Bruce Bridgeman: UC Santa Cruz Department of Psychology

Estimation of slope is an everyday tool for navigating the external world. Previous studies have found that slopes are overestimated more greatly with a verbal than with a proprioceptive measure. Since some neurons in the premotor cortex respond differently to objects within arm’s reach, we hypothesized that slope estimation may also be affected by neural pathways that respond differently to identical visual information at different distances. Alternatively, vision may be warning us about the greater effort required to walk up a slope. Verbal estimates greatly overestimated the actual slope, and increased logarithmically with distance from the participant, contradicting both theories. Proprioceptive estimates were more accurate. When participants experience a slope directly by walking up and down a hill prior to making estimates, their estimates remain unchanged. Increases in perceived slope with distance depend upon range of the segment judged, not length of the segment. The results can be interpreted as an implicit slope, previously measured only in darkness, modulated by depth cues available at near distances.
http://www.bec.ucla.edu/slopes09.pdf