INFLUENCE OF GROUND SURFACE ON SLANT PERCEPTION
In past studies surprisingly little attention has been given to the potential influence of ground inclination on perception of geographical slant (Proffitt, Bhalla, Gossweiler & Midgett, 1995). Bridgeman and Hoover (2008) had participants estimate the steepness of hills that they stood on, but the ground slope was not manipulated. Our first experiment investigated the effects of standing on a ramp on the visual perception of surface orientation. Our participants stood on one of three slopes while estimating a separate large ramp in front of them: a 12.5 degree incline, the horizontal ground, or a 12.5 degree decline. For visually inspected surfaces near horizontal there was minimal effect, but for steeper surfaces (e.g. 28 degrees) the visually perceived surface orientation was distorted as if in contrast with the stood-upon surface. More specifically, standing on an incline resulted in less overestimation of the largest ramp slope, than standing on the horizontal ground or on a decline. We hypothesized that perception of slope in this configuration is the result of the interactive effects of at least two sources of information: the kinesthetic information about orientation, and the visual information that specifies geographical slant. The second experiment was a test of how visual and haptic information about slant interact and influence perceived slope. Participants stood on the horizontal ground, or touched the ramp with one foot with eyes open or with a blindfold blocking eyesight. The haptic-only information resulted in highest exaggeration of reported slant as compared to the visual-only, and visual- haptic conditions. This confirms our previous findings (Hajnal, Abdul-Malak, & Durgin, in press) that ramps feel steeper than they look. Future studies will reveal whether additional sources of information (e.g. sense of balance or tilt) further influence estimates of geographical slant.