Retinal image quality during accommodation

Abstract

Purpose: We asked if retinal image quality is maximum during accommodation, or sub-optimal due to accommodative error, when subjects perform an acuity task.

Methods: Subjects viewed a monochromatic (552 nm), high-contrast letter target placed at various viewing distances. Wavefront aberrations of the accommodating eye were measured near the endpoint of an acuity staircase paradigm. Refractive state, defined as the optimum target vergence for maximising retinal image quality, was computed by through-focus wavefront analysis to find the power of the virtual correcting lens that maximizes visual Strehl ratio.

Results: Despite changes in ocular aberrations and pupil size during binocular viewing, retinal image quality and visual acuity typically remain high for all target vergences. When accommodative errors lead to sub-optimal retinal image quality, acuity and measured image quality both decline. However, the effect of accommodation errors of on visual acuity are mitigated by pupillary constriction associated with accommodation and binocular convergence and also to binocular summation of dissimilar retinal image blur. Under monocular viewing conditions some subjects displayed significant accommodative lag that reduced visual performance, an effect that was exacerbated by pharmacological dilation of the pupil.

Conclusions: Spurious measurement of accommodative error can be avoided when the image quality metric used to determine refractive state is compatible with the focusing criteria used by the visual system to control accommodation. Real focusing errors of the accommodating eye do not necessarily produce a reliably measurable loss of image quality or clinically significant loss of visual performance, probably because of increased depth-of-focus due to pupil constriction. When retinal image quality is close to maximum achievable (given the eye's higher-order aberrations), acuity is also near maximum. A combination of accommodative lag, reduced image quality, and reduced visual function may be a useful sign for diagnosing functionally-significant accommodative errors indicating the need for therapeutic intervention.

Figure 1

Classic accommodative response curve in which accommodative response (refractive state of relaxed eye – refractive state of accommodated eye) in dioptres is plotted as a function of accommodative demand (refractive state of relaxed eye – target vergence) in dioptres. For this example an accommodative demand of 4D elicits an accommodative response of 3D, indicating a lag (demand-response) of 1 D. Although this eye has the ability to accommodate 4D (small circle), lag persists even though improved focusing would improve retinal image quality by reducing defocus blur.

Figure 2

Schematic of the experimental apparatus and the location of several reference planes, all of which are conjugate to the retina according to some metric of image quality.

Figure 3

Example of accurate accommodation and normal acuity for observer JM. Upper panel shows variation of binocular visual acuity (black line, left ordinate, symbol = mean, error bar = +/− 1 standard error of the mean) and pupil diameter (blue line, right ordinate) with target vergence. Large values of negative target vergence correspond to a near target. Middle panel shows absolute refractive state of the accommodating eye + spectacle system computed three ways as a function of target vergence. Dashed line indicates ideal focusing (optimum target vergence matches the physical target vergence). Lower panel shows absolute image quality (VSMTF*) for the measured wavefront (circles) relative to the maximum possible value (triangles) achieved by optimum focusing of the wavefront. Dashed curve is the ratio of VSMTF* to maximum VSMTF*. Ordinate for the bottom panel is logarithmic.

Figure 4

Example of inaccurate accommodation (lag) and reduced acuity as the visual target approaches the eye. Graphical format and conventions are the same as in Fig. 3. Monocular viewing with phenylephrine by same subject (JM) as in Fig. 3.

Figure 5

Summary of the effect of accommodative error on retinal image quality and visual acuity during binocular viewing by all subjects. Each symbol in each quadrant represents the results for one viewing distance by one subject. Blue symbols indicated accommodative lead (error > 0) and red symbols indicate accommodative lag (error < 0). The blue box connects corresponding points in all 4 quadrants for which accommodative lead was greatest and the red box connects corresponding points when accommodative lag was greatest.

Figure 6

Summary of the effect of accommodative error on retinal image quality and visual acuity during monocular viewing by all subjects. Graphical conventions are the same as in Fig. 5.

Figure 7

Association between accommodative error and pupil diameter. Upper panel is for binocular viewing with normal pupil constriction. Middle panel is for monocular viewing with normal pupil constriction. Lower panel is for monocular viewing with pharmacologically dilated pupil. Each symbol represents the mean pupil diameter for 3 repetitions of one viewing distance by one subject.

Figure 8

Summary of the effect of accommodative error on retinal image quality and visual acuity during monocular viewing with pupils dilated pharmacologically. Graphical conventions are the same as in Fig. 5.