Safe and effective locomotion depends critically on judgements of the surface properties of the ground to be traversed. Little is known about the role of binocular vision in surface perception at distances relevant to visually guided locomotion in humans. Programmable arrays of illuminated targets were used to present sparsely textured surfaces with real depth at distances of 4.5 and 9.0 m. Psychophysical measurements of discrimination thresholds demonstrated a clear superiority for stereoscopic over monocular judgments of relative and absolute surface slant. Judgements of surface roughness in particular demonstrated a substantial binocular advantage. Binocular vision is thus shown to directly contribute to judgements of the layout of terrain up to at least 4.5 m, and its smoothness to at least 9.0 m. Hence binocular vision could support moment-to-moment wayfinding and path planning, especially when monocular cues are weak.

*d*′) and its variance were estimated for each condition using signal detection theory (Green & Swets, 1966; Macmillan & Creelman, 1991). Specifically,

*d*′ = z(H) − z(F), where z(x) is the z-score for rate x (from the inverse cumulative normal distribution). H is the hit rate and is estimated by the proportion of ‘single planar surface’ responses on trials when a single stimulus was in fact presented (i.e. across all of the 0° slant difference trials). H was estimated separately for each viewing distance by viewing condition (monocular vs. binocular) combination. Likewise, F is the false-alarm rate and is estimated by the proportion of ‘single planar surface’ responses on trials when a single planar surface was

*not*presented. For each observer, F and subsequently

*d*′ were calculated for each slant difference at each viewing distance by viewing condition combination.

*t*-test:

*t*(5) = 6.46, two-tailed

*p*= 0.0013). Figure 4, bottom shows measured binocular and monocular thresholds estimated from these slopes.

*t*-test:

*t*(5) = 10.40, two-tailed

*p*= 0.0005).

*d*′. In Figure 6 bottom, we plot the difference in binocular and monocular sensitivity or

*d*′ as a function of slant difference. The plot demonstrates that in all cases binocular viewing significantly improves sensitivity at both 4.5 and 9.0 m, particularly at large slant differences.

Distance | Slant Difference | Sensitivity ( d′) | |
---|---|---|---|

Monocular | Binocular | ||

4.5 m | 2° | 0.16 | 0.74 |

4° | 0.20 | 1.35 | |

6° | 0.46 | 1.90 | |

9.0 m | 2° | −0.06 | 0.31 |

4° | 0.23 | 1.36 | |

6° | 0.60 | 2.03 |