When contemplating the coos and screams of a fellow member of its species, the rhesus monkey, or macaque, makes use of brain regions that correspond to the two principal language centers in the human brain, according to research conducted by scientists at the National Institute on Deafness and Other Communication Disorders (NIDCD) and the National Institute of Mental Health (NIMH), two of the National Institutes of Health, Washington, DC. The finding, published July 23 in the online issue of Nature Neuroscience, bolsters the hypothesis that a shared ancestor to humans and present-day non-human primates may have possessed the key neural mechanisms upon which language was built.
Principal collaborators on the study are Allen Braun, MD, chief of NIDCD’s Language Section, Alex Martin, PhD, chief of NIMH’s Cognitive Neuropsychology Section, and Ricardo Gil-da-Costa, Gulbenkian Science Institute, Oeiras, Portugal, who conducted the study during a 3-year joint appointment at the NIDCD and NIMH.
“This intriguing finding brings us closer to understanding the point at which the building blocks of language appeared on the evolutionary timeline,” says James F. Battey, Jr, MD, PhD, director of the NIDCD. “While the fossil record cannot answer this question for us, we can turn to the here and now—through brain imaging of living non-human primates—for a glimpse into how language, or at least the neural circuitry required for language, came to be.”
While non-human primates do not possess language, they are able to communicate about such things as food, identity, or danger to members of their species by way of vocalizations that are interpreted and acted upon. In humans, the two main regions of the brain that are involved in encoding this type of information in language are known as Broca’s area and Wernicke’s area, named for the physician-researchers who discovered them. Both areas are located along the Sylvian fissure (and are therefore referred to as perisylvian areas) with Broca’s area located in the frontal lobe and Wernicke’s area located behind it in the temporal and parietal lobes. Scientists once believed that Broca’s area was chiefly involved in language production while Wernicke’s area dealt more with comprehension, however current thinking suggests that the two areas work in tandem with one another. Although monkeys are not able to perform the mental activities required for language, their brains possess regions that are structurally similar to the perisylvian areas in humans in both hemispheres. The functional significance of such similarities, however, has been unclear up to this point.
Although the coo of a monkey is acoustically very different from a high-pitched scream, the researchers found that both of these meaningful species-specific sounds elicited significantly more activity than the non-biological control stimuli in the same three regions of the macaque’s brain. Moreover, these regions correspond to the key language centers in humans, with the ventral premotor cortex (PMv) corresponding to Broca’s area, and the temporoparietal area (Tpt) and posterior parietal cortex (PPC) corresponding to Wernicke’s area. In contrast, the non-biological sounds—which were acoustically similar to the coos and screams but had no meaning for the animals—elicited significantly less activity in these regions; rather, they were associated with greater activation of the brain’s primary auditory areas.
Based on these findings, the researchers suggest that the communication centers in the brain of the last common ancestor to macaques and humans—particularly those centers used for interpreting species-specific vocalizations—may have been recruited during the evolution of language in humans.
Other institutions represented on the study include Harvard University, Cambridge, Mass; University College London/Institute of Child Health, London; and the University of Maryland, College Park. The work was supported by NIDCD, NIMH, and Fundação para a Ciência e Tecnologia, Portugal.
[SOURCE: NIDCD, July 23, 2006]