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Results for “ACCOMMODATION (OCULAR)”

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At least 1,135 records · Page 63Linked to original sources
PubMed1982

Meridional variations in acuity and CSF's in monkeys (Macaca nemestrina) reared with externally applied astigmatism.

Infant macaque monkeys have been raised under such conditions that all visual experience was obtained while looking through a cylindrical lens. When tested at older ages, these monkeys were found to have meridional differences in acuity and contrast sensitivity. The direction of the differences were predictable on the basis of the defocus produced by the rearing condition. The data support the theory, previously suggested on the basis of clinical and correlational studies, that astigmatism is a major cause factor in the occurrence of meridional amblyopia.

Accommodation, Ocular↗
PubMed1982

Model of the accommodative mechanism in the human eye.

The crystalline lens of the age 11 human eye has been modelled mathematically, using simplified assumptions about lens curvature, internal organization and elasticity. From this representation, expressions for description of strain and stress during accommodation have been obtained. Solution of these equations indicates that the lens capsule acts as a force distributor, spreading tension applied by the suspensory apparatus evenly over the surface of the underlying lens material. It also becomes clear that the vitreous body provides an essential support function during the accommodative process. Finally, the relative contribution of lens-associated structures has been determined for five different values of the Poisson ratio. In order for accommodation to occur by relaxation of zonular tension, this value must be greater than 0.38; with an additional constraint of the net axial force equalling zero during a small accommodative change, the Poisson ratio equals 0.46.

Accommodation, Ocular↗
PubMed1982

Effect of focus on visual acuity of human infants.

Recent theoretical arguments (Green et al. (1980) Vision Res. 20, 827-835) predict that young human infants should have large depths of focus compared to adults. If so, optical blur should have relatively little effect on the resolving power of infant subjects. We tested this hypothesis by measuring the influence of optical blur on acuity thresholds in 6-week old infants and adults. Using the forced-choice preferential looking (FPL) technique, we obtained acuity thresholds for each subject with five different lens powers (plano, -14 D, -3 D, + 6 D and +14 D). Acuity was differentially affected by lens power in all subjects, with the best acuity produced by high power lenses, both plus and minus, was considerably less for infants than for adults. Although the effects of negative lenses are difficult to interpret without knowing the infants' exact accommodative state, the results with positive lenses support the theoretical predictions.

Accommodation, Ocular↗
PubMed1982

Pupillary micro movements apparently related to pulse frequency.

Pupillary micro changes in diameter apparently related to pulse frequency are superimposed on the larger fluctuations called the pupillary unrest. This small component of the pupillary unrest waxes and wanes with each pulsation of the blood. The effect was found to vary inversely with the average pupil diameter which can be controlled by making the eye accommodate or allowing more or less light to enter the eye.

Accommodation, Ocular↗
PubMed1983

Photorefraction of normal and astigmatic infants during viewing of patterned stimuli.

Photorefraction was used to assess the state of accommodation of 3-month old infants with and without astigmatism while they viewed stimuli in an apparatus commonly used to test infant visual acuity. The stimuli were a 1.6 c/deg vertical grating, a 1.6 c/deg horizontal grating and a cross composed of orthogonal 19 min lines, which were presented 55 cm from the infants. Most infants' accommodation was appropriate for the vertical grating. Astigmatic infants did not change their focus when the orientation of the grating was changed.

Accommodation, Ocular↗
PubMed1983

Evoked potential estimates of visual accommodation in infants.

Pattern reversal visually evoked potentials (VEPs) were recorded from infants between 2 and 5 months of age in order to measure their accommodative performance over a 9 diopter range (10-150 cm). Accommodation was stimulated by placing minus spherical lenses of increasing power in front of the infant's eyes and by changing the infant's distance from the stimulus. VEP latency measurements indicated that the infant's ability to maintain a zone of clear vision changes with age and pattern element size. For large checks (60 min arc), most 2- to 4-month old infants were able to maintain clear vision over the range of 9 diopters. When small checks (15 min arc) were used, the majority of 4- to 5-month old infants were unable by VEP measures to accommodate beyond 5-6 diopters. This decline in the infant's zone of clear vision with decreasing check size is due primarily to the relatively large depth of focus and poor visual resolution of the infant eye. In a small group of infants, accommodation was assessed both electrophysiologically by VEP recording and behaviorally using a preferential looking technique. Results indicated better accommodation by VEP estimates than by behavioral measures.

Accommodation, Ocular↗
PubMed1983

Pupillary escape intensified by large pupillary size.

Pupil responses to light are greatly influenced by initial pupil size. Small pupils, operating under photopic conditions, show tonic responses to step increases of light and high gains; thus the pupil is a good regulator of light. Large pupils, operating under mesopic or scotopic conditions show phasic responses, "pupillary escape", and smaller gains; the pupil only transiently influences retinal flux. By using accommodation level to set the size of the pupil, the mechanism of the "pupil size effect" is shown to be dependent on retinal light level only so far as retinal activity sets pupil size.

Accommodation, Ocular↗
PubMed1983

Visual accommodation in the flying fox (Pteropus giganteus).

The flying fox, Pteropus giganteus, possesses a papillated retina and was previously thought to lack a mechanism for active accommodation. An investigation of the anatomy and physiology of accommodation in this megachiropteran bat revealed that it has a well developed ciliary muscle and that it can actively accommodate through a range of at least 3.12 D. In addition, it was found that the origin of the retinoscopic reflex does not reside in the receptor layer of the retina, because the focus of the eye is invariant for small changes of angle of refraction.

Accommodation, Ocular↗
PubMed1983

Spatial frequency dependence of accommodative responses in amblyopic eyes.

Monocular, steady-state accommodative responses were measured as a function of spatial frequency of simple sinusoidal gratings presented at high contrast and target vergence levels in amblyopes, as well as in strabismics without amblyopia and in visually-normal control subjects. In general, spatial frequency dependence of the accommodative response was the rule. However, the amblyopic eyes exhibited markedly reduced accommodative responses over most of the spatial frequency range tested, and this was attributed to reduced accommodative controller gain in the sensory pathways involved in the control of accommodation in the amblyopic eye. Due to the diversity of accommodative response spatial frequency profiles found across all groups, the results suggest that reflex, voluntary, and higher-level perceptual aspects of accommodation may interplay in a complex manner in the act of accommodation on a simple sinusoidal grating.

Accommodation, Ocular↗
PubMed1983

Dark vergence and dark accommodation in human infants.

Vergence angle and accommodative state were assessed photographically in 3-, 6-, and 12-month old infants. In the dark, fluctuations of vergence and accommodation were generally uncorrelated among all groups. The vergence-accommodation functions obtained in the dark had a mean slope of 0.04. These findings provide evidence in infants for an uncoupling of vergence and accommodation in the absence of patterned retinal input.

Accommodation, Ocular↗
PubMed1983

A model for accommodation in the young human eye: the effects of lens elastic anisotropy on the mechanism.

An explanation of the mechanism of visual accommodation depends heavily on understanding the mechanical properties of the lens, as well as the way in which its shape is altered in small accommodative changes. An initial attempt to relate these properties to a mechanism has already been performed (Koretz and Handelman, 1982) for the young (age 11 yr) human lens, using certain simplifying assumptions (spherical curvature on the anterior lens surface and elastic isotropy). However, since it has been shown that the lens behaves as an anisotropic body, the previous treatment has been extended to include the variation of lens elastic properties in the polar and radial directions. With this modified representation, it is found that only one combination of elastic constants is consistent with the generally accepted qualitative theory of accommodation and with clinical data on the accommodative range of the emmetropic age 11 human eye. For this unique solution of the equations, however, the general mechanism already suggested by us, which includes support by the vitreous and alteration of the magnitude and angle of application of zonular force with accommodation, remains little changed.

Accommodation, Ocular↗
PubMed1984

Behavioral measurements of accommodative amplitude in rhesus monkeys.

The extent of voluntary accommodation was investigated behaviorally in two young rhesus monkeys and an adult human using a modified minus-lens technique. Specifically, contrast sensitivity for a high spatial frequency grating was determined as a function of spectacle lens power. Accommodative amplitudes derived from contrast sensitivity vs lens power functions indicate that young monkeys have at least 17 to 18 D of voluntary accommodation and that the accommodative amplitudes of monkeys are larger than those of equivalent-aged humans.

Accommodation, Ocular↗