Relative eye height modulates perceived velocity of targets approaching along a horizontal plane.

Relative eye height, defined as the vertical distance between the observer's eye level and a target on the horizontal plane, provides a geometric cue to depth through the angle between eye level and the line of sight. In three-dimensional space, objects often move in depth at varying heights relative to the observer's eye level, requiring integration of multiple cues for accurate velocity judgments. However, the contribution of relative eye height to velocity perception remains unclear. This study examined how relative eye height influences perceived velocity for an approaching target moving along a horizontal plane. Participants performed a two-alternative forced-choice task in which they judged whether the current target appeared faster or slower than targets presented in previous trials, under two eye-height conditions (5 cm and 10 cm from the target level). Greater relative eye height led to faster perceived velocities and improved discrimination performance. Model comparisons further indicated that elevation angular velocity during the early phase of motion (100-400 ms post-onset), a direct cue derived from changes in elevation angle, was the key predictor of velocity judgments, suggesting that the effect of relative eye height is mediated through this cue. These findings highlight the critical role of spatial information, particularly relative eye height, in shaping motion-in-depth perception.