Levels of Processing in the Size-Distance Paradox

Abstract

Phenomena such as the moon illusion and accommodation-convergence microp-sia pose a paradox. Visual cues and oculomotor changes normally associated with a far distance may make objects appear both large and near, while those associated with a near distance have the opposite effect. The reported distance effects are paradoxical because they contradict the classical theory of size-distance in-variance, in which objects of the same angular size appear larger in linear size because they appear further away. One recent solution is to say that certain cues produce a change in perceived angular size, which in turn influences perceived distance and perceived linear size. An older solution is the 'further-larger-nearer' account, in which distance is first processed at a subconscious (automatic) level, and determines the perceived linear size; the perceived linear size is then inappropriately used as a cue for the conscious judgement of distance. Some authors dissociate size and distance processing, and some do not accept more than one type of perceived size. Several authors distinguish between cognitive judgements of distance (which may be biased by perceived size) and the automatic use of distance cues to scale size. Various experiments show that conflicting spatial values can sometimes be held at the same time in the visual and tactile-kinaesthetic systems. Tactile spatial judgements cannot, therefore, provide a reliable measure of visual perception. It remains difficult to find an empirical distinction between cognitive and automatic processes. "Paradoxes have no place in science. Their removal is the substitution of true for false statements and thoughts." William Thomson, Baron Kelvin of Largs. Address to the Royal Institution (1887). 8.1 The Size-Distance Paradox The size-distance paradox refers to those perceptual phenomena where objects appear both larger and nearer, or smaller and further, than would be predicted on the basis of size-distance invariance (SDI). The SDI hypothesis (see Sedgwick, 1986) is that an object's perceived linear size is determined in a geometrical manner by its true angular size and its perceived distance (Figure 8.1). On that basis,