Action to Language via the Mirror Neuron System: The Mirror System Hypothesis on the linkage of action and languages

Introduction Our progress towards an understanding of how the human brain evolved to be ready for language starts with the mirror neurons for grasping in the brain of the macaque monkey. Area F5 of the macaque brain is part of premotor cortex, i.e., F5 is part of the area of cerebral cortex just in front of the primary motor cortex shown as F1 in Fig. 1.1 (left). Different parts of F5 contain neurons active during manual and orofacial actions. Crucially for us, an anatomically segregated subset of these neurons are mirror neurons . Each such mirror neuron is active not only when the monkey performs actions of a certain kind (e.g., a precision pinch or a power grasp) but also when the monkey observes a human or another monkey perform a more or less similar action. In humans, we cannot measure the activity of single neurons (save when needed for testing during neurosurgery) but we can gather comparatively crude data on the relative blood flow through (and thus, presumably, the neural activity of) a brain region when the human performs one task or another. We may then ask whether the human brain also contains a “mirror system for grasping” in the sense of a region active for both execution and observation of manual actions as compared to some baseline task like simply observing an object. Remarkably, such sites were found in frontal, parietal, and temporal cortex of the human brain.