Reaction time differences in spatially constrained bilateral and unilateral movements

Previous investigations of bilateral-unilateral reaction time (RT) differences have reported equivocal findings. Studies where bilateral RT has been found longer than unilateral RT have often emphasized movement precision, while studies reporting non-significant differences have placed little emphasis on precision. To test the hypothesis that movement precision is an important factor in the bilateral unilateral difference, we investigated the influence of changes in spatial accuracy constraints on RT for unilateral and bilateral movements. Ten self-declared right-handed subjects performed fast and accurate 45 degrees unilateral and bilateral elbow flexion movements to small (1.5 degrees) and large (12.0 degrees) targets. For bilateral movements, spatial accuracy was emphasized for only one arm (the "aiming" arm), while the contralateral ("mirroring") arm performed a simultaneous flexion movement with no emphasis on movement accuracy. We found that while changes in target size had no significant effect on movement latency, changing the hand (right vs left) for which accuracy was emphasized did. When subjects performed right-arm aiming, bilateral movements, unilateral and bilateral RT did not differ significantly. In contrast, when subjects performed left-arm aiming, bilateral movements, RT was significantly longer than for unilateral movements. We conclude that while spatial accuracy per se (i.e., target size) does not differentiate bilateral and unilateral movements, the role of the left hand (i.e., aiming vs mirroring) does. Differences in the hemispheric control of right- and left-hand aiming movements are discussed.