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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

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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 shape 
constancy across rotation in two studies.  Rotation provides a useful test case because it allows 
examination of observer perception of stimuli at constant retinal eccentricity.   In the first study, we 
observed that 3D shapes inferred from texture cues are perceived to be deeper at vertical than at
oblique orientations, violating shape constancy.  We demonstrated that this depth bias is attributable
to 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 near 
horizontal orientations combined with cross-orientation inhibition explains the 
orientation-dependent angle distortion and hence the 3-D shape inconstancy.
In the second study, we present evidence that the configural (or shape) representation of dot 
stimuli facilitates the perception of rotation.  We employed a stimulus in which the direction of
 rotation cannot be inferred by pooling the strongest local apparent motions.  Next we 
demonstrated 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 direction 
of rotation under circumstances where there were no consistent local motion cues and the only 
evidence was orientation changes in axes of symmetry.  Taken together, while the results suggest
 that metric shape judgments are vulnerable to low-level anisotropies, evidence that observers 
can use form to disambiguate motion signals suggests that representation of global shape may
 be a ubiquitous strategy across visual tasks.


 Background Reading:

Dr. Elias Cohen