Interocular suppressive interactions in amblyopia depend on spatial frequency

Abstract

In amblyopia, there is an interocular suppressive imbalance that results in the fixing eye dominating perception. In this study, we aimed to determine whether these suppressive interactions were narrowband and tuned for spatial frequency or broadband and independent of spatial frequency.We measured the contrast sensitivity and masking functions of fifteen amblyopic subjects and seventeen control subjects using the quick Contrast Sensitivity Function (qCSF) approach (Lesmes, Lu, Baek, & Albright, 2010). We first measured the monocular sensitivity functions of each participant and thereafter corrected for it. We then measured masking sensitivity functions for low, mid and high spatial frequency masks, normalized to their visibility.In the control group, we observed that the strength of dichoptic masking is equivalent between the two eyes. It is also tuned such that masking by low spatial frequencies in one eye mainly affects low spatial frequencies in the other eye and masking by high spatial frequencies mainly affects high spatial frequencies. In amblyopes, although the interocular masking is also tuned for spatial frequency, it is not equivalent between the two eyes: the masking effect from the amblyopic to fixing eye is weaker than the other way around.The asymmetry observed in the strength of masking between the two eyes in amblyopia is tuned for spatial frequency. It is not the consequence of the contrast sensitivity deficit of the amblyopic eye nor is it the consequence of abnormally strong masking from the fixing eye. Rather it is due to an abnormally weak masking strength by the amblyopic eye per se.

Dichoptic Spatial Contrast Sensitivity Reflects Binocular Balance in Normal and Stereoanomalous Subjects

“…Thus, stimulation of the nontested eye, with identical background delivered to the tested eye, should modulate the monocular sensitivity of the tested eye, which was captured in the ratio termed DSD. The present data agree with previous reports, 35,55 as dichoptic condition indeed changed contrast sensitivity. The extent of the DSD was more or less pronounced based on binocular status: the data showed small but significant DSD in both eyes of stereonormal subjects, while a large DSD was evident in the amblyopic eye of foveally fixating, stereoanomalous amblyopic patients.…”

“…We hypothesize that amblyopic inputs do not provide enough suppressive modulation to the dominant eye as observed in subjects with normal binocular vision between nondominant and dominant eyes. These results are backed by a recent article published by Beylerian et al, 55 who have conducted a similar experiment with both monocular contrast sensitivity measurement and a dichoptic condition with a noise mask presented to the contralateral eye, called dichoptic masking, which is similar to our noise background seen by the contralateral eye. They showed that dichoptic masking asymmetrically influenced the amblyopic and the nonamblyopic eyes.…”

“…The second result, namely, that the amblyopic eye does not exert dichoptic masking onto the dominant eye in the low frequency range only, might be less readily explainable. One possible explanation would be a spatial frequency tuning of this dichoptic masking as has been shown by Beylerian et al, 55 which posits that the masking effect is strongest around the spatial frequency of the mask itself. They found that the high-frequency mask presented to one eye had a detrimental effect on contrast sensitivity of the contralateral eye in the high-frequency range, but not in the low-frequency range, and the opposite was true for the low-frequency mask.…”

Interocular Suppression as Revealed by Dichoptic Masking Is Orientation-Dependent and Imbalanced in Amblyopia

“…15,16 Several theoretical models of binocular interactions in amblyopia, for example the two-stage model, 17 multiple-pathway contrast gain control model, 18 and contrast gain control and gain enhancement model, 19 suggest that a disruption to the normally balanced dichoptic inhibitory circuits could provide the basis for understanding suppression in amblyopia. In this case a detailed study of dichoptic masking, which is thought to reflect these inhibitory interocular interactions, has been considered a potentially useful avenue to better understand the phenomenon that under conditions of binocular viewing, information from the amblyopic eye is suppressed [20][21][22][23] at an early stage in the visual cortex. 24 Using the dichoptic masking approach, Zhou et al 23 recently observed less suppression from the amblyopic eye to the fellow eye and a normal suppression from the fellow eye to the amblyopic eye in amblyopic adult patients, resulting in a net suppression of the amblyopic eye under binocular viewing.…”

“…To answer this question, we adopted a dichoptic masking paradigm based on our previous studies. 21,23 In contrast to the studies by Levi et al 40 and Harrad and Hess 22 and Gao et al, 39 we used different combinations of target and masks orientations rather than a fixed target orientation with varied mask orientation. We measured subjects' contrast sensitivity for detecting the target in isolation or under various masking conditions for each eye.…”