The corpus callosum (CC), the largest commissure in the human brain, is thought to play an essential part in the formation and maintenance of lateralized cognitive and motor functions. In particular, it has been suggested that inhibition of the subdominant hemisphere via commissural fiber tracts plays a crucial role for functional lateralization. However, many studies supporting this idea have either been conducted in nonhuman model species or used indirect measures of callosal functioning. Method: In order to directly assess the impact of the presence or absence of the CC on both the existence and the extent of functional hemispheric asymmetries, we investigated handedness and language lateralization, as well as interhemispheric information integration, in 6 high-functioning individuals with partial or complete agenesis of the CC (AgCC). Performance was compared with that of 30 IQ- and age-matched controls with intact CC. Results: We found a stronger predisposition for ambidexterity in individuals with AgCC. Similarly, the typical right ear advantage in the dichotic listening task was significantly reduced in AgCC. Furthermore, AgCC patients generally reacted slower than controls in a test of interhemispheric information integration, and showed reduced accuracy in trials that required interhemispheric integration. Conclusions: These findings indicate that reduced hemispheric specialization in the acallosal brain might be due to a higher degree of hemispheric autonomy in AgCC.
