Introduction

Anecdotal evidence abounds that blind people develop better capacities in their nondeprived sensory modalities than sighted individuals do. There are many examples of blind musicians. Louis Braille became blind at an early age and developed a tactile system for reading and writing that was later adopted the world over. Perhaps most famous is Helen Keller, who overcame deficits in both vision and hearing. As these examples suggest, the human brain seems to have the capacity to compensate for the loss of some sensory information by invoking other input channels.

However, the concept of cross-modal compensatory plasticity has not gone unchallenged. Early reports often claimed that individuals with blindness or deafness were generally impaired and had lower intelligence. Although such claims can in retrospect be dismissed as uncontrolled observations and most likely reflected additional neurological insults or birth defects that went undetected in prescientific medicine, another challenge to compensatory plasticity in the blind is more serious. A philosophical argument can be made that the concept of space in hearing (or perhaps any concept of space) depends largely on visual input (Fisher, 1964; Worchel, 1951). Supporters of such theories have often claimed an “innate dominance of vision over audition in the development and maintenance of sound localization” (Knudsen & Knudsen, 1989). Individuals who are blind from birth, therefore, should show impairments in sound localization, because the visual reference frame is missing.

Thus, we are left with two diametrically opposed viewpoints whose resolution requires empirical studies. In fact, the viewpoints seem almost mutually exclusive, so that a resolution one way or another should be straightforward. If the compensatory plasticity hypothesis holds, blind humans or animals should be just as good as sighted individuals (or even better) on any complex auditory task, and particularly on sound localization. If the general degradation hypothesis holds, blind individuals should show marked impairment in these capacities. An auxiliary argument can be made on the basis of comparisons between individuals who became blind early or late in life. Although early-blind individuals would be expected to have better-developed audition according to the compensatory plasticity hypothesis, late-blind individuals should have better audition according to the general degradation hypothesis, because they at least had a chance to build up an auditory space concept using vision while they were able to see early in their lives.