Lawrence R. Stark

Pupil size, mean accommodation response and the fluctuations of accommodation.

We wished to determine how pupil size and mean accommodation response level interact to influence the fluctuations of accommodation. A dynamic infra‐red optometer was used to record accommodation responses while subjects viewed a steady target at two stimulus levels (1.5 and 3 D) through four pupils (1, 2, 4 and 6 mm). It was found for most subjects that the fluctuations of accommodation increase at higher mean accommodation response levels, and small pupils lead to an increase in the low frequency (but not the high frequency) fluctuations of accommodation. The effects of mean accommodation response are independent of pupil size, and the effects of pupil size are independent of mean response level.

Accommodation to static chromatic simulations of blurred retinal images

The eye’s longitudinal chromatic aberration (LCA) is known to drive ‘reflex’ accommodation to moving objects, but the evidence is not as clear for stationary objects. The present study examined whether accommodation can be driven by static simulations of the effects of defocus and LCA. Accommodation was recorded continuously while each of 12 subjects viewed images (through a 0.75 mm pinhole) that simulated the appearances of blurred sine wave gratings (3.9 c.p.d.). In two experimental conditions, an eye with normal LCA was assumed and defocus of +1 D or −1 D was simulated. In a control condition, an eye with neutralised LCA was assumed and target defocus of 1 D was simulated. Subjects’ accommodation responses were consistent with the hypothesis that LCA provides a stimulus to accommodation. Chromatic aberration drives accommodation to both moving and stationary objects, and thus is an important stimulus for accommodation in everyday situations. The study findings are discussed in relation to colour vision, visual display terminals and emmetropization.

Dynamic accommodation response in the presence of astigmatism.

It has been suggested that in the presence of astigmatism some individuals make cyclic changes in focus over the astigmatic interval to obtain better visual performance. The aim in the present study was to identify such cyclic accommodative behavior and to characterize the variability of the response in the presence of astigmatism. The dynamic accommodation response in the presence of induced astigmatism was recorded objectively with an infrared optometer in seven young adults. Astigmatism led directly to increased accommodative variability in certain individuals. In two of seven participants there was evidence for aperiodic cyclic accommodative responses between different portions of the astigmatic interval. However, the amplitude of these tracking responses was much smaller than the astigmatic interval.

Accommodation and the Stiles–Crawford effect: theory and a case study

Purpose: Fincham (The accommodation reflex and its stimulus. Br. J. Ophthalmol. 35, 381–393) was the first to suggest that the Stiles–Crawford effect (Type I) might provide a stimulus for accommodation, but the possibility has not been investigated experimentally. The present paper outlines a theoretical basis for such a mechanism, and includes a case study on a subject with a nasally decentred Stiles–Crawford (S–C) function.

Small foveal targets for studies of accommodation and the Stiles-Crawford effect.

The properties of small monochromatic targets as accommodative stimuli are not well understood. We used a dynamic optometer to record accommodation responses to monochromatic disc targets (1.0-27.3 min arc) and to a Maltese cross. Accommodation responded adequately to points as small as 13.6 min arc. The response to these small targets is relevant to the question of whether the Stiles-Crawford (SC) effect could provide a stimulus to accommodation. Previous studies have used pupil apodizing filters to neutralise the natural SC function and so determine how visual performance or accommodation is influenced by the SC effect. However, these filters cannot correct for known inhomogeneities in the SC function across the retina for extended targets. Therefore, we calculated the SC function inhomogeneities across the retinal image of a smaller 13.6-min arc target. Unfortunately, even this small target is too large to permit a homogenous SC function across its extent. Alternatives to the apodizing filter approach are discussed.

Effect of an intervening screen on accommodation to a distant object.

Background: An intervening screen has been suggested to induce an inward shift of accommodation when viewing a distant object. This is an example of the Mandelbaum effect. However, there have been no objective measures of the magnitude of this effect in this particular situation.

Accommodation to wavefront vergence and chromatic aberration.

Purpose. Longitudinal chromatic aberration (LCA) provides a cue to accommodation with small pupils. However, large pupils increase monochromatic aberrations, which may obscure chromatic blur. In this study, we examined the effect of pupil size and LCA on accommodation. Methods. Accommodation was recorded by infrared optometer while observers (nine normal trichromats) viewed a sinusoidally moving Maltese cross target in a Badal stimulus system. There were two illumination conditions: white (3000 K; 20 cd/m2) and monochromatic (550 nm with 10 nm bandwidth; 20 cd/m2) and two artificial pupil conditions (3 and 5.7 mm). Separately, static measurements of wavefront aberration were made with the eye accommodating to targets between 0 and 4 D (COAS, Wavefront Sciences). Results. Large individual differences in accommodation to wavefront vergence and to LCA are a hallmark of accommodation. LCA continues to provide a signal at large pupil sizes despite higher levels of monochromatic aberrations. Conclusions. Monochromatic aberrations may defend against chromatic blur at high spatial frequencies, but accommodation responds best to optical vergence and to LCA at 3 c/deg where blur from higher order aberrations is less.

An evaluation of the M&S technologies smart system II for visual acuity measurement in young visually-normal adults.

Purpose. To compare visual acuity measures obtained with the M&S Technologies Smart System II (SSII) and the revised Early Treatment of Diabetic Retinopathy Study (ETDRS) charts in terms of accuracy and test–retest repeatability. Methods. Monocular visual acuities were taken in 57 young, visually normal adults on two separate visits in which both the SSII system and the ETDRS charts were tested in random order by two masked examiners. The eye to be tested throughout was chosen randomly at the initial visit. Measurements were made through an optimal phoropter correction, determined by a noncycloplegic refraction for a 10-foot distance. Both charts were presented at 10 feet, and were matched closely for luminance. Results. The mean visual acuity in the group was −0.16 log minimum angle of resolution (MAR) for the ETDRS chart and −0.18 log MAR for the SSII, a small but statistically significant difference. A 95% confidence interval for the mean difference in visual acuity between the two charts was −0.033 log MAR to −0.003 log MAR. The test–retest repeatability was not significantly different in the two tests. The 95% limits of agreement for test–retest repeatability were −0.13 log MAR to +0.17 log MAR for the SSII and −0.12 log MAR to +0.13 log MAR for the ETDRS charts. Conclusions. The SSII can provide an accurate (mean difference <0.033 log MAR) and repeatable alternative to the ETDRS charts for visual acuity measurement in young, visually normal, well-corrected individuals.

Chromatic stimulus for accommodation to stationary and moving targets

Experiments using moving targets (sinusoidal and sum-of-sine motion) suggest that the longitudinal chromatic aberration (LCA) of the eye provides directional information at edges that specifies ocular focus and drives reflex accommodation (Kruger, Mathews, Aggarwala, Sanchez, 1993). Recently two studies used stationary targets to stimulate accommodation and both concluded that chromatic aberration has no effect on accommodation when the target is stationary (Bobier, Campbell and Hindi, 1992; Kotulak, Morse and Billock, 1995). The issue was addressed in three experiments: First, subjects (8) viewed a stationary target (3.5 cycles per degree square-wave grating) through a 3 mm artificial pupil at three dioptric stimulus levels (0 D, 2.5 D, 5 D) in a Badal stimulus system. There were 3 experimental conditions including Normal LCA, Reversed LCA and Monochromatic (550 nm; 10 nm bandwidth) and the resting position of accommodation was monitored while subjects viewed the grating target through a pinhole pupil. The target remained stationary during randomized 40-second trials, and accommodation was monitored continuously. Most subjects accommodated accurately in the normal condition at all three stimulus distances, a few subjects (3 of 8) had difficulty maintaining focus in monochromatic light at the near (5 D) or far (0) stimulus distances, and most subjects (7 of 8) could not maintain focus at both the near and far distances with LCA reversed. When the target is stationary it must be considerably closer or further away than the (tonic) resting level to demonstrate the effect of LCA.

The value of training accommodative facility for ball sports

We address the issue of training accommodative facility as a means of improving performance in ball sports. Presenting examples from the game of baseball, we show that dynamic accommodation is unlikely to play an important role in ball skills. The movement speed of the ball combined with the last possible time at which visual information can be converted to motor action means that accommodation cannot provide useful information on the time to contact of the ball. In addition, we question whether altering accommodation to provide a clear retinal image of an approaching target is necessary for most ball sport skills.