Effect of target shape on movement time in a Fitts task

Experiments on the effect of target shape in a Fitts movement task are reported. Two experiments investigated the movement time vs. index of difficulty relationship for targets that were square, circular, diamond and triangular in shape. The ratio of standard deviation of hits in the vertical and horizontal directions was constant for all target shapes and thus the movement time could be expressed in terms of the constraint in the direction of motion. Correlation of movement time in terms of an Index of Difficulty based on the standard deviation of hits in the direction of motion showed all data fitting on a single line. Thus the constraint effects of the various shapes occurred from the way in which the hits were distributed within each shape. Various models were tried to account for the effect of the target shape. It was found that the experimental data were best accounted for by a 'cookie-cutter' model in which the target shape was cut into a bivariate normal distribution of hits on the target to obtain a 'shape factor' that defined the difficulty of the task relative to the usual rectangular target used in Fitts task experiments.