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PubMed1990

Influence of target color and vergence of light on ocular accommodation during binocular fixation.

In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.

Accommodation, Ocular↗
PubMed1989

Perceptual detectability of ocular accommodation microfluctuations.

When the eye fixates a stationary stimulus, the power of the lens is known to change rapidly and continuously. Although the basic characteristics of this fluctuating activity are known and the influencing factors have been identified, their exact role in the control of accommodation is uncertain. It is thought that, for the fluctuations to be useful, a detectable change in the retinal image is necessary, yet an accommodation response can be provoked by a stimulus change of only 0.1 D, well within the ocular depth of focus as conventionally measured. In this study we examine whether the fluctuations of accommodation are able to provide perceptual sign information or whether the presence of a sensorimotor mechanism utilizing subthreshold blur is required. Normal accommodation fluctuations for steady-state viewing were recorded using an infrared optometer and subsequently were used to drive a Badal stimulus optometer. The perceptual threshold of the fluctuation signal for an eye under cycloplegia was then determined. The threshold for the detection of the fluctuations at optimal focus was not significantly different from the root-mean-square value of the actual response. Hence a portion of the fluctuations more than spans the dead space (depth of focus) and is therefore capable of providing information to control accommodation without the need for a subthreshold mechanism.

Accommodation, Ocular↗
PubMed1996

[Selected problems with ocular accommodation in children and youth].

This paper intends to present some accommodative disturbances which cause difficulties in reading and should be differentiated from dyslexia. The work describes the way of evaluation of accommodative convergence to accommodation rate (AC/A ratio). Clinical forms of two types of nonrefractive accommodative convergence excess connected with high AC/A ratio, namely hyperkinetic and hypoaccommodative, are presented. In hyperkinetic type, disturbances of ocular movements coordination during reading prevail. Patients with hypoaccommodation disorders suffer from youth presbyopia. The paper describes the ways of treatment of these disorders.

Accommodation, Ocular↗
PubMed2001

Fatigue reduces tonic accommodation.

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Accommodation, Ocular↗
PubMed1991

Effect of a topical beta-adrenergic receptor antagonist on the dynamics of steady-state accommodation.

The ocular accommodation response to a stationary near target exhibits microfluctuations which are characterized by two dominant components of activity: a low-frequency component (LFC: less than 0.5 Hz) and a high-frequency component (HFC: 1.0-2.2 Hz). We have shown recently that there is a significant correlation between the frequency of arterial pulse (AP) and the peak frequency of the HFC. If the mechanisms by which AP induces the HFC are intraocular they are likely to involve both rhythmic variations in choroidal blood flow and intraocular pressure (IOP) pulse: the former inducing changes in ciliary ring diameter from pulsatile blood volume changes in the ciliary body, the latter producing changes in effective lens power following small shifts in lens position or, with each drop in IOP, reduced resistance to inherent lens substance and capsule elasticity. In addition to its well-known ocular hypotensive effect, topical instillation of the beta-adrenergic receptor antagonist timolol maleate has been shown to reduce significantly the magnitude of the IOP pulse, an effect that has been attributed to the vasoconstrictive action of the drug on the choroidal vasculature. Using a modified Canon IR optometer we report on the effect of 0.5% timolol maleate on the accommodative microfluctuations for a group of 10 emmetropic subjects viewing a target monocularly at a distance of 25 cm. Following a double-blind protocol against a saline control, timolol was shown to reduce the rms for both LFCs and HFCs. An experiment to separate local from systemic effects involved four subjects whose treated and untreated eyes were examined to determine the effects of the consensual reduction in IOP: the results on microfluctuations in the untreated eyes were equivocal but the correlation between arterial pulse frequency and location of the peak frequency of HFC was maintained. The results show that factors relating to IOP and vasculature will affect the magnitude of accommodative microfluctuations.

Accommodation, Ocular↗
PubMed2002

The effect of accommodation on ocular shape.

PURPOSE: Ocular shape is altered in myopia, and accommodation during nearwork is a proposed risk factor for myopia. Using relative peripheral refractive error (RPRE), ocular shape was assessed before, during, and after a period of sustained nearwork to determine whether accommodation affects ocular shape. METHODS: Measurements of RPRE at 30 degrees in the nasal visual field were obtained using the spherical equivalent calculated from Canon R-1 autorefraction. The RPRE of 41 young adults was measured on two separate occasions separated by at least 1 week to assess RPRE repeatability. Later, the RPRE of 22 young adults was measured at a 0 D accommodative stimulus and then at a 3 D stimulus level at 0, 1, and 2 h during which subjects performed sustained nearwork at 33 cm. After 2 h of nearwork, subjects had RPRE measured at prescribed time intervals over a 1-h period in which they looked in the distance (0 D stimulus). RESULTS: The measurement of RPRE had adequate repeatability (mean difference +/- SD, -0.05 +/- 0.35 D) with +/- 0.68 D as the 95% limits of agreement. The onset of accommodation produced an immediate hyperopic shift of RPRE relative to baseline (+0.37 +/- 0.44 D; p = 0.0007), indicating that ocular shape had become more prolate. This shape remained unchanged after 1 h of sustained accommodation (RPRE difference from baseline, +0.25 +/- 0.55 D; p = 0.04) and then returned to baseline dimensions after 2h of accommodation (RPRE difference from baseline, +0.11 +/- 0.39 D; p = 0.21). At the 0 D stimulus level one minute after the period of nearwork, RPRE became more myopic relative to baseline (RPRE difference from baseline, -0.28 +/- 0.50 D; p = 0.016). Ocular shape returned to baseline dimensions after 45 min of accommodative relaxation. CONCLUSIONS: Accommodation induced the ocular shape to become more prolate. The opposite occurred after accommodation was relaxed, namely a change toward a more oblate ocular shape. The transient nature of these changes suggests that tension on the choroid and choroidal hysteresis may play a role in influencing ocular shape.

Accommodation, Ocular↗
PubMed1990

Experimental studies of emmetropization in the chick.

The eyes of neonates grow from ametropia (refractive error) toward emmetropia. Whether or not this 'emmetropization' is visually guided is controversial. I describe experiments which demonstrate that in the chick refractive state is used to regulate the growth of the eye's vitreous chamber in order to achieve emmetropia from hyperopia or myopia that is induced by different visual deprivations. I discuss several studies that begin to examine the neural pathways that might be involved in the control of eye growth. Optic nerve section was used to examine the level of visual processing necessary for the control of eye growth. Eyes in which the optic nerve has been cut can still grow in the appropriate direction to correct induced hyperopia or myopia. Nevertheless, an intact optic nerve is necessary for normal refractions to be achieved; eyes with optic nerve section overshoot control levels and reverse the sign of the initial refractive error. These findings suggest that eye growth in chicks is controlled by an intraocular mechanism and possibly by a brain-mediated mechanism as well. The hypothesis that ocular accommodation is integral to the control of eye growth was also tested. Complete recovery from induced refractive errors was achieved even when accommodation had been abolished by lesions of the Edinger-Westphal nucleus. The elimination of accommodation did not prevent the ability of chick eyes to compensate for the defocus of spectacle lenses. These results suggest that accommodation is not necessary for the control of eye growth.

Accommodation, Ocular↗
PubMedDate not supplied

Pseudoaccommodation and progressive addition lenses.

The limitations of traditional multifocal lenses to replace normal ocular accommodation are well known. The recent development of progressive addition lenses circumvent many of those problems, furnishing smoothly varying power in desired amounts for various distances and allowing freedom from distracting lines. Optical characteristics, screening considerations, prescribing and fitting requirements, and the advantages and limitations of progressive addition lenses are discussed.

Accommodation, Ocular↗
PubMed1992

Neonatal eye growth and emmetropisation--a literature review.

The refractive development of the neonatal eye has been the subject of much study and debate. In this paper the hypothetical mechanisms of emmetropisation and their relationship to the development of refractive errors will be reviewed. The evidence supporting visual feedback control of eye growth will be described, and the role of ocular accommodation will be discussed.

Accommodation, Ocular↗
PubMed1983

A simple continuously recording infrared optometer.

The design of a continuously recording infrared optometer for measuring dynamic changes in ocular accommodation is described. The optometer operates on the Scheiner principle, provides a good signal-to-noise ratio, and has several practical advantages over other optometers described in vision literature. It is primarily designed for use in animal experimentation but can be used for humans if eye and pupil movements are controlled. The design of a schematic eye for assessing the dynamic response of the optometer is also presented. Several methods of calibrating the optometer in absolute units are discussed.

Accommodation, Ocular↗
PubMed1995

On the short-term variability of measurements of intraocular pressure.

Measurement of intraocular pressure (IOP) (tonometry) is conducted routinely in the eye clinic for the diagnosis and management of primary open-angle glaucoma. Mean IOP may be influenced by a number of factors including ocular accommodation, spontaneous pulsations of IOP, and the effect of repeated applanation. A review of some of the factors that influence IOP should help us gain a perspective on the reliability of diurnal tonometry in the diagnosis of primary open-angle glaucoma.

Accommodation, Ocular↗
PubMed1983

The effect of phenylephrine hydrochloride on the resting point of accommodation.

Previous models for ocular accommodation have suggested a dual sympathetic/parasympathetic innervation to the ciliary muscle. Such models would predict that sympathomimetic drugs should change the resting point or dark focus of accommodation. The effect of an alpha-adrenergic agonist (10% phenylephrine hydrochloride) on accommodation was examined; the results show a decrease in the near point of accommodation, without a change in the resting point of accommodation. These results suggest that the accommodative state of the eye is not the result of a simple balance between sympathetic and parasympathetic innervation as proposed by previous models of accommodation.

Accommodation, Ocular↗
PubMed2002

Changes of ocular aberration with accommodation.

PURPOSE: To study the effect of accommodation on ocular aberrations. DESIGN: Observational case series. METHODS: The ocular aberrations in 33 eyes of 33 young adults were measured with a Hartmann-Shack wavefront aberrometer before and during 3 diopters of accommodation. RESULTS: The root mean square values of wavefront error did not change for a 4-mm and a 6-mm zone; however, spherical aberration changed significantly toward negativity for both the 4-mm zone (P <.001) and 6-mm zone (P =.022). CONCLUSIONS: During accommodation, the significant change in spherical aberration occurs without a change of image quality. Therefore, for customized aberration, it may not be advantageous to eliminate the physiologic spherical aberration for far vision.

Accommodation, Ocular↗
PubMed1992

Steady-state accommodation and ocular biometry in late-onset myopia.

The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at -1, -3 and -5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.

Accommodation, Ocular↗
PubMed2000

An area for vergence eye movement in primate frontal cortex.

To view objects at different distances, humans rely on vergence eye movements to appropriately converge or diverge the eyes and on ocular accommodation to focus the object. Despite the importance of these coordinated eye movements (the 'near response') very little is known about the role of the cerebral cortex in their control. As near-response neurons exist within the nucleus reticularis tegmenti pontis, which receives input from the frontal eye field region of frontal cortex, and this cortical region is known to be involved in saccadic and smooth-pursuit eye movements, we propose that a nearby region might play a role in vergence and ocular accommodation. Here we provide evidence from rhesus monkeys that a region of frontal cortex located immediately anterior to the saccade-related frontal eye field region is involved in vergence and ocular accommodation, and in the sensorimotor transformations required for these eye movements. We conclude that the macaque frontal cortex is involved in the control of all voluntary eye movements, and suggest that the definition of the frontal eye fields should be expanded to include this region.

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↗
PubMed1996

The role of cerebro-ponto-cerebellar pathways in the control of vergence eye movements.

Single-unit recording and anatomical techniques have been used in Rhesus monkeys to investigate the central pathways involved in the neural control of vergence and ocular accommodation. Anatomical studies have revealed connections between the midbrain near-response region and the posterior interposed (IP) and fastigial nuclei of the cerebellum. Single-unit recording studies in the IP of alert, trained monkeys identified cells with activity that increased with increases in the amplitude of divergence and far accommodation, i.e. the far-response. Microstimulation at the site of these neurons often produced a far-response. Single-unit recording from a precerebellar nucleus, the nucleus reticularis tegmenti pontis (NRTP), identified some cells with activity that linearly increased with increases in the amplitude of the near-response, and some with activity that linearly increased with increases in the amplitude of the far-response. Microstimulation at the site of these near- or far-response neurons often produced changes in vergence angle and accommodation. Single-unit recording studies in the region of the frontal eye fields identified cells in the prearcuate cortex with activity that is modulated by either the near- or far-response. These results suggest that regions of the prearcuate cortex, the NRTP and the IP form part of a cerebro-ponto-cerebellar pathway modulating or controlling vergence and ocular accommodation.

Accommodation, Ocular↗
PubMed2002

Neural mechanisms for the control of vergence eye movements.

Our recent studies in non-human primates have identified and characterized cerebro-ponto-cerebellar pathways involved in the control of vergence eye movements. Specifically, within the deep cerebellar nuclei and nucleus reticularis tegmenti pontis, we have identified neurons that are related to either the near response (convergence and increased ocular accommodation) or the far response (divergence and decreased ocular accommodation). In addition, within the prearcuate region (area 8a), we have characterized neurons related not only to either the far response or the near response, but also to the sensorimotor transformations underlying these eye movements. Because the vergence-related prearcuate region abuts the frontal eye fields, we suggest that the extent of the frontal eye fields be expanded to include this region. We further suggest that with inclusion of this vergence-related region, the frontal eye fields are important for all voluntary eye movements.

Animals↗