses to show that both power and frequency of gamma oscillations exhibit substantial moment-by-moment variation. This has raised the question of whether these apparently random variations might limit the functional role of gamma-band synchronization for neural processing. Here, we studied the moment-by-moment variation in gamma oscillation power and frequency, as well as inter-areal gamma synchronization, by simultaneously recording local field potentials in V1 and V2 of two macaque monkeys. We additionally analyzed electrocorticographic V1 data from a third monkey. Our analyses confirm that gamma-band synchronization is not stationary and sustained but undergoes moment-by-moment variations in power and frequency. However, those variations are neither random and nor a possible obstacle to neural communication. Instead, the gamma power and frequency variations are highly structured, shared between areas and shaped by a microsaccade-related 3-4-Hz theta rhythm. Our findings provide experimental support for the suggestion that cross-frequency coupling might structure and facilitate the information flow between brain regions.
Contribution to the field
The idea of sustained gamma working as a clock has remained a dominant idea implicit in a lot of work and in chosen data analysis techniques, but it is challenged by a number of studies showing the importance of microsaccades in enabling neural communication. In the gamma band and at the level of spiking, microsaccades are followed by brief bursts of activity. This goes against the idea of continued and stable gamma activity over time, and opens the question how gamma then could be a vehicle of communication in the brain. A lower frequency shared across different areas that aligns the gamma bursts, would be one way in which gamma can be aligned among different areas. Interestingly, microsaccades provide that low, common frequency. The findings presented here suggest that the brief gamma bursts that are nested in a microsaccade-related 3-4Hz rhythm represent general processing windows in which not only feedforward, but also feedback processes need to take effect. In general, this paper contributes to ongoing research into the relationship between microsaccades, perception and attention.