Retino-cortical stimulus frequency-dependent gamma coupling: evidence and functional implications of oscillatory potentials.

Long-range gamma band EEG oscillations mediate information transmission between distant brain regions. Gamma band-based coupling may not be restricted to cortex-to-cortex communication but may include extracortical parts of the visual system. The retinogram and visual event-related evoked potentials exhibit time-locked, forward propagating oscillations that are candidates of gamma oscillatory coupling between the retina and the visual cortex. In this study, we tested if this gamma coupling is present as indicated by the coherence of gamma-range (70-200 Hz) oscillatory potentials (OPs) recorded simultaneously from the retina and the primary visual cortex in freely moving, adult rats. We found significant retino-cortical OP coherence in a wide range of stimulus duration (0.01-1000 msec), stimulus intensity (800-5000 mcd/mm), interstimulus interval (10-400 msec), and stimulus frequency (0.25-25 Hz). However, at low stimulus frequencies, the OPs were time-locked, flickering light at 25 Hz entrained continuous OP coherence (steady-state response, SSR). Our results suggest that the retina and the visual cortex exhibit oscillatory coupling at high-gamma frequency with precise time locking and synchronization of information transfer from the retina to the visual cortex, similar to cortico-cortical gamma coupling. The temporal fusion of retino-cortical gamma coherence at stimulus rates of theater movies may explain the mechanism of the visual illusion of continuity. How visual perception depends on early transformations of ascending sensory information is incompletely understood. By simultaneous measurement of flash-evoked potentials in the retina and the visual cortex in awake, freely moving rats, we demonstrate for the first time that time-locked gamma oscillatory potentials exhibit stable retino-cortical synchrony across a wide range of stimulus parameters and that the temporal continuity of coherence changes with stimulus frequency according to the expected change in the visual illusion of continuity.