Spatial Layout Perception, Psychophysical

The Spatial Framework

Humans are terrestrial animals, which means that they live mainly on the ground. The extended “ground plane” is the foundation for the perception of spatial layout. The floor of a room functions perceptually in a similar way to the ground plane. Other extended surfaces in the environment, such as the walls and ceiling of a room, join with the ground plane to form a visual framework surrounding the observer. Spatial layout is generally perceived in relation to this “spatial framework.”

The spatial framework occupies much or all of the visual field, and the surfaces composing it tend to subtend large angles (called “visual angles”) at the eye. Some important sources of visual information, such as linear perspective, are most perceptually salient at large visual angles. Also, some important perceptual effects of the spatial framework, such as the perception of being in a stable upright environment, depend on the large angular extent of the spatial framework.

Movement of the observer produces a continuous transformation of the entire optic array. There is a strong perceptual tendency to assume that such transformations are being produced by the observer's movement through an unmoving spatial framework rather than seeing the spatial framework as itself moving. This perceptual assumption is made even if the observer is not actively moving but is being passively carried through the environment (e.g., in a car or on a moving walkway). If an artificial visual environment is created that actually does move, while the observer remains stationary, the perceptual assumption of the stability of the spatial framework tends to produce the powerful perceptual illusion that it is the observer who is moving through a stationary environment.

Orientation

Many forms of visual information contribute to the perceived orientation of environmental surfaces, but some have a stronger effect on perception than others. Parallel receding lines on a surface, such as the floor of a room, project to the eye as lines converging toward a vanishing point on the horizon (Figure 1a). This is “linear perspective,” which can produce a powerful and accurate perception of the three-dimensional orientation of the surface. If, however, the parallel receding lines are replaced by a textured surface with more irregular markings, the perception of recession is weaker and the amount of recession is perceptually underestimated, even if the same linear perspective is still implicit in the projection of this irregular texture. On the same receding surface, evenly spaced lines parallel to the horizon project to the eye as lines that are compressed more closely together with increasing distance (Figure 1a). This compression also contributes to the perception of the surface's orientation in depth, but it produces a weaker perceptual effect than linear perspective, and taken by itself produces much less perception of recession.

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