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Jiawei Zhou, Rong Liu, Yifeng Zhou, Robert Hess; Binocular combination of second-order stimuli in normals and amblyopes. Invest. Ophthalmol. Vis. Sci. 2014;55(13):799.
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The nature of the binocular combination of stimuli defined by modulations in contrast, so-called second-order stimuli, is presently not clear. To address this issue, we asked four questions: first, does binocular phase combination also occur for second-order stimuli; if so, what is the difference between the binocular phase combination of first-order and second-order stimuli; does binocular combination before or after the extraction of second-order signals and finally how does combination differ in normals and amblyopes.
A dichoptic phase combination paradigm was used in our study. In this test, two horizontal sine-wave gratings of equal and opposite phase-shifts were dichoptically presented to the two eyes; the perceived phase of the binocular percept was measured when the contrast of the first-order gratings or the modulation depth of the second-order gratings fixed at 100% in the non-dominant eye, and the signal (the contrast of the first-order gratings or the modulation depth of the second-order gratings) in the dominant eye was varied for a range of interocular signal differences. To determine whether the binocular combination occurs before or after the extraction of second-order signals, three types of second-order dichoptic pairs were used in the study: the carriers in the two eyes could be correlated, anti-correlated or uncorrelated.
We found that the binocular perceived phase of second-order gratings depends on the interocular signal ratio as has been previously shown for their first order counterparts in all observers. The interocular signal ratios (dominant eye / non-dominant eye) when the two eyes were balanced were comparable in first- and second-order phase combinations: close to 1 in normals and less than 1 in amblyopes. Second-order combination is more linear than previously found for first-order combination in both normals and amblyopes. Furthermore, binocular combination of second-order stimuli was similar regardless of whether the carriers in the two eyes were correlated, anti-correlated, or uncorrelated.
This suggests that, the binocular phase combination of second-order stimuli occurs after the monocular extracting of the second-order modulations. The sensory balance associated with this second-order combination can be obtained from binocular phase combination measurements and is imbalanced in amblyopes similarly to that of comparable first-order stimuli.
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