Targeted Paced and Unpaced Movements in the Transverse and Sagittal Planes

Five experiments are reported related to control of arm movement in the sagittal and transverse planes when making paced and unpaced movements. A single group of 12 participants and the same equipment were used in the main experiments to allow comparisons across conditions. As well as the 2 different directions of movement, there were movements that were time-constrained (as in the W. D. A. Beggs & C. I. Howarth, 1971, 1972a,b paradigm) and movements that were constrained by the ending tolerance (as in Fitts' paradigm). Results showed that, for movement times as high as 900 ms, the Schmidt and Beggs and Howarth models appeared to describe the time for movements that had time constraint. Fitts' law (P. M. Fitts, 1954 ; P. M. Fitts & J. R. Peterson, 1964) applied to movements that were constrained by final accuracy. These results were independent of whether the target was in the aiming or stopping movement direction. A new interpretation of data for movements with time constraint is presented, based on the possible number of accuracy submovements available when near the target. This model suggests that the standard deviation of hits at the target is not dependent on the time spent in reaching the target region, but largely on the time remaining in order to produce final accuracy at the target.