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Huseyin Boyaci, Katja Doerschner, Scott O. Murray; Shape-dependence of a size illusion explained by spatial mapping in V1. Journal of Vision 2009;9(8):38. doi: https://doi.org/10.1167/9.8.38.
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© ARVO (1962-2015); The Authors (2016-present)
Perceived size of an object depends not only on its retinal image size but also on its perceived distance to the observer. This principle leads to well known visual effects such as the Ponzo illusion and the hallway illusion: holding retinal image constant, the angular size of an object is perceived larger for far as compared to near objects. Here we investigate the extent to which the size effect is modulated by the shape of the object.
In a behavioral experiment we measured the magnitude of the effect in a computer rendered hallway scene using three different shapes: a spherical object (ball), a narrow horizontal rod and a narrow vertical rod. Each shape was presented in five different absolute image sizes. We quantified the effect as the ratio of the image size (radius for the ball, length for the rods) of the front object to that of the back object at the point of subjective equal image size.
For the ball, we found that the magnitude of the effect depends on absolute image size: the perceived angular size difference between the near and far ball decreases as the radius of the ball increases. In contrast, there was no dependence on absolute size for either of the rod shaped objects. Furthermore, for horizontal rods we consistently observed a larger size effect than for balls.
We explain the pattern of results using a model based on a functional form of the mapping from visual space to cortical space (Schwartz, 1980) in primary visual cortex (V1). The model assumes that observers' image size estimation is based on the amount of V1 area recruited. This suggests that the pattern of cortical activity in V1 may constitute a basis for perceptual coding of angular image size.
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