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Michael Kramer, Lynn Olzak; The effects of collinearity on contrast discrimination tasks. Journal of Vision 2007;7(9):252. doi: 10.1167/7.9.252.
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The role of collinearity in psychophysical tasks involving contrast has not yielded clear answers. Collinear facilitation has been found in contrast detection tasks (Solomon & Morgan, 2000), while apparent contrast tasks have yielded no differences when using targets with collinear versus non-collinear surrounds (Cannon & Fullenkamp, 1991). The present study was performed to find out more about the effects of collinearity in regards to contrast discrimination tasks. It has been shown that other spatial discrimination tasks (orientation and spatial frequency) are more strongly influenced by collinear surround than by non-collinear surrounds, even when using surrounds of up to two-thirds the size of a full surround (Kramer & Olzak, 2006). However, this had yet to be shown for contrast discrimination tasks. The tasks used are fine contrast discriminations (centered around a contrast of 0.1) between 40-minarc, circular center patches of 4 cpd sinusoidal grating. Surrounds were vertical sinusoidal grating rings, also 4 cpd, with a ring width of 40-minarc. 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 around the target. Discriminability was measured using a 6 point response scale from which d' values were calculated. Our results suggest that, like results found with orientation and spatial frequency discriminations, collinear surrounds have a stronger inhibitory effect on contrast discrimination tasks when compared to the effects of non-collinear side-flanks of equal size.
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