Biomedical subjects

Niall C Strang

Publications and source records attributed to Niall C Strang.

10 recordsLinked to original sources

PubMed2006

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. METHODS: Thirty-six subjects were divided into three groups depending upon their refractive error and age of onset of their myopia; 12 emmetropes (EMMs), 12 early onset myopes (EOMs) and 12 late-onset myopes (LOMs). Steady-state accommodation responses were recorded continuously for 2 min using the Shin-Nippon SRW-5000 autorefractor at a sampling rate of 52 Hz while viewing targets at accommodation stimuli levels of 0, 1, 2, 3 and 4 D in a Badal (+5 D) optical system. RESULTS: The EMMs and EOMs showed systematic increases in the root mean square (r.m.s.) value of the microfluctuations with increasing accommodation stimulus. In contrast, no systematic variation with accommodation stimulus was found for the LOMs. Power spectrum analysis demonstrated that increases in the size of the microfluctuations were mediated by increases in the power of the low frequency components of the accommodation response. CONCLUSIONS: The magnitude of the microfluctuations in the EMMs and EOMs may be influenced primarily by accommodation response-induced zonular relaxation effects or to changes in the physical properties of the accommodation plant with increasing accommodation response. The LOMs may have an increased baseline neural blur threshold, which appears to modulate the magnitude of the accommodative microfluctuations for low accommodation levels. At higher accommodation demands, the changes in the physical properties of the accommodation plant or the zonular relaxation effects appear to exceed the blur threshold, and the known association between microfluctuations and accommodation stimulus level is restored.

Accommodation, Ocular↗

PubMed2005

A psychophysical investigation of ocular expansion in human eyes.

PURPOSE: To investigate spatial anisotropies in the peripheral visual field of axially myopic eyes, in an attempt to distinguish between models of isotropic and anisotropic ocular stretching. METHODS: Stimuli consisted of two high-contrast Gaussian patches presented in one of four orientations (90 degrees, 180 degrees, 45 degrees, or 135 degrees). For each orientation, perceived separation was established relative to that for all other orientations. The experiment was conducted with central fixation and at 15 degrees in the nasal and inferior visual fields. Eleven myopes and nine emmetropes participated in the study. Biometric data were collected from all subjects. RESULTS: For foveal fixation, the magnitude of the spatial anisotropy (approximately 5%) was consistent with the well-documented horizontal-vertical illusion (HVI), and unrelated to axial length. In the nasal visual field, much larger misperceptions were found (approximately 19%), the magnitude of which increased significantly with increasing axial length. Inferiorly, a reversal of the traditional HVI is found in most subjects (approximately 7%), with a tendency for a larger reversed illusion with increasing axial length. Differences between nasal and inferior misperceptions were significantly correlated with axial length. CONCLUSIONS: Isotropic stretching, such as globe expansion, should preserve the aspect ratios of receptive fields, predicting a separation misperception which is independent of axial length. In contrast, the magnitude of the misperception is significantly correlated with axial length, supporting anisotropic stretching models of myopic growth.

Elastic Tissue↗

PubMed2004

Dynamic accommodation responses to stationary colored targets.

PURPOSE: When the targets or the background in a display are different colors, longitudinal chromatic aberration ensures that there is no single correct accommodative response. The purpose of the present study was to determine whether the response becomes more variable when viewing certain multicolor displays. METHODS: Accommodative responses of five young participants were measured with a dynamic infrared optometer while they viewed steady targets at a nominal stimulus level of 3 D. Target-on-background color combinations were black on white, black on blue, black on red, blue on red, red on blue, dark blue on red, and dark red on blue. RESULTS: When compared with the standard black-on-white target, responses to targets with reduced spectral bandwidth were not significantly more variable. In most participants, responses to near-isoluminant targets (e.g., red on blue and blue on red) were not more variable than to the standard target. However, calculated confidence intervals cannot rule out moderate to large changes in variability near isoluminance. Responses to these multicolor targets tended to favor the blue focus. CONCLUSIONS: In most individuals, viewing multichromatic targets does not increase significantly the variation in accommodative response as compared with broadband black-and-white targets.

Accommodation, Ocular↗

PubMed2003

Open- and closed-loop regressions compared in myopic and emmetropic subjects.

Post-near task accommodation regressions have been measured in a number of studies to investigate the association between myopia and nearwork. The slow nature of the regression found when measuring both open- and closed-loop regressions in some subjects has been hypothesised to be associated with myopia development and progression. However, it is not known whether the speed and characteristics of the regression for an individual is the same under open- and closed-loop conditions. In this study we measured post-near task accommodation regression functions under open- and closed-loop conditions for myopic and emmetropic subjects. We compared the responses of each subject under the two conditions and found that some subjects responded differently depending on the condition (i.e. a fast regression under one condition and a slow regression under the other condition). When compared as a group, the myopic subjects showed significantly more prolonged open-loop regressions than closed-loop regressions. In terms of the effect on the post-task accommodation, the myopes appear to be more affected by the measurement conditions than the emmetropes in this study. The results highlight the importance of the type of accommodation stimulus when investigating the accommodation response.

Accommodation, Ocular↗

PubMed2003

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.

Accommodation, Ocular↗

PubMed2003

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, Ocular↗

PubMed2002

Effect of beta-adrenoceptor antagonists on autonomic control of ciliary smooth muscle.

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. METHODS: A continuously recording infra-red optometer was used to measure accommodation on a group of five visually normal emmetropic subjects under open- and closed-loop conditions. A double-blind protocol between saline, timolol and betaxolol was used to differentiate between the localised action on ciliary smooth muscle and effects induced by changes in stimulus conditions. Data were collected before and 45 min following the instillation of saline, timolol or betaxolol. Open-loop post-task decay was investigated following 3 min sustained near fixation of a stimulus placed 3 D above the subject's pre-task tonic accommodation level. Closed-loop dynamic responses were recorded for each treatment condition while subjects viewed sinusoidally (0.05-0.6 Hz) or stepwise vergence-modulated targets over a 2 D range (2-4 D). RESULTS: Open-loop data demonstrate a rapid post-task regression to pre-task tonic accommodation levels for saline and betaxolol control conditions. A slow positive post-task shift was induced by timolol indicating that sympathetic inhibition contributes to accommodative adaptation during sustained near vision. Closed-loop accommodation responses to temporally modulated sinusoidal stimuli showed characteristic features for both saline and betaxolol control conditions. Timolol induced a reduced gain for low- and mid-temporal frequencies (< 0.3 Hz) but did not affect the response at higher temporal frequencies. Response times to stepwise stimuli increased following the instillation of timolol for the near-to-far fixation condition compared with the controls and was related to the period of sustained prior fixation. CONCLUSIONS: Modulation of accommodation under open- and closed-loop conditions by a non-selective beta-blocker is consistent with the temporal and inhibitory features of sympathetic innervation to ciliary smooth muscle. Although parasympathetic innervation predominates there is evidence to support a role for sympathetic innervation in the control of ocular accommodation.

Accommodation, Ocular↗

PubMed2002

Nearwork induced transient myopia during myopia progression.

PURPOSE: Recent work has shown that differences exist in the accommodation response characteristics of myopes and emmetropes, although the exact nature of the differences has yet to be ascertained. This may in part be due to the classification of the refractive error employed in many studies. Previously it has been suggested that inaccuracies in the accommodation response occur predominantly during the progression of myopia.(1) To test this hypothesis we measure nearwork induced transient myopia (NITM) in progressing myopes (PMs), stable myopes (SMs) and emmetropes (EMMs). METHODS: A total of 41 young (18-27 years) subjects participated in the study (13 PMs, 14 SMs, 14 EMMs). Following a 10 minute near task (4 D) the change in accommodation response back to distance viewing was measured over a two minute time period. RESULTS: Grouped mean data revealed a significant nearwork after-effect in PMs in comparison to SMs and EMMs after 10 seconds (p < 0.01) and 30 seconds (p < 0.01) post-task. Significantly longer time constants were also found in the PMs group. CONCLUSIONS: Our results suggest that NITM is manifest during the progressive phase of myopia development.

Accommodation, Ocular↗

PubMed2002

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 eye's 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).

Contrast Sensitivity↗

PubMedDate not supplied

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. METHODS: Forty-seven subjects were randomly allocated spectacles using prescriptions determined either by an optometrist or from an autorefractor in a double-blind protocol. Subjects wore each prescription for two weeks and completed an oral questionnaire following each period of wear. The questionnaire assessed both pairs of spectacle lenses in terms of visual performance and ocular comfort. RESULTS: The level of negative responses to all questions was higher for spectacles based on the autorefractor findings than for those based on the optometrist's prescriptions. Having reported initial discomfort, many subjects indicated that they had adapted to the spectacle lenses over a two-week period. CONCLUSIONS: Our results suggest that, in a clinical environment, subjective refraction produces a more accurate and acceptable spectacle prescription than an autorefractor.

Journal Article↗