Walking And The Role Of Speed In The Perception Of Time To Contact

Many analyses of time-to-contact (TTC) emphasize that retinal information, independent of distal distance and speed, is used to compute TTC (e.g., Tau). However, our research indicates that speed information is also used and that TTC judgments are influenced by extra-retinal self-motion information. A stereo HMD and a wide-area tracking system were used to present TTC stimuli in an immersive virtual environment. In Experiment 1, stimulus approach rate was independent of observer motion. TTC judgments were made by nine naïve observers, while walking and while standing, for object speeds that bracketed standard walking speed (∼ 0.5, 1, and 2 m/s). Displays lasted 3.5 s, with TTC varying from 4 to 6.5 s from onset. The visual environment was untextured, so that there was no visual information specifying the speed of self-motion. When standing, TTC judgments were fairly accurate (mean error = +169 ms), but were earlier for slow objects (49 ms) and later for fast (288 ms). This influence of object speed despite equivalence of Tau is consistent with a misperception of object speed (poorer speed differentiation than distance differentiation). When walking, all TTC estimates were earlier (M = −186 ms), and the differential between slow (−393 ms) and fast (25 ms) objects was increased (p \u3c .05). The increased effect of perceived object speed might be a consequence of a greater misperception of speed. Extra-retinal information specifying self-motion speed might substitute for object speed. In Experiment 2, we studied TTC judgments while walking toward objects whose position was defined in absolute space, so that true TTC was a collision between the motion of the object and the observer. Stimuli were matched in initial retinal angle and approximate TTC, and drifted at varying rates. TTC judgments were again underestimated in all cases, but the estimates were most variable when the ratio of object approach speed to walking speed was most extreme