Figure 210

The Ames Room.

observers are asked to make. Kaneko and Uchikawa (1997) argued that the instructions given to observers in previous studies were not always clear. They distinguished between perceived linear size (what the actual size of the object seems to be) and perceived angular size (the apparent retinal size of the object). Kaneko and Uchikawa (1997) manipulated depth cues such as binocular disparity. Overall, they found much more evidence for size constancy with linear-size instructions than with angular-size instructions. There was a closer approximation to size constancy with linear-size instructions when depth could be perceived more accurately, but this was less so with angular-size instructions. Thus, the size-distance invariance hypothesis is more applicable to judgements of linear size than of angular size.

Size judgements can depend on factors other than perceived distance. For example, we can use information about familiar size to make accurate assessments of size regardless of whether the retinal image is very large or very small. Evidence of the importance of familiar size was obtained by Schiffman (1967). Observers viewed familiar objects at various distances in the presence or absence of depth cues. Their size estimates were accurate even when depth cues were not available, because they made use of their knowledge of familiar size.

There is also evidence that the horizon is sometimes used in size estimation. The horizon is generally sufficiently far away that, "the line connecting the point of observation with the horizon is parallel to the ground" (Bertamini, Yang, & Proffitt, 1998, p. 673). As a result, an object that is on the line between a standing observer and the horizon is about 1.50 to 1.75 metres tall. Bertamini et al. (1998) obtained size judgements from standing and sitting observers. These judgements were most accurate when the objects being judged were at about eye-level height, suggesting that the horizon can be used as a reference point for size estimation.

In sum, size constancy depends on various factors including perceived distance, size familiarity, the horizon, and so on. As yet, we do not have a theory providing a coherent account of how these factors combine to produce size judgements.

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