Macaques have neural homologue of Broca's area

A paper by Michael Petrides (McGill University) and colleagues reports that a brain region on the left side of the macaque brain, in the same general area as Broca's area in the human brain, is associated with functions of the mouth and face. The authors consider some uncertainty about the full extent of Broca's area, particularly as related to speech function, and consider it equivalent to area 44. They then express the problem as applied to monkeys, particularly in relation to the famous "mirror neurons":

Although it is not disputed that, 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 confusion and debate as to 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 (also known as area F5c) 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, it has been suggested that the homologue of Broca's area in the monkey might be the ventral premotor cortical area F5 (part of area 6) within which the mirror neurons were discovered. However, this area is agranular, whereas area 44 (Broca's area) in the human brain is a dysgranular architectonic entity that lies rostral to the ventral agranular premotor cortex. The key question is therefore: Is there a cortical area immediately in front of premotor cortical area 6 of the macaque monkey brain that is comparable to area 44 of the human brain? Such an area should have the following three properties: first, it should exhibit the key architectonic characteristics of area 44 of the human brain; second, it should be bounded topographically by the same architectonic areas as in the human brain; and third, it should be involved, at least, with the orofacial musculature. (Petrides et al. 2005:1236, citations omitted).

They went looking for the area, recording neuron activity with electrodes and stimulating brain regions to observe whether an effect was observed on the mouth or face musculature.

In short, they find it:

The present study has established that a cortical area comparable in architecture to human area 44 exists in the macaque monkey immediately in front of premotor cortical area 6V and that it is involved with the orofacial musculature. The lack of an outgroup comparison limits our ability to provide further inferential evidence that area 44 in the human and the macaque monkey brain reflect shared common ancestry. Area 44 in the monkey lies rostral to the convexity of the premotor cortex where the mirror neurons were recorded (area F5c). Thus, the involvement (if any) of area 44 in the mirror neuron system remains to be established. Furthermore, it has recently been argued that mirror neurons cannot provide a basis for an essential structural relation in human language, namely the bi-directional arbitrary mapping between sound and meanings. In the human brain, area 44 is involved with the motor aspects of speech production. Studies of the effects of lesions that are more or less 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 (Petrides et al. 2005:1237, citations omitted).

If you don't know about mirror neurons, they're pretty cool. They are neurons in monkey brains (and in human brains as well) that are active both when an individual does an action, and also when an individual watches someone else doing the same action. They have been suggested to be the basis of imitative learning, in that merely watching another individual do an action might prime the brain -- by strengthening neural connections -- to take the same actions itself in the same contexts.

The study does not comment on topics of comparative interest, such as the relative size of this presumed area 44 in the monkey brains compared to humans, or the previous literature on the comparative neuroanatomy of Broca's area in humans and chimpanzees. But this is interesting in its own right: it is a demonstration of a deep homology in language-related brain anatomy between humans and other anthropoids. Clearly there is no functional analogue in macaques for the full array of processes influenced by Broca's area in humans, but the fact that similar functional systems exist in both species gives a clear indication that human language has developed by the elaboration of many previously evolved neural systems.

It remains to be seen whether other aspects of neural architecture related to language may have arisen only within the hominid lineage; indeed, most of the neural functions related to language as yet have no precisely defined neuroanatomical correlates. This may be suggestive in itself, particularly if it indicates (as some think) that the neural basis of language is significantly self-constructed in the brains of different individuals.

References:

Petrides M, Cadoret G, Mackey S. 2005. Orofacial somatomotor responses in the macaque monkey homologue of Broca's area. Nature 435:1235-1238. Full text online

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