A Role for the Right
A counterintuitive but essential feature of Petkov’s results, similar to the corresponding findings in the human brain, is that voice-selective activity was stronger in the right hemisphere. Furthermore, the identity-specific neuronal adaptation was observed only in the right hemisphere of the macaque brain, exactly as in the human studies. This finding means that the right hemisphere may well have played a major role in how speech appeared in our ancestors and that a response to the puzzle of speech evolution may lie not only in the left hemisphere.
We have much work ahead before we can attain a complete understanding of the functional role of the voice area, in macaques as well as in humans. Several alternative hypotheses remain to be tested: Does the voice area represent a hardwired preference for the particular acoustical structure of vocalizations from one’s own species? Or is it more simply a “formant” detector, a structure specialized in detecting vocal features in general? Another possibility is that this voice area is actually a “social” structure, tuned to vocalizations because they are cues for social interaction and not because they share a particular acoustical structure.
In conclusion, Petkov’s findings provide an exciting common substrate for high-level, or complex, auditory cognition that can be studied in parallel in humans and in macaques. Now that the location of the voice area in the macaque brain has been established, researchers will obtain critical additional information in the near future by exploring the monkey’s voice area using more conventional electrophysiological techniques, such as recording directly from neurons. Even more important, this seminal work opens the road for comparative neuroimaging studies in which humans and other animals perform similar tasks using similar methodologies, and the results can be analyzed using similar strategies.
Note: This story was originally published with the name, "Monkeys Hear Voices".