The hollow face illusion.

• Additivity: all the information from different cues is simply added together.

• Selection: information from a single cue is used, with information from the other cue or cues being ignored.

• Multiplication: information from different cues interacts in a multiplicative fashion.

Bruno and Cutting studied relative distance in studies in which three untextured parallel flat surfaces were arranged in depth. The observers viewed the displays monocularly, and there were four sources of information about depth: relative size; height in the projection plane; interposition; and motion parallax. The findings supported the additivity notion (Bruno & Cutting, 1988, p. 161): "Information is gathered by separate visual subsystems...and it is added together in the simplest manner." There is growing evidence that many visual processes operate in parallel (see the section entitled "Brain systems"), and the notion of additivity is entirely consistent with such evidence. However, it should be noted that the visual system may well make use of weighted addition. In other words, information from different depth cues is combined, but more weight is attached to some cues than to others.

It is advantageous to have a visual system that combines information from different depth cues in an additive fashion. Any depth cue provides inaccurate information under some circumstances, and so relying exclusively on any one cue would often lead to error. In contrast, taking equal account of all the available information (or using weighted addition) is often the best way of ensuring that depth perception is accurate. Another advantage of having additive, independent mechanisms involved in depth perception can be seen in infants, because each mechanism can develop in its own time without having to wait for other mechanisms to develop before it can be used. As Bruno and Cutting (1988) pointed out, infants can use motion parallax before the age of three months, even though many of the other mechanisms involved in depth perception have not developed at that stage.

Bruno and Cutting (1988) did not study what happens when two cues provide conflicting information about depth. However, it follows from their general theoretical orientation that observers would combine information from both cues in their depth perception. Support for this position was obtained by Rogers and Collett (1989). They set up a complex display in which binocular disparity and motion parallax cues provided conflicting information about depth, and found that the conflict was resolved by taking both cues into account.

The evidence indicates that observers typically use information from all the available depth cues when trying to judge relative or absolute distance. However, there are some exceptions. Woodworth and Schlosberg (1954) described a situation in which two normal playing cards of the same size were attached vertically to stands, with one card being closer to the observer than the other (see Figure 2.9). The observer viewed the two cards monocularly, and the further card looked more distant. In the next phase of the study, a corner was clipped from the nearer card, and the two cards were arranged so that in the observer's retinal image the edges of the more distant card exactly fitted the cutout edges of the nearer card. With monocular vision, the more distant card seemed to be in front of, and partially obscuring, the nearer card. In this case, the cue of interposition (which normally provides very powerful evidence about relative depth) completely overwhelmed the cue of familiar size.

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