Analysing the impact of myopia on the Stiles‐Crawford effect of the first kind using a digital micromirror device

Abstract

Purpose Photoreceptor light acceptance is closely tied to the Stiles‐Crawford effect of the first kind (SCE‐I). Whether the SCE‐I plays a role in myopic development remains unclear although a reduction in directionality has been predicted for high myopia. The purpose of this study is to analyse the relationship between foveal SCE‐I directionality, axial eye length, and defocus for emmetropic subjects wearing ophthalmic trial lenses during psychophysical measurements and for myopic subjects with their natural correction. Method A novel uniaxial flicker system has been implemented making use of a Digital Micromirror Device (DMD) to flicker between a 2.3 visual degrees circular reference and a set of circular test patterns in a monocular Maxwellian view at 0.5 Hz. The brightness of the test is adjusted by the duty cycle of the projected light to an upper limit of 22 727 Hz. The wavelength and bandwidth are set by a tuneable liquid‐crystal filter centred at 550 nm. A total of four measurement series for 11 pupil entrance points have been realized for the right eye of 6 emmetropic and 10 myopic subjects whose pupils were dilated with tropicamide. Five of the emmetropic subjects wore ophthalmic trial lenses in the range of −3 to +9 dioptres to mimic hyperopic to highly myopic vision and resulting visibility plots have been fitted to a Gaussian SCE‐I function. In turn, the myopic subjects wore their natural correction during the analysis of the SCE‐I. All subjects had their axial eye length determined with an ultrasound device. Results A SCE‐I directionality parameter in the range of 0.03 to 0.06/mm2 was found for the emmetropic subjects with corrected vision in fair agreement to values in the literature. The results also revealed a marked reduction in directionality in the range from 16% to 30% with every 3 dioptre increase of simulated myopia, as well as a 10% increased directionality in simulated hyperopic eyes. For both emmetropic and myopic subjects, a decrease in directionality with increase in axial length was found in agreement with theoretical expectations. Conclusion The study confirms a clear link between SCE‐I directionality, uncorrected defocus, and axial eye length. This may play a role for emmetropization and thus myopic progression as cone photoreceptors capture light from a wider pupil area in elongated eyes due to a geometrical scaling.

Factors associated with reduced visual acuity in myopes with and without ocular pathologies after optical correction

“…Other possible reasons for reduced visual acuity in myopes in the absence of any pathologic myopia lesions are: (a) reduced contrast sensitivity, 35 (b) higher‐order aberrations, 21,22,36–38 (c) minification of the retinal image and the poor optical quality of the spectacle lenses, 16,17 (d) crowding effect that is more evident with increasing myopia, 15,39,40 (e) fixation stability that correlates with visual function and affects performance of a visually demanding task, 41 (f) an error in the magnitude or amplitude of microsaccades or the amplitude of slow drifts that may affect the dynamics and the spatial allocation of selective attention contributing to the reduction in visual acuity in amblyopia, 42,43 (g) larger spacing between the photoreceptors and neurons in the myopic retina resulting in neural deficits and reduced sensitivity in myopes compared with emmetropes 17,21,22,36,44 and (h) reduced directionality of photoreceptors (Stiles–Crawford effect) in myopes due to axial elongation 45 …”

Subjective measurement of the Stiles‐Crawford effect of the first kind with variation in accommodation

“…The SCE‐I can be measured using subjective 1,6,7 and objective techniques, 8–10 but the directionality reported using the latter method is approximately twice that of the former technique due to the double‐passage of light through the photoreceptors 11–13 …”

“…Various factors influence the directionality and peak location, such as wavelength, 14–16 luminance, 16,17 eccentricity 16,18,19 and ocular conditions such as ocular disease, 20–22 phototropism, 23–25 refractive errors, 6,7 aberrations 6,7 and accommodation 6,26 …”

Measuring Ocular Aberrations Sequentially Using a Digital Micromirror Device

“…Digital micromirror devices (DMDs) can operate at speeds in the 10′s of kHz range, while sequential scanning provides a large dynamic range due to the lack of a lenslet array; therefore, avoiding the appearance of cross-talk, as recently proposed by the authors in a DMD-WFS [13]. Additionally, DMDs have recently been used for ophthalmic applications in retinal imaging [14] and psychophysical measurements [15]. A somewhat related technique uses scanning of an incident beam of light in the pupil plane to capture multiple retinal images [16].…”