Intrinsic, light-independent and visual activity-dependent mechanisms cooperate in the shaping of the field response in rat visual cortex.

Do light intensity and behavioral state regulate synaptic plasticity in the visual cortex? We have shown previously that synaptic transmission in the visual cortex oscillates between elevated and depressed levels in accordance with the diurnal light-dark cycle. In this study, we examined the role of intrinsic, light-independent, and visual activity-driven sensory information on the field response during diurnal fluctuations, and examined the plasticity properties of the visual cortex under both conditions. Recordings were obtained from layer 2/3 of the primary visual cortex, of adult freely moving Long Evans rats, after stimulation of the dorsal lateral geniculate nucleus. We observed that visual experience during different states of vigilance leads to increased responsiveness, and plastic changes, in the strength of connections among neurons, consistent with a naturalistic shift in the induction thresholds for synaptic plasticity. We identified this phenomenon as BDNF-dependent. We also found that gamma oscillatory activity, which increases during active visual exploration, is tightly associated with suppression of cortical field potentials, suggesting that coincident changes in synaptic responsiveness and gamma oscillatory levels may reflect mechanisms for optimal stimulus-feature encoding. Translating into an increased signal-to-noise ratio, field depression could thus alter the efficacy of cortical visual processing. These data indicate that the adult visual cortex serves as a synaptic network, where the ability to process visual stimuli is dynamically modified by active visual exploration and arousal states.