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Results for “Accommodation, Ocular”

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At least 19 recordsLinked to original sources
PubMed1990

Arterial pulse modulates steady-state ocular accommodation.

Rapid and continuous fluctuations in ocular focus are known to occur when the eye views a stationary target. The advent of high-speed infra-red optometers has established that these microfluctuations of ocular accommodation have two dominant components: low frequency of less than 0.6 Hz and high frequency between 1.0-2.3Hz. Although the retinal image blur associated with microfluctuations has the potential to guide and maintain optimum accommodation levels, there is no consensus with regard to the respective contribution of each of the dominant frequency components. Using a newly-designed measurement and recording system we show that, when viewing a stationary target located at 25cm, individuals exhibit little variation in the frequency of low frequency components but significant variation in high frequency components. Simultaneous measurements of ocular accommodation and systemic arterial pulse demonstrate that the variation in high frequency component is significantly correlated with arterial pulse frequency. Since control experiments indicate that the microfluctuations are derived from activity of the crystalline lens our observations could provide the basis for a non-invasive method of assessing the effects of arterial pulse on ciliary body/choroidal vasculature, the vitreous/lens interface and intraocular pressure. Further, it is feasible that under certain conditions an abnormal increase in the magnitude of arterial pulse may affect the aggregate nature of accommodative microfluctuations to an extent that disrupts the normal control processes maintaining optimum retinal contrast during sustained near vision.

Accommodation, Ocular↗
PubMed1982

Ocular accommodation neurons in brain stem of the alert cat.

Ten "ocular accommodation" units in the brain stem reticular formation of alert, unanesthetized cats were studied. While the cat's head was immobilized with a skull mount, ocular accommodation was induced by stimuli presented to the right eye. The consensual accommodation response was measured in the left eye with an infrared optometer. There was a close association between the accommodative demand of the visual stimulus, the discharge frequency of brain stem accommodation neurons, and consequent ocular accommodation. Controls for eye movements, pupil size, et., were applied. The accommodation neurons described were found in brain stem areas not traditionally associated with the accommodative mechanism.

Accommodation, Ocular↗
PubMed1982

Ocular accommodation in the cat after electrical stimulation of mesencephalic reticular formation.

We measured ocular accommodation in the cat with a continuously recording infrared optometer. The brain stem reticular formation in an anesthetized cat was stimulated electrically with microelectrodes to evoke accommodation. Sites in the mesencephalic reticular formation caudal and ventral to the III nerve nucleus near the midline gave large positive accommodation responses. The velocity and amplitude of postive accommodation was graded, depending on the applied current and frequency of stimulation. Relaxation of accommodation proceeded at a fairly constant rate upon cessation of electrical stimulation. These findings provide new information on the amplitude and dynamic aspects of ocular accommodation in the cat. Also, the location of the stimulating electrodes suggest that in addition to the Edinger-Westphal nucleus, areas in the mesencephalic reticular formation between the cerebellum and the oculomotor nucleus are involved in ocular accommodation in the cat.

Accommodation, Ocular↗
PubMed1986

A review of the role of sympathetic innervation of the ciliary muscle in ocular accommodation.

Although it is well established that autonomic control of ocular accommodation is predominantly parasympathetic, many investigators have, over the last 150 years, proposed that a supplementary sympathetic innervation should be considered. Particular attention is directed in this review to previous literature providing anatomical, physiological and pharmacological evidence for dual innervation of the ciliary muscle. Clinical and psychological evidence is shown to be equivocal. A review of recent laser optometry studies of tonic ("dark-focus") resting positions of accommodation indicates that the inhibitory nature of sympathetic innervation suggested by the majority of previous studies can be further defined with respect to specific adrenergic receptors. The implications of dual innervation relating to ocular accommodation during sustained near-vision tasks is discussed.

Accommodation, Ocular↗
PubMed1983

The structural basis of ocular accommodation in the chick.

The structures involved in ocular accommodation were studied with scanning electron microscopy in 5-day old chicken. Particular attention was paid to the ciliary muscle that was found to be subdivided into an anterior and a posterior portion. Since the anterior portion inserted on the wall of canal of Schlemm, we postulate that it could somehow control the flow of aqueous humour. According to its insertions, the posterior muscle could stretch the choroid coat. Thus, changes in the refractive power of the eye would be brought about by an indirect mechanism.

Accommodation, Ocular↗
PubMed2002

[Effect of ocular accommodation on refractive components in children].

PURPOSE: To study the correlation between ocular accommodation and ametropia, especially myopia by analyzing the effect of accommodation on refractive components in children. METHODS: The anterior chamber depth (AD), lens thickness (LT), vitreous chamber length (VL) and ocular axial length (AL) were measured for 135 cases (270 eyes) of children before and after cycloplegia. The corneal shapes of 136 eyes selected from the 270 eyes were studied before and after cycloplegia by a computerized video keratoscope. RESULTS: AD increased (P < 0. 001) while both LT (P < 0. 001) and VL decreased (P < 0. 01) significantly after cycloplegia regardless of their refractive state. However, AL increased for hyperopic eyes (P = 0.027) and decreased for myopic eyes (P = 0.008) after cycloplegia. For hyperopic eyes, the major keratometer K1 increased (P = 0.025) while for myopic eyes, the major keratometer K1 (P = 0.039) and minor keratometer K2 (P = 0.003) decreased after cycloplegia. Similarly, mean corneal power of zones 3 mm (MD3), 5 mm (MD5) and 7 mm (MD7) decreased dramatically (P < 0.05) with myopic eyes after cycloplegia, while MD3 (P = 0.009) increased with hyperopic eyes. The mean refractive power of the lower half vertical meridian (LMD) is greater than that of upper half (UMD); and the mean refractive power of the lateral half of the horizontal (LaMD) is greater than that of the medial half (MMD). CONCLUSION: The ocular accommodation has a great influence on refractive components in children. It is not only the process by which the refractive power of the lens is increased. Furthermore, the lens itself moved forward relatively. There is asymmetry of the corneal surface within the same horizontal or vertical meridian.

Accommodation, Ocular↗
PubMed1986

Ocular accommodation in chickens: corneal vs lenticular accommodation and effect of age.

The avian accommodative response has long been suspected of having a corneal component resulting from contraction of a limbal extension of the ciliary muscle. Efforts to confirm the existence of such a mechanism have been sporadic and the results contradictory. In addition, while the accommodative mechanism of the bird eye is commonly considered to be very effective, the effect of lenticular aging is not known. The accommodative responses of excised chicken eyes of various age, were measured by photographing changes in focal effects on incident fine parallel laser beams in response to electrical stimulation of the intraocular muscles. An opening at the posterior pole of the eye enabled the beams to exit in order to facilitate measurement of refractive change. The question of a possible corneal contribution was examined by taking advantage of the fact that corneal refractive power is neutralized when the surface of the cornea is in water. Accommodation was measured with the cornea in air and in water. The results indicate that there is little or no corneal accommodation and accommodative ability drops off substantially with age. The results may also indicate the existence of serious inter-breed differences in chicken accommodation.

Accommodation, Ocular↗
PubMed2005

Acceleration characteristics of human ocular accommodation.

Position and velocity of accommodation are known to increase with stimulus magnitude, however, little is known about acceleration properties. We investigated three acceleration properties: peak acceleration, time-to-peak acceleration and total duration of acceleration to step changes in defocus. Peak velocity and total duration of acceleration increased with response magnitude. Peak acceleration and time-to-peak acceleration remained independent of response magnitude. Independent first-order and second-order dynamic components of accommodation demonstrate that neural control of accommodation has an initial open-loop component that is independent of response magnitude and a closed-loop component that increases with response magnitude.

Accommodation, Ocular↗
PubMed2005

Pulse-step models of control strategies for dynamic ocular accommodation and disaccommodation.

Dynamic properties and control strategies of step responses by accommodation and disaccommodation differ from one another. Peak velocity of accommodation increases with response magnitude, while peak velocity and peak acceleration of disaccommodation increase with starting position. These dynamic properties can be modeled as control strategies that use independent acceleration-pulse and velocity-step components that are integrated respectively into phasic-velocity signals that control movement and tonic-position signals that control magnitude. Accommodation is initiated toward its final destination by an acceleration-pulse whose width increases with response magnitude to increase peak velocity. Disaccommodation is initiated toward a default destination (the far point) by an acceleration-pulse whose height increases with dioptric distance of the starting position to increase peak velocity and peak acceleration. Both responses are completed and maintained by tonic-position signals whose amplitudes are proportional to the final destination. Mismatched amplitudes of phasic-velocity and tonic-position signals in disaccommodation produce unstable step responses.

Accommodation, Ocular↗
PubMed2001

The influence of tinted lenses upon ocular accommodation.

We determined the effect upon accommodative responses of tinted lenses prescribed for the relief of visual discomfort in a group of five long term lens wearers. Static and dynamic responses were measured under four viewing conditions (1) prescribed tinted lens (2) neutral density filter (3) tinted lens of complementary colour and (4) no absorptive lens. While similarity and normality of the mean stimulus-response functions between the four viewing conditions were evident, the low frequency component of the accommodation microfluctuations was significantly greater while viewing the target in the 'no lens' viewing condition. These increases in the low frequency components (LFC) of the accommodation may be a subtle indicator of visual stress in these patients. Colour specificity is not supported by this finding.

Accommodation, Ocular↗