Origin and modulation of the narrowband gamma oscillation in the mouse visual system

Visual cortex (V1) exhibits two types of gamma oscillation: a well-characterized broadband (30-90Hz) rhythm, and a narrowband oscillation occurring at frequencies close to 60 Hz in mice. We investigated the source of narrowband gamma, the factors modulating its strength, and its relationship to broadband gamma. Narrowband and broadband gamma power were uncorrelated. Increasing visual contrast had opposite effects on the two kinds of gamma activity: it increased broadband power, but suppressed the narrowband oscillation. Narrowband power was strongest in layer 4, and was mediated primarily by excitatory currents entrained by rhythmically firing neuronal ensembles in the lateral geniculate nucleus (LGN). Silencing the cortex optogenetically did not affect narrowband rhythmicity in either LGN spike trains or cortical EPSCs, suggesting that this oscillation reflects unidirectional flow of information from LGN to V1.