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Elias H. Cohen, Manish Singh; Perceived orientation of complex shapes reflects graded part decomposition. Journal of Vision 2005;5(8):214. doi: 10.1167/5.8.214.
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Although orientation of line segments and simple shapes is a well-studied area of vision, little is known about the factors that influence perceived orientation of complex shapes. Study of these factors is of great interest because it allows for a better understanding of how local shape attributes are integrated into a global shape description. We studied the perceived global orientation of two-part shapes using an adjustment method and a 2AFC task. In particular, we investigated the influence of (i) the perceptual distinctiveness of a part — as defined by the turning angles at the negative minima of curvature defining its boundaries, and (ii) its area relative to the main ‘base’ part. Results exhibited systematic deviations of perceived orientation from the axis of greatest elongation of the shape (i.e., the principal axis with the greatest eigenvalue). For sharp part boundaries, perceived global orientation deviated maximally from the principal axis, and was approximated instead by the axis of the main ‘base’ part of the shape (i.e., observers largely ignored the attached part in estimating overall orientation). However, with weakening part boundaries, the perceived orientation gradually approached the principal axis of the entire shape (albeit with weaker precision) — reflecting that both parts were taken into account in estimating orientation. The results allow us to quantify a largely ignored characteristic of complex shape representation: part independence. Previously we showed that increase in part-boundary strength leads to systematically better performance in a 4AFC part-identification task (Cohen & Singh, VSS 2003). The current results show that geometric factors such as turning angles at part boundaries determine not only the strength of a part's representation, but also the extent to which it is represented as an independent unit, i.e., the extent to which its influence is separable from the rest of the shape.
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