Is Suppression Just Normal Dichoptic Masking? Suprathreshold Considerations

Abstract

PURPOSE. Amblyopic patients have a deficit in visual acuity and contrast sensitivity in their amblyopic eye as well as suppression of the amblyopic eye input under binocular viewing conditions. In this study we wanted to assess the origin of the amblyopic suppression by studying the contrast perception of the amblyopic eye at suprathreshold levels under binocular and monocular viewing.METHODS. Using a suprathreshold contrast matching task in which the reference and target stimuli were presented to different eyes either simultaneously or successively, we measured interocular contrast matching in 10 controls and 11 amblyopes (mean age 35 6 15; 5 strabismics; 3 anisometropes; 3 mixed). This was then used as an index of the binocular balance across spatial frequency and compared against the contrast sensitivity ratio measured with the same stimuli.RESULTS. We observed that binocular matching becomes more imbalanced at high spatial frequency for amblyopes, compared with controls; that this imbalance did not depend in either group on whether the stimuli were presented simultaneously or successively; and that for both modes of presentation the matching balance correlates well with the interocular contrast sensitivity ratio (mean correlation coefficient of the slopes R ¼ 0.7125). CONCLUSIONS.The results from our amblyopes show comparable losses of contrast perception at and above threshold under these binocular viewing conditions across a wide spatial frequency range, much stronger than that observed for our controls. This occurs under conditions in which there should be no dichoptic masking. Furthermore, the matching contrast could be well predicted by the monocular contrast sensitivity. Altogether, this suggests that amblyopic suppression cannot be explained by normal dichoptic masking but rather an attenuation of the input.Keywords: interocular matching, amblyopia, qCSF, contrast constancy, suppression A mblyopia involves a loss of monocular vision and a suppression of the amblyopic eye's function under binocular viewing. There is debate about the causal relationship between these two factors. One possibility is that the suppression seen in amblyopia is a consequence of combining normal dichoptic masking with elevated contrast threshold in the amblyopic eye. Dichoptic masking depends on the level of suprathreshold contrast, so an elevated threshold in one eye (e.g., the amblyopic eye) would automatically result in imbalanced masking at all contrast levels, such that one eye (e.g., the fellow fixing eye) will always dominate over the other. We refer to this as the threshold hypothesis, as implemented by the offset subtraction model (see Appendix) where the primary deficit is the amblyopia and the secondary consequence is the binocular dysfunction.1,2 An alternate explanation is that the suppression results from anomalously large dichoptic inhibition of the amblyopic eye's input by the fellow fixing eye's input that in the long term results in a contrast threshold elevation in the amblyopic eye. We refer to this as the...

An Unexpected Spontaneous Motion-In-Depth Pulfrich Phenomenon in Amblyopia

“…Hence, the absolute reduction in rPSE observed could be driven by spatial summation mechanisms. This effect of spatial frequency could be supported by the fact that, in amblyopia, the two eyes are more balanced at low spatial frequency [44][45][46][47]. The absolute |rPSE| might be reduced regardless of its initial polarity because the amblyopic delay might be reduced due to the eyes being more balanced, and the blur effect [41] might also be decreased due to blurring having less of an effect on low-spatial-frequency images.…”

“…The contrast sensitivities of the amblyopic eye (AE) and the non-amblyopic eye (NAE) of all amblyopic subjects as a function of the spatial frequency are presented in Figures A1a,b. They were measured with the quick contrast sensitivity function (qCSF) [48] with the same apparatus and procedure as used in Reference [44]. Then, the ratio between the monocular sensitivity AE/NAE was calculated; it is plotted as a function of the spatial frequency in Figure A1c.…”

Two Patterns of Interocular Delay Revealed by Spontaneous Motion-in-Depth Pulfrich Phenomenon in Amblyopes with Stereopsis

“…In binocular spatial processes, there is evidence that the two eyes of the amblyopes are generally more imbalanced at higher spatial frequency for contrast thresholds, [24][25][26] and suprathreshold contrasts. 27,28 In this study, we show that the spatial frequency-dependent imbalance of amblyopes is also applicable to the temporal aspect of interocular processing. Our results thus provide additional support for the current notion that amblyopes are more binocularly imbalanced than controls, especially at higher spatial frequencies.…”

Interocular correlation sensitivity and its relationship with stereopsis

“…The procedure used to measure the interocular balance was similar to the simultaneous matching experiment described in Reynaud and Hess ( 2016 ). An alignment task was performed beforehand.…”