A robust cerebral asymmetry in the infant brain: the rightward superior temporal sulcus.

In order to understand how genetic mutations might have favored language development in our species, we need a better description of the human brain at the beginning of life. As the linguistic network mainly involves the left perisylvian regions in adults, we used anatomical MRI to study the structural asymmetries of these regions in 14 preverbal infants. Our results show four significant asymmetries. First and foremost, they stress an important but little-known asymmetry: the larger depth of the right superior temporal sulcus (STS) at the base of Heschl's gyrus. Then, we characterized the early forward and upward shift of the posterior end of the right Sylvian fissure, the elongation of the left planum temporale as well as the thickening of the left Heschl's gyrus. The rightward bias of the STS is robust and large, and is not correlated with the leftward asymmetries of the planum and Heschl's gyrus, suggesting that different morphogenetic factors drive these asymmetries. As this sulcus is engaged in multiple high-level functions (e.g. language and theory of mind), and has been spotted as abnormal in several developmental disorders (e.g. schizophrenia, autism), this early rightward asymmetry should be further explored as a target for a genetic evolutionary pressure.