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Sarah J. Waugh, Sarah J. H. Lalor, M. Izzuddin Hairol; Binocular summation for luminance- and contrast-modulated noise stimuli. Journal of Vision 2009;9(8):1012. doi: 10.1167/9.8.1012.
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The magnitude of measured binocular summation can provide information about the locus of processing for different tasks or for different types of stimuli.
We measured 1) monocular and binocular detection and discrimination thresholds (i.e. dipper functions) for luminance-modulated and contrast-modulated Gaussian blobs (σ=0.25 deg) and 2) monocular and binocular detection thresholds for luminance-modulated and contrast-modulated Gabors (σ=1.0 deg; modulations of 0.5, 1, 2, 4, 8 and 16 cpd). Luminance- and contrast-modulated Gaussian blobs were constructed by adding or multiplying a Gaussian profile to a binary, random-dot, dynamic noise background. Gabors were constructed from the same noise background. Data were collected using a method of constant stimuli and temporal 2AFC paradigm. Four adults with normal vision participated.
For both luminance-modulated and contrast-modulated Gaussian blobs, we obtained detection thresholds that were binocularly on average 45% lower than monocular thresholds and on average 55% lower in the facilitation region. Discrimination thresholds for higher pedestal visibilities were similar monocularly and binocularly for luminance-modulated Gaussian blobs but slightly better binocularly for contrast-modulated Gaussian blobs. Slopes in the masking region were similar for all stimuli with exponents around 0.7.
For luminance-modulated Gabors (0.5 cpd) binocular summation reached around 70%, decreasing substantially for higher spatial frequency modulations. For contrast-modulated Gabors, binocular summation was more consistent across modulation frequency. Binocular summation ratios for these stimuli were equal or higher than those measured for luminance-modulated Gabors for all modulation frequencies above 0.5cpd.
In combination with our previous results, where dichoptic viewing had a greater effect on the discrimination of contrast-modulated Gaussian stimuli (Waugh & Hairol, VSS2008) and indirect evidence of others (Wong, Levi & McGraw, Vis Res, 2001), we propose that contrast-modulated stimuli are likely processed at a more binocular site than luminance-modulated stimuli.
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