On re-scaleability of force and time in aiming movements

Summary Evidence for invariant relative timing is mostly based on comparisons between experimental conditions which differ in the average speed of the movements under study. The present experiment examines whether spontaneous variability of acceleration-time curves of aiming movements within a single experimental condition can also be described in terms of variable scaling parameters for both dimensions. This requires that the correlations between peak acceleration and peak deceleration, as well as those between time intervals defined by peak values and zero crossings, are high. In extensions of the elbow joint on a horizontal plane, the latter correlations turned out to be small or even negative. These results cannot be explained as being due to artifacts of several origins. To reconcile them with those based on comparisons between experimental conditions, a dual-level hypothesis of control in aiming movements is suggested. Only variability on one level can be described in terms of variable scaling parameters, and whether or not variability of acceleration-time curves can be described in this way depends on the relative contributions of both levels of control.