Global-flash mfERG responses to local differences in spherical and astigmatic defocus across the human retina

Emmetropisation is essentially a visually guided, within-eye process. We investigated differences in global-flash multifocal electroretinogram (gmfERG) responses to naturally occurring differences in spherical and astigmatic defocus across the retina, which might provide a basis for guiding eye growth.

Experiment 1: The gmfERG responses (direct, DC, and induced, IC, amplitudes and latencies) recorded simultaneously from six retinal areas (15° eccentricity, spaced at 60°, areas 3.2°² ) were correlated with the uncorrected retinal defocus measured at the six corresponding retinal locations in 20 adults with foveal refractive errors (-4.75 to +1.25D). No correcting lenses were used to avoid introduction of lens-induced aberrations and magnification. Experiment 2 investigated the effect of superimposing astigmatic defocus (+2.00/-4.00D Jackson Cross Cylinder presented at four orientations) on gmfERG responses.

Experiment 1: DC and IC response amplitudes were greater in retinal regions naturally exposed to more hyperopic spherical defocus (DC: rho = 0.26, p = 0.005; IC: rho = 0.29, p = 0.001), but response latencies were unaffected by sign or magnitude of spherical defocus (DC: p = 0.34; IC: p = 0.40). Response amplitudes and latencies were unaffected by astigmatic defocus. Experiment 2: Rotating the JCC axis to four different orientations had no effect on the gmfERG responses (DC amplitude, p = 0.39; DC latency, p = 0.10; IC amplitude, p = 0.51; IC latency, p = 0.64).

The gmfERG responses from discrete retinal areas varied with the sign and magnitude of local spherical defocus, but we found no evidence that retinal responses were affected by astigmatic defocus. Therefore, local astigmatism is unlikely to provide cues for controlling eye growth, whereas differences in response to spherical defocus between different retinal regions could potentially provide cues for controlling eye growth in emmetropisation.