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As the shape turns: Rotation and shape (in) constancy
Dr. Elias Cohen
Monday, April 21, 2008, 01:00pm - 02:00pm
SUNY College of Optometry, Vision Sciences Department
Shape is the geometric attribute of an object that is invariant to location, rotation, and scale effects.In order to test if visual perception of shape is invariant to these transformations, we examined shapeconstancy across rotation in two studies. Rotation provides a useful test case because it allowsexamination of observer perception of stimuli at constant retinal eccentricity. In the first study, weobserved that 3D shapes inferred from texture cues are perceived to be deeper at vertical than atoblique orientations, violating shape constancy. We demonstrated that this depth bias is attributableto a more fundamental orientation bias for the magnitude of two-dimensional obtuse angles.Using an optimal stimulus decoding model, we demonstrated that narrower tuning of cells nearhorizontal orientations combined with cross-orientation inhibition explains theorientation-dependent angle distortion and hence the 3-D shape inconstancy.
In the second study, we present evidence that the configural (or shape) representation of dotstimuli facilitates the perception of rotation. We employed a stimulus in which the direction ofrotation cannot be inferred by pooling the strongest local apparent motions. Next wedemonstrated that properties of the dot configuration (order and magnitude of shape variation)facilitate rotation detection. Finally, we showed that subjects were able to determine the directionof rotation under circumstances where there were no consistent local motion cues and the onlyevidence was orientation changes in axes of symmetry. Taken together, while the results suggestthat metric shape judgments are vulnerable to low-level anisotropies, evidence that observerscan use form to disambiguate motion signals suggests that representation of global shape maybe a ubiquitous strategy across visual tasks.
Background Reading: http://www.sunyopt.edu/research/ecohen/CohenZaidi2007b.pdf