Academic journal article Attention, Perception and Psychophysics

Body Ownership Affects Visual Perception of Object Size by Rescaling the Visual Representation of External Space

Academic journal article Attention, Perception and Psychophysics

Body Ownership Affects Visual Perception of Object Size by Rescaling the Visual Representation of External Space

Article excerpt

Published online: 8 May 2014

# The Author(s) 2014. This article is published with open access at Springerlink.com

Abstract Size perception is most often explained by a combination of cues derived from the visual system. However, this traditional cue approach neglects the role of the observer's body beyond mere visual comparison. In a previous study, we used a full-body illusion to show that objects appear larger and farther away when participants experience a small artificial body as their own and that objects appear smaller and closer when they assume ownership of a large artificial body ("Barbie-doll illusion"; van der Hoort, Guterstam, & Ehrsson, PLoS ONE, 6(5), e20195, 2011). The first aim of the present study was to test the hypothesis that this own-body-size effect is distinct from the role of the seen body as a direct familiar-size cue. To this end, we developed a novel setup that allowed for occlusion of the artificial body during the presentation of test objects. Our results demonstrate that the feeling of ownership of an artificial body can alter the perceived sizes of objects without the need for a visible body. Second, we demonstrate that fixation shifts do not contribute to the own-body-size effect. Third, we show that the effect exists in both peri-personal space and distant extra-personal space. Finally, through a meta-analysis, we demonstrate that the own-body-size effect is independent of and adds to the classical visual familiar-size cue effect. Our results suggest that, by changing body size, the entire spatial layout rescales and new objects are now perceived according to this rescaling, without the need to see the body.

Keywords 3D perception . Cue integration . Multisensory processing . Visual perception . Illusion

Introduction

When adults see a toy that they have not seen since their childhood, they tend to be surprised by how small it appears, because they remembered it to be much larger. Anecdotes like this suggest that objects are perceived to be larger to smaller observers and smaller to larger observers. In line with this idea, Poincaré (1952) proposed a thought experiment in which the entire world, including your body, becomes a thousand times larger during sleep. Upon awakening, you would not notice this tremendous change because "our body serves us as a systemofaxesofcoordinates" (Poincaré, 1952,p.100).A recently published experimental study used a body illusion to confirm Poincaré'snotionthatchangingbodysizechangesthe perceived size of objects ("Barbie-doll illusion"; van der Hoort, Guterstam, & Ehrsson, 2011). The present study sought to examine the possible mechanisms of this own- body-size effect by isolating the role of the multisensory experience of owning a body from the mere use of classical visual cues. Because size perception and depth perception are closely related (size is the distance between two sides of an object, and depth is the distance between the object and the observer), we will use the term space perception to refer to both.

Psychology textbooks typically explain the visual percep- tion of space on the basis of the visual cue approach (e.g., Cutting & Vishton, 1995; Goldstein, 1999; Schwartz, 2010). According to this approach, the perceived size of an object depends on a combination of its size on the retina and its perceived distance from the observer. A multitude of visual cues can be used to perceive object distance. Pictorial cues (e.g., familiar size, relative size, texture gradient, and linear perspective), motion-produced cues (depth from motion and motion parallax), and binocular disparity are all visual cues derived from retinal information. In addition, oculomotor cues (accommodation and eye convergence) are considered visual distance cues because the muscles from which they derive are part of the visual system. The visual cue approach depicts visual perception as being much like a video camera, because it only emphasizes visual cues without accounting for the presence of a body. …

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