Monkeys, like humans, use the left side of the brain when they communicate with each other, according to a recent study in Nature (Vol. 427, No. 6973). The finding suggests that an area of the left brain in rhesus monkeys might be a precursor for human language, says psychologist Amy Poremba, PhD.
Poremba and her colleagues at the National Institute of Mental Health (NIMH) used positron emission tomography (PET) scans to examine the brains of eight rhesus monkeys as they listened to the calls of other monkeys. They found that as the monkeys listened to vocalizations made by members of their own species, a portion of a brain region called the superior temporal gyrus showed more activity in the left hemisphere than in the right hemisphere.
"That's very similar to humans listening to speech," says Poremba, who is now at the University of Iowa. Although monkey calls are not language or speech, the animals do use grunts, coos, barks and other sounds to communicate information like the location of a predator or a good food source.
The scientists didn't find the same lateralization--unequal use of brain hemispheres--in this region when the monkeys listened to other sounds, such as human speech or random noises.
Next, Poremba and her colleagues used the PET scans to examine the reasons that such lateralization might occur. They were surprised to find, Poremba says, that the monkeys' left hemispheres were not, in fact, more active when listening to monkey calls than when listening to other sounds. Instead, the right hemispheres were less active.
In other words, the monkeys had a certain baseline, equal, level of activity in the right and the left hemispheres when they listened to a random sound, like background noise. In that case, there was no lateralization. When the monkeys listened to monkey calls, lateralization appeared. But it wasn't because the left hemisphere became more active; instead, it was because the right hemisphere became less active, making the left hemisphere seem relatively more active in comparison. "We would not have predicted that," Poremba says.
The finding suggests that the left hemisphere was actually suppressing the right hemisphere in the monkeys' brains, Poremba says. To test that theory, the researchers severed the corpus callosum, which connects the two sides of the brain, in several monkeys. Afterward, these monkeys no longer showed lateralization when listening to monkey calls.
The study, Poremba says, might help shed light on how lateralization occurs in both humans and animals. Lateralization is key to human speech processing, she adds, and the fact that monkeys also use lateralization suggests that studying the way they process auditory communication could help elucidate human speech processing systems.
The researchers' next step will be to use electrodes to record signals from individual neurons in the superior temporal gyrus to see what types of information they're encoding and what parts of the monkey calls they're responding to. "We want to get a more complete picture of what the auditory processing system looks like," Poremba says.