Monkeys, like people, have a brain region that responds selectively to the voices of other monkeys, according to new research. The finding should pave the way for studies on the neural basis of voice recognition and may help shed light on how the human brain changed as speech and language evolved.
In people, a small patch of the temporal lobe of the brain revs up in response to human speech but not to other sounds. This region is thought to play a role in recognizing individuals by voice, a talent we share with many animals. Yet little is known about the neural basis of voice recognition in any species. Recent studies with monkeys have claimed to demonstrate a voice region analogous to that in humans, but not all scientists have been convinced.
In the new study, neuroscientists led by Christopher Petkov and Nikos Logothetis, both of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, used functional magnetic resonance imaging to monitor brain activity in macaques while the monkeys listened to a variety of sounds. Those included coos, grunts, and other macaque vocalizations, as well as the calls of other animals and natural noises such as thunder and running water. A small region of the monkeys' temporal lobes became active in response to monkey voices but not to other sounds, the researchers report online this week in Nature Neuroscience. The team also found that this brain region can distinguish the voices of individual monkeys: Responses diminished when the researchers played one monkey's voice repeatedly but perked up again when a new voice was played.
The monkey voice region may provide clues to the evolution of speech, Petkov says. One possibility, he notes, is that neural circuits in the brains of our distant primate ancestors, used for recognizing and evaluating the calls of other individuals, became the precursors for the neural circuits that evolved to handle more complex verbal communication such as speech.
"They've done a really nice job" of demonstrating that monkeys have a voice region very similar to that in the human brain, says neuroscientist Asif Ghazanfar of Princeton University. It's noteworthy, however, that the monkey and human voice regions are positioned somewhat differently in their respective temporal lobes, Ghazanfar says. He suspects that the voice-recognition region of our primate ancestors' brains migrated toward more recently evolved regions that decode and produce speech in the human brain--an example of the kind of neural reorganization that happened as language evolved.