Functional neuroimaging studies of Braille reading: cross-modal reorganization and its implications

A frequent criticism of functional neuroimaging is that it offers limited additional information to that obtained with lesion studies. However, there are many cases in which lesion studies are not sufficient to study principles of cortical organization. One such case is cross-modal reorganization in the human brain after sensory deprivation. Vision is the dominant human sense and its loss leads to a major disability. However, the advent of modern information technology has made life easier for blind people. Spatial orientation in natural environments can be aided by a novel mobile satellite based Global Positioning System with a voice synthesizer, allowing blind subjects to reach a location by electronic guidance through the auditory modality. Another, well known sensory substitution is the tactile letter system Braille. Although attributed to Louis Braille, this system has its roots in the military field. In the early 18th century, Charles Barbier, a French soldier, invented a tactile reading and writing system that coded sounds as tactile patterns. Its intended use was silent communication in complete darkness along trenches. However, it quickly transpired that the system was too complicated to be of practical use. In 1821, Charles Barbier met Louis Braille, who was blind from the age of 4 years. Louis Braille immediately realized how useful this system of raised dots could be and simplified it from the original 12 to six dots. From then on it only took 6 more years until the first book in Braille was published. Early research on Braille reading in blind subjects from the 1960s to the 80s has mainly focused on hand preference. The first experimental data on possible cortical reorganization in blind subjects came from experiments conducted by WanetDefalqueet al. (1988). They used PET and [ 18F]fluorodeoxyglucose to investigate the effects of early onset blindness on cortical metabolism. Surprisingly, they observed vastly elevated glucose metabolism in the visually deprived occipital cortex of early blind subjects which was comparable to metabolism seen in sighted subjects studied with their eyes open. The most interesting question arising from this result was whether metabolic activity is task dependent. Unfortunately this question could not be answered by this initial experiment. Several groups (Sadato et al., 1996; Büchel et al., 1998) followed up on this question, now using H 2O PET which allowed assessment of more subtle differences in regional activity. Both studies showed that occipital cortex activation is indeed task dependent and is related to tactile