Introduction: It is well accepted that reduced visual function occurs in physiological (non-pathological) myopia. However, it is less well understood whether this is due to true dysfunction in the retinal cells, or simply a result of reduced density of functionally normal cells secondary to myopic eye growth and retinal stretch. To investigate this, we considered spatial and temporal summation characteristics in physiological myopia, and compared this to our previous work in glaucoma, a pathological eye condition that causes reduced cell density (cell death) and cell dysfunction. Spatial and temporal summation are core visual functions which refer to how the perceptive fields of the visual system summate light over space and time respectively.

Aim: To investigate spatial and temporal summation in physiological myopia

Method: Spatial summation was investigated in 24 myopes (mean: -4.14DS, range: -0.50DS, -9.75DS) and 20 non-myopic controls (mean: +0.71DS, range: +1.75DS, -0.25DS) by measuring achromatic contrast thresholds for six stimuli varying in area (0.01–2.07 deg², 200ms). The effects of refractive error induced variations in retinal image size (RIS) were considered by correcting refractive error separately with (i) trial lenses placed at the anterior focal point (constant RIS in mm for all participants), and (ii) contact lenses (RIS increases in line with eye length). Temporal summation was investigated in a similar cohort (24 myopes, mean: -4.65DS, range: -1.00DS, -11.25DS; 21 controls, mean: +0.87DS, range -0.25D, +2.00D) by measuring achromatic contrast thresholds for six stimuli varying in duration (1.1 – 187.8ms, 0.43° diameter). RIS was kept constant. The upper limit of complete summation (‘Ricco’s Area’ (RA) and ‘Critical Duration’ (CD) for spatial and temporal summation respectively) was estimated from the data with iterative two-phase regression analysis. Retinal temporal summation was also measured objectively using electrophysiology and analysing how the amplitude of response changed for stimuli of constant energy (3cd/m²) but varying duration (0.5-100ms).

Results: With spectacle correction, RA was significantly larger (p=0.02, Mann Whitney U-test) in the myopes compared to controls (myopes median: -0.92 log deg², IQR: -1.10, -0.78; controls median: -1.14 log deg², IQR: -1.29, -1.07). However, for contact lens correction, there was no significant difference in RA (p=0.44) between groups (myopes median: -1.19 log deg², IQR: -1.28, -0.96; controls median: -1.14 log deg², IQR: -1.24, -0.87). There was also no significant difference in CD between groups measured psychophysically (myopes median: 44.3ms, IQR: 26.5, 51.2; controls median: 41.6ms, IQR: 27.3, 48.5), nor objectively with the electroretinogram.

Conclusions: The area of complete spatial summation was altered in myopia when differences in projected RIS were accounted for, likely a compensation for reduced cell density secondary to axial elongation. However, when RIS was allowed to increase in line with axial length, there was no measurable difference in spatial visual function in myopia. In addition, temporal summation was unchanged in myopia.  This contrasts to glaucoma, where both spatial and temporal summation are altered. Taken together, these results suggest that reduced visual function in myopia is due to reduced cell density, rather than dysfunction in the cells themselves.