Somatosensory cortex shapes perceptual decision bias via the superior colliculus.

Sensory inputs reach cortical and subcortical areas in parallel, but how they coordinate to guide decisions is not well understood. Here, we examined primary somatosensory cortex (S1) and superior colliculus (SC) in mice performing a tactile detection task. Inactivating S1 contralateral to the stimulus decreased the detection probability as expected. Surprisingly, inactivating ipsilateral S1 increased detection probability, even when the stimulus was absent. These opposite effects reflected shifts in choice bias, rather than changes in sensitivity. SC inactivation produced similar directional bias shifts, and SC lesions abolished the effects of S1 inactivation. Neural recordings showed that although S1 inactivation decreased SC sensory responses, choice-activity remained robust in SC, but fell to chance in S1, suggesting that behavioral impairment was better explained by changes in SC-dependent choice signals rather than the loss of S1 sensory encoding. Thus, S1 shapes tactile behavior by modulating SC-dependent circuits that help transform sensory evidence into a decision.