Carrie Armel and Vilayanur Ramachandran describe just such a case in which an individual progressively lost vision starting in childhood until he became totally blind at age 40.Source:( Acquired Synesthesia in retinitis pigmentosa,Neurocase 5(4):293-296)
A few years later he experience touch as spots of light. It required a firmer touch (above the detection threshold) to evoke synesthesia, and the synesthetic threshold were constant over weeks, indicating the effect was genuine. A likely explanation is that visual deprivation allows tactile input to activate visual areas by letting existing connection between the areas become hyperactive. The connectivity does not depend on the growth of new connections, as evidenced by the rapid changes in experiments featuring blind or blindfolded subjects. For example, as newly blind individuals learn Braille, the brain area corresponding to the reading finger expands as unused V1 changes its functional assignment from sight to the feeling and “reading” of Braille. More to the point, when sighted individuals are blindfolded for only two days, their visual cortex activates when they perform tasks with their fingers or when they hear either tones or words. Two days is too short a time for new synapses to grow from touch and hearing areas into V1. More importantly, removing the blindfold for just 12 hours reverts V1 so that it responds again only to visual input. Thus, the brain’s sudden ability to “see” with the fingers and ears may depend upon connections from other senses that are already there but are not used so long as the eyes input a signal.
Finally sensory substitution experiments underscore that latent cross-sensory connections exist in everyone. We normally think of the tongue as a taste organ, but it is loaded with touch receptors, making it an excellent brain-machine interface that can learn to “see” when a tingling electrode grid is laid on its surface. Source(Bach-y Rita, P, Collins CC, Saunders F, White B, Scadden L 1969. Vision substitution by tactile image projection. Nature 221: 963-964, Bach-y-Rita, P.2004. Tactile sensory substitution studies. The grid translates a video input into patterns of touch, allowing the tongue to discern qualities usually ascribed to vision, such as distance, shape, direction of movement, and size. The apparatus reminds us that we do not see with our eyes but rather with our brain.The technique was originally developed to assist the blind; a more recent application, however, feeds infrared or sonar input to the tongue grid so tat divers can “see” in murky water and soldiers can have 360 degree vision in the dark. The implication of these applications is that synesthesia is a latent capacity in everyone. The difference between the synesthetic and non- synesthetic brain therefore is not whether there is cross talk, but rather how much there is.