Traditionally, the sensory areas of the brain were believed to be exclusively devoted for processing of sensory information from one set of sensory organs, i.e., uni-modal. Increasing evidence from both animal and human studies now suggest that even the very primary sensory cortices have the capacity to process information from the other modalities. Greater understanding of the neural mechanisms underlying cross-modal plasticity is crucial for the rehabilitation of sensory deprived humans and development of neural prostheses. Recent imaging studies on congenitally deaf adults have shown that the deaf """"""""auditory"""""""" cortex can process both visual and tactile information. However, the functional organization of the deaf """"""""auditory"""""""" cortex is not known, as spatiotemporal processing of both visual and tactile information has not been investigated in the same subjects. Furthermore, the mere fact that the auditory cortices of the congenitally deaf can be activated by stimuli of different sensory modalities tells us very little about the level of the auditory pathway where these plastic changes take place. The measured cortical activations could reflect reorganization of the thalamic nuclei, and/or plasticity in thalamo-cortical as well as in cortico-cortical connections.
In Specific Aim 1 the investigators will delineate structural plasticity both at the cortical and sub-cortical level. They will use anatomical and diffusion tensor magnetic resonance imaging in a cohort of congenitally deaf and age-matched hearing subjects to reveal putative thalamic and axonal connectivity differences.
In Specific Aim 2 the investigators will combine functional magnetic resonance imaging with electromagnetic recordings to reveal the possible spatiotemporal differences in the activation of the deaf """"""""auditory"""""""" cortex during visual and vibratory tactile stimulation. This will allow the investigators to distinguish whether overlapping or separate neuronal networks support the cross-modal plasticity of the deaf 'auditory' cortex.