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PubMed2006

The contribution of accommodation and the ocular surface to the microfluctuations of wavefront aberrations of the eye.

We have used videokeratoscopy and wavefront sensing to investigate the contribution of the ocular surface and the effect of stimulus vergence on the microfluctuations of the wavefront aberrations of the eye. The fluctuations of the wavefront aberrations were quantified by their variations around the mean and by using power spectrum analysis. Integrated power was determined in two regions: 0.1-0.7 Hz (low frequencies) and 0.8-1.8 Hz (high frequencies). Changes in the ocular surface topography were measured using high-speed videokeratoscopy and variations in the ocular wavefront aberrations were measured with a wavefront sensor. The microfluctuations of wavefront aberrations of the ocular surface were found to be considerably smaller than the microfluctuations of the wavefront aberrations of the total eye. The fluctuations in defocus while viewing a closer target at 2 or 4 D were found to be significantly greater than fluctuations in defocus when viewing a far target. This increase in defocus fluctuations (p < or = 0.001) occurred in both the low- and high-frequency regions of the power spectra.

Accommodation, Ocular↗
PubMed1997

Changes in equivalent and gradient refractive index of the crystalline lens with accommodation.

The ocular dimensions and refraction of the eye were measured for accommodation stimulus levels of 0.0, 1.5, 3.5, 5.5, and 8.0 D for 11 subjects aged 18 to 28 years, mean 21.2 +/- 2.62 years using keratometry, autorefraction, A-Scan ultrasonography, and video phakometry techniques. The subjects had refractive errors in the range + 0.50 to -4.25 D, mean -1.88 +/- 1.64 D. With the maximum level of accommodation the anterior chamber depth decreased by 0.23 +/- 0.09 mm, the lens thickness increased by 0.28 +/- 0.09 mm, and no significant differences were recorded in axial length or vitreous chamber depth. The radius of curvature of the anterior surface of the crystalline lens decreased from 11.54 +/- 1.27 to 6.59 +/- 0.97 mm and the posterior surface from -6.67 +/- 0.97 to -5.30 +/- 0.4 mm. We determined the equivalent refractive index to be 1.4277 +/- 0.0011, with no significant differences at different levels of accommodation. When the crystalline lens was modeled as a gradient refractive index (GRIN) structure with elliptical iso-indicial lines, the mean surface refractive index of the lens was 1.3859 +/- 0.0009 for an assumed central refractive index of 1.406. The power of the anterior surface of the lens increased from 4.38 +/- 0.49 to 7.59 +/- 0.34 D, the posterior surface increased from 7.67 +/- 1.28 to 9.32 +/- 0.64 D, and the GRIN power increased from 9.70 +/- 1.31 to 13.74 +/- 0.77 D for the maximum accommodative stimulus of 8.00 D. On the basis of the model used, a substantial part of the increase in power of the crystalline lens with accommodation resulted from the change in refractive index distribution within the lens.

Accommodation, Ocular↗
PubMed1990

[Aging changes in ocular tissues and their influences on accommodative functions].

It is well known that the amplitude of accommodation deteriorates with aging. However, it is not clear what kinds of morphological changes of age-dependent deterioration of amplitude of accommodation accompany in the crystalline lens and the ciliary muscle, which are the most important involved tissues in the accommodation mechanism. In the present study morphological changes of these tissues were investigated in relation to the deterioration of the accommodative amplitude. Age-dependent deterioration of the subjective amplitude of accommodation obtained by an accommodometer (VDT Accommodometer NP-200, Toyo-Medical, Nagoya) and the objective amplitude by an autorefractometer with a built-in infrared optometer (NIDEK AA 2000 Accommodometer, NIDEK, Gamagori, Japan) were compared. The objective amplitude was 2-3 dptr less than the subjective amplitude for all ages, but change of both curves with aging were parallel. The crystalline lens measured by an ultrasonic A-scan instrument, Alpha 20/20 (Storz, U.S.A.), showed continuous increase in thickness with aging and by accommodative contraction. The crystalline lens in the fifties was as same in thickness as the accommodatively contracted lens in the twenties but the increasing ratio of lens thickness in accommodative contraction decreased with aging and it reached a minimum in the sixties. Scanning electron microscopy (JSM F-15, JOEL Co, Tokyo) revealed three types of characteristic substructures on the cell surface of the crystalline lens fiber, ie, interlocking protrusion, ball-and-socket junction and microplica or the tongue-and-groove junction. In the lenticular nucleus the microplica was prominent on the cell surface throughout all ages but the ball-and-socket junction was sparse in number. On the other hand, the ball-and-socket junction was well developed on the cell surface in the superficial cortex of the lens. The microplica on the cell surface in the superficial cortex was recognizable but indistinct. The microplica and ball-and-socket junction seemed to shift relative to each other in the deeper cortex of the lens. The interlocking protrusion was recognized throughout the whole lens, but was prominent in the lenticular nucleus. The crystalline lens fibers of advanced age were irregural in shape and the spherical substructure, large and small in size, and frequently formed suggesting the cataractous change. Delicate shift of the ciliary processes after conjunctival application of 4% pilocarpine was confirmed by an angioendoscope (Angio Fiber Imaging System FCA-8000. Fukuda Electronics K.K., Tokyo) which was inserted into the vitreous space of cynomorgus monkey eyes.(ABSTRACT TRUNCATED AT 400 WORDS)

Accommodation, Ocular↗
PubMed1997

Changes in ocular dimensions and refraction with accommodation.

The purpose of this study was to measure the changes in ocular dimensions with accommodation, with particular reference to the radius of curvature of the posterior surface of the crystalline lens. The increase in power of the eye with accommodation is considered to arise primarily from a decrease in the radius of curvature of the anterior surface of the lens, with the role of the posterior surface somewhat unclear. We measured the axial dimensions (A-Scan ultrasonography), corneal radius of curvature (keratometry), refractive error (auto-refractor) and radii of curvature of the lenticular surfaces (video phakometry) for 11 subjects, mean age 21.2 +/- 2.6 years, for five levels of ocular accommodation up to 8.00 D. At maximum accommodation the mean changes were a decrease in anterior chamber depth of 0.24 mm, an increase in lens thickness of 0.28 mm, a decrease in radius of curvature of the anterior surface of the lens of 4.95 mm and 1.34 mm for the posterior surface. The corresponding increase in power of the lenticular surfaces for an equivalent refractive index of 1.422 for the lens was 5.53 D and 3.10 D for the anterior and posterior surfaces respectively. No significant changes were recorded in axial length or vitreous chamber depth. We conclude that when crystalline lens power is calculated on the basis of an equivalent refractive index, changes in the posterior surface of the lens contribute around one third of the increase in lens power associated with 8.00 D of ocular accommodation.

Accommodation, Ocular↗
PubMed1991

Changes in ocular astigmatism over the whole range of accommodation.

The refraction of 20 subjects was measured over the whole range of accommodation using an objective automated refractometer in order to investigate the astigmatic change whose origin is presumed to be at the lens. For this purpose, the corneal vertex was used as the plane of reference to avoid changes in the optical effectively of a cylindrical lens at near. The change in the astigmatism with accommodation was observed in each subject. Subjects were then divided into three groups: (1) seven subjects showed an increase in astigmatism with accommodation; (2) eight subjects showed constant astigmatism up to the accommodative nearpoint; and (3) five subjects showed other types of change, such as reduced astigmatism in the intermediate accommodative state. These variations suggest that accommodative astigmatism is affected by many elements such as lens, zonule, and muscle. Two of 20 subjects could see the vertical line of the visual target composed of many directional lines clearer than the horizontal line at distance. Conversely, they could see the horizontal line clearer than the vertical line at near. We also studied the correspondence between the dark focus of accommodation and the dioptric distance where astigmatism is at a minimum, as suggested previously. The results showed that no such direct relation existed.

Accommodation, Ocular↗
PubMed1987

Vision therapy for the child with cerebral palsy.

There has been an abundance of data collected on the high prevalence of visual anomalies in cerebral palsy. Relatively little has been written about remediation. This article discusses the problems encountered in the CP population, and therapy programs which can be utilized to deal with them. Heavy emphasis is placed on ocular motility and accommodation because deficits in these two visual skills are encountered very frequently, are most amenable to therapy, and show the greatest improvement as a result of vision therapy.

Accommodation, Ocular↗