Functional connectivity between somatosensory and visual cortex in early blind humans.

Crossmodal plasticity occurs when loss of input in one sensory modality leads to reorganization in brain representations of other sensory modalities. In congenital blindness the visual cortex becomes responsive to somatosensory input such as occurs during Braille reading. The route by which somatosensory information reaches the visual cortex is not known. Here, we used repetitive transcranial magnetic stimulation (rTMS) to probe the connection between primary somatosensory cortex (S1) and early visual cortex (V1 and neighboring areas), combining rTMS with positron emission tomography (PET). We applied stimulation over S1 in sighted, early blind and late blind individuals. Baseline regional cerebral blood flow in occipital cortex was highest in early blind and lowest in late blind individuals. Only the early blind group showed significant activation of early visual areas when rTMS was delivered over S1. This activation was significantly higher in early than in late blind, but not relative to sighted controls. These results are consistent with the hypothesis that tactile information may reach early visual areas in early blind humans through cortico-cortical pathways, possibly supporting enhanced tactile information processing.