Rapid sequential activation from A1 to V1 in congenitally blind and sighted subjects

Tactile information in congenitally blind (CB) individuals is funneled to the visual cortex through both via a fast thalamo-cortical and a strengthened polysynaptic cortico-cortical pathway. Auditory inputs can also activate the visual cortex in CB individuals, but whether these signals rely on the same dual route remains unclear. We used magnetoencephalography (MEG) to map the spatiotemporal dynamics and measure directed functional connectivity between regional brain responses to monaural auditory cues in eight CB and eight sex and age-matched sighted control (SC) participants. In both groups, a distinct sequential activation pattern was observed, initiating in the thalamus 10-20 ms after stimulus onset, followed by auditory cortex (A1) at approximately 35 ms, and visual cortex (V1) at 45-50 ms post-cue. Because monosynaptic cortico-cortical transmission typically occurs within about 10-15 ms, this temporal sequence suggests that auditory inputs reach the visual cortex through a direct, likely monosynaptic pathway between A1 and V1 in both CB and SC participants. Effective connectivity in the alpha band (8-12 Hz) was stronger in CB, birectionally between the thalamus and V1 and unidirectionally from A1 to V1, indicating functional strengthening of these pathways in congenital blindness. These findings suggest that, in CB, auditory signals are relayed to the occipital cortex primarily through an enhanced monosynaptic A1-to-V1 pathway that is also present, though weaker, in sighted individuals. This organization contrasts with tactile information processing in CB, which additionally engages a novel thalamo-cortical route to V1. Our results provide MEG-based evidence for a rapid, likely monosynaptic, A1-to-V1 pathway that is strengthened in congenital blindness.