Sensory Substitution and the Neural Correlates of Navigation in Blindness
This chapter reviews the most recent advances in sensory substitution and the neural correlates of navigation in congenital blindness Studies have established the superior ability of congenitally blind (CB) participants with the aid of Sensory Substitution Devices (SSDs) to navigate new environments and detect the size and shape of obstacles in order to avoid them. These studies suggest that with training, CB can achieve a representation of space that is equivalent to that of the sighted. From a phenomenological point of view, sensation and perception provided by SSDs have been likened to real vision, but the question remains as to the subjective sensations (qualia) felt by users. We review recent theories on the phenomenological properties of sensory substitution and the recent literature on spatial abilities of participants using SSDs. From these different sources of research, we conclude that training induced plastic changes enable task specific brain activations. The recruitment of the primary visual cortex by nonvisual SSD stimulations and, the subsequent activations of associative visual cortices in the congenitally blind, suggest that the sensory information is treated in an amodal fashion; i.e. in terms of the task being performed rather than the sensory modality. These anatomical changes enable the embodiment of nonvisual information allowing SSD users to accomplish a multitude of "visual" tasks. We will emphasize here the abilities of CB individuals to navigate in real and virtual environments in spite of a large volumetric reduction in the posterior segment of the hippocampus a key area involved in navigation In addition, the superior behavioral performance of CB in a variety of sensory and cognitive tasks, combined with anatomical and functional MRI, underlines the susceptibility of the brain to training induced plasticity.