The most marked limitation of this study consists of the mismatch in age between the cohorts (
Table). Additionally, the normal cohort was also more myopic and exhibited better visual acuity, although these may be attributed to the age difference.
52,53 The difference in age is likely due to older age constituting a risk factor for glaucoma.
54 It has been previously suggested that the spatial summation characteristic does not change significantly with age
24,55 and accordingly, the difference in age should not alter the conclusion of the study. Despite the disparity in age, the actual effect on the DLS and GCL thickness/GCc is arguably small. For instance, between the age of 35 and 65 (3 decades), the average GIII sensitivity within the central 12.7° is expected to alter by 1.7 dB on average (minimum: 1.65 dB; maximum: 2.1 dB)
45 while the average grid GCL thickness within the equivalent measurement area may alter by 0.3 dB or 0.9 μm (minimum: 0 μm; maximum: 1.7 μm).
21 The difference imposed by age between these cohorts is within the test-retest variability range of the instruments,
44–56 and even before correction, there were notable overlaps between the prediction interval for the regression curves of the two cohorts (data not shown). Regardless, DLS and GCc for the normal and glaucomatous cohort were adjusted to a common age equivalent using a well-established age-correction method, frequently used in prior studies by different investigators
11,21,24–26,31,32,45,47 and regularly utilized by clinical test algorithms for the Humphrey Field Analyzer; SITA.
48 A subanalysis was conducted to further confirm the validity of the age-correction technique, and good fidelity was demonstrated between the glaucomatous subgroup, and the normal age-corrected and age-matched cohorts (
Supplementary Fig. S3). The age-correction models utilized for this study
21,45 exhibited differences in the age-normal regression of DLS and GCL thickness (and therefore GCc); specifically, the DLS regressed at a faster rate. Such difference may be attributed to other ocular effects associated with aging that may affect the DLS, such as cataract,
57 and highlights the importance of age correction for investigations of visual function.