Researchers from Cornell University, New York, may have identified a new model of emotion in the brain, which can explain why a form of treatment for mental health problems may not work on left-handed people.

The study titled “Approach motivation in human cerebral cortex” was published in Philosophical Transactions of the Royal Society B on June 18.

Old model on brain hemispheres

One of the theories in studies emerging since the 1970s was that each half of the brain is specialized for different emotions i.e. the left side of your brain processes approach-oriented emotions (happiness, pride, anger) while the right side of your brain processes avoidance-oriented emotions (disgust, fear). 

But these studies were flawed since they involved only right-handed participants in most cases.

"The old model suggests that each hemisphere is specialized for one type of emotion, but that’s not true," said Daniel Casasanto, associate professor of human development and of psychology at Cornell.

The sword and shield hypothesis

The new study suggested this model is actually reversed in the brains of left-handed people. The findings may lead to a major shift in our understanding of how brain processes emotion.

Casasanto observed people use their dominant hand for approach-oriented actions and their non-dominant hand for avoidance. One would wield a sword in their dominant hand to make approach-related actions (like stabbing) but would hold the shield in their non-dominant hand to avoid attacks.

“Your dominant hand gets the thing you want and your non-dominant hand pushes away the thing you don’t,” he explained.

Electrical stimulation experiment

As part of the study, 25 participants without health problems were recruited for an experiment where a painless electric current was used to stimulate both hemispheres of their brain.

The brain-zapping was performed for 20 minutes every day for five days, after which they reported how strongly they felt approach-related emotions like enthusiasm, pride, alertness, etc.

Positive emotions were reported by strong right-handers zapped in the left hemisphere and strong left-handers zapped in the right hemisphere. But when right-handers were zapped in the right hemisphere or when left-handers were zapped in the left hemisphere, they either reported no change or a decrease in such emotions.

Implications for mental health treatment

The authors pointed out an FDA-approved treatment called neural therapy used for recalcitrant anxiety and depression. They explained it uses a similar technique as the brain-zapping experiment, involving a mild electrical stimulation of the left hemisphere of the brain to boost approach-related emotions.

Since most people are not strongly right- nor left-handed, the stimulation may not have a major effect on them. Casasanto attributed this to their approach-related emotions being distributed across both hemispheres. The findings suggested this treatment may provide no benefit or even be detrimental if used by strongly left-handed patients. 

“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,” he said.

However, since the study was very small and only involved healthy participants, further research is needed to know if the results can be replicated in a clinical setting.