The Mechanisms of Size Constancy

Abstract

Size constancy is the result of cognitive scaling operations that enable us to perceive an object as having the same size when presented at different viewing distances. In this article, we review the literature on size and distance perception to form an overarching synthesis of how the brain might combine retinal images and distance cues of retinal and extra-retinal origin to produce a percep tual visual experience of a world where objects have a constant size. A convergence of evidence from visual psychophysics, neurophysiology, neuropsychology, electrophysiology and neuroimaging highlight the primary visual cortex (VI) as an important node in mediating size-distance scaling. It is now evident that this brain area is involved in the integration of multiple signals for the purposes of size perception and does much more than fulfil the role of an entry position in a series of hierarchical cortical events. We also discuss how information from other sensory modalities can also contribute to size-distance scaling and shape our perceptual visual experience.

Why do animals differ in their susceptibility to geometrical illusions?

“…The results revealed that monkeys were susceptible to the illusion and that their V1 neurons responded in a way that more reflected the perceptual outcome as opposed to the retinal input. This is highly suggestive of top-down contextual influences, given that V1 is the first cortical weigh station of visual processing (Sperandio & Chouinard, 2015). It would be interesting to compare firing responses between a species that succumbs to this illusion versus one that does not.…”

“…They are framed by a set of converging lines, such that the upper line, closer to the point of convergence, appears longer than the lower line (Table 1). The converging lines are thought to be perceived as depth cues, resulting in inappropriate perceptual rescaling (Gregory, 1963;Sperandio & Chouinard, 2015;see Newman &Newman, 1974 andFineman &Carlson, 1973 for anternative viewpoints). That is, if the top line were further away and projected the same sized image on the retina, based on our understanding of size constancy, the top line must be longer (Ward et al, 1977).…”

“…Researchers suggest that, for humans, many illusions are rule-based, reflecting application of rules that we apply to visual inputs and that generally aid in perception of our surroundings (Eagleman, 2001;Helmholtz, Rev (2017) 24:262-2761867; Gregory, 1980). For example, Gregory's (1963) Inappropriate Constancy-Scaling theory suggests that illusions such as the Ponzo illusion occur due to the presence of apparent depth cues and the subsequent use of size-constancy rules that help us understand that objects do not normally become physically smaller with increasing distance (see also Sperandio & Chouinard, 2015; for alternative viewpoints, see Humphrey &Morgan, 1965 andNewman &Newman, 1974).…”

“…Animals with eyes located on the sides of their heads have limited binocular vision, suggesting that they must rely more heavily on monocular visual cues to determine how far away objects are. This is of interest in the context of geometrical illusions, as many common examples are argued to be due to humans' misapplication of monocular depth cues (Gregory, 1963;Sperandio & Chouinard, 2015; but see Newman &Newman, 1974 andFineman &Carlson, 1973 for alternative viewpoints). Animals that naturally rely more heavily on monocular depth cues might be expected to show greater susceptibility to these illusions or, alternatively, may not be susceptible to them at all.…”

Do Dogs Demonstrate Susceptibility to a Vertically Presented Ponzo Illusion?

“…Firstly, the illusory stimuli were presented horizontally. Gregory's (1963) Inappropriate Constancy-Scaling theory proposes that the Ponzo illusion is driven by a misapplication of size constancy arising from the brain's interpretation of the converging lines as depth cues (for review of multiple theories see Humphrey & Morgan, 1965;Newman & Newman, 1974;Sperandio & Chouinard, 2015). Thus, the stimulus nearest to the apex of the converging lines appears larger as it is perceived as being further away, which causes the viewer to inappropriately compensate for this distance in order to maintain size constancy.…”