Mental health treatments that involve electrical or magnetic stimulation to the brain could be ineffective or even harmful to psychiatric patients who are not strongly right-handed, according to a new model of human emotion demonstrated by researcher Dr. Daniel Casasanto from Cornell University.

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

Since the 1970s, hundreds of studies have suggested that specific emotions are housed in either the right or left side of the brain. Emotions associated with approaching and engaging with the world, like happiness, pride and anger, are in the left side of the brain, while emotions linked to avoidance like disgust and fear are housed in the right.

But those studies were done almost exclusively on right-handed people. This has given us a skewed understanding of how emotion works in the brain, according to Casasanto, an associate professor of human development and psychology at Cornell University in New York.

He suggests that this long-standing model is, in fact, reversed in left-handed people, whose outward emotions such as alertness and determination are housed in the right side of their brains. In fact, the new model shows that the location of a person’s neural systems for emotion depends on whether they are left-handed, right-handed or somewhere in between.

The new theory is called the “sword and shield hypothesis.” It posits that, based on the way we perform actions with our hands determines how emotions are organized in our brains. For example, sword fighters of old would wield their swords in their dominant hand to attack the enemy — an approach action — and raise their shields with their non-dominant hand to fend off attack — an avoidance action.

Consistent with these action habits, the study findings show that approach emotions rely on the hemisphere of the brain that controls the dominant “sword” hand, while avoidance emotions depend on the hemisphere that controls the non-dominant “shield” hand.

The findings have significant implications for neural therapy used for hard-to-treat anxiety and depression. It involves a mild electrical stimulation or a magnetic stimulation to the left side of the brain, to encourage approach-related emotions.

But Casasanto’s work suggests that this treatment could be harmful for left-handers. Specifically, stimulation on the left would reduce life-affirming approach emotions.

“If you give left-handers the standard treatment, you’re probably going to make them worse,” Casasanto said. “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.”

“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.”

Since the study focused on healthy participants only, more research is needed to extend these findings to a clinical setting, researchers said.

Source: Cornell University