Brains of Lefties Organized Differently

EMBARGOED until March 4, 2002 at 4 p.m. EST

Elaine Schmidt ([email protected])(310) 794-2265

Brains of Lefties Organized Differently Than Right-handers', UCLA Scientists Discover

For the first time, UCLA researchers have determined that genetics plays a significant role in shaping brain structure and influences the brains of left-handed and right-handed people differently. Reported in March's Proceedings of the National Academy of Sciences, the findings may offer insights into autism, dyslexia and stuttering -- language disorders more prevalent in left-handers.

Scientists have long known that the left side of the brain typically controls the right side of the body and vice versa. That's why the language control center for most right-handed people operates from the left side of the brain. But the organization of left-handed people's brains has never been fully understood.

"Ninety percent of the world is right-handed," explained Dr. Daniel Geschwind, UCLA assistant professor of neurology and principal investigator. "Close to 99 percent of these people have language localized in the left hemispheres of their brains. But previous studies have shown that left-handed people don't mirror this model. We wanted to determine whether genetics explained this difference."

Geschwind and colleagues Drs. Dorit Carmelli, Charlie DeCarli and Bruce Miller used MRI scans to compare the brain size and structure of 72 sets of identical twins to 67 pairs of fraternal twins. The twins were all male World War II veterans ages 75 to 85, and part of a larger study led by Carmelli.

Identical right-handed twins displayed very similar brain structure sizes, while twins with at least one left-hander showed less similar brains. Fraternal twins -- who share only 50 percent of their genes -- did not demonstrate this difference, suggesting that the explanation could be genetic.

"Identical twins share 100 percent of their genes, so this study shows us that brain structure is highly influenced by genetics, even later in life," Geschwind observed. "This implies that aging-related changes to the brain also possess a strong genetic basis."

Regardless of whether the twins were left-handed or right-handed, the UCLA team noticed a significantly greater environmental influence on certain regions of the left side of the brain.

"This is important, because language resides in this hemisphere in most people," said Geschwind. "We believe this influence takes place before birth, when the twins shared a womb environment."

According to Geschwind, environmental influences, such as hormones or other factors, may play a greater role in shaping language than cognitive functions housed in the right hemisphere of the brain. His theory will focus future UCLA research to identify the genes underlying the structural differences that may contribute to language.

The National Heart, Lung and Blood Institute supported the UCLA study. Other funding for Geschwind's research came from the National Institute of Neurological Disease and Stroke, the National Institute of Mental Health and the McDonnel-Pew Foundation.

-UCLA-