Visual speech circuits in profound acquired deafness: a possible role for latent multimodal connectivity

It is commonly held that losing one sense provokes cross-modal takeover of deprived cortical areas, and therefore results in a benefit for the remaining modalities. Using functional magnetic resonance imaging (fMRI), we assessed the impact of acquired deafness on the brain network related to speechreading and teased apart cortical areas with responses showing long-term reorganization, i.e. time-dependent plasticity over 4-48 months of deafness, from those expressing compensation, i.e. performance-related activity. Nine deaf patients (7 women, age; mean +/- SE. = 50.2 +/- 4.8) and control subjects performed equally well in a visual speechreading task but deaf patients activated the left posterior superior temporal cortex more than controls. This effect correlated with speechreading fluency but not with the duration of sensory deprivation, thus arguing against long-term reorganization as the source of these cross-modal effects. To the contrary, cross-modal activation in the left posterior superior temporal cortex of deaf patients decreased with deafness duration. Our observation that cross-modal effects were most pronounced right after deafness onset is at odds with the classical view on brain reorganization. We suggest that functional compensation of sensory deprivation does not require slowly progressive colonization of superior temporal regions by visual inputs, but can exploit a switch to pre-existing latent multimodal connectivity.