Effective locomotion depends on judgements of the support, passability and effort to traverse provided by terrain several metres away. Elementary texts commonly assert that stereopsis per se is ineffective in these judgements beyond modest distances. He et al. (Perception, 2004, 33: 789) proposed that vergence and stereopsis calibrate and anchor depth percepts in near space that are then extended to larger distances by integrating monocular cues over the continuous ground plane. However, stereopsis has a much larger theoretical range and we have shown binocular performance improvements to at least 18.0m (VSS2007). Here we evaluate the contribution of binocular vision to judgements of ground surface properties.

A computer-controlled constellation of LEDs was distributed throughout a volume of space centred 4.5 or 9.0 metres from the subject. LEDs could be selectively lit to create a single ground plane or two planes either adjacent or interleaved (simulating uneven terrain). In separate 2AFC experiments subjects discriminated: 1) the absolute slant of a single plane; 2) the relative slant between two adjacent planes; or 3) whether all the lights lay in a single plane or not (surface smoothness). Viewing was binocular or monocular.

Binocular discrimination of absolute and relative slant showed less bias and was more precise than monocular discrimination for all tasks at both distances. Judgements of surface smoothness were very difficult monocularly compared to binocularly, as reflected in substantial differences in sensitivity (d').

Binocular vision is useful for judgements of the layout and regularity of terrain to at least 9.0 metres (an important range for moment-to-moment path planning during walking, running and assisted travel). In sum, binocular vision can contribute to precise judgements of ground surface properties. This contribution is not simply limited to calibration and anchoring of monocular cues in personal space.