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A Theory of Relativity

Family history of left-handedness sheds light on memory function

FRIDAY, Nov. 30, 2001 (HealthDayNews) -- Is mom or dad left-handed? How about sis or bro? If so, you might have a better memory for events -- no matter what hand you favor, says new research reported in the journal Neuropsychology.

The key lies in a part of the brain called the corpus callosum, which seems to be larger and more developed in folks with a family history of left-handedness, says study author Steven Christman.

"The corpus callosum is the large bundle of nerve fibers that connects the two sides of our brain and is one of the last major brain structures to mature," says Christman, a researcher at Ohio's University of Toledo, where the study took place.

A larger corpus callosum allows for more and better communication between both halves of the brain, and he says that translates into a better memory for episodic events.

Christman says memory is divided into two categories. The first type is called semantic or implicit memory: We don't really remember things as much as we just "know" them -- such as a fact we learned in school.

"For example, you probably know that Mt. Everest is the tallest mountain on earth but are unable to recall when and where you first learned that fact; thus, it would be a semantic memory," says Christman.

Implicit or semantic recall uses just one side of the brain.

The second type of recall is known as episodic memory: We remember not only a fact, but how and when we learned that fact.

"You remember what you had for dinner last night and can also remember where and when you acquired that memory -- Sunday evening in the kitchen, for example," says Christman.

Episodic memory relies on both halves of the brain working together -- one side recalling the fact, the other recalling the circumstances or episodes around that fact.

While all of us have the ability to recall both episodic and semantic memories, Christman says his study shows that people with left-handed relatives appear to use both sides of their brains more efficiently. Oddly enough, they don't have to be left-handed themselves to reap the benefits.

For Dr. Melinda Lantz, the finding is important because it increases understanding of memory loss, and one day that may help us better understand how to stimulate memory in people with a variety of illnesses.

"This research has great potential for helping stroke victims, as well as those who suffer with dementia [severe mental decline], and even Alzheimer's disease. If the findings are duplicated, then I think we have a wonderful new basis for discovering how we can better stimulate memory in those who are suffering a loss," says Lantz, chief of psychiatry at the Jewish Home and Hospital in New York City.

Christman's two-part study included a group of 84 right-handed undergraduates. They were presented with strings of words and letters designed to stimulate only one side of the brain -- for implicit memory --or interactions between the two sides of the brain -- for episodic memory.

When the stimuli was geared to only one half of the brain, the subjects remembered facts better, but not events. When they were exposed to stimuli geared towards both sides of the brain, participants remembered both facts and the events surrounding those facts.

Christman says the results illustrate that communication between both halves of the brain are required to recall episodic memories.

In the second part of the study, Christman tested 180 right-handed Air Force recruits, using various types of word-memory tasks. He found that subjects with a family history of left-handedness showed superior episodic memory.

"Our results showed that episodic memory is specifically enhanced by increased interaction between the two sides of the brain, which is mediated and handled via the corpus callosum," says Christman. And those who are left-handed appear to have a larger, more well-developed corpus callosum and better inter-brain communication -- and ultimately, a better episodic memory, he says.

The study also helped shed light on why children normally don't have any episodic memory until after age 4. Since his research indicates that the corpus callosum is required for memory activity involving both halves of the brain, and since that section of the brain doesn't mature until after age 4, episodic memories can't form before then, he says.

If the study has any flaw, Lantz says it's that all the subjects were healthy, with normal memories.

"What we don't know is if these same results would hold true if we were testing people who had memory problems, whether from disease such as Alzheimer's, a stroke, or dementia," she says.

What To Do

For answers to common questions about memory loss, visit Rutgers University's Memory Disorder Project.

For some facts about what affects memory and some things you can do to protect against memory loss, click here. Check here to see the corpus callosum.

For a fun page of experiments that help improve your memory, click here.

SOURCES: Interviews with Steven Christman, Ph.D., researcher, University of Toledo, Ohio; Melinda Lantz, M.D., chief of psychiatry, Jewish Home and Hospital, New York City; October 2001 Neuropsychology
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