Gamma rhythm and its relationship with neuronal activity in early visual cortex
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The gamma components (30-50 Hz) of the local field potentials (LFPs) are correlated with numerous perceptual and cognitive phenomena, and have been proposed to have important functions in sensory processing. Despite its interesting response properties and the theories which these have spawned, the relationship between gamma and neuronal spiking activity remains unclear.;I focused on elucidating this relationship by simultaneously recorded LFPs and spiking activity with multi-electrode arrays in the primary visual cortex and area V2 of anesthetized macaque monkeys. We've made several important observations. First, the tuning properties and spatial extent of gamma are flexible and the relationship between gamma rhythm and neuronal activity is dependent on stimulus conditions. This is because gamma power arises from two sources that reflect different spatial scales of neural ensemble activity: a 'global' rhythm that is spatially coherent, well-tuned tuned with shared stimulus preference across millimeters of cortex and extremely sensitive to prolonged orientation adaptation; and a broadband increase in LFP power that reflects local spiking activity. The relative contribution of these two components depends on stimulus conditions, which can thus alter gamma tuning and spatial coherence. To have a better understanding of the proposed functions of gamma, we explored the interaction between gamma and spiking timing. We find that spiking activity is phase modulated and thus more coordinated when a global gamma rhythm is induced in a large neuronal population. To test the consequence of this coordinated V1 activity on downstream networks, we paired our V1 recordings with recordings in the middle layers of V2. The coordination of VI spiking activity results in an enhanced probability that each V1 spike that will be followed by a spike in downstream neurons in V2, in a retinotopically specific and gamma-phase dependent manner.;Together my thesis provides a more complete understanding of the properties of gamma rhythm in the LFP and its relationship with spiking activity, providing important constraints on its proposed function and on the mechanisms that generate it.
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