Rebalancing binocular vision in amblyopia

Abstract

Purpose: Humans with amblyopia have an asymmetry in binocular vision: neural signals from the amblyopic eye are suppressed in the cortex by the fellow eye. The purpose of this study was to develop new models and methods for rebalancing this asymmetric binocular vision by manipulating the contrast and luminance in the two eyes.

Methods: We measured the perceived phase of a cyclopean sinewave by asking normal and amblyopic observers to indicate the apparent location (phase) of the dark trough in the horizontal cyclopean sine wave relative to a black horizontal reference line, and used the same stimuli to measure perceived contrast by matching the binocular combined contrast to a standard contrast presented to one eye. We varied both the relative contrast and luminance of the two eyes' inputs, in order to rebalance the asymmetric binocular vision.

Results: Amblyopic binocular vision becomes more and more asymmetric the higher the stimulus contrast or spatial frequency. Reanalysing our previous data, we found that, at a given spatial frequency, the binocular asymmetry could be described by a log-linear formula with two parameters, one for the maximum asymmetry and one for the rate at which the binocular system becomes asymmetric as the contrast increases. Our new data demonstrates that reducing the dominant eye's mean luminance reduces its suppression of the non-dominant eye, and therefore rebalances the asymmetric binocular vision.

Conclusions: While the binocular asymmetry in amblyopic vision can be rebalanced by manipulating the relative contrast or luminance of the two eyes at a given spatial frequency and contrast, it is very difficult or even impossible to rebalance the asymmetry for all visual conditions. Nonetheless, wearing a neutral density filter before the dominant eye (or increasing the mean luminance in the non-dominant eye) may be more beneficial than the traditional method of patching the dominant eye for treating amblyopia.

Keywords: amblyopia treatment; binocular asymmetry; gain control; imbalanced vision; interocular suppression; neutral density filter.

© 2014 The Authors Ophthalmic & Physiological Optics © 2014 The College of Optometrists.

Figures

Figure 1

Stimuli. A dichoptic nonius cross surrounded by a high contrast frame (above) and sinewave gratings presented to the two eyes (bottom). An observer’s task was to indicate the apparent location of the centre of the dark stripe in the perceived cyclopean sinewave grating relative to reference horizontal lines adjacent to its edge. The physical position of the reference lines was fixed, and its relative vertical position to the cyclopean sinewave grating varied from trial to trial in a staircase by shifting the phases of the two eyes’ sinewaves correspondingly.

Figure 2

Ding-Sperling model, a gain-control model for binocular combination , , . The model consists of left and right eye channels, each containing two gain control mechanisms: one is selective for orientation and spatial frequency in the signal layer (black) and the other is non-selective for those dimensions, and is based on total gain-control energy (TGE) summed across all dimensions in the gain-control layer (blue). The two TGE components exert reciprocal inhibition on one another, in the gain-control layers (blue) in proportion to their respective TGE outputs, and the outputs of those TGE components exert gain control on the other eye’s signal layer (black). The outputs are summed linearly to determine the binocular signal.

Figure 3

Results from our previous studies , . Perceived phase θ̂ of binocularly-combined cyclopean sine waves as a function of the right eye/left eye (RE/LE) or non-dominant eye/ dominant eye (NDE/DE) contrast ratio (δ ) for two normal observers (A) or two amblyopic observers (B), when the base contrast m is 96% (*), 48% (x), 24% (○), 12% (∇), or 6% (□). The phase difference of the two eyes’ sinewave gratings was fixed at 90 deg; LE’s (or DE’s) was −45 deg indicated by arrows in the left side and RE’s (or NDE’s) was 45 deg indicated by arrows in the right side. When δ ≤ 1 the DE’s grating contrast was fixed at base contrast m and δ was increased by increasing the NDE’s contrast (δ m). When δ ≥ 1 the NDE’s contrast remained constant at the base contrast m, and δ was increased by decreasing the DE’s contrast (m/δ). The solid curves are the best fits from the DSKL model (a modified Ding-Sperling model). The black dashed curve is the prediction of linear summation, the asymptote of the DSKL model at zero gain-control energy. The short black bars indicate contrast threshold ratios. Error bars: ±SE.

Figure 4

A. Binocular asymmetry (δB = non-dominant eye/ dominant eye or NDE/DE contrast ratio at rebalanced vision) as a function of NDE contrast for two amblyopic observers GD (left) and GJ (right), adapted from Ding, Klein & Levi . For one spatial frequency, the data can be fit by a straight line, the rebalance line (solid coloured lines), at which the imbalanced binocular vision is rebalanced. The horizontal dashed line at δB = 1 shows the symmetry line for normal vision, and the black markers indicate the right eye/left eye (RE/LE) contrast ratio for symmetric binocular vision. B. The going-away-from-symmetry rate (the slope of a rebalance line in A) as a function of spatial frequency. The horizontal black dashed line at 0 indicates that normal vision always remains symmetric (never going away from the symmetry). C. Maximum asymmetry (when NDE’s contrast = 100%) as a function of spatial frequency. The horizontal black dashed line at 1 indicates no asymmetry in normal vision.

Figure 5

A. Contrast of dominant eye vs. non- dominant eye (DE vs. NDE) at rebalanced vision for two amblyopic observers measured at three spatial frequencies. Black markers show the contrast of the left eye vs. right eye (LE vs. RE) at balanced vision for normal observers. B. The same as in A but in contrast threshold units (CTU).

Figure 6

Perceived phase θ̂ of cyclopean sinewaves as a function of the non-dominant eye/ dominant eye (NDE/DE) contrast ratio (δ) for amblyopic observer GD with a neutral density (ND) filter placed in front of her DE. A. The ND filter varied from 0 (no filter) to 2.0 log unit and the base contrast was fixed at 24%. B. The ND filter was fixed at 1.5 log unit and the base contrast varied from 6% to 96%. The mean luminance without an ND filter was 26.2 cd/m2. Error bars: ±SE.

Figure 7

A. Binocular equal contrast contour for amblyopic observer GD when a neutral density (ND) filter was placed in front of her DE. A. The ND filter varied from 0 (no filter) to 2.0 log unit and the base contrast was fixed at 24%. B. The ND filter was fixed at 1.5 log unit and the base contrast varied from 6% to 48% (the data for 24% was shown in A). The binocularly-combined contrast was measured by matching it with a standard that was always in the non-dominant eye (without wearing an ND filter). The mean luminance without an ND filter was 26.2 cd/m2. Error bars: ±SE.

Figure 8

Results from modelling. A. Dominant eye (DE) gain-control threshold as a function of DE luminance. The non-dominant eye (NDE)’s gain-control threshold is indicated by a red circle and its luminance is fixed at 26.2 cd/m2. B. DE apparent contrast as a function of DE luminance when the standard contrast in the NDE was 24% and the NDE’s mean luminance was fixed at 26.2 cd/m2 (red circle). C. DE gain-control energy as a function of DE luminance. NDE luminance (26.2 cd/m2) and gain-control energy are indicated by the red circle. The physical contrasts of the two eyes were identical at 24%. D. DE apparent contrast as a function of the standard contrast in the NDE when the DE luminance was 0.83 cd/m2 (wearing 1.5 ND filter) and the NDE luminance was 26.2 cd/m2.

Figure 9

A. Gain-control energy as a function of contrast when the dominant eye (DE) luminance was 26.2 cd/m2 (black star) or 0.83 cd/m2 (blue diamond), and the non-dominant eye (NDE) luminance was 26.2 cd/m2 (red circle) as calculated from Eq. 3 using best fit model parameters. B. Contrast of DE vs. NDE at rebalanced vision for amblyopic observer GD when the DE luminance was 0.83 cd/m2 (blue) and 26.2 cd/m2 (red), and the NDE luminance was fixed at 26.2 cd/m2.