There were two recent items in ScienceDaily reporting two sides of the brain’s flexibility. In one paper S. Lomber and others reported that in cats born deaf, vision is reorganized to use the neural space that is usually used by hearing. This gives better localization in peripheral vision and better motion detection. (here) In the other paper J. Rauschecker and others found that people blind from birth show fMRI activity in visual modules from sound and touch stimuli. These modules analyzed spatial location, patterns and motion. (here)
There are some interesting aspects of this plasticity. First, the ‘born’ in born deaf and born blind is probably very important. If the eyes are not signaling then the visual cortex is not undergoing the developmental stage that ties it to vision. If the ears are not signaling then the auditory cortex is not tied to hearing. This lack of the normal connection probably allows unusual connections to be made more easily.
Second, parts of the cortex that do not get normal stimulation take on stimulation from neighbouring parts of the cortex rather than being idle or dieing.
Third, aspects of perception like the position of something in three dimensional space can involve more than one sense mode. I can tell where something is by the sound it makes or if trained by the echoes from it; I can tell from the position of my body when I touch it where it is; and I can see where the light is reflected off the object. Location and movement in space is so important to perception that we can think of it as the pre-existing framework that objects are placed in. Location is not so much a product of vision as vision is a tool of location and not its only tool.
Fourth, consciousness has this spatial framework no matter what sense is feeding it. Thus born blind can ‘see’ what they hear and the born deaf can ‘hear’ what they see. Seeing really has two meanings, one that means sensory information that comes via the eye and one that means the usual conscious awareness of the world from a viewpoint similar to the eye’s. Hearing also has two meanings. Hearing is what we get from our ears and it can also mean the part of our consciousness that we know as sound.