Science Watch
Psychologist Stephen Christman, PhD, is mixed-handed: He uses his left hand for most everyday activities, including writing, but can use his right hand for some things, like throwing a ball. Christman's wife, whom he met in college, is strongly right-handed: She uses her right hand for everything.
There's another difference between the two. Christman says he has a better memory for those early college days than his wife does: "There are lots of times I'll say, 'Do you remember that party?' and she'll say, 'What party?'"
Those two facts don't sound related, but Christman suspects that they might be. For nearly two decades, he's been pursuing a line of research that suggests that mixed-handed people have more interaction between the left and right hemispheres of their brains, and that this increased interaction is linked to more reliable and accurate episodic memories--recollections involving personal life events.
Now, in a study in the May issue of Neuropsychology (Vol. 20, No. 3), Christman has extended those findings to the earliest of childhood memories. Everyone has "childhood amnesia" to some degree: No one can recall memories from infancy or early toddlerhood. But Christman's study suggests that mixed-handed people may be able to recall slightly earlier childhood memories than people who are strongly right-handed.
A winding research road
Christman, a psychology professor at the University of Toledo, first began thinking about the connection between interhemispheric interaction and early childhood memories nearly 20 years ago. At the time, he says, he realized that two things happen around age 4 or 5: The corpus callosum--the bundle of nerve cells that connects the two hemispheres of the brain--starts to become functional, and childhood amnesia goes away.
"I thought, 'That seems interesting,'" Christman says. "So I wrote a little note about it to myself and taped it to the wall of my office. But at that point the only evidence I had was this temporal coincidence, and that didn't seem strong enough to justify research."
When, six years later, Christman attended a talk by psychologist Endel Tulving, PhD, the note was still taped to the wall. At the talk, Tulving presented research showing that for semantic memories--memories for names, dates and facts--encoding and retrieval both happen in the left hemisphere. But for episodic memories--such as memories of early childhood or other events--encoding happens in the left hemisphere, and retrieval in the right.
"I had to stop myself from jumping up and down," Christman says. "Because here was evidence that both hemispheres were involved in episodic memory, and so there had to be some interhemispheric communication. And that had to involve the corpus callosum."
Another piece of evidence that the corpus callosum plays a key role in episodic memory comes from studies of split-brain patients--people whose corpus callosi have been cut as a last-resort treatment for epilepsy. One 1996 Neuropsychology study (Vol. 10, No. 2, pages 254-262) by psychologist Alice Cronin-Golomb, PhD, found that these patients had impaired episodic memories.
Still, Christman says, jumping to studying childhood amnesia right away seemed like too big a leap. So he and his colleagues began to study episodic memory in mixed-handed adults. In a 2004 study published in Brain and Cognition (Vol. 56, No. 3, pages 313-319), for example, they found that mixed-handed adults were less likely to falsely remember words than strong right-handers in a word-list memorization task.
In his most recent study, Christman made his way back to the question inspired by the note on his office wall: Do mixed-handed adults, who have a bigger corpus callosum and more interaction between their brain hemispheres, also have earlier first memories from childhood?
To answer this, he asked 103 college-age men and women--37 mixed-handers and 63 strong right-handers--to write down two stories from their early childhood. He did not specifically ask them for their earliest memories--just for an early memory. One story, designed to measure episodic memory, was supposed to be an event that they actually remembered. The other, designed to measure semantic memory, was supposed to be something that they had heard from their parents or another person. Then Christman contacted the participants' parents to verify that the stories, which involved experiences such as a trip to Disney World or a mother's second wedding, were actually true.
On average, he found that the age of the personally remembered stories recalled by the mixed-handers was significantly lower than the average age of the personally remembered stories recalled by the right-handers--5.21 years and 5.94 years, respectively.
Somewhat puzzlingly, he also found that the age of the secondhand stories given by the mixed-handers--2.48 years--was also significantly lower than the right-handers average age of 3.28 years. In early childhood memories, the line between what we personally remember and what we've been told by others is fuzzy, Christman says, and so he suspects that even the secondhand stories might involve some episodic memory.
Support and dissent
Neuropsychologist Joseph Hellige, PhD, who studies brain hemisphere specialization and interaction at the University of Southern California, says he finds Christman's work intriguing. "What makes Steve's work most interesting," he says, "is the novel way that he's taken a number of phenomena in the literature that might be disconnected and put together a logical chain and a story that makes them all fit."
Hellige says that there is "reasonable circumstantial evidence" for each link in that logical chain--that mixed-handers have better interaction between their hemispheres, that better interaction is linked to better episodic memory, and that better episodic memory includes memories from early childhood.
"There are several critical arguments, and if any were off the mark, he wouldn't be able to get the results he's predicting," Hellige says.
Still, some of links remain controversial. For example, University of California, Los Angeles neuropsychologist Dahlia Zaidel, PhD, who has studied memory in split-brain patients, says that she's not convinced that there's enough evidence, particularly physical evidence, to say that mixed-handedness is associated with better interhemispheric communication.
"One of the biggest mysteries we have is the nature of interhemispheric interaction," she says. "We know what the left hemisphere can do, and we know what the right hemisphere can do, but how they communicate is a mystery."
Christman agrees that there is still much left to learn about how the two hemispheres of the brain communicate and the relationship between that communication and handedness. Still, he says that his studies of adult and now early childhood memory provide enough evidence to say that the link exists.
