Sensory origin of visually evoked activity in auditory cortex: evidence for true cross-modal processing.

The meaning and functions of cross-modal sensory processing in the cortex is at the center of an ongoing debate. While some studies claim that such responses reflect genuine multisensory integration, others argue they are mere artifacts of stimulus-evoked movement or changes in internal state. We examined this issue by measuring face movements and neural activity in awake mouse primary auditory cortex (A1) and primary visual cortex (V1) during visual and auditory stimulation. Visual stimuli rarely evoked face movements, A1 responses to visual input remained robust even in the absence of movement, and optogenetic silencing of V1 reduced A1 visual responses, confirming a sensory origin of these cross-modal responses. These findings directly challenge the argument that cross- modal responses are purely movement-driven and emphasize that rather than assuming all cross-modal effects are artifactual, researchers must rigorously test each case. Highlights: Unlike auditory stimuli, visual stimuli rarely evoke face movementsFace movements explain sound-evoked firing in VC but not visually evoked firing in ACSilencing VC suppresses visually evoked firing in ACThese results confirm a sensory (rather than motor) origin of these responses.