Somesthetic discrimination thresholds in the absence of the corpus callosum.

The aim of this study was to investigate how the absence of the corpus callosum affects somesthetic sensation on the axial midline and in proximal and distal body regions. For this purpose, two-point discrimination ability was evaluated in four acallosal subjects, four callosotomized subjects, six IQ-matched subjects and 10 control subjects with average and above average IQ. Sensory thresholds were established in the distal (index, palm), proximal (forearm), cranio-axial (forehead) and axial (dorsal trunk) body regions. The threshold was defined as the smallest separation at which the two points were perceived at a 70% accuracy level. Results showed that the thresholds of the acallosal and the callosotomized subjects were not significantly different from those of the IQ-matched control groups in the distal, proximal and cranio-axial body regions. However, thresholds in the dorsal trunk were significantly higher in the two experimental groups. It thus appears that the axial regions of the body that are normally densely represented in the corpus callosum function abnormally when this structure is absent or transected. Moreover, compensatory mechanisms normally seen in cases of early brain injury do not seem to apply in the present case since the acallosals showed the same impairments as the callosotomized subjects.