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Kathy T. Mullen, Jose M. Medina; Isotropic orientation tuning for masking in human color vision. Journal of Vision 2009;9(8):316. doi: 10.1167/9.8.316.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Cross-orientation masking (XOM) is defined psychophysically as the phenomenon whereby detection of a test grating is masked by a superimposed stimulus at an orthogonal orientation and is thought to be part of a system of gain control that modulates detection and visibility of the test stimulus. Here we investigate the chromatic gain control mechanisms, particularly their orientation tuning. Methods: Horizontal Gabor stimuli (spatial envelope, σ=2 degrees) were presented in nine combinations of three spatial (0.375, 0.75, 1.5cpd) and three temporal frequencies (2, 4, 8Hz). The mask had the same spatio-temporal frequency and chromaticity as the test but was superimposed with a range of orientations (15-90degs) relative to the test. Binocular contrast detection thresholds were determined using a temporal 2AFC staircase method over a wide range of mask contrasts, scaled in multiples of detection threshold. We used isoluminant red-green or achromatic stimuli.
Results: We find that chromatic XOM (mask at 90 degs) is significantly greater than luminance XOM at equivalent mask contrasts. Chromatic XOM is invariant across all spatiotemporal conditions, unlike luminance XOM that is greatest in the high temporal, low spatial frequency range. We also find that chromatic masking is invariant across orientation difference between test and mask, and remains isotropic for both high and low mask contrasts. This differs from luminance masking, which shows orientation tuning, as previously reported.
Conclusions: The results indicate distinct physiological origins for chromatic and luminance cross-channel masking. We argue for a predominantly cortical site for chromatic XOM masking, whereas previous studies have proposed subcortical M-cell influences for luminance XOM.
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