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Fundamental failures of shape constancy resulting from cortical anisotropy.

Cohen EH, Zaidi Q

Vision Science Department, State University of New York, College of Optometry, New York, New York 10036, USA.

Contrary to the conventional assumption that humans perceive shapes of rigid objects as constant despite retinal-image variations caused by changes in orientation and position, we show that the depths of three-dimensional (3-D) textured shapes appear to vary when the image is rotated. In paired comparisons of static stimuli, depth amplitude was perceived to be greater at vertical than at oblique orientations. A similar oblique bias was found for simple two-dimensional (2-D) obtuse angles. Using projective geometry to link angle magnitude to the orientation flows that convey 3-D shape from texture cues, we show quantitatively that the 2-D bias predicts the 3-D bias. Our finding that perception of angular magnitude is dependent on orientation has broad implications for shape constancy because orientation flows have also been implicated in 3-D perception from reflections and shading, and contour curvature is fundamental to uncovering depth and part-structure of shapes. We examined the roles played in the observed biases by anisotropies in numbers and tuning widths of orientation-tuned cells in striate cortex as well as the distribution of oriented energy in natural scenes. An optimal stimulus decoding model for 2-D angles revealed that the narrower tuning of cells for horizontal orientations combined with cross-orientation inhibition explains the orientation-dependent angle distortion and hence the 3-D shape inconstancy. Variations in properties within neural populations can thus have direct effects on visual perceptions and need to be included in neural decoding models.

Published 16 November 2007 in J Neurosci, 27(46): 12540-5.
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