The visual perception of distance in action space.

This work examines our perception of distance within action space (about 2m ~ 30m), an ability that is important for various actions. Two general problems are addressed: what information can be used to judge distance accurately and how is it processed? The dissertation is in two parts. The first part considers the what question. Subjects\u27 distance judgment was examined in real, altered and virtual environments by using perceptual tasks or actions to assess the role of a variety of intrinsic and environmental depth cues. The findings show that the perception of angular declination, or height in the visual field, is largely veridical and a target is visually located on the projection line from the observer\u27s eyes to it. It is also shown that a continuous ground texture is essential for veridical space perception. Of multiple textural cues, linear perspective is a strong cue for representing the ground and hence judging distance but compression is a relatively ineffective cue. In the second part, the sequential surface integration process (SSIP) hypothesis is proposed to understand the processing of depth information. The hypothesis asserts that an accurate representation of the ground surface is critical for veridical space perception and a global ground representation is formed by an integrative process that samples and combines local information over space and time. Confirming this, the experiments found that information from an extended ground area is necessary for judging distance accurately and distance was underestimated when an observer\u27s view was restricted to the local ground area about the target. The SSIP hypothesis also suggests that, to build an accurate ground representation, the integrative process might start from near space where rich depth cues can provide for a reliable initial representation and then progressively extend to distant areas. This is also confirmed by the finding that subjects could judge distance accurately by scanning local patches of the ground surface from near to far, but not in the reverse direction