Abstract: This study aims to explore the impact of speed of motion and direction of motion on human performance in time to contact (TTC) estimation. Based on the occlusion paradigm, a testing software of TTC estimation was developed, in which a ball first appears at a peripheral position of the computer screen, and then moves with a constant speed toward the center of the screen, which is marked by small red cycle whose diameter is the same as the moving ball. The ball is visible at first, after moving for a period, it enters a dark circle and becomes invisible. The participant has to imagine the motion of the ball under the dark circle, and judge when it will reach the center for the screen, and press a key to indicate the judgment. The visible distance, the occlusion distance (radius of the dark circle), the speed of motion, the direction of motion can all be configured in the testing software. 34 participants, aged between 21 and 30 years, volunteered to complete the TTC estimation experiment, the experiment used a 3-level speeds of motion (relatively slow, relatively medium, and relatively fast, being 50, 100, 150 pixels/s respectively) × 4-level directions of motion (upward, downward, leftward, and rightward) design. The visible distance, the occlusion distance were kept constant during the experiment, being 170 pixels and 180 pixels, respectively. The four directions may also be categorized into two directions in the analyses: the vertical direction and the horizontal direction, when no significant difference was found between the upward motion and the downward motion, and between the leftward motion and the rightward motion. Performance indices such as deviation rate, absolute deviation rate, underestimate rate, overestimate rate and hit rate in TTC estimation were analyzed. Deviation rate (DR) is defined as (TE-TR)/TR, where TR (short for time required) is the time interval for the ball to move from the edge of the dark circle to the center of that circle, it is the value of occlusion distance divided by speed; TE (short for time estimated) means the time interval from the time when the ball first enters the dark circle to the time when the participant presses the key to indicate his estimation. Absolute deviation rate (ADR) is defined as |(TE-TR)|/TR. When DR is between -0.05 and 0.05 for a trial, the estimation is defined as a hit; when DR is higher than 0.05, it is defined an overestimate of TTC; when DR is smaller than -0.05, it is defined an underestimate of TTC. Hence, the hit rate, overestimate rate and underestimate rate can be calculated. Results showed that the accuracy of TTC estimation, indicated by the absolute deviation rate, was significantly lower in the relatively slow speed condition compared to that in the two faster conditions. In the relatively slow speed condition, the absolute deviation rate of the participants in TTC estimation of the vertical motion was significantly greater than that of the horizontal motion; meanwhile, the underestimation rate of TTC was significantly higher than that of the overestimation rate. In the two faster speed conditions, no significant differences were found in the absolute deviation rate between vertical and horizontal motion; no significant differences were found between participants' underestimation rate of TTC and overestimation rate of TTC. Those results suggest that in the settings of the current experiment, the impact of speed of motion on performance indices of TTC estimation was relatively strong, while the impact of direction of motion was relatively weak. Meanwhile, the impact of speed of motion and direction of motion showed significant interactions.

Key words: occlusion paradigm, speed of motion, direction of motion, TTC estimation

摘要： 为探索运动速度和方向对碰撞时间（TTC）估计的影响，采用3运动速度×4运动方向的双因素设计开展了实验研究。结果发现，在相对慢速条件下，受试者TTC估计的准确性显著差于相对中速和相对快速条件；仅在相对慢速条件下受试者对竖直方向的TTC估计准确度显著差于水平方向、低估TTC的比例显著高于高估TTC的比例。结果表明，运动速度对TTC估计绩效指标的影响相对较强，运动方向对其影响相对较弱；两个因素的影响存在交互作用。

关键词: 遮挡范式, 运动速度, 运动方向, 碰撞时间估计