Brain lateralization and the emergence of language
The question of the rules by which left-hemisphere specialization (HS) is established for language is connected to issues related to the origins of language. Exploration of the relationships between variability in linguistic neural networks and HS allows one to examine the relationships between the neural support for language, HS, and the emergence of speech. Studies of aphasiology, presurgical exploration, and intraoperative stimulation have shown a strong correlation between right-handedness and left-hemisphere dominance for language. The hypothesis that the neural bases of language developed in the dominant hemisphere for right-hand motor control coincides with motor theories of language evolution. The fact that exceptions to this rule exist, such as aphasias after right-hemisphere lesions in right-handers or functional imaging observations of healthy right-handers with rightward asymmetries during linguistic tasks, suggests that factors other than handedness may be at play in the development of HS. For example, the observation of increased lateralization of activation for speech listening with increased brain volume suggests that constraints related to the processing speed of speech sounds also play a role in the development of HS. This role of brain volume is consistent with the perceptual theory of the origin of HS, which postulates that language develops in the left hemisphere because of its aptitude for processing rapid temporal signals, which is indispensable for the analysis of speech sounds. Anatomical factors related to this perceptual processing likely combined with the influence of handedness in the course of language and language network evolution.