Brain plasticity and multi-sensory experience in early blind humans.

During the first years of life, the human brain undergoes repetitive modifications in its anatomical, functional, and synaptic construction to reach the complex functional organization of the adult central nervous system. This is also the case for the visual cortex that receives visual stimuli from the environment and matures during infancy and childhood. But what about this maturation when the human is affected by early (or congenital) blindness? The "neural Darwinism" theory would predict that when one sensory modality is lacking, as in congenital blindness, the target structures would be taken over by the afferent inputs from other senses that would promote and control their functional maturation (Edelman, 1993). This view receives support from both cross-modal plasticity experiments in animal models and functional imaging studies in man. On the one hand, a reorganization of sensory representations with cross-modal expansion of nonvisual modalities into normally visual brain areas has been demonstrated, using electrode recordings, in early visually deprived animals (Rauschecker, 1995). These physiological modifications might be partly related to behavioral abilities of congenitally blind cats, who are, for instance, faster in learning tactile exploration of space or sound localization than blindfolded controls (Rauschecker & Korte, 1993). On the other hand, over the last decade, functional brain imaging like positron emission tomography (PET) has provided further insight in the maturation processes in humans and helped elucidate the pathophysiological processes involved in brain plasticity in the absence of vision. In this chapter, we present some research applications of PET and rehabilitation of vision to investigate sensory substitution procedures in human blindness.