Magnetic pulses can sway the hand you use, briefly

• Brain • • Neurology • Sep 28, 10

Whether your left or right hand reaches for the phone, elevator button or cup of coffee is typically decided unconsciously. Now, a new study suggests that magnetic pulses sent into your brain could alter that choice.

The finding is preliminary, but it brings to mind past efforts to “correct” the handedness of lefty children. In Germany, for example, such “conversions” were standard practice until the 1970s, according to Dr. Stefan Kloppel of the University of Freiburg.

Kloppel, who has studied how the brain makes these decisions, was not involved in the current study and noted that such conversion is no longer recommended.

“People make this decision many, many times throughout the day,” lead researcher Flavio Oliveira of University of California, Berkeley, told Reuters Health, speaking of the choice of which hand to use for a particular task, not handedness conversions. “We wanted to explore how this process emerges in the brain, and which parts of the brain are involved.”

Hand preference is known to be influenced by past experience and the current position of the hands in relation to the target. But just how the decision process is carried out remains unclear.

Alien hand syndrome, a curious condition in which people report losing conscious control over one of their limbs, inspires one possible explanation.

“Often when a patient reaches out with one hand towards an object, the other hand follows, and they will deny attempting that movement,” said Oliveira. “Maybe this is some evidence to suggest that there is a competition that occurs in the brain.”

“For example, in healthy individuals, once one hand wins the competition it inhibits the other,” he explained. “But in these patients, maybe that doesn’t happen.”

To see if the idea held, Oliveira and his colleagues put 33 people, all right-handed, through a series of experiments. Each round, participants were presented with an object at varying locations on a table and told to reach for it as quickly as possible.

Participants showed an overall strong-hand bias, targeting objects midway between their hands more frequently with the right hand until about 15 degrees left of the mid-point.

Compared to reaching for objects positioned at right and left extremes, reaction times were longer for objects closer to the center, report the researchers in the Proceedings of the National Academy of Sciences.

When they stimulated the left posterior parietal cortex with magnetic pulses, temporarily disrupting activity in this region of the brain’s left hemisphere, participants were more apt to reach with their left hand than they had during previous rounds without stimulation. In fact, when objects were positioned in the ambiguous middle territory, they used their left an average of 14 percent more often than they had earlier.

No significant changes were seen in hand preference when the corresponding right part of the brain was stimulated.

“We found that we could modify people’s preference,” Oliveira noted, and this “gave us strong evidence that there was this competition going on.”

In other words, the brain might be simultaneously devising action plans for each hand until one hits a threshold and is actually performed. “You can think of it like a race between two horses,” he added. “With stimulation, you’re handicapping one of the horses—slowing it down such that the other has a better chance to win. So in situations where the horse running for the right hand would normally win, we’re slowing it down a little bit so that the horse for left hand wins more often.”

While well done, Kloppel said the study was not enough to suggest a way to “convert” left-handers - and that such conversions are no longer recommended anyway.

“I think the study by Oliveira provides convincing evidence for the role of the left posterior parietal cortex in hand choice. A study on left-handers would be required to conclude a possible role when converting left-handers,” Kloppel said.

Further, he added that the current study was concerned with hand choice in ambiguous situations: “These differ from most tasks that are used to define handedness, such as writing or using a toothbrush, where the choice of hand should be stable.”

For now, the researchers are more focused on the hope that it could help people who struggle with this basic decision-making as a result of stroke, Parkinson’s or alien hand syndrome.

Regarding stroke, for example, Oliveira noted that recovering patients still tend to prefer to use the limb that wasn’t affected. “By understanding a little more how these decisions happen,” he said, “there might be potential to develop better rehabilitation regimens to work with these patients.”

Parkinson’s patients, he further noted, tend to persist in the same behavior, such as using a particular hand. Oliveira suggested this new understanding could also help in the development of new ways to treat these patients as well.

SOURCE: Proceedings of the National Academy of Sciences, online September 27, 2010.