Academic journal article Perception and Psychophysics

The Effect of Pictorial Depth Information on Projected Size Judgments

Academic journal article Perception and Psychophysics

The Effect of Pictorial Depth Information on Projected Size Judgments

Article excerpt

When full depth cues are available, size judgments are dominated by physical size. However, with reduced depth cues, size judgments are influenced less by physical size and more by projected size. By manipulating monocularly presented pictorial depth cues only, in this study we reduced depth cues further than had previous size judgment studies. Participants were presented monocularly with two shapes against a background of zero (control), one, two, or three pictorial depth cues. Each cue was added progressively in the following order: height in the visual field, linear perspective, and texture gradient. Participants made a same/different judgment regarding the projected size of the two shapes (i.e., ignoring any depth cues). As was expected, accuracy increased and response times decreased as the ratio between the projected size of the two shapes increased (range of projected size ratios, 1:1-1:5). In addition, with the exception of the larger size ratios (1:4 and 1:5), detection of projected size difference grew poorer as depth cues were added. One- and two-cue conditions had the most weighting in this performance decrement, with little weighting from the three-cue condition. We conclude that even minimal depth information is difficult to inhibit, which indicates that depth perception requires little focused attention.

When they are available, depth cues combine to enable us to perceive the physical size of a static object in depth. This perception of physical size remains constant, despite changes in the distance of the object from the observer and the visual angle that it subtends (i.e., size constancy; see Sedgwick, 1986). In contrast to physical size, the projected size of an object is established by the visual angle that it subtends on the retina. When judgments of size are made, the relative influence of physical size and projected size is dependent on the amount of depth information available; that is, when depth is perceived, we expect a more distant object to have a smaller projected size, but to have the same physical size, than when it is closer in depth (Epstein, 1973). Depth can be determined by using the binocular cues of vergence and retinal disparity and by using monocular information, available through accommodation, motion parallax, and pictorial cues. We know that when adults are asked to make size judgments, such judgments are dominated by physical size over projected size, even in the absence of binocular cues, accommodation, and motion parallax-that is, when only pictorial cues are available (Uhlarik, Pringle, Jordan, & Misceo, 1980; Yonas & Hagen, 1973). However, little is known about the effect that reducing the number of pictorial cues has on the perception of size. In this study, our aim was to determine the point at which depth cues begin to influence size judgments-that is, how much depth information is required for the perception of size to be influenced by physical size information.

Yonas and Hagen (1973) investigated depth perception by manipulating accommodation (3-D vs. 2-D presentation) and motion parallax (present vs. absent) depth cues in a size judgment task. Two real triangles of different physical size, presented monocularly, were positioned in a real textured alley, and participants were asked which triangle was larger. The visual angle (projected size) subtended by the large triangle either was equal to that subtended by the small triangle or was 70% or 80% of the projected size of the smaller triangle. Adult participants used the available cues and gave judgments according to physical size. Three- and 7-year-olds responded to physical size when the projected size of the two triangles was equal. However, when the projected and physical size differences were incongruent, children required at least 3-D depth information in order to respond according to physical size, and on the hardest trials (70% projected size difference), 3-year-olds also required the additional depth cue of motion parallax. …

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