If one had to persist with the idea of a substitution, the notions of "sensory-motor substitution system", or "perceptual substitution", would be preferable to "sensory substitution". However, we now wish to argue that the second reason why the phrase "sensory substitution" is misleading and unfortunate, is that what is at stake is not a substitution. The warning came from the visually handicapped persons themselves, who expressed disappointment at the very time when they began to discover this novel mode of access to objects situated at a distance in space. Certainly, these devices made it possible to carry out certain tasks which would otherwise have been impossible for them. However, this was not the fundamental desire which motivated the blind persons who lent themselves to these experiments. A blind person can well find personal fulfilment irrespective of these tasks for which vision is necessary. What a blind person who accepts to undergo the learning of a coupling device is really looking for, is rather the sort of knowledge and experience that sighted persons tell him so much about: The marvels of the visible world. What the blind person hopes for is the joy of this experiential domain which has hitherto remained beyond his ken.
Now the problem is that this is not what the device procures. In fact, there are a large number of differences between this artificial coupling device and normal vision: There is no color, a small number of points, a camera whose movements are limited and clumsy, all of which slows down the recognition of a situation. This novel sensori-motor coupling resembles vision in many ways, but the quality of lived experience that it procures is quite different - as can be readily appreciated by sighted subjects who are blindfolded for the purposes of the experiment. The device of Bach y Rita does not produce a sensory substitution, but rather an addition, the creation of a new space of coupling between a human being and the world. The sensory substitution devices upset the classical definitions of the diverse sensory modalities.
It would be vain to believe that one has alleviated the suffering of a blind person just by giving him access to a sort of information. What is always at stake is the insertion of the person in a world of shared meanings, which depend on a personal history whose coherence must not be brutally shattered. Now what is cruelly missing in this new perceptual modality is what Bach y Rita calls the qualia, i.e. the values and the quality of lived experience associated with perceived entities. If one shows a person blind from birth an image of his wife, or if one shows some students pictures of nude women, the disappointment is complete: Their perception does not convey any emotion. But after the event, it is clear that it is the reverse which would have been astonishing. Meaning or emotional significance are not things that are already there, in the world, just waiting to be picked up like a piece of information. Here again, by the failure of its initial ambition, the device of Bach y Rita provides a crucial empirical proof: An isolated subject cannot attribute an existential meaning to objects and events that he perceives simply on the basis of a new perception. Does it follow that something essential is lacking in these devices? Unable to give a "content" to the perception (color, value), they demonstrate what distinguishes natural perception from a simple capacity to discriminate and categorize. There is a striking similarity between these observations, and reports of the absence of emotion and meaning felt by persons blind from birth who recover sight by removal of a cataract. In other words, it is not the principle of sensory substitution as such which is responsible for the impossibility of gaining access to qualia (Gregory 1990).
It is remarkable that in all the observations reported in the literature, it is always a question of a purely individual use of these devices. The user is surrounded by sighted persons, but is isolated in his particular mode of perception. Now it is plausible to suppose that perceptual values are closely linked to the existence of a shared history and collective memory, a memory which can only emerge in the course of interactions between several subjects in a common environment. This suggests possible experiments in the future. In any event, it seems to us that the term "perceptual supplementation" is more appropriate than "sensory substitution". This new term implies that these devices do not exactly remedy a deficit, but rather that they introduce perceptual modalities that are quite original.
The sensori-motor coupling devices thus give rise to experimental research into a deep problem, classically restricted to philosophy and psychology, concerning the origin and the nature of the value attached to things. A purely intellectual link inferred between a perceived form, and a feeling of pleasure or pain in another sensory modality, does not seem to be immediately sufficient to confer an emotional value to the form in question.
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