Date Published: December 16, 2009
Publisher: Public Library of Science
Author(s): Andrea Li, Qasim Zaidi, David Whitney. http://doi.org/10.1371/journal.pone.0008333
Abstract: Cross-orientation suppression (COS) in striate cortex has been implicated in the efficient encoding of visual stimuli. We show that release from COS facilitates the decoding of 3-D shape. In planar surfaces overlaid with textures, slanting the surface can increase the visibility of the component parallel to the slant. Since this component provides the orientation flows that signify 3-D shape, the enhancement of visibility facilitates 3-D slant perception. Contrast thresholds reveal that this enhancement results from a decrease in COS when 3-D slant creates a frequency mismatch between texture components. We show that coupling compressive nonlinearities in LGN neurons with expansive nonlinearities in cortical neurons can model the frequency-specific component of suppression.
Partial Text: In the perspective image of a slanted textured surface, oriented components of the texture that are aligned with the 3-D slant converge to form orientation flows (, , ), while components orthogonal to the slant increase in frequency (Figure 1a). On casual observation, the horizontal component appears perceptually more salient than other components when a surface is slanted (Figure 1a, top left and right) than it does when the surface is parallel to the frontal plane (Figure 1a, top center). The increase in saliency is more pronounced in complex texture patterns, e.g. the octotropic plaid, which consists of eight gratings of the same frequency, equally spaced in orientation (Figure 1a bottom). Since these converging orientation flows play a critical role in conveying the perceived 3-D slant and shape of the surface (, , , ), an increase in their saliency should enhance the 3-D perceived slant. The goal of this work is to examine the neural mechanisms that enhance the visibility of orientation flows.
All research followed the tenets of the World Medical Association Declaration of Helsinki and informed consent was obtained from the subjects after explanation of the nature and possible consequences of the study. The research was approved by the Queens College Institutional Review Board.
The suggested roles of COS in visual encoding have included orientation tuning (, , ), contrast gain control (, , , , ), and redundancy reduction in the coding of natural images (, , ). Here we postulate a potential role for COS in the decoding of 3-D slant. We have shown that when textured surfaces are slanted, the release of COS makes the critical orientation flows more visible, which correlates with better perception of 3-D slant. We have shown that COS is frequency specific, and that this specificity can arise in simple feed-forward models of COS. To our knowledge, feed-forward explanations of frequency selectivity of COS have not been suggested previously. The LGN models of COS were formulated on the basis of cat physiology, where almost all cell response functions are compressive as a function of contrast. In primate LGN, M-cells are compressive, but P-cells are fairly linear. Our model thus provides the M-cell component of COS. Since V1 cells get input from P and M-cells, some component of COS involves cortical interactions ().