Academic journal article Attention, Perception and Psychophysics

Slant Perception in near Space Is Categorically Biased: Evidence for a Vertical Tendency

Academic journal article Attention, Perception and Psychophysics

Slant Perception in near Space Is Categorically Biased: Evidence for a Vertical Tendency

Article excerpt

The geographical slants of hills are known to appear quite exaggerated. Here, we examine the visual and haptic perception of the geographical slant of surfaces within reach under full-cue conditions and show that the perceived orientation of even these surfaces is biased. An exaggeration with respect to deviations from horizontal is shown to be present cross-modally. Experiment 1 employed numerical estimation to show the effect for visually observed surfaces, while controlling for verbal numerical bias. Experiment 2 demonstrated that the bias is present even when manual measures show good calibration. Experiment 3 controlled for direction of gaze. Experiment 4 measured the same bias for haptic surfaces. Experiment 5 showed that the bias can also be observed using the nonnumeric task of angle bisection. These results constrain theories of geographical slant perception and appear most consistent with functional scale expansion of deviations from horizontal.

Gibson (1950) proposed that surfaces, rather than spaces, were the objects of perception. Here, we show that the perceived geographical slants of surfaces in near space are systematically biased. Geographical slant refers to the slant of a surface relative to the horizontal plane (Sedgwick, 1986). Perceived geographical slant is known to be exaggerated for large distal surfaces, such as hills (Kammann, 1967; Proffitt, Bhalla, Gossweiler, & Midgett, 1995; Ross, 1974). We have recently found that ramps feel very steep under foot (Hajnal, Abdul-Malak, & Durgin, in press). Our present investigation shows that the perceived slants of small surfaces within reach of the hand are also exaggerated. We will argue that these biases cannot be explained by the frontal tendency observed by Gibson (1950; see also Ooi, Wu, & He, 2006) but seem to be due to systematic spatial coding distortions, relative to the categorical references of horizontal and vertical.

Many investigators have considered how various visual factors and sources of information, relevant to depth and shape perception, may affect the perception of slant (M. S. Banks, Hooge, & Backus, 2001; Bridgeman & Hoover, 2008; Clark, Smith, & Rabe, 1956; Flock, 1965; Gibson, 1950; Gibson & Cornsweet, 1952; Gruber & Clark, 1956; Howard & Kaneko, 1994; Kaneko & Howard, 1997; Knill, 1998; Knill & Saunders, 2003; Li & Durgin, 2009; Norman, Crabtree, Bartholomew, & Ferrell, 2009; O'Shea & Ross, 2007; Perrone, 1982). Our concern is not with the specific sources of visual (or nonvisual) information, but with perceptual experience as reported by our participants. Our (real) surfaces are presented under full-cue conditions, in the absence of any cue conflict.

Gibson and Cornsweet (1952) defined optical slant as the orientation of a surface relative to the axis of gaze (i.e., relative to the set of planes to which the line of sight forms a normal vector). Using a vertical palm board measure, Gibson (1950) observed evidence of a frontal tendency in estimates of slant from texture gradients. That is, his texture-gradient-defined surfaces appeared more frontal to gaze than they were. Consistent with modern views, Gibson noted that such effects might be partly caused by conflicting cues to flatness (such as the lack of accommodative blur in his monocular stimuli) and partly by the aperture through which his participants looked at the stimuli, a factor that has also later been confirmed (Eby & Braunstein, 1995). Although it was only later that Gibson and Cornsweet explicitly distinguished optical slant from geographical slant experimentally, the frontal tendency has been assumed to reference optical slant or egocentric slant: In the absence of depth information indicating otherwise (e.g., from contour, surface texture, binocular disparity, etc.), a surface in the visual field appears frontal to gaze.

In Gibson's (1950) early framework, a frontal surface was defined as having zero optical slant, so that the frontal tendency was described as the underestimation of slant (see also Norman et al. …

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