Niall C. Strang

Improving vision: neural compensation for optical defocus

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur, or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for thedefocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid–range (5–25) cycles deg−1 spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two–stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid–range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter–ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Wavefront aberration and its relationship to the accommodative stimulus-response function in myopic subjects.

Background. Autorefractors are increasingly used in myopia research because they are convenient tools to investigate aspects of the accommodation response. The degree to which the autorefractor measures are affected by ocular aberrations has been highlighted by studies that have shown changes in aberration levels through different parts of the pupil and with accommodation. We have compared accommodative accuracy as measured with a Shin-Nippon SRW 5000 autorefractor with wavefront error as measured with a Hartmann-Shack wavefront sensor to investigate how factors such as accommodation demand, ocular aberrations, and pupil size can influence autorefractor measures. Methods. Accommodation stimulus-response curves were determined (using negative lenses) for 30 young healthy subjects (20 myopic [−0.75 to −6.00 D] and 10 emmetropic). Accommodation levels ranged from 0 to 4 D in 1 D steps. Wavefront aberrations were also determined for the same accommodation levels using a Hartmann-Shack wavefront sensor for both the subjects’ natural pupil sizes and for a 2.9-mm pupil. Results. For all subjects, there was a consistent increase in negative spherical aberration with increases in accommodative stimulus. However, there was no consistent change in paraxial spherocylindrical refractive correction with accommodation stimulus. For the emmetropic subjects, accommodation error as measured with the autorefractor was statistically similar to the total spherocylindrical correction for the eye as estimated by the Hartmann-Shack wavefront sensor, but only for a 2.9-mm pupil (the pupil size utilized by the autorefractor). For the myopic subjects, accommodation error as measured with the autorefractor was statistically similar to the higher-order aberrations, but only when measured for a natural pupil size. Conclusions. The relationship between the accommodation accuracy as measured with the autorefractor and the total wavefront aberration as measured with a Hartmann-Shack wavefront sensor is largely influenced by the higher-order (fourth and above) aberration levels. For the emmetropic subjects, the errors measured by the two methods agree when adjusted to measure at similar pupil sizes. For the myopic subjects with similar pupil sizes, however, the Hartmann-Shack wavefront sensor underestimates the accommodation error at higher accommodation levels (2 to 4 D) compared with the autorefractor.

Hyperopia is predominantly axial in nature.

PURPOSE Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes.

The role of neural and optical factors in limiting visual resolution in myopia.

The myopic growth process has the potential to modify both the optical and neural performance of the eye. We provide three simple models, based on different types of retinal stretching, to predict changes in neural resolution resulting from axial length increases in myopia. These predictions are compared to visual acuity (VA) measures in 34 subjects with refractive errors ranging from plano to -14 D. Our results show a reduction in VA with increasing myopia but not in a manner predicted by our models. We discuss the relative contribution of optical and neural factors to the reduction in visual resolution in myopia.

Refractive group differences in accommodation microfluctuations with changing accommodation stimulus

Purpose:  Microfluctuations of accommodation are known to increase in magnitude with increasing accommodation stimulus. Reduced sensitivity to blur in myopic subjects could also lead to increases in the magnitude of the microfluctuations. The aim of this study is to examine the effect of variations in accommodation stimulus upon the microfluctuations in different refractive groups.

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia

The characteristics of post‐task regression of accommodation to pre‐task ionic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within‐task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post‐task regression occurs in late‐onset myopes (LOMs) when compared with emmetropes(EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late‐onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post‐task regression may have some value in predicting those individuals who may be susceptible to post‐task accommodative hysteresis. A pre‐requisite for such a predictive value is that for a given individual the variation in inter‐trial regression patterns is not significant. The aim of this study is principally to Investigate the inter‐trial variability of post‐task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post‐task regressions, A modified Canon Rl infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post‐task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subjects baseline TA. Repeatability of post‐task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions, A significant difference in the post‐task regression patterns between EMMs and LOMs was observed which supports previously reported work; the rate of regression to pre‐task TA being slower in the LOMs. Within‐subject analysis showed that for both EMMs and LOMs the differences between time‐course of post‐task regressions were not significant. The repeatability of post‐task regression patterns thus justifies their use as indices of within‐task adaptation.

Influence of Stiles–Crawford apodization on visual acuity

The Stiles-Crawford effect (SCE) of the first kind has often been considered to be important to spatial visual performance in that it ameliorates the influence of defocus and aberrations. We investigated the influence of SCE apodization on visual acuity as a function of defocus (out to +/-2 D) in four subjects. We used optical filters, conjugate with the eyes entrance pupil, that neutralized or doubled the existing SCE. With an illiterate-E task, the influence of the SCE was more noticeable for myopic defocus than for hypermetropic defocus, was generally more noticeable for high-contrast than for low-contrast letters, and increased with increase in pupil size. The greatest influence on visual acuity of neutralizing the SCE, across the subjects and range of conditions, was deterioration of 0.06 (4-mm pupil), 0.16 (6-mm pupil), and 0.29 log unit (7.6-mm pupil).

Effects of defocus and pupil size on human contrast sensitivity

Defocus lowers the contrast sensitivity function (CSF), producing a complex function with local dips and peaks. Previously, we were able to predict the shape of the CSF with large pupils from measured transverse aberrations with hypermetropic defocus but not with myopic defocus ( Atchison et al., 1998c , J. Opt. Soc. Am. A.15, 2536). As there is no reason that myopic defocus should be more difficult to predict than hypermetropic defocus, we modified the procedure to try to improve CSF predictions with myopic defocus. Also, we extended the study to consider a range of pupil sizes. CSFs were measured for three subjects at three defocus levels (in‐focus, −2D and +2D) and three pupil sizes (2  mm, 4  mm and 6  mm). Using a diffraction optics model, transverse aberration measures and in‐focus CSF measures, we predicted the defocused CSFs. The predicted defocused CSFs were lower than the in‐focus CSF as expected, and had complex shapes that varied with defocus and pupil size and between subjects. While a few predictions were poor, generally, the overall magnitude and shape of the defocused CSFs were well predicted and similarly so for myopic and hypermetropic defocus. Some further improvements in technique are indicated.

Clinical evaluation of patient tolerance to autorefractor prescriptions.

Background: In clinical optometric practice, autorefractors are used as an objective measure of refractive error prior to subjective refraction. We evaluate the clinical efficacy of autorefractor measurements by determining whether spectacles can be prescribed from autorefractor results.

Effect of β-adrenoceptor antagonists on autonomic control of ciliary smooth muscle

Abstract Purpose: Pharmacological intervention with peripheral sympathetic transmission at ciliary smooth muscle neuro‐receptor junctions has been used against a background of controlled parasympathetic activity to investigate the characteristics of autonomic control of ocular accommodation.