Direction-dependent effects of gravity on speed-accuracy trade-off during vertical pointing movements.

This study aimed to clarify the effects of gravity on the speed-accuracy trade-off (SAT) for vertical pointing movements. For downward movements, gravity assists the initial acceleration phase and opposes the later deceleration phase; for upward movements, it opposes the initial acceleration and assists the later deceleration. We hypothesized that gravity influences the SAT asymmetry in vertical pointing movements depending on movement direction, which would be observable as temporal kinematic differences during the acceleration and deceleration phases. Twelve participants engaged in vertical pointing movements toward targets of different directions, sizes, and distances. The movement time (MT) obtained was fitted using Fitts's equations: MT = a + b × ID and ID = log₂(2A/W), where ID, A, W, a, and b represent the index of difficulty, distance, target size, intercept, and slope factor, respectively. The results showed that the MTs were longer for downward movements than for upward movements. In addition, the slope factor b, which indicates the changing ratio of the MT relative to the term ID, was larger for downward movements than that for upward movements, indicating that the MTs for downward movements changed largely as the target size and distance changed. Furthermore, the temporal properties of pointing movements changed asymmetrically, depending on the movement direction. These results suggest that gravity asymmetrically affects the initial and later phases of vertical pointing movements depending on the movement direction.