Drawing under visuomotor incongruence

Six human subjects were asked to draw ellipses presented on a screen by moving a manipulandum that controlled the position of a cursor. Six visual templates were used, which comprised three different ellipses displayed either horizontally or vertically; the ratio between the major and minor axes was 2, 4, or 5. For each visual template, the gains were set such that the movement trajectories required to trace the template with the cursor corresponded to one of six ellipses. Thus these movement ellipses were horizontal or vertical with a ratio between major and minor axes of 2, 4, or 5. All 36 combinations of six visual ellipses and six required movement ellipses were used. Therefore, in some conditions the required movement ellipse had a different orientation (with respect to the major axis) than the visual template. These conditions were called orientation incongruent whereas, when the orientation of the required movement ellipse matched the orientation of the visual template, the conditions were called orientation congruent. Similarly, eccentricity incongruent referred to conditions where the eccentricities of the visual ellipse and the required movement ellipse were different, as opposed to eccentricity congruent. The main results were as follows: (a) The perimeter of the traced ellipse always tended to be larger than that of the visual template. In addition, it was significantly larger in the orientation incongruent conditions than in the orientation congruent conditions. Nevertheless, the perimeter of the traced figure increased with the template in both orientation congruent and incongruent conditions. (b) The shape of the traced figure varied appropriately with the visual template, but differed significantly between the orientation congruent and incongruent conditions. It was closer to the one of the template in the orientation congruent than in the incongruent conditions. Finally, (c) the instantaneous speed was significantly correlated with curvature but more tightly so in the orientation congruent than in the orientation incongruent conditions. The parameters defining the relation between speed and curvature were affected by the required movement ellipse, but not by the particular visuomotor condition. These results showed that although spatial motor performance was affected by changes in the correspondence between visual and movement coordinates, the relation between the speed and curvature of the movement trajectory was stable despite drastic changes in this correspondence.