Lateralized effects of self-induced sadness and happiness on corticospinal excitability

We studied the changes in excitability of the corticospinal projection evoked by self-induced sad and happy thoughts. Corticospinal excitability was probed using focal, single-pulse transcranial magnetic stimulation (TMS) applied to the optimal scalp position for evoking motor potentials in the contralateral first dorsal interosseus muscle. Fourteen right-handed subjects were studied while counting mentally, thinking sad thoughts, or thinking happy thoughts. In each of these three conditions TMS was applied in each subject randomly, 20 times to the right and 20 times to the left hemisphere. Sad thoughts resulted in a significant facilitation of the motor potentials evoked by left-hemispheric stimulation, while happy thoughts facilitated motor potentials evoked by right-hemispheric TMS, but decreased the amplitude of those evoked by left-hemispheric TMS. In two subjects an additional experiment using H-reflex measurements suggests that these changes are caused by changes in cortical rather than spinal excitability. These results further illustrate the lateralized control of mood in normal volunteers.