Visual Expertise and the Log-Polar Transform Explain Image Inversion Effects

Martha Gahl, Shubham Kulkarni, Nikhil Pathak, Alex Russell, Garrison W. Cottrell

Abstract: Visual expertise can be defined as the ability to discriminate among subordinate-level objects in homogeneous classes, such as identities of faces within the class "face". Despite being able to discriminate many faces, subjects perform poorly at recognizing even familiar faces once inverted. This face-inversion effect is in contrast to subjects’ performance identifying inverted objects for which their experience is at a basic level, which results in less impairment. Experimental results have suggested that when identifying mono-oriented objects, such as cars, car novices' performance is between that of faces and other objects. We build an anatomically-inspired neurocomputational model to explore this effect. Our model includes a foveated retina and the log-polar mapping from the visual field to V1. This transformation causes changes in scale to appear as horizontal translations, leading to scale equivariance. Rotation is similarly equivariant, leading to vertical translations. When fed into a standard convolutional network, this provides rotation and scale invariance. It may be surprising that a rotation-invariant network shows any inversion effect at all. This is because there is a crucial topological difference between scale and rotation: Rotational invariance is discontinuous, with V1 ranging from 90 degrees (vertically up) to 270 degrees (vertically down). Hence when a face is inverted, the configural information in the face is disrupted while feature information is relatively unaffected. We show that the inversion effect arises as a result of visual expertise, where configural information becomes relevant as more identities are learned at the subordinate level. Our model matches the classic result: faces suffer more from inversion than mono-oriented objects, which are more disrupted than non-mono-oriented objects when objects are only familiar at a basic level.

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