Crossmodal reorganisation in deafness: Mechanisms for functional preservation and functional change.

The study of deafness and blindness has contributed unique knowledge to our understanding of the brain, showing that environmental experience critically shapes neural structure and function. Nevertheless, the most prevalent theories of crossmodal plasticity propose opposing views about the function of reorganised cortical regions. Some theories agree on functional preservation, where in the absence of early sensory stimulation, cortical regions respond to a different sensory modality, but perform the same function. Others propose that the absence of sensory stimulation from birth results in cortical regions changing their "typical" sensory processing function to higher-order cognition. Both deafness and blindness have provided vast evidence in support of each of these theories. Here we use examples from the study of deafness to explore organisational mechanisms that would allow functional preservation and functional change to co-exist either in the same or adjacent regions. We provide a set of predictions and testable hypotheses that support each of these accounts, and lay out some steps that could move us towards more specific theories of cortical reorganisation.