Right but not left hemispheric discrimination of faces in infancy.

The ontogeny of the functional asymmetries of the human brain is poorly understood. Are they a consequence of differential development based on competition mechanisms, or are they constitutive of the human brain architecture from the start? Using structural magnetic resonance imaging and a face-discrimination electroencephalography paradigm with lateralized presentation of faces, we studied face perception in infants over the first postnatal semester. We showed that the corpus callosum is sufficiently mature to transfer visual information across hemispheres, but the inter-hemispheric transfer time of early visual responses is modulated by callosal fibre myelination. We also revealed that only the right hemisphere shows evidence of face discrimination when presented in the left visual hemifield. This capability improved throughout the first semester with no evidence of discrimination in the left hemisphere. Face-processing lateralization is thus a characteristic of the infant's extra-striate visual cortex, highlighting the differential left-right organization of the human brain already established in infanthood.