2 The shape constancy of disparity-defined objects moving in depth
2.4 Discussion:
2.4.3 Summary: perceiving a stable world
In summary, the results provide a striking demonstration of the perceptual consequences of the failure to obtain shape constancy. Although sufficient information for the recovery of geometrically correct 3-D shape was available to the observer, physically constant disparity defined cylinders are perceived to expand in depth extent when moving towards the observer and to contract in depth when moving away (Figure 2.1b). Conversely, in order to be perceived as constant in shape, cylinders needed to modulate their shape to counter the effects of incorrect distance scaling, by contracting in depth extent when moving toward the observer and expanding in depth extent when moving away (Figure 2.1c, 2.7). This is consistent with recent research demonstrating that observers are unable to exploit information from disparity and motion in an immersive virtual reality environment to determine whether objects are moving relative to them as they move (Tcheang, Gilson & Glennerster, 2005), or that the room in which they are walking is expanding dramatically (Glennerster, Tcheang, Gilson, Fitzgibbon & Parker, 2006b).
Tcheang et al. (2005) got observers to move laterally back and forth
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±1m relative to isolated rendered football viewed at 1.5m in an otherwise blank environment. The task was to judge whether the football remained static or rotated with or against their lateral movement. The football rotated about its vertical axis yoked to the observer’s lateral movement with a gain between -1 and +1. A gain of +1 meant that the football
rotated with the observer’s movement so that the same surface of the ball always faced the observer, a negative gain meant that the football rotated against the observer’s movement and a gain of zero that the ball remained stationary. In nearly all instances a football needed to have a positive gain, such that it rotated with the observers movement to be judged as static. This bias was greatly reduced by presenting other static footballs in the room or by giving the room lattice grid walls and a floor (Tcheang et al., 2005)
An even more dramatic demonstration of the visual system’s inability to perceive a stable environment was found by manipulating the size of a virtual room as an observer moved within it (Glennerster et al., 2006b). Observers stood to the left of a virtual room with brick rendered walls and a checkerboard floor. While standing in this position the observer viewed a ‘standard’ cube positioned at 0.75m, they then walked horizontally across the room to view a ‘comparison’ cube. The task was to judge whether the comparison cube was larger or smaller than the standard. Each cube was only viewable when the observer was in zones to the left and the right of the room, and the room was otherwise empty. This meant that at no point in time were observers able to see both cubes at once, or any other objects. During the observer’s horizontal movement across the room, whilst neither cube was visible, the room smoothly expanded in size by a factor of four around the cyclopean point mid way between the eyes, this included the floor and wall textures scaling. Subjects failed to notice this expansion, and for the comparison cube to be judged as the same size as the standard cube, it too had to expand in size. The expansion needed in the comparison cube varied from around a factor 2 when the comparison cube was viewed at 1.5m, to nearly 4 when viewed at 6m (Glennerster et al., 2006b).
The visual information available from the texture of the floor and walls of the room was ambiguous as to the room’s size throughout the experiment, whereas stereo, motion and height in the field indicated the changing size of the room. Texture information could however be used to judge the sizes of the standard and comparison cubes. This is because observers could judge the size of the standard and comparison cubes relative to the texture of the walls and floor in the two zones where each cube was visible. There were therefore two broad categories of information that could be used by observers in judging the size of the two cubes. These are (1) absolute
information about distance and shape from stereo, motion and height in the field, and (2) relativeinformation from the comparison of the cubes size relative to the wall and floor textures (Glennerster et al., 2006b).
The authors demonstrate that a model with a single free parameter, which represents the relative weighting given to absolute information compared to relative information, predicts observer’s size matching performance well. In this model an increase in viewing distance results in an increase in the weight given relative information about cube size, and a decrease in the weight given absolute information about cube size, as would be predicted from geometry. The fact that observers fail to notice the changing size of the room is likely to be due to the low weighting given stereo, motion and height in the field, compared to texture, given the room’s dimensions. The weighting of these cues may also be reduced as they conflicted with the observer’s proprioceptive information about the position of their body in the room.
Proprioceptive information suggests that the observer is standing on the ground but stereo, motion and height in the field specify that the room has expanded four fold, and that the observer is now floating above the room’s floor. Texture on the other hand is ambiguous as to the size of the room, so does not conflict with preoperception. Observers in the study were to some extent aware that the room was changing in size as they felt the length of their strides to be increasing and decreasing as they moved across the room (Glennerster et al., 2006b). This suggests that observers had access to the conflicting preoperceptive and visual information (see also Ernst, 2006, Hillis et al., 2002).
The results of the current experiment complement those from Tcheang et al. (2005) and Glennerster et al. (2006b) by demonstrating that within a distance range where vergence information should be maximally reliable, observers are unable to scale disparity with an estimate of the true viewing distance to correctly estimate the shape constancy of objects moving in depth. These studies serve to demonstrate that biases in perceived shape can come about both directly, through the transformation inherent in the use of information from a single visual cue, and more indirectly, through the way in which different sources of information are integrated to form a single percept (Knill & Richards, 1996, Mamassian et al., 2002).
It is justifiable to ask why, given well-documented biases in perception (Todd & Norman, 2003), even in the natural environment (Wagner, 1985), we perceive the world to be a relatively stable place. Much will depend on the type and quality of the visual cues available (Knill & Richards, 1996). However, it may be that the visual system embodies a strong assumption toward world stability (Glennerster et al., 2006b, Koenderink, 1998). The visual system may simply not need to estimate the absolute metric structure of the environment during our everyday interactions (Bradshaw et al., 2000, Garding et al., 1995). Consequently, the visual system would routinely avoid any conflict or distortion inherent in making decisions about metric structure by using alternative processing strategies. The experimental situation might force the visual system to make an inaccurate decision regarding metric structure, but this may bear little cost in everyday life.