Temporal summation in myopia and its implications for the investigation of glaucoma

We have previously demonstrated the upper limit of complete spatial summation (Ricco's area) to increase in non-pathological axial myopia compared to non-myopic controls. This study sought to investigate whether temporal summation is also altered in axial myopia to determine if this aspect of visual function, like in glaucoma, is influenced by reductions in retinal ganglion cell (RGC) density.

Achromatic contrast thresholds were measured for a GIII-equivalent stimulus (0.43° diameter) of six different stimulus durations (1-24 frames, 1.1-187.8 ms) in 24 participants with axial myopia (mean spherical refractive error: -4.65D, range: -1.00D to -11.25D, mean age: 34.1, range: 21-57 years) and 21 age-similar non-myopic controls (mean spherical refractive error: +0.87D, range: -0.25D to +2.00D, mean age: 31.0, range: 18-55 years). Measurements were performed at 10° eccentricity along the 90°, 180°, 270° and 360° meridians on an achromatic 10 cd/m² background. The upper limit of complete temporal summation (critical duration, CD) was estimated from the data with iterative two-phase regression analysis.

There was no significant difference (p = 0.90, Mann-Whitney U-test) in median CD between myopes (median: 44.3 ms; IQR: 26.5, 51.2) and non-myopes (median: 41.6 ms; IQR: 27.3, 48.5). Despite RGC numbers underlying the stimulus being significantly lower in the myopic group (p < 0.001), no relationship was observed between the CD estimate and co-localised RGC number (Pearson's r = -0.13, p = 0.43) or ocular length (Pearson's r = -0.08, p = 0.61).

Unlike spatial summation, temporal summation is unchanged in myopia. This contrasts with glaucoma where both temporal and spatial summation are altered. As such, perimetric methods optimised to test for anomalies of temporal summation may provide a means to differentiate between conditions causing only a reduced RGC density (e.g., myopia), and pathological processes causing both a reduced RGC density and RGC dysfunction (e.g., glaucoma).