Why being a righty or lefty matters in mental health treatment

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ITHACA, N. Y. — Since the 1970s, it has been generally understood by neurologists and psychologists that different emotions come from different hemispheres of the brain. “Approach” emotions linked to how we engage with the world — happiness, pride, or anger, for example — usually come from the left side of the brain. On the other hand, those associated with avoidance, such as disgust and fear, come from the right side. Decades of research into this concept has informed mental health treatment, but there’s one problem: it was conducted almost entirely on right-handed people.

As it turns out, long-discovered emotional structure of the brain is reversed in left-handed people, affecting and calling into question how they respond to certain treatments, according to recent research out of Cornell University.

The study by Daniel Casasanto, an associate professor of human development and psychology, not only describes the reversal in left-handed people, but it suggests that many are actually somewhere in between being right- and left-handed. Researchers say their emotional centers are correspondingly in different areas of the brain than previously thought.

“The old model suggests that each hemisphere is specialized for one type of emotion, but that’s not true,” Casasanto explains in a university release. “Approach emotions are smeared over both hemispheres according to the direction and degree of your handedness … . The big theoretical shift is, we’re saying emotion in the brain isn’t its own system. Emotion in the cerebral cortex is built upon neural systems for motor action.”

For the research, Casasanto and his team stimulated the two brain hemispheres of 25 healthy patients with a painless electrical current for 20 minutes daily over five days. The research team wanted to see if they could cause their subjects to experience approach emotions like enthusiasm, interest, and excitement through stimulation depending on whether they were right-handed, left-handed, or in-between. Participants were asked to report levels of positive emotions like pride and happiness at the start and end of the study period.

As hypothesized, right-handed participants who were zapped in the left hemisphere reported greater levels of positivity, as did lefties when their right hemisphere was stimulated. When they were zapped in the same hemisphere as their dominant hand, there was no change in such emotions.

Casasanto says the findings impact a current electrical stimulation treatment for people suffering from anxiety or depression. Because the treatment involves zapping the left side of the brain in order to boost positive emotions, the new findings would suggest that the treatment could be detrimental for left-handed patients.

“If you give left-handers the standard treatment, you’re probably going to make them worse. And because many people are neither strongly right- nor left-handed, the stimulation won’t make any difference for them, because their approach emotions are distributed across both hemispheres,” he explains. “This suggests strong righties should get the normal treatment, but they make up only 50 percent of the population. Strong lefties should get the opposite treatment, and people in the middle shouldn’t get the treatment at all.”

The study is published in the journal Philosophical Transactions of the Royal Society B: Biological Sciences.

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