Selection of action: can transcranial magnetic stimulation influence hand choice in reaching?

Konkle, Marchant, Verstynen, Diedrichsen, & Ivry

When reaching for an object, how do you decide which hand to use? We theorized that competing action plans are generated for both hands before one response is selected. This competitive process is evidenced in reaction times: reaches to central locations, where there is high response conflict between the hands, take longer to initiate than reaches to peripheral locations, where there is low response conflict. This reaction time difference is absent when only one hand is used to perform all of the movements. In the current study, we explore the neural locus of this competition. Participants reached with either hand to a target circle presented at one of ten locations on a semi-circular array. For half of the reaches, a TMS pulse was delivered 140 ms after target presentation to either the left or right lateral premotor cortex. As a control condition, trials with no stimulation were interleaved. Each subject preferred to use the hand closest to the target for eccentric locations, with a soft decision boundary for central targets. We predicted that participants would be biased to use the hand ipsilateral to the stimulation on TMS trials. However no shift in hand preference was observed. TMS did lead to a significant decrease in reaction time. This effect was independent of target position and the side of stimulation, suggesting a general arousal and auditory cueing effect. It is unclear whether we are disrupting the lateral premotor cortex at the incorrect time, or if stimulation over another cortical area would be more effective. To address this, we are currently investigating a range of stimulation times over premotor cortex, and also primary motor and parietal cortex.

Konkle, T., Marchant, N., Verstynen, T., Diedrichsen, J., & Ivry, R. B. Selection of action: can transcranial magnetic stimulation influence hand choice in reaching? Program No. 980.12. 2005 Neuroscience Meeting Planner. Washington, DC: Society for Neuroscience, 2005. Online.