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People with cerebral palsy (CP) are known to rely heavily on visual guidance when making targeted upper-limb movements. In the present study, we examined whether being able to visually monitor the moving limb forms a precondition for people with CP to make accurate upper-limb movements. Eight participants with tetraparetic CP and eight controls were asked to produce large-amplitude, straight-line drawing movements on a digitizing tablet. In half the trials, vision of the moving limb was blocked. Accuracy constraints were manipulated by varying the width of the target and by imposing a maximum width of the movement path. Surprisingly, when vision was blocked movement accuracy was comparable in the two groups. Thus, people with tetraparetic CP do not strictly require constant vision of their moving limb to make accurate upper-limb movements. They compensated for the lack of visual information, however, by prolonging movement time. Using a high pen force proved a general strategic adaptation, possibly to filter out unwanted noise from the motor system or to enhance proprioceptive input.
The authors are with the Nijmegen Institute for Cognition and Information, Radboud University, P.O. Box 9104, 6500 HE Nijmegen, the Netherlands.