Glaucoma has been shown to affect monocular contrast sensitivity over a wide range of spatial frequencies (see Bierings et al. for a recent review).
39 The difference between monocular and binocular contrast sensitivity has been addressed extensively in healthy subjects, but is, to our knowledge, not so well understood in glaucoma. In healthy subjects, binocular contrast sensitivity is typically greater than the monocular contrast sensitivity of either eye. Campbell and Green
12 reported, for contrast sensitivity on a linear scale, a binocular contrast summation ratio of 1.4 (on average, a monocularly presented stimulus requires a contrast 1.4 times higher than the same stimulus presented binocularly in order to be equally detectable). After this publication, a similar difference in monocular and binocular contrast sensitivity has been reported several times.
11,14,40–42 In a recent meta-analysis involving 65 studies, Baker et al.
27 found binocular contrast summation ratios ranging from approximately 1.4 to 2, with a weighted average of 1.5, close to, but significantly larger than the abovementioned value of 1.4. Among others, the ratio was influenced by differences between the eyes (imbalance) and the spatial and temporal frequency of the stimulus. The binocular contrast summation ratio found in our controls was 1.20. However, this value is based on the AULCSF, that is, on the area under the CSF on a log-log scale. Thus far, only one study used the quick CSF method to determine the binocular contrast summation ratio, in young healthy subjects.
43 Their ratio was 1.15 (based on binocular viewing versus the dominant eye). Following the recommendations of Baker et al.,
27 we used the mean of both monocular viewing conditions rather than the dominant eye. For the dominant eye, our ratio would have been 1.19, which is in good agreement with the results of Dorr et al.
43 If we would have used a linear scale, for example applied to the peak CS as displayed in
Fig. 1B, then the binocular contrast summation ratio of the controls would have been 2.02, which is at the upper limit of the range as reported in the abovementioned meta-analysis.
27 In glaucoma, the ratios between binocular and monocular contrast sensitivity were previously reported by El-Gohary et al.
44 using contrast grating charts. They found a decreased ratio in glaucoma at spatial frequencies of 1.5, 3, and 18 cpd. On the contrary, Essock et al.,
5 using a Pelli-Robson chart and various temporally modulated stimuli, did not find any significant decrease in binocular summation ratios in early glaucoma. We were not able to find any other study that has reported on contrast summation in glaucoma ever since. In our study, glaucoma patients showed a binocular advantage similar to that of controls. This seems counterintuitive, given that they had nonoverlapping VF defects, hampering the ability to integrate information from two eyes from overlapping points in space. However, their binocular VF was intact whereas the monocular VFs were not, suggesting that their binocular advantage was based on integration of information from different points in space. This explanation agrees with previous research linking stimulus size to contrast sensitivity.
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