Slow and fast gamma oscillations show phase-amplitude coupling with distinct high-frequency bands in macaque primary visual cortex

Gamma oscillations (25-70 Hz) can be induced in the visual cortex by presenting stimuli like gratings. Large stimuli produce two distinct gamma oscillations in primate primary visual cortex (V1) - slow (25-40 Hz) and fast (40-70 Hz), possibly due to different interneuronal networks. While fast-gamma has been shown to strongly lock spikes, slow-gamma does not, and hence its role in cortical processing is unclear. Here, we studied potential influence of gamma rhythms on neural activity using phase-amplitude coupling (PAC). We recorded spikes, local field potential and electrocorticogram (ECoG) from V1 of two adult female monkeys while presenting stimuli that simultaneously induced both gamma rhythms, and developed a novel method that reduces the influence of spike-related-transient on PAC. Interestingly, while fast-gamma showed coupling with frequencies above 150 Hz, reflecting spike-locking, slow-gamma showed PAC in a distinct frequency range between 80-150 Hz, which was especially prominent in ECoG. PAC varied with orientation and spatial frequency in the same way as power in the respective frequency bands, confirming dissociation in the coupling of the two gamma rhythms. Thus, fast-gamma could be more involved in spike-locking, while slow-gamma could represent a modulatory signal acting by amplitude modulation between 80-150 Hz at a more global scale. Significance StatementGamma oscillations in the visual cortex can be induced by stimuli like gratings, producing two distinct gamma oscillations: slow (20-40 Hz) and fast (40-70 Hz). While fast-gamma strongly locks spikes, the role of slow gamma is unclear. Oscillations have been proposed to influence processing through phase-amplitude coupling (PAC). We recorded spikes, local field potential (LFP) and electrocorticogram (ECoG) from female monkeys and developed a new method to study PAC. While fast-gamma showed PAC with 150-500 Hz, reflecting spike-locking, we found PAC between slow-gamma and 80-150 Hz, which was especially strong in ECoG. The two PAC signatures varied differently with stimulus, reflecting distinct origins. Thus, while fast-gamma could lock spikes, slow-gamma could modulate amplitudes between 80-150 Hz at a global scale.