The Mechanisms of Size Constancy

in Multisensory Research
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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 perceptual 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 (V1) 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.

Multisensory Research

A Journal of Scientific Research on All Aspects of Multisensory Processing



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  • Retinotopic mapping of human visual areas using the phase-encoding approach. This approach to mapping retinal responses in the visual cortex is based on the principle that when a stimulus is presented in a cyclical manner, the BOLD signal will be modulated in a similar cyclical pattern (A). A movie of a stimulus moving across different parts of the visual field is played to the participant in a repeating loop (B: for polar angle mapping; C: for eccentricity mapping) while the BOLD signal is collected over time. The BOLD signal is then analysed using a Fourier analysis to determine when and where in the visual field the response is the highest. As shown in (A), the BOLD signal is highest as the stimulus passes through the receptive field for a specific cortical region while the BOLD signal in this same cortical region is lowest when the stimulus is no longer in the receptive field. The results from these kinds of stimulation typically give rise to what is shown in (D) and (E). As shown in (D), the white lines denote the vertical meridians defining the boundaries of V1. The upper visual field is represented in inferior structures while the lower visual field is represented in superior structures. As shown in (E), the fovea is represented in the more posterior parts of the visual cortex, while the peripheral parts of the retina are represented in the more anterior parts of the visual cortex. Note how the fovea is also represented in proportionally more cortex relative to the more peripheral parts of the retina. This figure is published in colour in the online version.

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  • The relationship between retinal image size and distance. If the same object is placed at different distances, such as the mouse illustrated in (A), then its image on the retina will vary in size following Euclid’s law. According to Euclid’s law, the size of the image on the retina will decrease proportionally as a function of distance. Different sized objects, such as the mouse and the elephant in (B), can have the same retinal image size if the relative distance between the two is just right. Namely, a mouse close by can have the same retinal image size as an elephant far away. An inducing stimulus, such as the circle in (C), will bleach a certain part of the retina. The result of this bleaching will cause a subsequent afterimage to appear several seconds later. According to Emmert’s law, the perceived size of the afterimage will increase proportionally as a function of the distance with which one projects the afterimage on. The diagrams are not drawn to scale. This figure is published in colour in the online version.

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  • Examples of optical illusions. The Ebbinghaus illusion (A) is an example of a size-contrast illusion whereby the size of the inner circles is perceived relative to the size of the surrounding context. Specifically, the inner circle looks bigger when surrounded by smaller circles and smaller when surrounded by bigger circles. Figure adapted from Sperandio et al. (2012a). In this example of the Ponzo illusion (B), the length of the two red lines is identical but the converging lines in the background make the upper one appear longer. These types of pictorial cues about distance are frequently present in our environment. For example, they can be observed when we look at railroad tracks, hall corridors, roads, and sidewalks. This figure is published in colour in the online version.

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  • The moon illusion. The moon appears larger when it is located on the horizon then when it is in its zenith position in the night sky, even though we know that the moon does not change in size. Interestingly, the larger-looking moon on the horizon will not appear so large any more if the moon is observed through a peephole such that the rest of the visual scene is occluded from view. Although this phenomenon has been studied for centuries, there is still no consensus on its explanation. The diagram is not drawn to scale. This figure is published in colour in the online version.

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  • Holway and Boring’s experiment. In a classic study, Holway and Boring (1941) had participants judge the size of circles at different viewing distances. The circles were always presented with a visual angle of one degree. The physical size of the circles increased proportionally as a function of distance such that the retinal image size was always constant. As the availability of distance cues was reduced, participants began to judge the size of the circles according to their retinal image size as opposed to their actual size. Many of the studies we review in this paper used a similar paradigm in order to present stimuli with the same retinal image size at different viewing distances (e.g. Combe and Wexler, 2009; Dobbins et al., 1998; Humphrey and Weiskrantz, 1969; Servos, 2006; Sperandio et al., 2010; Ungerleider et al., 1977; Weiskrantz, 1996). The diagram is not drawn to scale. This figure is published in colour in the online version.

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  • The Taylor illusion. To experience the illusion, an afterimage of the hand or of a hand-held object (e.g., a ring of lights as illustrated here) is generated in complete darkness by means of a flash of light. The hand is then moved right after the induction of the afterimage either towards or away from the body in a continuous and smooth fashion. One can readily observe that the afterimage will progressively decrease in size as the hand approaches the body (B) and vice versa increase in size as the hand is moved away from the body (C). Figure adapted from Sperandio et al. (2013a).

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  • Firing properties of neurons that reflect perceived size. The study performed by Ni and colleagues (2014) examined receptive field responses in V1 neurons in the monkey while they looked at rings over a Ponzo illusory background like the one shown here. In these simplified examples, the neuron in either upper or lower panel would fire equally poorly to the presentation of either ring presented without the illusory background. When the smaller ring was presented in the top position of the illusory background (upper panel), the authors showed that this would produce a stronger response because the context made it look bigger and its apparent size would cover a greater surface area of the neuron’s receptive field. When the bigger ring was presented in the bottom position of the illusory background (lower panel), the authors showed that it would produce a stronger response because the context made it look smaller and its apparent size would cover a greater surface area of the neuron’s receptive field. This figure is published in colour in the online version.

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  • An fMRI study on the Ponzo illusion. The study performed by Murray and colleagues (2006) examined BOLD responses to the presentation of flickering checkerboard spheres on a Ponzo illusory background like the one shown here (A). The BOLD responses were extracted from various pre-defined eccentric ROIs in V1. The top circle, which appeared larger to participants, invoked larger BOLD responses in an ROI for a larger eccentricity (drawn schematically in B) than the one for the retinal image size of the stimulus (drawn schematically in B). The bottom circle, which appeared smaller to participants, invoked larger BOLD responses in an ROI for a smaller eccentricity (drawn schematically in B) than the one for the retinal image size of the stimulus (drawn schematically in B). This figure is published in colour in the online version.

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  • An fMRI study on afterimages. In our fMRI study presented in this review, we examined the role of V1 in the size perception of afterimages (Sperandio et al., 2012c). (A) BOLD signal in V1 was extracted from eccentricity ROIs for the closest viewing distance (distance 1) vs. the furthest viewing distance (distance 5). See the legend on the inflated brain in (B) for colour coding of each ROI along the calcarine sulcus. At the beginning of the time course, when the inducing light was presented, we observed the greatest BOLD response in the smallest eccentricity ROI for both viewing distances. In fact, this ROI subtended 4.1° which corresponded to the retinal size of the inducing light. In contrast, during the perception of the afterimage (indicated by the two solid lines), we observed the greatest BOLD response in the biggest eccentricity ROIs at the furthest viewing distance, when participants experienced the biggest afterimages. The emergence of activity in regions that were never stimulated by light is a clear demonstration that V1 responds to the perceived size of an afterimage. (C) Correlation between perceived duration of the afterimage at the furthest viewing distances and area under the curve of the BOLD signal extracted from the biggest eccentricity ROIs. Every individual is represented as a data point while the solid line indicates the best-fitting line for the observed data. Figures adapted from Sperandio et al. (2012c). This figure is published in colour in the online version.

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  • Craik–O’Brien–Cornsweet circles. In an adaptation study, Pooresmaeili et al. (2013) presented Craik–O’Brien–Cornsweet circles similar to the ones shown here. When presenting an adapter followed by a smaller test stimulus, participants perceived the test stimulus as being smaller, and vice versa. In Craik–O’Brien–Cornsweet circles, the overall luminance inside is the same as the background yet we perceive the inside as having a slightly different luminance. In this case, it is perceived as lighter. This figure is published in colour in the online version.

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