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Michael Kramer, Lynn Olzak; The absence of a collinearity effect in second-order, contrast-modulation discrimination tasks. Journal of Vision 2008;8(6):275. doi: 10.1167/8.6.275.
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Second-order stimuli are stimuli that are not detectable by mechanisms sensitive to luminance or color changes. Second-order cues can be created by modulating local first-order cues. There is evidence to suggest that first- and second-order cues are processed via separate mechanisms (Ledgeway & Smith, 1994; Schofield & Georgeson, 1999). The present study was performed to investigate how similar these separate mechanisms may or may not be. The specific intent of this study was to look at the role of collinearity in surround suppression for contrast-modulated (CM), second-order stimuli. It has been shown previously that in first-order contrast discrimination tasks, the effect of a suppressive surround seems to lie largely in the collinear regions of the surround (Kramer & Olzak, 2007). We set up a similar experiment with second-order stimuli to see if the second-order mechanism showed a similarly strong collinearity effect. The task was a fine second-order contrast modulation discrimination of circular, sinusoidal patches of CM binary noise (while holding spatial frequency and orientation constant). Surrounds were also sinusoidal patches of CM binary noise abutting the circular target (with the contrast modulation level, m, for the surrounds centered between the m-values of the two central patches being discriminated while the orientation and spatial frequency of the surrounds were identical to the central patches). BOW-TIE stimuli (Cannon & Fullenkamp, 1991) were used to modulate surround size and location in order to create both collinear and non-collinear surrounds of varying sizes as used by Kramer and Olzak (2007). Discriminability was measured using a 6-point response scale from which d' values were calculated. Our results suggest that there is no effect of collinearity on surround suppression for this type of second-order stimulus.
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