Backward masking in mice requires visual cortex

Visual masking is used to infer the timescale of conscious perception in humans; yet the underlying circuit mechanisms are not understood. Here we describe a two-alternative choice backward masking task in mice and humans, in which the location of a briefly presented grating is effectively masked within a 50 ms window after stimulus onset. Importantly, human subjects reported reduced perceptual visibility during masking that closely corresponded with behavior deficits. In mice, optogenetic silencing of visual cortex reduces performance over a similar time course as masking. However, response rates and accuracy do not match masking, demonstrating that cortical silencing and masking are distinct phenomena. Spiking responses recorded in primary visual cortex (V1) are consistent with masked behavior when quantified over long, but not short, time windows, indicating masking involves further downstream processing. Behavioral performance can be quantitatively recapitulated by a dual accumulator model constrained by V1 activity. The model and the animals performance for the earliest decisions imply that the initial spike arriving from the periphery can trigger a correct response, but subsequent V1 spikes, evoked by the mask, degrade performance for longer decisions. To test the necessity of visual cortex for backward masking, we optogenetically silenced mask-evoked cortical activity which fully restored discrimination of target location. Together, these results demonstrate that mice, like humans, are susceptible to backward visual masking and that visual cortex has a crucial role in this process.