title: Fundamental failures of shape constancy resulting from cortical anisotropy.
abstract: 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.
authors.affiliation: Vision Science Department, State University of New York, College of Optometry, New York, New York 10036, USA.
authors.forename: Elias H
authors.initials: EH
authors.lastname: Cohen
authors.forename: Qasim
authors.initials: Q
authors.lastname: Zaidi
mesh_terms: D000465:Algorithms; D016880:Anisotropy; D015982:Bias; D003463:Cues; D006801:Humans; D007088:Illusions / Q000502:physiology*; D008959:Models, Neurological; D009433:Neural Inhibition / Q000502:physiology; D009434:Neural Pathways / Q000166:cytology / Q000502:physiology; D009474:Neurons / Q000502:physiology; D009483:Neuropsychological Tests; D009949:Orientation / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010775:Photic Stimulation; D011594:Psychometrics; D013028:Space Perception / Q000502:physiology*; D014793:Visual Cortex / Q000166:cytology / Q000502:physiology*