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 remains unknown. We examined the roles of the primary somatosensory cortex (S1) and the superior colliculus (SC) in mice performing whisker-mediated tactile detection. While inactivating S1 contralateral to the stimulus increased detection threshold, inactivating ipsilateral S1 decreased it. These opposing effects reflected shifts in choice bias rather than sensitivity. Unilateral SC inactivation produced similar directional effects, whereas bilateral inactivation of either S1 or SC neutralized bias. Furthermore, SC lesions impaired detection performance, and combined S1 inactivation caused no further impairment, indicating that S1-mediated shifts in bias were SC-dependent. S1 inactivation also decreased sensory-evoked neural activity in SC, and choice decoding revealed that behavioral impairment was more closely associated with changes in SC population activity than with loss of sensory encoding in S1. Thus, S1 shapes perceptual bias by modulating SC-dependent circuits that transform sensory evidence into perceptual decisions.