Absence of disconnexion syndrome in callosal agenesis and early callosotomy: brain reorganization or lack of structural specificity during ontogeny?

Four acallosal subjects, one child, aged 5, and three adults, as well as five epileptic patients who underwent callosotomy between the ages of 6-21 years, were tested on a variety of intra- and intermanual tasks in a study aimed at elucidating the developmental aspects of callosal plasticity. The performance of the clinical sample was compared to that of 48 normal children, aged 5-12 years, an age span generally considered to coincide with the final stages of callosal maturation. As previously reported, interhemispheric integration improved with increasing age in the normal sample. The two patients having undergone callosotomy in childhood performed as well as their normal peers, whereas the three others who had the operation in late adolescence or adulthood showed the typical disconnexion deficits reported in the literature. The acallosal subjects, including the youngest one, outperformed all groups. We speculate that the remarkable plasticity seen in the acallosals and the young callosotomized patients may be related to a critical period in development coinciding with a phase of synaptic overproduction and redundancy that would favor the reinforcement of alternative neural pathways. The compensatory mechanisms appear to become more limited in late adolescence when synaptic distribution presumably assumes adult patterns.