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PubMed1985

Modification of pattern reversal VERs by ocular accommodation.

The amplitudes and implicit times of the P-1 component of monocular transient pattern VERs elicited by reversing checkerboard targets were measured in 6 conditions: (1) Ocular accommodation (Ao) elicited by minus power ophthalmic lenses; (2) Ao stimulated by minus lenses with the natural pupil of the test eye dilated and replaced by an artificial pupil; (3) accommodative demand and pupil size fixed with retinal image size (Ir) changed by afocal minifying lenses, (4) accommodative demand and Ir fixed but pupil size changed; (5) Ao elicited by minus lenses for three sizes of reversing checkerboards, and (6) Ao elicited by minus lenses for a constant checkerboard size with three check sizes. The observed decrease in VER amplitude with Ao could not be attributed entirely to the effects of pupil size or Ir. The rate at which the amplitude of P-1 decreased with Ao was not affected by checkerboard check size, but was closely correlated with checkerboard field size. The implicit time of P-1 first decreased slightly below baseline values at low and moderate levels of Ao and then tended to increase significantly at higher levels. The increase in implicit time at high levels of Ao could be explained adequately by the effects of pupil size alone on the VER. However, where Ao occurred without pupillary miosis, the increase in implicit time could only be explained by focusing error.

Accommodation, Ocular↗

PubMed2005

Cognition, ocular accommodation, and cardiovascular function in emmetropes and late-onset myopes.

PURPOSE: To investigate objectively and noninvasively the role of cognitive demand on autonomic control of systemic cardiovascular and ocular accommodative responses in emmetropes and myopes of late-onset. METHODS: Sixteen subjects (10 men, 6 women) aged between 18 and 34 years (mean +/- SD: 22.6 +/- 4.4 years), eight emmetropes (EMMs; mean spherical equivalent [MSE] refractive error +/- SD: 0.05 +/- 0.24 D) and eight with late-onset myopia (LOMs; MSE +/- SD: -3.66 +/- 2.31 D) participated in the study. Subjects viewed stationary numerical digits monocularly within a Badal optical system (at both 0.0 and -3.0 D) while performing a two-alternative, forced-choice paradigm that matched cognitive loading across subjects. Five individually matched cognitive levels of increasing difficulty were used in random order for each subject. Five 20-second, continuous-objective recordings of the accommodative response measured with an open-view infrared autorefractor were obtained for each cognitive level, whereas simultaneous measurement of heart rate was continuously recorded with a finger-mounted piezoelectric pulse transducer for 5 minutes. Fast Fourier transformation of cardiovascular function allowed the relative power of the autonomic components to be assessed in the frequency domain, whereas heart period gave an indication of the time-domain response. RESULTS: Increasing the cognitive demand led to a significant reduction in the accommodative response in all subjects (0.0 D: by -0.35 +/- 0.33 D; -3.0 D: by -0.31 +/- 0.40 D, P < 0.001). The greater lag of LOMs compared with EMMs was not significant (P = 0.07) at both distance (0.38 +/- 0.35 D) and near (0.14 +/- 0.42 D). Mean heart period reduced with increasing levels of workload (P < 0.0005). LOMs exhibited a relative elevation in sympathetic system activity compared to EMMs. Within refractive groups, however, accommodative shifts with increasing cognition correlated with parasympathetic activity (r = 0.99, P < 0.001), more than with sympathetic activity (r = 0.62, P > 0.05). CONCLUSIONS: In an equivalent workload paradigm, increasing cognitive demand caused a reduction in accommodative response that was attributable principally to a concurrent reduction in the relative power of the parasympathetic component of the autonomic nervous system (ANS). The disparity in accommodative response between EMMs and LOMs, however, appears to be augmented by changes in the sympathetic nervous component of the systemic ANS.

Accommodation, Ocular↗

PubMed1995

Evolution of control system models of ocular accommodation, vergence and their interaction.

This paper reviews the evolution of inferential models describing ocular accommodation and vergence, which have been developed using the techniques of control engineering. The models are developed by inferring that the observed behaviour of the ocular systems could be produced by particular types of feedback control systems. The models are subsequently tested and improved by modifications resulting from the comparison of model predictions with physiological experimentation. Current models of accommodation and vergence have had considerable success in describing both the steady-state and dynamic behaviours of the individual systems. However, controversy currently exists among researchers, particularly with respect to the composition of models that describe the interaction of the two systems.

Accommodation, Ocular↗

PubMed1994

Behavior of identified Edinger-Westphal neurons during ocular accommodation.

1. The present study used single-unit recording and antidromic activation techniques in alert rhesus monkeys to examine the static and dynamic behavior of 21 parasympathetic, preganglionic neurons of the Edinger-Westphal nucleus (EW) during ocular accommodation. 2. All identified EW neurons were active when viewing at optical infinity with an average firing rate of 11.6 spikes/s. During near viewing, there was a linear relationship between firing rate and accommodation with an overall gain for the population of preganglionic EW neurons of 3.3 (spikes/s)/diopter. 3. The activity of eight identified EW neurons was studied during viewing of targets with conflicting vergence and accommodative demands to dissociate their vergence and accommodation responses. With normal viewing these responses are so closely matched that it cannot be determined if the activity of a cell is related to vergence or to accommodation, but with dissociated viewing these relationships can be determined. Under this viewing condition, six preganglionic EW neurons showed the same relationship to accommodation as they did during normal viewing. However, the activity of two cells could not be explained solely by accommodation, and they showed some activity related to vergence. 4. Microstimulation at the sites of identified EW neurons produced accommodation in the ipsilateral eye. Repeated measures of the effect of microstimulation yielded a value of 75 ms for the latency of the response. This latency was essentially the same in both animals tested. 5. The activity of identified EW neurons is related to the velocity of accommodation as well as to static accommodation. The relationship between accommodation velocity and firing rate was studied for 15 identified EW neurons during sine-wave tracking of targets moving in depth. All of these cells showed a clear relationship between firing rate and accommodation velocity. Overall, this group of identified EW neurons showed a velocity sensitivity of 1.2 (spikes/s)/(diopter/s) and an estimated neural time constant of 380 ms. 6. Eleven neurons encountered near to preganglionic EW neurons could not be antidromically activated by stimulation of the oculomotor nerve. These neurons had statistically higher gains with respect to the near response; indeed, there was no overlap between the gains of these neurons and the gains of preganglionic EW neurons. Upon dissociation of vergence from accommodation, they were found to be related to either vergence or to vergence and accommodation but not solely to accommodation.

Accommodation, Ocular↗

PubMed2005

Changes in ocular accommodation when shifting between global and local attention.

BACKGROUND: The link between cognitive changes in attentional focus and the physiological parameters of the eye is not well understood. The aim of the current work was to examine the role of ocular accommodation, that is, the process of changing the shape of the crystalline lens in order to focus an image onto the retina, in active shifts of visual attention between global and local information. METHOD: Ten adult participants (aged 19 to 27) viewed sequences of complex global/local figures presented at the same location. They were asked to identify either a global or local red target letter and to look for either a global or local letter x (probe) in the sequence following. Target and probe items were separated by a temporal gap of approximately one second. Refraction was measured using a Canon Autorefractor R1 at the time of target appearance and again at the time of probe presentation. Particular interest was paid to trials where participants were required to shift attention either from a local to global level or in the reverse direction. The difference between the two measurements of refraction gave a 'change score' which provided an indication of the change of accommodation. RESULTS: No significant change in refraction was observed when shifting from the local to the global condition. A mean change in refraction of -0.128 dioptres was observed for the global to local condition. The 95 per cent confidence interval for this difference did not overlap zero, indicating a significant change in refraction, which was attributed to an increase in accommodation. DISCUSSION: The results suggest that while a change in accommodation occurs in shifting attention from the global to the local aspect of the complex figures, a similar change is not observed in the reverse direction. These results are consistent with the hypothesis that it is more difficult to restrict attentional focus than it is to widen it.

Accommodation, Ocular↗

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↗

PubMed2004

Spatially variant changes in lens power during ocular accommodation in a rhesus monkey eye.

This study investigated the changes in ocular aberrations that occur over the entire lens equatorial diameter during accommodation in iridectomized rhesus monkey eyes to understand the nature of accommodative lenticular deformation. Accommodation was centrally stimulated to a range of different response amplitudes (0 D to approximately 11 D), and ocular aberrations were measured with a Shack-Hartmann wavefront sensor in both eyes of one previously iridectomized 10-year-old rhesus monkey. At the highest amplitude in the two eyes, aberrations were analyzed over entrance pupil diameters ranging from 3 to 8 mm in steps of 1 mm. Root mean square error of the total measured aberrations, excluding defocus, increased systematically with increasing accommodation from about 1 to 3.5 microns. Spherical aberration became systematically more negative, and vertical coma increased significantly in magnitude with accommodation. There was a strong accommodative change in power near the center of the lens and little change in power at the periphery. At the highest accommodative state, decreasing the analyzed entrance pupil diameter from 8 to 3 mm considerably reduced the wavefront error. The greater increase in optical power near the central region of the lens, combined with an accommodative pupillary miosis, would serve to maximize accommodative refractive change while maintaining acceptable image quality.

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↗

PubMed2003

Refractive error, cognitive demand and nearwork-induced transient myopia.

PURPOSE: Whereas many previous studies have identified the association between sustained near work and myopia, few have assessed the influence of concomitant levels of cognitive effort. This study investigates the effect of cognitive effort on near-work induced transient myopia (NITM). METHODS: Subjects comprised of six early onset myopes (EOM; mean age 23.7 yrs; mean onset 10.8 yrs), six late-onset myopes (LOM; mean age 23.2 yrs; mean onset 20.0 yrs) and six emmetropes (EMM; mean age 23.8 yrs). Dynamic, monocular, ocular accommodation was measured with the Shin-Nippon SRW-5000 autorefractor. Subjects engaged passively or actively in a 5 minute arithmetic sum checking task presented monocularly on an LCD monitor via a Badal optical system. In all conditions the task was initially located at near (4.50 D) and immediately following the task instantaneously changed to far (0.00 D) for a further 5 minutes. The combinations of active (A) and passive (P) cognition were randomly allocated as P:P; A:P; A:A; P:A. RESULTS: For the initial near task, LOMs were shown to have a significantly less accurate accommodative response than either EOMs or EMMs (p < 0.001). For the far task, post hoc analyses for refraction identified EOMs as demonstrating significant NITM compared to LOMs (p < 0.05), who in turn showed greater NITM than EMMs (p < 0.001). The data show that for EOMs the level of cognitive activity operating during the near and far tasks determines the persistence of NITM; persistence being maximal when active cognition at near is followed by passive cognition at far. CONCLUSIONS: Compared with EMMs, EOMs and LOMs are particularly susceptible to NITM such that sustained near vision reduces subsequent accommodative accuracy for far vision. It is speculated that the marked NITM found in EOM may be a consequence of the crystalline lens thinning shown to be a developmental feature of EOM. Whereas the role of small amounts of retinal defocus in myopigenesis remains equivocal, the results show that account needs to be taken of cognitive demand in assessing phenomena such as NITM.

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↗