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Lynn A. Olzak, Patrick J. Hibbeler; Second-order mechanisms do not process contrast-modulated orientation information optimally. Journal of Vision 2008;8(6):279. doi: 10.1167/8.6.279.
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There is considerable argument over whether second-order mechanisms (those responding to changes in textural contrast, grain, or orientation when mean luminance is contrast) constitute a single class of mechanism or whether different mechanisms underlie the processing of different types of second-order patterns. There is also argument as to the relative sensitivity of first order mechanisms (responding to luminance-defined patterns) vs. second-order mechanisms. Regan (2000) argues that the relative sensitivity of first and second order mechanisms is comparable for all hyperacuity tasks, although his measurements were confined to orientation-defined second-order patterns. We previously measured spatial frequency, contrast, and orientation discrimination thresholds for contrast-modulated second-order patterns. Our initial results indicated that while spatial frequency and contrast thresholds were comparable to those obtained with luminance-defined patterns, orientation judgments were much worse and observers refused to continue the experiment. We have now re-measured orientation discrimination thresholds and find them to be on the order of 10x values obtained with first-order stimuli. Our results imply that the mechanisms mediating contrast-modulated, second-order judgments are a) not well-suited to mediate judgments about orientation ; b) considerably less sensitive to orientation than first-order mechanisms; and c) different mechanisms than those mediating orientation-modulated judgments. We conclude that an understanding of the second-order systems is by no means complete.
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