Kanizsa's (1976) illusory square.

A further cue is interposition, in which a nearer object hides part of a more distant object from view. Some evidence of how powerful interposition can be is provided by Kanizsa's (1976) illusory square (see Figure 2.7). There is a strong impression of a white square in front of four black circles, in spite of the fact that most of the contours of the white square are missing. Thus, the visual system makes sense of the four sectored black discs by perceiving an illusory interpolated white square.

Another cue to depth is provided by shading. Flat, two-dimensional surfaces do not cast shadows, and so the presence of shading generally provides good evidence for the presence of a three-dimensional object. Ramachandran (1988) presented observers with a visual display consisting of numerous very similar shaded circular patches, some of which were illuminated by one light source and the remainder of which were illuminated by a different light source. The observers incorrectly assumed that the visual display was lit by a single light source above the display. This led them to assign different depths to different parts of the display (i.e., some "dents" were seen as bumps).

The sun was easily the major source of light until fairly recently in our evolutionary history, and this might explain why people assume that visual scenes are generally illuminated from above. Howard, Bergstrem, and Masao (1990) pointed out that the notion of "above" is ambiguous, in that it can be above with reference to gravity (as is assumed in the explanation just given), or it can be above with reference to the position of the person's head. Accordingly, they persuaded their participants to view displays like those of Ramachandran (1988) with their heads upside down! The perceived source of light was determined with reference to head position rather than gravity, indicating that the location of the sun is not relevant to decisions about the direction of illumination. However, head orientation is normally upright, and so the assumption that the sun is above is probably closely associated with head position.

Another cue to depth is provided by familiar size. It is possible to use the retinal image size of an object to provide an accurate estimate of its distance, but only when you know the object's actual size. Ittelson (1951) had participants look at playing cards through a peep-hole that restricted them to monocular vision and largely eliminated cues to depth other than familiar size. There were three playing cards (normal size, half-size, and double-size), and they were presented one at a time at a distance of 2.28 metres from the observer. On the basis of familiar size, the judged distance of the normal card should have been 2.28 metres, that of the half-size card 4.56 metres, and that of the double-size card 1.14 metres. The actual judged distances were 2.28 metres, 4.56 metres, and 1.38 metres, indicating that familar size can be a powerful determinant of distance judgements.

Another cue to depth is image blur. As Mather (1997, p. 1147) pointed out, "if one image region contains sharply focused texture, and another contains blurred texture, then the two regions may be perceived at different depths, even in the absence of other depth cues." He discussed some of his findings on ambiguous stimuli consisting of two regions of texture (one sharp and one blurred), which were separated by a wavy boundary. When the boundary was sharp, the sharp texture was seen as nearer, whereas the opposite was the case when the boundary was blurred. Thus, the boundary is seen as part of the nearer region.

The final monocular cue we will discuss is motion parallax, which refers to the movement of an object's image over the retina. Consider, for example, two objects moving left to right across the line of vision at the same speed, but one object is much further away from the observer than is the other. In that case, the image cast by the nearer object would move much further across the retina than would the image cast by the more distant object.

Motion parallax is also involved if there are two stationary objects at different distances from the observer, and the observer moves sideways. It would again be the case that the image of the nearer object would travel a greater distance across the retina. Some of the properties of motion parallax can be seen through the windows of a moving train. Look into the far distance, and you will notice that the apparent speed of objects passing by seems faster the nearer they are to you.

Convincing evidence that motion parallax can generate depth information in the absence of all other cues was obtained by Rogers and Graham (1979). Their participants looked at a display containing about 2000 random dots with only one eye. When there was relative movement of a section of the display (motion parallax) to simulate the movement produced by a three-dimensional surface, the participants reported a three-dimensional surface standing out in depth from its surroundings. As Rogers and Graham (1979, p. 134) concluded, "it has been clearly demonstrated that parallax information can be a subtle and powerful cue to the shape and relative depth of three-dimensional surfaces."

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