Accuracy of aiming in linear hand-movements

Six subjects took part in an experiment which consisted of drawing, in a horizontal plane, a number of lines of different lengths and in different directions. The lines were drawn from a starting-point towards a target-point, and the instructions and previous practice were designed to ensure that the lines were drawn in one movement without appreciable amendment during the drawing. The direction and length of the lines were both found to affect the accuracy of aiming, and certain directions showed a persistent bias in the distribution of their errors. Closing the eyes just prior to and during the drawing of the lines had the result of diminishing the effect of changes in direction and length, but it increased the bias. In attempting a theoretical analysis of the results, some of the limitations imposed by human anatomy have been considered in an elementary way. The general conclusion is that, in this task, the effects of altering length, of altering direction and of opening or closing the eyes can, to a great extent, be plausibly ascribed to the particular anatomical nature of the human operator, provided one makes certain simple assumptions about the theory of movement control.