Scans have pinpointed circuits in the monkey brain that could be precursors of those in humans for speech and language. As in humans, an area specialized for processing species-specific vocalizations is on the left side of the brain, report Drs. Amy Poremba, Mortimer Mishkin, and colleagues in NIH's National Institute of Mental Health (NIMH), Warren G. Magnuson Clinical Center (CC), components of the National Institutes of Health (NIH), and the University of Iowa. An area near the left temple responded significantly more than the same area on the right only to monkey calls, not to other animal calls, human voices or various other sounds. The researchers published their findings in the January 29, 2004 Nature.
"Since it's in the left temporal lobe and specialized for vocalizations only, it bears intriguing similarities to human language," noted Mishkin. "Assuming this is an adaptive mechanism, it suggests that vocalizations can be deciphered better if they are processed by only one temporal pole rather than by both."
Scientists have known for years that the human brain processes speech on the left side of the brain, but they only had hints that this is also the case for non-human primates. For example, when a monkey hears a call from behind, it characteristically turns its head to the right, suggesting that the primitive vocalizations are being processed in the left hemisphere, which receives greater input from the right ear than from the left. Also, a monkey's ability to perceive such calls is impaired if it lacks the left auditory cortex, but not the right.
To find out how this works, the researchers used PET (positron emission tomography) scanning. A radioactive tracer visualized the parts of the brain that were active while different types of sounds were being processed. After eight healthy monkeys heard a series of monkey calls, an area just below the left temple, at the front of the left temporal lobe (left dorsal temporal pole), activated significantly more than its mate on the right. Yet, this area failed to similarly activate when the animals heard a variety of other sounds - bells, tones, dog barks, bird tweets, a human voice, scrambled monkey calls, etc. Instead, significant activation was seen in a different temporal lobe area on the right side of the brain, which seems to process virtually every sound.
To gain insight into how the brain achieves this hemispheric specialization, three monkeys surgically-altered to lack connecting links between the hemispheres were also scanned after listening to the sounds. With communication between the hemispheres thusly severed, the asymmetrical pattern vanished. Conspicuously, no significant difference in activation was seen in the two temporal poles when the animals heard monkey calls.
This suggests that monkey calls normally stimulate interactions between brain hemispheres that suppress the corresponding right temporal lobe area, focusing auditory processing within the left area. "Our results open up the possibility of characterizing such neuronal responses in a cortical region of the monkey that is not only a higher-order auditory processing area, but also one that could be a precursor for an acoustic language area in humans," note the researchers.
"This study provides neuroscientists with new biological clues for studying how communication evolved," said Poremba, who left NIMH a few years ago and is now at the University of Iowa.
In addition to NIMH, the research was supported, in part, by the University of Iowa.
Also participating in the study were Megan Malloy, NIMH, Dr. Richard Saunders, NIMH, Dr. Richard Carson, CC, Dr. Peter Herscovitch, CC.
The National Institute of Mental Health (NIMH) and the Warren G. Magnuson Clinical Center are parts of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research. NIH is a component of the U.S. Department of Health and Human Services.