Adapting Movement Amplitude for Temporal Constraints: Further Tests of the Reciprocity of Space-Time Error

The speed-accuracy tradeoff has historically been framed through spatial variables such as movement amplitude and spatial error, yet time can be an equally salient performance constraint in goal-directed action. In this study, we examined whether the reciprocal of Fitts' Law - where time, rather than space, serves as the task criterion - produces systematic change in movement spatial outcome. Twenty right-handed adults performed discrete line drawing movements under two manipulations: (1) target movement time (T) with a fixed temporal bandwidth; and (2) varying time bandwidths (W_T) at a fixed T. These manipulations produced distinct spatial and temporal variability patterns, with target T exerting a stronger and more consistent influence than W_T. Temporal variability increased with ID_T under the target T conditions but decreased with ID_T under narrowing W_T, indicating a reciprocal and constraint-specific modulation of movement performance. These findings reflect the reciprocal nature of space and time outcome measures in discrete aiming tasks, in which spatial and temporal constraints conjointly modulate performance outcome.