Lateralization of the Human Mirror Neuron System

TLDR

A cortical network comprising inferior frontal, rostral inferior parietal, and posterior superior temporal cortices underlies action representation and imitation, may be an evolutionary precursor to language systems, yet its lateralization in humans remains unclear. The study used fMRI to examine finger‑movement imitation with lateralized visual stimuli and responses. Imitation elicited bilateral but ipsilateral‑biased activity in IFG and IPL, with right‑hemisphere STS activation for both sides, indicating that the human mirror system’s visual and motor components are not left‑lateralized, suggesting language lateralization arises from other mechanisms.

Abstract

A cortical network consisting of the inferior frontal, rostral inferior parietal, and posterior superior temporal cortices has been implicated in representing actions in the primate brain and is critical to imitation in humans. This neural circuitry may be an evolutionary precursor of neural systems associated with language. However, language is predominantly lateralized to the left hemisphere, whereas the degree of lateralization of the imitation circuitry in humans is unclear. We conducted a functional magnetic resonance imaging study of imitation of finger movements with lateralized stimuli and responses. During imitation, activity in the inferior frontal and rostral inferior parietal cortex, although fairly bilateral, was stronger in the hemisphere ipsilateral to the visual stimulus and response hand. This ipsilateral pattern is at variance with the typical contralateral activity of primary visual and motor areas. Reliably increased signal in the right superior temporal sulcus (STS) was observed for both left-sided and right-sided imitation tasks, although subthreshold activity was also observed in the left STS. Overall, the data indicate that visual and motor components of the human mirror system are not left-lateralized. The left hemisphere superiority for language, then, must be have been favored by other types of language precursors, perhaps auditory or multimodal action representations.