Motor Asymmetries Predict Neural Organization of Emotion

According to decades of research in affective neuroscience, approach and avoidance motivation are supported by the left and right hemispheres, respectively. With the Sword and Shield Hypothesis (SSH), we challenge this conclusion, and propose a novel principle underlying the organization of emotion in the brain: the hemispheric lateralization of motivation depends on the neural locus of motor control for the dominant hand (used preferentially for approach actions) and the non-dominant hand (used preferentially for avoidance actions). The SSH predicts that the laterality of approach motivation should vary continuously with the laterality of circuits used for planning and executing approach-related actions. To test this prediction, we measured mood before and after 5 sessions of tDCS applied bilaterally to DLPFC in rightand lefthanders. Results in right-handers show that positive emotions increased after left-excitatory stimulation, but decreased after right-excitatory stimulation. In non-right-handers, however, the opposite pattern was found: Positive emotions decreased after left-excitatory stimulation, but increased after right-excitatory stimulation. These findings reveal continuous covariation between the neural systems for action and emotion, supporting the SSH.