Origin of the thalamic projection to dorsal auditory cortex in hearing and deafness

While it is now well accepted that the brain reorganizes following sensory loss, the neural mechanisms that give rise to this plasticity are not well understood. Anatomical tract tracing studies have begun to shed light on the structural underpinnings of cross-modal reorganization by comparing cerebral connectivity in sensory-deprived animals to that of their non-deprived counterparts. However, so far, full documentation of connectional patterns within hearing, congenitally deaf, as well as animals deafened early versus later in life exist only for primary auditory cortex, a region not known to undergo cross-modal reorganization in the deaf. The purpose of the present investigation was to examine thalamo-cortical patterns of connectivity in hearing, late- and early-deafened cats to the dorsal zone (DZ), a region of auditory cortex that cross-modally reorganizes to mediate enhanced visual motion perception following deafness. In hearing cats, the largest projections to DZ arose from the dorsal division of the medial geniculate body (MGB) with lesser projections originating in the medial and ventral MGB and from the suprageniculate and the lateral posterior nuclei. In general, while some variations in the strength of specific thalamic projections were noted, the pattern of projections arising from the thalamus in early- and late-deafened animals remained consistent with that of hearing subjects. These results complement the existing thalamic connectivity data described for congenitally deaf animals, which together demonstrate that thalamo-cortical connectivity patterns to DZ are conserved following deafness, irrespective of the time of onset and etiology of deafness.