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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↗
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↗
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↗
PubMed1999

Saccades reduce latency and increase velocity of ocular accommodation.

Horizontal vergence can be stimulated binocularly with disparity (disparity vergence) or monocularly with accommodation (accommodative vergence). The latter results from a neural cross-coupling that causes both horizontal vergence and accommodation to respond when either one is stimulated [Alpern, M., & Ellen, P. (1956). American Journal of Ophthalmology, 42, 289-303]. The velocity of disparity and accommodative vergence is enhanced when accompanied by saccades [Enright, J. T. (1984). Journal of Physiology (London) 350, 9-31; Enright, J. T. (1986). Journal of Physiology (London) 371, 69-89]. Based upon the coupling between accommodation and vergence, we predicted that accommodation should also be facilitated by saccades. An SRI Dual Purkinje Eyetracker was used to measure left and right eye position, and the accommodation of the left eye, in response to stimulation. Horizontal saccades were stimulated by targets separated by 2-6 degrees and accommodation was stimulated monocularly over a range of +/- 2 diopters (D). When saccades occurred within 0-400 ms following a monocular step stimulus to accommodation, latency of accommodation decreased and the associated accommodative-vergence response was synchronized with the saccade. Saccades also enhanced the velocity of accommodation and accommodative-vergence, and this facilitation increased with saccade amplitude. Transient vergence responses that are normally associated with saccades [Erkelens, C. J., Steinman, R. M., & Collewijn, H. (1989). Proceedings of the Royal Society of London B. Biological Sciences, 236, 441-465; Maxwell, J. S., & King, W. M. (1992). Journal of Neurophysiology, 68 (4), 1248-1260] did not affect accommodation when it was not stimulated by defocus. Because saccades and accommodation utilize separate plants and final common pathways, the synchronization of saccades and accommodation and the enhanced velocity of accommodation and accommodative-vergence must occur at more central sites. Possibilities include the superior colliculus, which represents both accommodation and saccades [Nagasaka, Y., & Ohtsuka, K., (1998). Investigative Ophthalmology AVRO supplement], vestibular nuclei which project to regions near the oculomotor nuclei [Lang, W., Buttner-Ennever, J. A., & Buttner, U. (1979). Brain Research, 177, 3-17], and interactions between omni pause neurons and near response cells of the mesencephalic reticular formation (MRF) [Mays, L. E., & Gamlin, P. D. R. (1995a). Current Opinions in Neurobiology, 5, 763-768; Mays, L. E., & Gamlin, P. D. R. (1995b). Eye movement research: Mechanisms, processes and applications. New York: Elsevier] which represent both accommodation and vergence [Judge, S. J., & Cumming, B. G. (1986). Journal of Neurophysiology, 55, 915-930; Zhang, Y., Mays, L. E., & Gamli, P. D. R. (1992). Journal of Neurophysiology, 67, 944-960].

Accommodation, Ocular↗
PubMed1999

Subcortical neural circuits for ocular accommodation and vergence in primates.

Our current knowledge of the neural bases of vergence and accommodation has increased significantly over the past few years. The behavior of medial rectus motoneurons during vergence, which has been reported by a number of investigators, is described. The behavior of Edinger-Westphal neurons during accommodation is also described, as are the characteristics of midbrain near-response neurons in the supraoculomotor area. Evidence that some of these near-response neurons provide the vergence input to medial rectus motoneurons and possibly the accommodation input to Edinger-Westphal neurons is reviewed. Anatomical studies have shown that the midbrain near-response region receives input from two deep cerebellar nuclei, the posterior interposed and the fastigial nucleus. Single-unit recording in the posterior interposed nucleus has revealed cells that increase their activity during the far-response, and the behavior of these neurons is reviewed. In addition, studies of a precerebellar nucleus, the nucleus reticularis tegmenti pontis, have revealed some cells that increase their activity during the near-response and others that do so during the far-response. The behavior of these neurons is reviewed. This review documents the great strides that are occurring in our understanding of the anatomy and physiology of the neural pathways controlling vergence and accommodation in the primate.

Accommodation, Ocular↗
PubMed1983

Ocular accommodation, personality, and autonomic balance.

The autonomic nervous system is made up of two subsystems: the parasympathetic (PNS) and the sympathetic (SNS). The balance between these systems regulates bodily functions during routine (PNS-dominant) and crisis (SNS-dominant) situations. It may also control visual accommodation: PNS activation for inward focus, and SNS activation for outward focus. Near- and far-sighted persons have been characterized as introverted and extraverted, respectively. These personality characteristics have been linked to autonomic balance: introversion and PNS-dominance, extraversion and SNS-dominance. Because autonomic balance may influence accommodation, the introversion of near-sighted and the extraversion of far-sighted persons may be related to the more general PNS-SNS balance rather than only to the difference in visual functioning. Research relevant to this hypothesis is reviewed.

Accommodation, Ocular↗
PubMed1984

Cortical indices of impaired ocular accommodation and associated convergence mechanisms.

The accommodative and fusional vergence mechanisms were examined electrophysiologically in a juvenile suffering from a severe reduction in focusing ability. We used the visually evoked response (VER) to diagnose the accommodation-vergence insufficiency. The procedure is shown to be effective for monitoring longitudinally the ability of simple orthoptic techniques to restore normal accommodative function.

Accommodation, Ocular↗
PubMed1985

Ultrasound measures of vitreous chamber depth during ocular accommodation.

The vitreous chamber decreased in depth about 0.12 mm as accommodative demand increased from 0.2 to 7 D in nine accommodative steps. The depth decreases matched movement of the posterior lens surface toward the retina. Accommodation caused no significant increases in axial length compared to cyclopleged controls. These results do not favor the active role for the vitreous in accommodation proposed by Coleman in 1970. Computer analysis of A-scan interval counts demonstrated that multiple retinal complex peaks might erroneously point to vitreous or axial lengthening. About 30% of total lens thickening during accommodation was accounted for by posterior lens surface displacement.

Accommodation, Ocular↗