Steven Mathews

Scleral expansion surgery does not restore accommodation in human presbyopia.

OBJECTIVE To determine the efficacy of scleral expansion surgery in the treatment of human presbyopia. DESIGN Experimental study. PARTICIPANTS Three preoperative presbyopic subjects, three postoperative presbyopic subjects, and three young control subjects. METHODS Accommodative responses were recorded by an infrared dynamic optometer while subjects viewed an accommodative stimulus that stepped from 0 to 4 diopters in 1-diopter steps. MAIN OUTCOME MEASURES Dynamic infrared optometer recordings of accommodation. RESULTS Presbyopic patients showed no evidence of accommodation after scleral expansion surgery. CONCLUSIONS If presbyopic patients read smaller letters at near after scleral expansion surgery than they were able to read prior to the surgery, then an explanation other than the restoration of accommodation needs to be found.

Accommodation Without Feedback Suggests Directional Signals Specify Ocular Focus

Accommodation was monitored continuously under open-loop conditions while subjects viewed a sinusoidally oscillating sine-wave grating (0.2 Hz; +/- 1 D; 2.7 c/d; 0.56 contrast) in a Badal optometer. The target was illuminated by monochromatic light (590 nm) or white light (3000 K) with longitudinal chromatic aberration (LCA) normal, doubled, neutralized and reversed. Subjects (12) accommodated well in white light with LCA normal and doubled (mean gains = 0.85 and 0.94), gain was reduced in the neutralized condition (0.54), in monochromatic light (0.43), and especially when LCA was reversed (0.30). The results suggest that accommodation responds to changes in the relative contrast of spectral components of the retinal image and perhaps to the vergence of light.

Accommodation responds to changing contrast of long, middle and short spectral-waveband components of the retinal image.

We simulated the effects of longitudinal (axial) chromatic aberration and defocus on contrast of the long-, middle- and short-wavelength components of the retinal image to determine whether the effects of chromatic aberration are sufficient to drive accommodation. Accommodation was monitored continuously while subjects (12) viewed a 3 c/deg white sine-wave grating (0.92 contrast) in a Badal stimulus system. The contrasts (amplitudes) of the red, green and blue components of the white grating changed independently to simulate a grating oscillating from 1 D behind the retina to 1 D in front of the retina at 0.2 Hz. Subjects responded strongly to the chromatic simulation but poorly to a luminance control. The results support the hypothesis that focus is specified by the contrast of spectral-wavebands of the retinal image, and that conventional color mechanisms, monitoring chromatic contrast at luminance borders (1-8 c/deg), mediate the signals that specify dioptric vergence.

Pupil size and the ablation zone in laser refractive surgery: considerations based on geometric optics

Purpose: To determine whether the currently accepted method of selecting a minimum ablation zone size for refractive surgery based on dark‐adapted pupil diameter is substantiated by geometric optical analysis. Setting: Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA. Methods: An optical model of the anterior segment was developed to calculate the effective corneal refractive diameter (ECRD), which is the diameter of the area of cornea that refracts all incident light rays arising from objects along the line of sight though the physical pupil (PP). The concept of the entrance pupil (EP) was reexamined and developed, and the ECRD was calculated over a range of corneal curvatures (K), anterior chamber depths (ACDs), and EP sizes. The model was generalized to include oblique light rays. Calculations were performed using MatLab Optimization Toolbox® software (The MathWorks). Results: For a given EP size, the ECRD was significantly influenced by K and slightly influenced by ACD. For objects on the line of sight, the ECRD was smaller than the EP in all cases. Regarding rays from objects in the periphery, the ECRD expanded rapidly as the angle of oblique incidence increased. Conclusions: For objects on the line of sight, the ECRD is always smaller than the clinically measured pupil (EP) because the EP is substantially magnified relative to the PP. Ablation zones larger than the EP should, in theory, prevent scattered or defocused light rays from contributing to the foveal image. When considering objects in the periphery, the increase in ECRD is sufficiently rapid that current refractive procedures cannot stop scattered light from these objects from contributing to the retinal image.

Spectral bandwidth and ocular accommodation

Previous studies have suggested that targets illuminated by monochromatic (narrow-band) light are less effective in stimulating the eye to change its focus than are black-white (broadband) targets. The present study investigates the influence of target spectral bandwidth on the dynamic accommodation response in eight subjects. The fixation target was a 3.5-cycle/deg square-wave grating illuminated by midspectral light of various bandwidths [10, 40, and 80 nm and white (CIE Illuminant B)]. The target was moved sinusoidally toward and away from the eye, and accommodation responses were recorded and Fourier analyzed. Accommodative gain increases, and phase lag decreases, with increasing spectral bandwidth. Thus the eye focuses more accurately on targets of wider spectral bandwidth. The visual system appears to have the ability to analyze polychromatic blur to determine the state of focus of the eye for the purpose of guiding the accommodation response.

Small Amounts of Chromatic Aberration Influence Dynamic Accommodation

The prevailing view of accommodation is that the eye changes focus to maximize luminance contrast by trial and error. Negative feedback is considered essential in this view because luminance contrast provides no directional information. Fincham proposed an alternate view in which longitudinal (axial) chromatic aberration (LCA) provides a directional stimulus for accommodation. For spatial frequencies above approximately 0.5 cpd contrast of the retinal image is different for long, middle, and short spectral waveband components of the image. We varied the amount of LCA in small steps (0.25 D) to determine how much LCA is needed to enhance or impair the response. An infrared optometer monitored accommodation continuously while subjects viewed a yellow/ black square-wave grating (3.5 cpd) in a Badal stimulus system. The yellow/black grating was produced by superimposing red (600 nm) and green (520 nm) gratings, and LCA was increased, decreased, neutralized, and reversed by repositioning the red grating component along the axis of the optical system. Target vergence was modulated sinusoidally (0.2 Hz) over a 1 D range (1.5 to 2.5 D) and gain and phase-lag of the accommodation response were determined by Fourier analysis. Subjects accommodated well as long as a normal amount of LCA was present—0.5 D in the correct direction enhanced accommodative gain, and 0.25 D in the reverse direction markedly inhibited the response. We conclude that the contrast of the retinal image in different spectral wavebands specifies focus of the eye, and provides a powerful directional stimulus for reflex accommodation.

Accommodation to stationary and moving targets

Purpose. To test the hypothesis that the contrast of spectral components of the retinal image specifies ocular focus and controls reflex accommodation. Methods. Eight subjects viewed a stationary target at 0, 2.5, and 5 D in a Badal optometer, with longitudinal chromatic aberration (LCA) normal and reversed and in monochromatic (550 nm) light. Accommodation was monitored continuously during 40-s trials. Subjects also viewed the grating target as it moved sinusoidally (1.5 to 2.5 D) at 0.2 Hz under the same three conditions. Results. Subjects accommodated relatively accurately at all distances in the normal condition; three subjects had difficulty accommodating in monochromatic light at 5 or 0 D, and seven subjects could not maintain focus with LCA reversed. The accommodative response differed significantly in the three chromatic conditions both for stationary and moving targets. Conclusions. Relative contrast of long-, middle-, and short-wavelength components of the retinal image specifies ocular focus and drives reflex accommodation.

Accommodation and chromatic aberration: effect of spatial frequency

When subjects view an edge in while light, a colour fringe, produced by longitudinal chromatic aberration (LCA) of the eye, is formed at the edge. The colour fringe changes with changes in focus, and serves as a complex colour‐coded cue for reflex accommodation. Fincham found that 60% of his subjects failed to accommodate appropriately when the colour fringe was removed with an achromatizing lens or by the use of monochromatic light. Our experiment sought to determine the spatial frequencies al which LCA is most effective. We monitored accommodation in 10 subjects while they viewed sinusoidally moving sine‐wave gratings (1–3 D at 0.2 Hz; l–10.5c/deg) in a Badal optometer. The targets were “while” gratings with LCA normal, doubled, neutralized or reversed. Doubling the aberration has minimal effect, removing the aberration reduces gain and increases phase‐lag, and reversing the aberration severely disrupts accommodation. Sensitivity to these chromatic cues exists at all spatial frequencies tested, but is most prominent between 3 and 5 c/deg. These results support the view that the system monitors focus by comparing contrast in red‐green and perhaps blue‐yellow colour‐opponent mechanisms.

Opaque contact lens treatment for older children with amblyopia.

Purpose. To investigate the usefulness of daily-wear opaque contact lens treatment for older children with amblyopia. Methods. A retrospective chart review of all children undergoing opaque contact lens therapy for amblyopia between 2000 and 2003 was performed. All patients were included except for those younger than 5 years of age, those undergoing maintenance treatment only, those lost to follow-up, or those unable to cooperate with Snellen visual acuity testing. Results. Thirteen children were identified, and seven met the inclusion criteria. The seven patients had a mean age of 7.5 years (range, 5.7–8.7 years). The causes of amblyopia were anisometropia (five patients), cataract (one patient), and strabismus (one patient). For all seven patients, the mean logMAR improvement in visual acuity was 0.52 (range, 0.26–1.22). The average duration of opaque contact lens use was 9.3 months (range, 2–21 months). Compliance problems were encountered in three patients. No major complications occurred, but one patient had an episode of mild superficial punctate keratitis. Conclusions. Daily-wear opaque contact lens treatment is a useful occlusion method for amblyopia treatment in older children with various practical and social impediments to skin patching. Meaningful improvement in visual acuity can be obtained, even in children older than 8 years of age.

Accommodation and the third spatial harmonic

Purpose Mathews and Kruger recently reported that accommodative tracking of sine wave grating targets is best between 1 and 7 cpd. Their study suggests that the spatial third harmonic might not enhance the accommodative response if the fundamental is higher than roughly 2 cpd. This study tested the effect of adding the third harmonic with targets that included 1, 3, and 5 cpd fundamental sine wave gratings, compound gratings with the third harmonic of each of these fundamentals added in either peaks-add or peaks-subtract phase, as well as each third harmonic presented alone at one-third of the contrast of the fundamental gratings. Methods Accommodation was monitored continuously with a dynamic infrared optometer while the subject viewed the gratings in a Badal stimulus system. Dioptric vergence changed sinusoidally over a 2.00 or 0.50 D range at 0.2 Hz. A Fast Fourier Transform extracted the amplitude and phase of each response at the temporal frequency of the stimulus. Results and Conclusions The third harmonic enhanced the accommodative response only when the fundamental was 1 cpd. This was true at both amplitudes of target motion. This contradicts models that suggest that high spatial frequencies are used to refine the accommodative response. Second, the responses to the peaks-add and peaks-subtract phase conditions were essentially the same for each fundamental. This suggests that the effect of harmonics cannot be described by any simple linear relationship between the increased luminance gradient of the grating target and the accommodative response. Third, the accommodative response to gratings with a 5 cpd fundamental were smaller than the responses to gratings with the 3 cpd fundamental. This contradicts the gradient hypothesis of accommodative control inasmuch as the luminance gradient is steeper in the gratings with the 5 cpd fundamental. Alternatively, it seems that harmonics will enhance an accommodative response regardless of phase if they are effective accommodative stimuli on their own.