Stimulus-dependent gamma (30-50 Hz) oscillations in simple and complex fast rhythmic bursting cells in primary visual cortex.

Oscillatory activity is generated by many neural systems. gamma band (approximately 40 Hz) oscillations in the thalamus and cortex occur spontaneously and in response to sensory stimuli. Fast rhythmic bursting (FRB) cells (also called chattering cells) comprise a unique class of cortical neurons that, during depolarization by current injection, intrinsically generate bursts of high-frequency action potentials with an interburst frequency between 30 and 50 Hz. In the present study, we show for the first time that FRB cells in the primary visual cortex can be either simple or complex and are distributed throughout all cortical layers. Strikingly, both simple and complex FRB cells generate spike bursts at gamma frequencies in response to depolarizing current pulses, but only simple FRB cells exhibit a selective, stimulus feature-dependent increase in gamma oscillations in response to visual stimulation. In addition, we find that hyperpolarization does not reduce the relative power of visually evoked gamma oscillations in the V(m) response of FRB cells. Our results thus indicate that visually evoked gamma activity in individual simple and complex FRB cells is generated in large part by rhythmic synaptic input, rather than by depolarization-dependent activation of intrinsic properties. Finally, the presence of FRB cells in layer 6 suggests a role for corticothalamic feedback in potentiating thalamic oscillations and facilitating the generation of a corticothalamocortical oscillatory loop. We propose that rather than functioning as pacemakers, FRB cells amplify and distribute stimulus-driven gamma oscillations in the neocortex.