Handedness and index finger movements performed on a small touchscreen.

Many studies of right/left differences in motor performance related to handedness have employed tasks that use arm movements or combined arm and hand movements rather than movements of the fingers per se, the well-known exception being rhythmic finger tapping. We therefore explored four simple tasks performed on a small touchscreen with relatively isolated movements of the index finger. Each task revealed a different right/left performance asymmetry. In a step-tracking Target Task, left-handed subjects showed greater accuracy with the index finger of the dominant left hand than with the nondominant right hand. In a Center-Out Task, right-handed subjects produced trajectories with the nondominant left hand that had greater curvature than those produced with the dominant right hand. In a continuous Circle Tracking Task, slips of the nondominant left index finger showed higher jerk than slips of the dominant right index finger. And in a continuous Complex Tracking Task, the nondominant left index finger showed shorter time lags in tracking the relatively unpredictable target than the dominant right index finger. Our findings are broadly consistent with previous studies indicating left hemisphere specialization for dynamic control and predictable situations vs. right hemisphere specialization for impedance control and unpredictable situations, the specialized contributions of the two hemispheres being combined to different degrees in the right vs. left hands of right-handed vs. left-handed individuals.