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Why Being Left Handed Matters for Mental Health

Summary: Researchers believe some treatments for mental health conditions may be ineffective, or even dangerous for left handed people. A new study reports areas of the brain housing alertness and determination may be on the right side for left dominant people. The new theory suggests the location of a person’s neural system for emotion depends on their handedness.

Source: Cornell University.

Treatment for the most common mental health problems could be ineffective or even detrimental to about 50 percent of the population, according to a radical new model of emotion in the brain.

Since the 1970s, hundreds of studies have suggested that each hemisphere of the brain is home to a specific type of emotion. Emotions linked to approaching and engaging with the world – like happiness, pride and anger – lives in the left side of the brain, while emotions associated with avoidance – like disgust and fear – are housed in the right.

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

That longstanding model is, in fact, reversed in left-handed people, whose emotions like alertness and determination are housed in the right side of their brains, Casasanto suggests in a new study. Even more radical: The location of a person’s neural systems for emotion depends on whether they are left-handed, right-handed or somewhere in between, the research shows.

The study, “Approach motivation in human cerebral cortex,” is published in Philosophical Transactions of the Royal Society B: Biological Sciences.

According to the new theory, called the “sword and shield hypothesis,” the way we perform actions with our hands determines how emotions are organized in our brains. 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, results show that approach emotions depend on the hemisphere of the brain that controls the dominant “sword” hand, and avoidance emotions on the hemisphere that controls the non-dominant “shield” hand.

The work has implications for a current treatment for recalcitrant anxiety and depression called neural therapy. Similar to the technique used in the study and approved by the Food and Drug Administration, 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 the treatment could be damaging for left-handed patients. Stimulation on the left would decrease 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,” he said.

a persons left hand

That longstanding model is, in fact, reversed in left-handed people, whose emotions like alertness and determination are housed in the right side of their brains, Casasanto suggests in a new study. NeuroscienceNews.com image is in the public domain.

“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.”

However, Casasanto cautions that this research studied only healthy participants and more work is needed to extend these findings to a clinical setting.

About this neuroscience research article

Funding: The research was funded by a James S. McDonnell Foundation Scholar Award and the National Science Foundation.

Source: Cornell University
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is in the public domain.
Original Research: Abstract for “Approach motivation in human cerebral cortex” by Geoffrey Brookshire and Daniel Casasanto in Philosophical Transactions of the Royal Society B: Biological Sciences. Published June 18 2018
doi:10.1098/rstb.2017.0141

Cite This NeuroscienceNews.com Article
Cornell University “Why Being Left Handed Matters for Mental Health.” NeuroscienceNews. NeuroscienceNews, 18 June 2018.
<http://neurosciencenews.com/left-handed-mental-health-9376/>.
Cornell University (2018, June 18). Why Being Left Handed Matters for Mental Health. NeuroscienceNews. Retrieved June 18, 2018 from http://neurosciencenews.com/left-handed-mental-health-9376/
Cornell University “Why Being Left Handed Matters for Mental Health.” http://neurosciencenews.com/left-handed-mental-health-9376/ (accessed June 18, 2018).

Abstract

Approach motivation in human cerebral cortex

Different regions of the human cerebral cortex are specialized for different emotions, but the principles underlying this specialization have remained unknown. According to the sword and shield hypothesis, hemispheric specialization for affective motivation, a basic dimension of human emotion, varies across individuals according to the way they use their hands to perform approach- and avoidance-related actions. In a test of this hypothesis, here we measured approach motivation before and after five sessions of transcranial direct current stimulation to increase excitation in the left or right dorsolateral prefrontal cortex, in healthy adults whose handedness ranged from strongly left-handed to strongly right-handed. The strength and direction of participants’ handedness predicted whether electrical stimulation to frontal cortex caused an increase or decrease in their experience of approach-related emotions. The organization of approach motivation in the human cerebral cortex varies across individuals as predicted by the organization of the individuals’ motor systems. These results show that the large-scale cortical organization of abstract concepts corresponds with the way people use their hands to interact with the world. Affective motivation may re-use neural circuits that evolved for performing approach- and avoidance-related motor actions.

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