Patients with central vision loss experience binocular inhibition and binocular summation in equal proportions, according to a newly published prospective observational case study. Researchers say they found no binocular reading advantage for patients with central vision loss, but did observe patients with binocular reading inhibition have poorer reading performance and different clinical characteristics than those with binocular reading summation. The research stresses the importance of evaluating binocular function when creating a plan for reading rehabilitation for patients with visual impairment due to central vision loss.
To investigate whether binocular reading has an advantage over monocular reading with the better eye in patients with age-related macular degeneration (AMD), the team observed 71 patients with bilateral central vision loss (mean age 63) due to either AMD or Stargardt’s disease. All participants had visual acuity scores of 1.3 logMAR or better, no cognitive impairments, and no comorbid eye diseases.
The team measured the visual acuity, reading performance, contrast sensitivity, stereo acuity, fixation stability, and preferred retinal locus (PRL) location of each participant. Differences in measures recorded during binocular and monocular viewing were examined with paired-samples t-tests.
Overall, the team found no significant binocular advantage in any measures. Similarly, they report nearly identical reading speeds during monocular and binocular viewing for the group. More specifically, the findings revealed that overall, binocular visual acuity and monocular visual acuity were about the same (P =.30). Contrast sensitivity was 1.12 (S.D. = 0.4) logCS during binocular viewing and 1.09 (S.D. = 0.4) logCS during monocular viewing (t70 = 2.50, P =.02). Reading performance, measured via maximum reading speed, critical print size, and reading acuity, did not differ between the two viewing conditions (smallest P =.4).
While binocular and monocular reading performances were similar, 41% of patients experienced binocular inhibition, 42% binocular summation, and 17% equality. The inhibition group had the lowest binocular reading speed, yet the best visual acuity and the largest difference between visual and reading acuity. Researchers noted that these patients generally had monocular PRLs in non-corresponding locations, the PRL in the better eye located temporal or nasal to the scotoma, the largest interocular acuity difference, and lacked residual stereopsis.
For the inhibition group, there was a binocular disadvantage in reading speed of 16 words per minute (wpm), while in the summation group, there was a binocular advantage of 20 wpm. Importantly, for the inhibition group, binocular reading speed was about 30 wpm slower than those of the equality and summation groups. The study found no difference in reading acuity and critical print size for the three groups, but surprisingly, binocular and monocular visual acuities were significantly better in the inhibition group than for the summation group, highlighting the potential for reading rehabilitation for this patient pool.
The performance of the better eye is typically used to predict binocular performance during vision rehabilitation, but this research suggests that the reserve from visual acuity to reading acuity and interocular acuity differences are key elements that may influence maximum reading speed during binocular viewing. Researchers recommend that reading performance for patients with central vision loss be measured with one eye as well as with two eyes, since outcomes can vary depending on the viewing condition and the patient’s clinical characteristics.
This study was subject to several limitations. First, reading performance with the worse eye was not measured. Second, stereo acuity was measured with the Stereo Fly Test, which provides non-stereoscopic cues only up to a point. Lastly, the team encourages further research regarding the role the dominant eye (which may not always be the eye with the better acuity) plays in binocular reading summation for patients with central vision loss.
Silvestri V, Sasso P, Piscopo P, Amore F, Rizzo S, Devenyi RG, Tarita-Nistor L. Reading with central vision loss: binocular summation and inhibition. Ophthalmic Physiol Opt. 2020. doi: 10.1111/opo.12726.