Differential Burst dynamics of Slow and Fast gamma rhythms in Macaque primary visual cortex

Gamma oscillations have been ubiquitously observed across a wide spectrum of brain areas in multiple species. They tend to occur intermittently in the form of bursts, rather than being produced as sustained and continuous rhythmic activity. Recent studies have shown that large visual sinusoidal gratings elicit two distinct gamma rhythms, namely, slow ({approx} 20-35 Hz) and fast gamma ({approx} 40-65 Hz), in the primary visual cortex (V1) of non-human primates. However, their mechanisms of generation and potential functional role in cortical processing remain unclear. Details of their burst signatures could potentially provide crucial insights about how the two rhythms influence network dynamics. Therefore, we computed burst statistics (durations and latencies) of simultaneously induced slow and fast gamma rhythms in the local field potential (LFP) recorded from area V1 of two adult female bonnet monkeys using several burst estimation methods. We found that slow gamma rhythm exhibited significantly longer burst durations and longer latencies as compared to fast gamma. Slow gamma exhibited higher long-range synchrony compared to fast gamma, as estimated by coherence and weighted phase lag index (WPLI), which could aid in enhanced global coordination in neocortex. Interestingly, longer burst length of slow-gamma could be replicated in a recently-developed noisy Wilson-Cowan network model by simply changing the firing-rate time-constant of the corresponding inhibitory interneuronal population, which leads to both slower and longer bursts. These results are consistent with the hypothesis that the two oscillations are generated by different inter-neuronal classes that operate over different temporal and spatial scales of integration. Significance statementSlow ({approx} 20-35 Hz) and fast gamma ({approx} 40-65 Hz) are two distinct rhythms known to be induced by large visual gratings in the primary visual cortex (V1). Interestingly, gamma oscillations manifest in the form of transient and stochastic epochs, generally termed as "bursts". We estimated the durations of stimulus-induced slow and fast gamma bursts generated in the local field potential (LFP) recorded from V1 of monkeys. Slow gamma bursts had significantly longer durations and increased latency to onset compared to fast gamma, which was replicated in a noisy Wilson-Cowan model by changing the time-constant associated with inhibitory neuronal population. These results suggest that slow and fast gamma are generated by different inter-neuronal networks operating at different spatio-temporal scales.