Scientific Understanding of Consciousness
Mirror Neurons and Language Evolution
Nature 435, 1235-1238 (30 June 2005)
Orofacial somatomotor responses in the macaque monkey homologue of Broca's area
Michael Petrides, Geneviève Cadoret & Scott Mackey
Montreal Neurological Institute, McGill University, 3801 University Street, Montreal, Quebec H3A 2B4, Canada
In the ventrolateral frontal lobe of the human brain there is a distinct entity, cytoarchitectonic area 44 (Broca's area), which is crucial in speech production. There has been controversy over whether monkeys possess an area comparable to human area 44. We have addressed this question in the macaque monkey by combining quantitative architectonic analysis of the cortical areas within the ventrolateral frontal region with electrophysiological recording of neuron activity and electrical intracortical microstimulation. Here we show that, immediately in front of the ventral part of the agranular premotor cortical area 6, there is a distinct cortical area that is architectonically comparable to human area 44 and that this monkey area 44 is involved with the orofacial musculature. We suggest that area 44 might have evolved originally as an area exercising high-level control over orofacial actions, including those related to communicative acts, and that, in the human brain, area 44 eventually also came to control certain aspects of speech.
In the macaque monkey as in the human, the most ventral part of the precentral gyrus (areas 4 and 6) is involved with the motor control of the orofacial musculature. There is considerable debate about whether, just rostral to the premotor area 6, there might be a cortical region corresponding to area 44 of the human brain. This debate has become of particular concern in recent years with the discovery of a class of neurons known as 'mirror neurons' in part of the ventral premotor cortical area 6 of the macaque monkey. These neurons become active both when the monkey performs a particular action and when the monkey observes a similar action being performed by another individual. Because there has been considerable theoretical interest in the possibility that the mirror-neuron system might be important for the evolution of language, the homologue of Broca's area in the monkey might be the ventral premotor cortical area F5 within which the mirror neurons were discovered.
The present study established that a cortical area comparable in architecture to human area 44 exists in the macaque monkey immediately in front of the premotor cortical area that is involved with orofacial musculature. In the human brain, area 44 is involved with the motor aspects of speech production. Studies of the effects of lesions that are restricted to area 44 have yielded an apraxia of speech (that is, a problem with the motor aspects of speech production), and not the classic full-blown aphasic syndrome including a major disruption of syntax, which was previously thought to be the result of damage to Broca's area.
The position of area 44 in the monkey and the human brain immediately in front of the premotor cortex and its orofacial responses to microstimulation suggest that it is an essential link in a series of cortical areas that control the orofacial musculature. Area 44 is in a unique position to communicate directly both with the agranular premotor cortex that is controlling the orofacial musculature and with area 45, which has been shown to be involved in the active retrieval of information from memory in both the monkey and the human brain (including verbal episodic information and semantic information in the left hemisphere of the human brain). We suggest that area 44 might have evolved originally as an area exercising high-level control over orofacial and other actions, including those related to communicative acts, and that, in the human brain, with the evolution of language, area 44 in the left hemisphere eventually also came to control the motor aspects of speech
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"Mirror neurons", (Damasio; Looking for Spinoza, 115)