Synaptic dynamics govern spatial integration in mouse visual cortex

Neurons in primary visual cortex are often suppressed by stimuli extending beyond their receptive fields. This surround suppression is proposed to reduce the redundancy of encoding large stimuli and support scene segmentation. We find that surround suppression decreases firing rates in mouse primary visual cortex by accelerating the decay of visually-evoked responses and reducing response duration. The rapid decay of visual responses at large sizes is enhanced by increased contrast, reduced by locomotion, and invariant to stimulus orientation, consistent with the engagement of a network mechanism. While fast-spiking interneurons have faster dynamics relative to neighboring pyramidal cells, the dynamics of somatostatin-expressing interneurons are delayed. At the subthreshold level, the rapid decay of visual responses with increasing size is due to a delayed removal of both synaptic excitation and inhibition below baseline levels following visual input. We propose that the delayed activation of somatostatin-expressing interneurons drives a network-wide suppression and accelerates the decay of the visual response. Thus, these data identify a key role for synaptic network dynamics in regulating both spatial and temporal integration in mouse visual cortex.