Dissociating cortical regions activated by semantic and phonological tasks: a FMRI study in blind and sighted people.

Abstract

Previous neuroimaging studies of language processing in blind individuals described cortical activation of primary (V1) and higher tier visual areas, irrespective of the age of blindness onset. Specifically, participants were given nouns and asked to generate an associated verb. These results confirmed the presence of adaptations in the visual cortex of blind people and suggested that these responses represented linguistic operations. The present functional magnetic resonance imaging study attempted to further characterize these responses as being preferential for semantic or phonological processing. Three groups of participants (sighted, early onset, and late-onset blind) heard lists of related words and attended to either a common meaning (semantic task) or common rhyme (phonological task) that linked the words. In all three groups, the semantic task elicited stronger activity in the left anterior inferior frontal gyrus and the phonological task evoked stronger activity bilaterally in the inferior parietal cortex and posterior aspects of the left inferior frontal gyrus. Only blind individuals showed activity in occipital, temporal, and parietal components of visual cortex. The spatial extent of visual cortex activity was greatest in early blind, who exhibited activation in all ventral and dorsal visual cortex subdivisions (V1 through MT) for both tasks. Preferential activation appeared for the semantic task. Late blind individuals exhibited responses in ventral and dorsal V1, ventral V2, VP and V8, but only for the semantic task. Our findings support prior evidence of visual cortex activity in blind people engaged in auditory language processing and suggest that this activity may be related to semantic processing.