Thalamic Afferents to Reorganized Auditory Cortices in Postnatally Deafened Cats

Deafness affects approximately 40 million people in the United States. However, little is known about how the brain reorganizes itself in response to this major loss of inputs. Preliminary studies of neonatally deafened cats reveals that the auditory cortical area, the auditory field of the anterior ectosylvian sulcus (FAES), is reorganized as a visual area and is involved in the control of visual orientation behaviors. The plastic changes in neuronal connectivity that underlie this cortical reorganization are not known, but it is our hypothesis that sensory driving via thalamocortical inputs must change from auditory to visual thalamic origins. The present study used neuroanatomical tracing techniques in two hearing adult cats and two adult cats deafened at birth to determine the thalamic origin of projections to the FAES. When tracer was injected into the FAES of hearing animals, MGm, MGv, Pom, and dorsal thalamic nuclei showed retrogradely labeled cell bodies indicative of their projection to the FAES. When tracer was injected into the FAES of the neonatally deafened animals, MGm, MGv, Sgl, Pom, and dorsal thalamic nuclei also showed retrogradely labeled cells. In the deafened animals, no retrogradely labeled neurons were identified in the primary visual thalamic areas. Because essentially the same thalamic regions project to the FAES but relay different sensory messages in hearing and deafened animals, it must be concluded that neuronal plasticity occurred prior to the thalamocorticals projection. Therefore, therapeutic efforts to ameliorate the effects of deafness might best address thalamic rather than cortical mechanisms of plasticity and neuronal reorganization