Approach motivation in right- and left-handers.
The way we use our hands may determine how emotions are organized in our brains, according to a new study led by Geoffrey Brookshire, a Ph.D. student in my lab at the New School for Social Research, published in PLoS ONE .
Motivation, the drive to approach or avoid physical and social stimuli, is a basic building block of human emotion. For decades, scientists have believed that approach motivation is computed mainly in the left hemisphere of the brain, and avoidance motivation in the right hemisphere . Brookshire’s study challenges this idea, showing that a well-established pattern of brain activity, found across dozens of studies in right-handers, completely reverses in left-handers.
The study used electroencephalography (EEG) to compare activity in participants’ right and left hemispheres during rest. After having their brain waves measured, participants completed a survey measuring their level of approach motivation, a core aspect of our personalities. In right-handers, stronger approach motivation was associated with greater activity in the left hemisphere than the right, consistent with previous studies. But left-handers showed the opposite pattern: Approach motivation was associated with greater activity in the right hemisphere than the left.
A new link between motor action and emotion: The Sword and Shield Hypothesis
Most cognitive functions do not reverse with handedness. Language, for example, is mainly in the left hemisphere for the majority of right- and left-handers. So why would the wiring of motivation depend on handedness?
We predicted this hemispheric reversal because we observed that people tend to use different hands to perform approach- and avoidance-related actions. Approach actions are often performed with the dominant hand, and avoidance actions with the nondominant hand .
In centuries past, sword fighters approached their enemy wielding the sword in their dominant hand, and avoided injury by raising the shield with the nondominant hand. This “sword and shield” pattern is evident in more ordinary motor actions, as well. Imagine picking an apple from a tree. You’re likely to pull the fruit toward you with your dominant hand (an approach action) and push away the branch with your nondominant hand (an avoidance action).
In right-handers, approach motivation is computed by the hemisphere that controls the right hand, and avoidance motivation by the hemisphere that controls the left hand. This may be no mere coincidence. Neural circuits underlying motivation may be functionally related to circuits that control hand actions—emotion may be built upon neural circuits for action, either in evolutionary or developmental time.
By showing that the right hemisphere computes approach motivation in left-handers, the same hemisphere that controls the left hand, Brookshire’s results provide the first evidence for the link we propose between action and emotion, which we call the Sword and Shield Hypothesis. Motivation is organized in the brain consistent with—and perhaps because of—the way people tend to use their dominant and nondominant hands to perform approach and avoidance actions.
These data are only a first step, establishing a correlation between emotional motivation and motor control. Further studies are needed to establish a causal link between action and emotion in the brain.
The “right” treatment for depression and anxiety disorders?
To treat depression and anxiety disorders, brain stimulation is used to increase neural activity in the patient’s left hemisphere , long believed to be the “approach hemisphere.” Given what we show here, however, it appears that the usual treatment, which is helpful to right-handers, may be detrimental to left-handers — the exact opposite of what they need. The discovery that approach motivation reverses with handedness may lead to safer, more effective neural therapies for left-handers.
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1. Brookshire, G. & Casasanto, D. (2012). Motivation and Motor Control: Hemispheric specialization for approach motivation reverses with handedness. PLoS ONE, 7(4): e36036.
To download paper: http://dx.plos.org/10.1371/journal.pone.0036036
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