Comparison of monkeys and humans reveals superior temporal sulcus as the region that has evolved for human language processing
It has been often remarked that speech and language represent a highly specialized skill that is unique to humans. It is, however, increasingly recognized that animals do also use acoustic signals to communicate with conspecifics. This suggests that humans and certain other species are closer to each other with respect to evolution of language than what has been traditionally assumed, even though the human language is much more complex and refined than animal communication calls. The species-specific vocalizations of non-human primates constitute a prime example of this, however, there have been relatively few attempts to compare foci of brain responses to non-speech/communicative sounds vs. speech and communication calls in humans vs. non-human primates.
In their recent functional magnetic resonance imaging (fMRI) study, Olivier Joly et al. (2012), presented humans and macaque monkeys with monkey vocalizations, human emotional non-linguistic vocalizations, intelligible speech, non-intelligible speech, bird songs, as well as scrambled control sounds. The authors observed widespread hemodynamic responses in temporal, frontal and parietal cortical areas to vocalizations and scrambled control sounds in both species. Further, non-primary auditory areas in the temporal cortex preferentially responded to the intact sounds. Interestingly, parabelt areas extending into superior temporal gyrus responded to monkey vocalizations in macaques matching areas activated by unintelligible speech and emotional sounds in humans. Further, monkey superior temporal sulcus appeared as not responding to species-specific sounds, thus sharply contrasting with the human superior temporal sulcus (and Broca’s area) that specifically responded to intelligible speech.
Taken together, the results of this highly interesting study suggest that evolution of language in humans has recruited most of the superior temporal sulcus, whereas in monkeys the much simpler species-specific vocalizations have not required corresponding involvement of this area. Methodologically, this pioneering study very nicely demonstrates how macaque and human brain function can be compared at multiple levels of processing using non-invasive functional magnetic resonance imaging, in addition to shedding light on the highly intriguing question of which brain areas have developed in humans to enable our rich language skills that have to a large part made it possible for human societies to emerge and develop.
Reference: Joly O, Pallier C, Ramus F, Pressnitzer D, Vanduffel W, Orban GA. Processing of vocalizations in humans and monkeys: a comparative fMRI study. Neuroimage (2012) 62: 1376-1389. http://dx.doi.org/ 10.1016/j.neuroimage.2012.05.070