Cortical Dynamics in Visual Processing
Recurrent circuits are a hallmark of mammalian sensory cortex. How they impact dynamics of sensory representation is not understood. Because recurrent circuits provide a majority of the synaptic excitation to cortical neurons in response to sensory stimulation, the intrinsic dynamics of these cortical recurrent circuits are expected to be a critical determinant of the timing of the sensory response in cortex. Previous methods could not isolate dynamics of these intra-cortical recurrent circuits from those of thalamic afferents during sensory processing. I now accomplish this by developing an approach to optogenetically silence thalamus in a model system: the mouse visual pathway. Silencing thalamus revealed the time course over which visually evoked activity in visual cortex was maintained by the intra-cortical recurrent circuits themselves, in isolation from thalamic input. I found that, at all time points during the cortical sensory-evoked response, optogenetically silencing thalamus led to a fast decay of sensory-evoked activity in cortical recurrent circuits. This activity decay time course was fit by a 10 ms network time constant, similar to a neuron’s integration time window. This decay time course was invariant across all tested visual stimulation conditions and behavioral states but depended on cortical inhibition. In awake mice, the dynamics of this time course predicted the time-locking of cortical activity to thalamic input at frequencies <15 Hz and the attenuation of the cortical response to higher frequencies. Under anesthesia, however, dynamics of depression at thalamocortical synapses disrupted the fidelity of sensory transmission. Thus, I determine sensory-evoked dynamics intrinsic to the intra-cortical recurrent circuits in isolation from thalamus and show how these dynamics transform afferent input in time.