A link between the brains of mice and men - brain lateralization

A LINK BETWEEN THE BRAINS OF MICE AND MEN

In man, spatial perception is the business of the right side of the brain, speech recognition the domain of the left. This division of labor, known in neuro psychological circles as brain lateralization, is thought to be found only in primates. But Gunter Ehret, a zoologist at the University of Constance in West Germany, has concluded from studies of the house mouse that animals were lateralizing long before they were capable of verbalizing, moralizing, or fantasizing. And he thinks he knows why.

When a mouse pup meanders too far from the litter, it emits an ultrasonic cry in a very narrow frequency range -- around 50 kHz. Upon hearing this plaintive call, the mother drops what she's doing, runs to her pup, and hauls it back to the nest. Nature has apparently wired her to react to the sound.

Actually, any 50 kHz sound will set a mother mouse to running. When Ehret simultaneously sounded a 50 kHz tone to one side of the mouse nest and a 20 kHz tone to the other, the mothers headed straight for the 50 kHz tone. If the mothers' left ears were plugged, they responded the same way. But if their right ears were plugged, tey ran to either pitch.

Since the auditory nerves of both mice and men are mainly contralateral (the left hemisphere is connected to the right ear and vice versa), Ehret's experiment demonstrated that the left hemisphere dominates when it comes to a mother mouse's hearing and responding to a distress call. Says Ehret, ''Left hemisphere dominance has been around since the beginnings of mammals and isn't a specialty of man.''

But what exactly is the mouse's left hemisphere in charge of -- merely favoring one pitch over another or understanding that one is a distress call? To find out, Ehret took virgin mice that had no experience with distressed pups and trained them to discriminate between the two tones: if they headed for the 50 kHz tone, they would get a drink of water; if they went for the 20 kHz tone, they would get none. They were then tested with one or the other of their ears plugged. Unlike the moth- ers, these mice headed for the 50 kHz tone regardless of which ear was covered.

Apparently, either hemisphere can discriminate between sounds. But when it comes to interpreting sounds as meaningful, the left hemisphere dominates. ''In a communicative context,'' says Ehret, ''the mother's brain is lateralized, but in a non- communicative context, it's not.''

This lateralization is probably determined by genetics, not by environment, he says. Yet it's environmental necessity that motivates a creature to interpret a sound correctly. Mice must learn to connect a 50 kHz sound to a lost pup. And man must learn to connect certain sounds to certain meanings in speech.

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