According to decades of research on affective motivation in the human
brain, approach motivational states are supported primarily by the
left hemisphere and avoidance states by the right hemisphere. The
underlying cause of this specialization, however, has remained
unknown. Here we conducted a first test of the Sword and Shield
Hypothesis (SSH), according to which the hemispheric laterality of
affective motivation depends on the laterality of motor control for
the dominant hand (i.e., the “sword hand," used preferentially to
perform approach actions) and the nondominant hand (i.e., the “shield
hand," used preferentially to perform avoidance actions).
To determine whether the laterality of approach motivation varies with
handedness, we measured alpha-band power (an inverse index of neural
activity) in right- and left-handers during resting-state
electroencephalography and analyzed hemispheric alpha-power
asymmetries as a function of the participants' trait approach
motivational tendencies. Stronger approach motivation was associated
with more left-hemisphere activity in right-handers, but with more
right-hemisphere activity in left-handers.
The hemispheric correlates of approach motivation reversed between
right- and left-handers, consistent with the way they typically use
their dominant and nondominant hands to perform approach and avoidance
actions. In both right- and left-handers, approach motivation was
lateralized to the same hemisphere that controls the dominant hand.
This covariation between neural systems for action and emotion
provides initial support for the SSH.
Citation: Brookshire G, Casasanto D (2012) Motivation and Motor Control: Hemispheric Specialization for Approach Motivation Reverses with Handedness. PLoS ONE 7(4): e36036. doi:10.1371/journal.pone.0036036