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Jason D. Forte; Binocular summation of color and luminance contrast gratings. Journal of Vision 2005;5(8):795. doi: 10.1167/5.8.795.
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© ARVO (1962-2015); The Authors (2016-present)
Binocular luminance contrast thresholds for stimuli with the same spatial configuration are lower than monocular thresholds. Binocular color contrast thresholds follow a similar pattern but have not been studied in great detail. The current experiment measured binocular summation for luminance gratings that modulate the L and M cones together (L+M gratings), and color gratings that modulate the L and M cones with opposite sign (L−M gratings).
Five subjects used the method of adjustment to determine monocular and binocular detection thresholds for contrast reversed (at 4 Hz) 1 cycle per degree gabor patches (with a 1 degree space constant). Thresholds were determined for horizontal and vertical gratings, with dichoptic cone contrast modulated either in-phase or out-of-phase. Thresholds were also determined for gratings with crossed orientations in the two eyes. A binocular summation index was computed from the ratio of monocular thresholds to binocular thresholds (a value > 1 indicates binocular summation).
Summation patterns were similar for color and luminance gratings when averaged across observers. The binocular summation index was 1.80 for in-phase gratings; greater than the 1.41 value expected for probability summation. Crossed orientation gratings showed only small levels of binocular summation (a binocular summation index of 1.16). The binocular summation index for out-of-phase gratings was 0.76, indicating that binocular out-of-phase gratings were less visible than the monocular components. This effect was only evident for color gratings in two observers.
The results suggest there is binocular neural summation for dichoptic luminance and color gratings with the same orientation and spatial phase. Luminance and color gratings show destructive binocular interactions for dichoptic gratings with opposite phase in the two eyes. The pattern of binocular summation is consistent with the existence of spatially selective binocular mechanisms for luminance and color.
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