Left brain hammers out tool use

From New York City, at a meeting of the Cognitive Neuroscience Society

Pounding nails with a hammer, sawing wood, and wielding other familiar tools are feats coordinated by the brain’s left hemisphere, new studies suggest.

“The left hemisphere may maintain knowledge about how to use objects that serve as extensions of our bodies,” says Scott H. Johnson-Frey of Dartmouth College in Hanover, N.H.

Johnson-Frey and his coworkers first studied 12 adults with healthy brains. A so-called functional magnetic resonance imaging scanner measured neural blood-flow changes–an indirect marker of brain-cell activity–as each volunteer first pantomimed using a hammer and other common tools and then made random hand movements.

The scans revealed increases in brain-cell activity unique to tool use only in parts of the left hemisphere, near the front and toward the back of the brain. These results held whether the right-handed participants pantomimed with their right or left hands.

The left-brain association with tool use was strengthened by studies of two people who previously had the nerve fibers connecting their left and right hemispheres surgically severed to control severe epilepsy. In these split-brain individuals (SN: 2/24/96, p. 124), the right hemisphere controls the body’s left side and handles information flashed in the left visual field, while the left hemisphere runs right-sided affairs. One patient was right-handed and the other was left-handed.

After seeing pictures of tools flashed on the right for left-brain inspection, both patients accurately demonstrated how to use the implements. After seeing pictures flashed on the left for right-brain scrutiny, however, their portrayals became confused and inappropriate.

The new findings add to prior observations of tool-use difficulties in people with various types of left-brain damage.


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Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.