The Necessity of Visual Cortex for Backward Masking in Mice

Written By Tiny Blue Dot Foundation

INSTITUTION: MindScope Program at the Allen Institute

PROJECT LEADERS: Christof Koch, PhD, Shawn Olsen, PhD, Sam Gale, PhD

PROJECT DURATION: 2019 - 2022

LAB WEBSITE: AllenInstitute.org

To find the neural correlates of consciousness, experimenters compare the neural response to a visual stimulus on trials when it is consciously perceived by the subject to trials when it is not perceived (but still present to the retina). In backward visual masking, subjects respond to a brief target stimulus followed by a “mask” stimulus at the same location. When the mask stimulus follows the target by less than ~50 ms, subjects no longer consciously perceive the target (the target is ‘masked’) but in some conditions can still respond to it with above chance accuracy (unconscious processing). This paradigm allows experimenters to isolate the neural correlates of perception by comparing the neural response during two types of trials: those in which the subject perceives the target and responds correctly and those in which the subject reports no perception of the target. Masking has been carried out in humans and monkeys but not in rodents. 

We developed a backward visual masking task in mice, in which the location of a briefly flashed grating is effectively masked within a 50 ms window after stimulus onset. Optogenetic silencing of visual cortex likewise reduces performance in this window, but response rates and accuracy do not match masking, demonstrating 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. Accuracy and performance can be quantitatively recapitulated by a dual accumulator model constrained by V1 activity. The model and the animal’s performance for the earliest decisions imply that the initial spike or two arriving from the periphery trigger a correct response, but subsequent V1 spikes, evoked by the mask, degrade performance for later 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 causally contributes to this process.

Figure. Backward masking task in mice

Broader Impact:

A visual stimulus can be rendered perceptually invisible when a second stimulus follows it in quick succession (e.g., SOA = 17 ms). This ‘masking’ effect can be eliminated when visual cortex is optogenetically silenced. When more than 50 ms passes between the onset of the target and the mask stimulus, the animal can correctly detect the grating. Understanding the brain mechanisms of such backward masking help reveal neural activity patterns that support our consciousness awareness of the world around us.  

Publications:

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