The effect of whole body translation on hand choice

R.H.A. Weijer, R.S. Bakker, L.S. Selen and W.P. Medendorp

We frequently choose which hand we use to interact with objects in our surroundings. Three major factors have been found to influence the decision process: the energetic cost of the movement, the likelihood of accurately reaching the target and a bias that shifts the decision in favor of the dominant hand. Previous literature has mostly focused on this decision process under body stationary conditions. Here we test the hypothesis that the decision governing hand choice is affected by whole body translation. Participants were seated in a vestibular sled, which accelerated sideways in a sinusoidal motion, while they had to move to a body fixed target. They were free to choose the hand they preferred. We recorded hand choices over a range of targets and fitted a psychometric line to estimate the point of subjective equivalence (PSE) which served as a measurement of the amount of right hand usage. Targets were presented at 4 different phases during the sled motion: maximum right- and leftwards velocity and maximum right- and leftwards acceleration. Results showed that hand usage was only significantly affected when targets were presented at maximum acceleration, not at maximum velocity. At maximum rightwards acceleration participants increased their left hand usage; an opposite effect was found for maximum leftwards acceleration. Results regarding reaction times were inconclusive as to how the brain anticipates inertial forces on the hand. However, we argue that it is possible that the brain can predict the path of the sled motion and hence, predict the inertial forces acting on the arm during the reach.