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 NIHs 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.
Area 5 (left dorsal temporal pole) at the front of the brains temporal lobe in the left hemisphere, activated significantly more than its counterpart in the right hemisphere only when monkeys heard monkey calls. A right hemisphere area activated more for a variety of other sounds. This suggests that the left hemisphere site is specialized for species-specific vocalizations and may hold clues to the evolution of human speech and language. Front of brain is at right.
Source: NIMH Laboratory of Neuropsychology
PET scan of monkey brain cross-section, showing greater activity in left dorsal temporal pole than in same area of right hemisphere (arrows) while animal listens to monkey vocalizations.
Source: NIMH Laboratory of Neuropsychology
"Since its 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 monkeys ability to perceive such calls is impaired if it lacks the left auditory cortex, but not the right.
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