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The blind get a taste of vision

Maurice Ptito, Daniel-Robert Chebat and Ronald E. Kupers


Introduction

Sensory substitution and cross-modal plasticity In sensory substitution, a given sensory modality acquires the functional properties of a missing one. This phenomenon is due to a reorganization of the sensory systems that are deprived of their normal input through a process called cross-modal plasticity [1]. ‘Rewiring’ studies carried out on ferrets [2] and hamsters [3] provided strong support for these phenomena. For example, lesions of central retinal targets induce the formation of new and permanent retinofugal projections into non-visual thalamic sites such as the auditory nucleus [3]. Single neurons in the auditory cortex of these rewired animals respond to visual stimuli and some of them respond equally well to auditory as to visual stimuli. Moreover, those cells that respond to visual stimuli show properties (e.g., orientation selectivity, motion and direction sensitivity) similar to those encountered in the visual cortex of normal hamsters. At the behavioral level, rewired hamsters can learn visual discrimination tasks as well as normal animals and a lesion of their auditory cortex provokes cortical blindness [4]. These data raise the question whether similar processes happen in congenitally blind humans. The absence of visual input from birth leads to the recruitment of the visual cortex by other sensory modalities such as touch or audition [5]. Most studies in man have been carried out in blind subjects who have had many years of experience with Braille reading and it is difficult to conclude on brain reorganization since the extensive reliance of these subjects on tactile or auditory stimulation may by itself result in enhanced activity in the occipital cortex [6, 7]. To avoid this bias, we took advantage of the tongue display unit, a tactile to vision sensory substitution device which does not use the fingertips or the auditory system and which is therefore equally novel for early blind, late blind and healthy controls. This device translates visual information into electrotactile stimulation which is delivered to the tongue by means of an electrode array.
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