Different origins of gamma rhythm and high-gamma activity in macaque visual cortex

During cognitive tasks electrical activity in the brain shows changes in power in specific frequency ranges,such as the alpha (8-12 hz) or gamma (30-80 hz) bands,as well as in a broad range above ~80 hz,called the high-gamma band. the role or significance of this broadband high-gamma activity is unclear. one hypothesis states that high-gamma oscillations serve just like gamma oscillations,operating at a higher frequency and consequently at a faster timescale. another hypothesis states that high-gamma power is related to spiking activity. because gamma power and spiking activity tend to co-vary during most stimulus manipulations (such as contrast modulations) or cognitive tasks (such as attentional modulation),it is difficult to dissociate these two hypotheses. we studied the relationship between high-gamma power,gamma rhythm,and spiking activity in the primary visual cortex (v1) of awake monkeys while varying the stimulus size,which increased the gamma power but decreased the firing rate,permitting a dissociation. we found that gamma power became anti-correlated with the high-gamma power,suggesting that the two phenomena are distinct and have different origins. on the other hand,high-gamma power remained tightly correlated with spiking activity under a wide range of stimulus manipulations. we studied this relationship using a signal processing technique called matching pursuit and found that action potentials are associated with sharp transients in the lfp with broadband power,which is visible at frequencies as low as ~50 hz. these results distinguish broadband high-gamma activity from gamma rhythms as an easily obtained and reliable electrophysiological index of neuronal firing near the microelectrode. further,they highlight the importance of making a careful dissociation between gamma rhythms and spike-related transients that could be incorrectly decomposed as rhythms using traditional signal processing methods. © 2011 ray,maunsell.