There is a current debate concerning whether people's physiological or behavioral potential alters their perception of slanted surfaces. One way to directly test this is to physiologically change people's potential by lowering their blood sugar and comparing their estimates of slant to those with normal blood sugar. In the first investigation of this (Schnall, Zadra, & Proffitt, 2010), it was shown that people with low blood sugar gave higher estimates of slanted surfaces than people with normal blood sugar. The question that arises is whether these higher estimates are due to lower blood sugar, per se, or experimental demand created by other aspects of the experiment. Here evidence was collected from 120 observers showing that directly manipulating physiological potential, while controlling for experimental demand effects, does not alter the perception of slant. Indeed, when experimental demand went against behavioral potential, it produced judgmental biases opposite to those predicted by behavioral potential in the low blood sugar condition. It is suggested that low blood sugar only affects slant judgments by making participants more susceptible to judgmental biases.

People verbally overestimate hill slant by ~15–25° whereas manual estimates (e.g., palm board measures) are thought to be more accurate. The relative accuracy of palm boards has contributed to the widely cited theoretical claim that they tap into an accurate, but unconscious motor representation of locomotor space. In the current work, four replications (total N = 204) carried out by two different laboratories tested an alternative, anchoring hypothesis that manual action measures give low estimates because they are always initiated from horizontal. The results of all four replications indicate that the bias from response anchoring can entirely account for the difference between manual and verbal estimates. Moreover consistent correlations between manual and verbal estimates given by the same observers support the conclusion that both measures are based on the same visual representation. Concepts from the study of judgment under uncertainty apply even to action measures in information rich environments.

People verbally overestimate hill slant by~15°-25°, whereas manual estimates (e.g., palm board measures) are thought to be more accurate. The relative accuracy of palm boards has contributed to the widely cited theoretical claim that they tap into an accurate, but unconscious, motor representation of locomotor space. Recently, it was shown that a bias that stems from anchoring the hand at horizontal prior to the estimate can quantitatively account for the difference between manual and verbal estimates of hill slant. The present work extends this observation to manual estimates of nearsurface slant, to test whether the bias derives from manual or visual uncertainty. As with far surfaces, strong manual anchoring effects were obtained for a large range of near-surface slants, including 45°. Moreover, correlations between participants' manual and verbal estimates further support the conclusion that both measures are based on the same visual representation.

In the current work we investigate people's perception of their own body tilt in the pitch direction. In Experiment 1, we tilted people backward at 1 of 5 different randomly assigned angles using an inversion table. People significantly overestimated the angle at which they were tilted backward at angles from 8°to 45°. The slope of the plotted average overestimates had a gain of 1.46, fitting nicely with previously reported gains of verbal overestimates of visually perceived slant of natural outdoor geographically oriented slopes as well as man-made wooden slopes within and outside of reach in the laboratory. In Experiment 2, we showed participants a 45°line and asked them to indicate when they were positioned at that orientation. Participants again significantly overestimated the angle at which they were tilted backward. This extends work showing that a scale-expanded theory of visual space is multisensory, results in equivalent estimates for both verbal and nonverbal/nonnumeric methods, and can now be expanded to include the perceived orientation of one's own body. Keywords Slant perception . Spatial orientation . PitchFor the last two decades, a wealth of evidence shows that people overestimate the slant of both geographical and manmade slopes by between 5°and 25° (Bhalla & Proffitt, 1999;Bridgeman & Hoover, 2008;Creem & Proffitt, 1998; CreemRegehr, Gooch, Sahm, & Thompson, 2004;Durgin & Li, 2011;Durgin, Li, & Hajnal, 2010;Hajnal, Abdul-Malak, & Durgin, 2011;Proffitt, Bhalla, Gossweiler, & Midgett, 1995;Proffitt, Creem & Zosh, 2001;Shaffer & Flint, 2011;Shaffer & McManama, 2015;Stefanucci, Proffitt, Clore, & Parekh, 2008;Witt & Proffitt, 2007). Much less work has been performed on people's perception of their own body orientation in the pitch dimension, and the results of some of this work are difficult to interpret. For instance, Cohen and Larson (1974) had participants adjust the pitch of their own body every 15°from a supine position to a prone position while restrained in a motorized hospital bed. They found systematic errors of underestimation of body tilt. For instance, when asked to place themselves at 15°backward from a vertical position, they placed themselves at 29°back-ward, and when asked to place themselves at 15°forward, they placed themselves at 23°forward. These errors were consistent but smaller as they moved in either direction in 15°increments from 15°to 60°, at which point there was almost no error. Of the studies to investigate pitch perception, this seems to be the only one where people underestimate pitch. We feel there are at least two reasons for this. First, participants were moved backward in 15°increments until they were prone, and then forward in 15°increments until they were in a supine position. They did this back and forth a total of eight times (four backward, four forward). Carryover effects from each previous estimate likely affected their subsequent estimate. Second, they were giving estimates of, say, 15°backward when they were either erect (straight up and down) o...

Previous work has shown that overestimates of geographic slant depend on the modality used (verbal or haptic). Recently, that line of reasoning has come into question for many reasons, not the least of which is that the typical method used for measuring "action" has been the use of a palm board, which is not well calibrated to any type of action toward slanted surfaces. In the present work, we investigated how a remote haptic task that has been well calibrated to action in previous work is related to verbal overestimates of slanted surfaces that are out of reach. The results show that haptic estimates are perceptually equivalent to the verbal overestimates that have been found in numerous previous studies. This work shows that the haptic perceptual system is scaled in the same way as the visual perceptual system for estimating the orientation of slanted surfaces that are out of reach.