The visual system's intrinsic bias influences space perception in the impoverished environment.

A blindfolded observer can walk accurately to a remembered target location on a continuous ground surface in a cue-rich environment, but fails to do so when the ground surface is interrupted by a gap, texture boundary, or an obstacle (e.g., He et al, 2004; Loomis, et al, 1992, 1996; Loomi & Philbeck, 2008; Philbeck, O'Leary, & Lew, 2004; Rieser, Ashmead, Talor, & Youngquist, 1990; Sinai et al, 1998; Thomson, 1983; Wu et al, 2004). These findings support the tenet that the visual system uses an uninterrupted ground surface as a reference frame for space perception beyond the near distance range (>2-3 m) (e.g., Bian & Andersen, 2011; Bian, Braunstein, & Andersen,2005, 2006; Feria, Braunstein, & Andersen, 2003; Gibson, 1950, 1979; He et al., 2004; Lappin, Shelton, & Rieser, 2006; Meng & Sedgwick, 2001, 2002;Ooi, Wu, & He, 2001, 2006; Ooi & He, 2007; Philbeck & Loomis, 1997; Sedgwick, 1986; Sinai, et al., 1998; Sugar, McBeath, Wang, 2006; B. Wu, He, & Ooi, 2004, 2007ab J. Wu, He, & Ooi, 2005, 2008). Adoption of the ground surface strategy for space perception is likely associated with several factors (Gibson, 1950, 1979; He & Ooi, 2000; Sedgwick, 1986). First, we normally stand, navigate and interact with both animate and inanimate objects on the ground surface. Second, the ground carries rich and reliable depth information (e.g., texture gradients) that often extends from our feet to the far distance. Third, by coding 3-D object locations with reference to the common 2-D ground surface, the visual system can improve its coding efficiency (by reducing one dimension). We proposed a Sequential Surface Integration Process (SSIP) hypothesis to elaborate on how the visual system constructs the ground surface representation as a reference frame for space perception in the intermediate distance range (He et al, 2004; Wu et al, 2004). It posits that in the normal full cue viewing condition, the SSIP begins by representing the near ground surface region using the accurately coded near depth information, such as absolute motion parallax, binocular disparity, etc. This leads to a locally accurate near ground representation, which the visual system capitalizes on as an anchoring surface to sequentially integrate with the farther ground regions that predominantly carry texture gradient information. The outcome of the SSIP is a global surface representation of the ground expanse that is used as the reference frame for space perception. However, when the visual system cannot access the depth information on the ground, the visual system employs the intrinsic bias as its reference frame (Gogel & Tietz, 1973; Ooi et al, 2001, 2006). The intrinsic bias is revealed in the dark environment where the ground cannot be seen and other depth information is absent. In this situation, a dimly lit target on the horizontal ground is perceived at the intersection between its projection line from the eye and an implicit slanted surface, rather than at its physical location or any other random locations (figure 1a). We have proposed that the implicit surface reflects the intrinsic bias, which represents the visual system's internal model of the ground surface (Ooi et al, 2001, 2006). Further supporting the hypothesis, it has been shown that the intrinsic bias determines the perceived shape and size of a dimly lit plane target in the dark (Aznar-Casanova et al, 2011; Ooi et al, 2006; Zhou et al, 2010, in press). For example, as illustrated in figure 1b, we found that the slant derived from the perceived aspect ratio (width/length in depth) of a horizontal L-shaped target in the dark is similar to that of the intrinsic bias (Ooi et al, 2006). In other words, it is as if the L-shaped target is perceived as resting on the intrinsic bias (implicit surface) in perceptual space. Figure 1 Space perception based on the visual system's intrinsic bias and external depth information. (a) A dimly lit target in an otherwise dark environment is perceived at the intersection between its projection line and the visual system's intrinsic bias (slanted ... The intrinsic bias possibly originates from one's past experience with the statistical properties of the natural scenes, where humans, animate and inanimate objects are either directly, or indirectly, supported by the ground (Yang & Purves, 2003). Consequently, when the ground is not visible, the visual system capitalizes on the intrinsic bias to make an “educated” guess of the target's location. Extending this idea, we hypothesize that the visual system also capitalizes on the intrinsic bias for space perception when the depth information on the horizontal ground is weak and/or ambiguous, i.e., in a reduced cue condition (impoverished environment). Thus, in addition to the SSIP that integrates the weak texture gradient (depth) cue, the intrinsic bias influences the process of ground surface representation. The outcome is a slanted surface representation of the horizontal ground whose slope is less steep than that of the intrinsic bias (figure 1c). Figure 2a (left) shows a reduced cue condition in the dark where the horizontal ground surface is sparsely delineated by a 2×3 array of texture elements to provide a weak texture gradient depth information. Our hypothesis predicts that the global ground surface representation will be slanted upward, resulting in the target on the floor being perceived above the floor at the intersection of the eye-to-target projection line and the slanted ground representation (figure 2a, right). This prediction opposes an alternative hypothesis that the intrinsic bias only contributes to space perception in the dark but not when there exists texture gradient information on the ground. Ooi & He (2007) argued that such an alternative “horizontal compression hypothesis” would predict that the ground surface representation is horizontally compressed (figure 2c). In other words, the alternative hypothesis predicts that the perceived angular declination is increased (α′> α, figure 2c) rather than remains unchanged (α′= α, figure 2a and 2b). Clearly, one approach to verifying between these competing hypotheses is to empirically obtain the perceived surface slant and the angular declination of the target on the ground. Figure 2 Predictions of space perception in a reduced cue condition where a 2×3 array of dimly lit texture elements delineates the horizontal ground in the dark. (a) The SSIP hypothesis predicts the ground surface representation that acts as a reference ... We thus conducted two experiments using discrete (figure 2a) and plane (figure 2b) targets with perceptual- and action-based tasks in the dark and reduced cue conditions. Our first experiment measured the observer's judged egocentric locations of discrete targets using the blind walking-gesturing task (Ooi et al, 2001). The profile of the judged target locations from this task reveals the shape of ground surface representation and the perceived angular declination. Complementing this, our second experiment reveals the slant of the ground surface representation by measuring judged exocentric distance of an L-shaped target placed horizontally on the ground (figure 2b). We used a perceptual task based on aspect ratio judgment (Loomis et al, 2002; Loomis & Philbeck, 1999; Ooi et al, 2006; Wu et al, 2004; Wu J et al, 2008) and visually directed tasks using the palm board (He et al, 2007; Proffitt et al, 1995; Witt & Proffitt, 2007) and blind walking tasks (Creem-Regehr et al, 2005; Loomis et al 1992, 1996; Ooi et al, 2001; Philbeck et al, 2004, 2008; Rieser et al, 1990; Sinai et al, 1998; Thompson, Dilda, & Creem-Regehr, 2007; Thomson 1983; Wraga, Creem, & Proffitt, 2000).