A part of the brain that’s involved in sound processing shows pronounced activity when rhesus monkeys hear their comrades vocalizing but not when the same animals hear other sounds, a new brain-scan investigation finds.
In human evolution, this call-oriented, left-hemisphere region served as a precursor of the left-brain structures that now contribute to speech understanding, propose Amy Poremba of the University of Iowa in Iowa City and her coworkers.
The researchers used a positron emission tomography scanner to measure glucose metabolism, a sign of neural activity, in the brains of eight monkeys. Measurements focused on two strips of tissue, one on each side of the brain, that regulate sound processing. During the scans, animals listened to recorded calls of other rhesus monkeys, including screams and barks. They also listened to recordings of people talking, water dripping, and other sounds unrelated to monkey communication.
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Only monkey calls elicited marked activity in the left-brain region, which handles information about sounds that has already been processed by adjacent acoustic tissue. Other sounds evoked particularly intense activity in right-brain areas. The findings appear in the Jan. 29 Nature.
When a monkey hears another’s call, auditory tissues on both sides of the brain react comparably to the sounds, but ensuing interactions between the hemispheres trigger the enhanced left-brain activity, Poremba’s group theorizes. In support of that idea, three monkeys that had surgery to sever the neural fibers that carry messages across the hemispheres displayed comparable left- and right-brain responses to monkey calls, rather than the left-brain emphasis, the scientists note.