Mice get an earful from left brain.

Mice get an earful from left brain

A mouse pup strays too far from thelitter and lets out a distinctive ultrasonic squeak. Immediately its mother comes to the rescue, dragging the misplaced tyke back to the nest.

There is, however, more to the mother'sperception of her pup's emergency call than meets the ear. The "communication" call is preferentially processed by the left hemisphere of her brain, according to a report in the Jan. 15 NATURE, just as language is predominantly handled by the left hemisphere in humans. This suggests, says Gunter Ehret of the University of Konstanz, West Germany, that a left-hemisphere advantage in recognizing communication sounds evolved early in mammals.

Until now, the only animal known tohave this type of lateralized brain function was the macaque monkey.

In his first experiment, Ehret removedpups from the litters of 44 female mice and placed the youngsters along a running board that extended across a central nest. Pups were quickly retrieved and a comparable level of "maternal motivation" was created among the mothers. Two loudspeakers, one at either end of the running board, were then switched on. One emitted signals resembling a pup's natural call for help (around 50 kilohertz) and the other gave off 20-kHz tone bursts. The mothers headed for the 50-kHz sound source when both ears were clear and when their left ears were plugged. When their right ears were plugged, the mice showed no preference.

But were females merely unable to pindown the location of the sound sources with only the left ear functioning, or was the meaning of the artificial calls blocked? In a second experiment, virgin female mice with no pup experience were placed in the central nest and trained to approach the 50-kHz tone bursts for a reward of drinking water. This response persisted when either the left or right ears were plugged, suggesting that in the mothers' case, the response was a result of recognizing the pups' call, rather than some extraneous, physiological phenomenon.

The two experiments, says Ehret, showthat mothers with pups have a right-ear, left-hemisphere advantage in recognizing pup calls that does not occur when females with no pup experience are conditioned to respond to the same ultrasounds.

He suggests that brain specializationfor the perception of communication sounds among mice "should be considered as a possible basis of the left-hemisphere advantage for speech sound recognition in man."

Ehret's demonstration of a left-hemisphereadvantage in processing communication sounds among animals that rank far below humans on the evolutionary scale is important, points out psychologist John C. Marshall in an accompanying editorial, but lateralized brain functions are far more complicated in humans than in mice. In the human brain, the right hemisphere appears to be in charge of the perception of emotional types of communication, either seen or heard, says Marshall, of The Radcliffe Infirmary in Oxford, England. The recognition and interpretation of facial expressions is one example. There is also evidence, he notes, that the right hemisphere can process nonspoken language and may have access to the meaning of spoken words.

Furthermore, the linguistic specializationof the left hemisphere in humans covers more than spoken words. A recent study found that the ability to use and understand sign language, in which hand movements and their manipulation in space are critical to meaning, appears to be rooted in the left brain hemisphere (SN: 8/2/86, p.70).

The customary explanation for lateralizedcontrol of human speech, says Marshall, is that it would be too difficult to synchronize timing in complex brain centers that were duplicated in two hemispheres. But this account, he contends, "seems not to apply to the perception of 50-kHz tone bursts . . . why nature should choose an asymmetrical [brain] location for critical biological functions remains as mysterious as ever."