Incomplete Prism Adaptation in Throwing K Spang, S Wischhusen, A-K Heppner, M Fahle |
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Human Neurobiology, University of Bremen, Germany
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| Subjects wearing prisms adapt to the resulting shift of the optical image within a few movements according to common knowledge. We scrutinized this notion and found that prism adaptation is incomplete even after more than one hundred throws. In several set-ups we tested more than 50 subjects with prisms shifting the visual image by 17 degrees either to the right or to the left. Overhead throwing movements employed softballs towards a Velcro plated screen. Ball position was recorded by means of a camera connected to a computer. In addition we tested pointing movements performed underneath a table, with terminal visual feedback. Finger trajectories were recorded by an ultrasound device (Zebris). Deviation from target decreased with increasing number of movements, almost following an exponential function. While deviations from target did not differ significantly from zero after 100 pointing movements, ball positions still deviated significantly from the target in the throwing experiment. After removal of the prisms, typical aftereffects in the opposite direction emerged in both experiments. We interpret this incomplete adaptation exclusively for throwing as indicating different mechanisms for prism adaptation in pointing versus throwing movements, possibly reflecting basic differences between the neuronal representation of near versus farther extrapersonal space. |
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