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H C Howland

Publications and source records attributed to H C Howland.

At least 19 recordsLinked to original sources
PubMed1996

A history of studies of visual accommodation in birds.

Since 1813, when Crampton first described the ciliary muscle of the avian eye, there has been little agreement on how birds are able to change the focus of their eyes. Numerous later studies on the eyes of a variety of bird species contradicted earlier findings or proposed new accommodative mechanisms. The resulting confusion persists today, and a number of significant works on the avian eye perpetuate many of the myths developed during the 1800s. There is little consensus on avian accommodation; the early literature contains many accurate descriptions of the mechanisms, along with elegant experimental evidence to support them. Much of the early literature, however, is in German and has remained obscure. Further, among the mechanistic descriptions of avian accommodation are many that are incorrect. The current confusion can be attributed in part to the fact that some birds have both corneal and lenticular accommodation. It is unclear to what extent different bird species employ both mechanisms, or depend on one mechanism or the other. These facts, together with the diversity of bird species, their range of visual requirements, and the numerous anatomical differences in their eyes, make it impossible to describe a single avian mechanism of accommodation. Our own experience in studying accommodation in the chick eye has led us to review the historical literature in an attempt to provide a new foundation for future studies on visual accommodation in birds. While in relation to the anatomical arrangements [of the bird eye], these have led our knowledge pretty much to a conclusion.... There is among all these works no real difference of opinion. However, in terms of the accommodative mechanism of the bird eye, at the present time there are many different views of what is going on. Here the circumstances are very similar to what happened a few decades ago in the study of accommodation generally, but particularly with humans, when Helmholtz made the remark. "There is no other portion of physiological optics in which one finds so many differing and contradictory ideas us in the accommodation of the eye, where we have only recently actually made observations on what previously was left to the play of hypotheses".

Accommodation, Ocular↗
PubMed1996

Corneal aberrations increase with the magnitude of radial keratotomy refractive correction.

BACKGROUND: Refractive surgery induces optically abrupt changes in shape in the midperiphery of the cornea. The abruptness of this change is in part dependent on the magnitude of the surgically induced refractive change. Therefore, the optical aberrations of the cornea, as quantified by wavefront variance (WFV), may be expected to increase as the surgically induced change in the refraction increases. PURPOSE: It is the purpose of this study to test the hypothesis that as the surgery-induced change in refraction increases, so does the WFV of the cornea. METHODS: Fourteen radial keratotomy (RK) patients and seven normal patients served as subjects. Measurements were made before and 2 years after RK surgery. To quantify the WFV of the cornea, we used corneal topography measurements to calculate the surgically induced change in corneal WFV with respect to two different reference surfaces, a sphere and the presurgical cornea. To quantify the surgically induced change in the equivalent spherical correction (ESC), cycloplegic refractions were performed. The measurements were summarized by regressing the surgically induced change in the WFV against the surgically induced change in the ESC. RESULTS: For large pupils (7 mm diameter), the correlation between the change in the WFV referenced to a sphere and the change in the ESC was significant (p < 0.0001, r2 = 0.745) and dominated by fourth order aberrations. Similar results were found for the surgical lens. For small pupils (3 mm diameter), the effects were markedly reduced. CONCLUSIONS: (1) As the magnitude of the surgically induced refractive change increases so does the WFV of the cornea, particularly for large pupils. (2) The increase in corneal WFV for large pupils is dominated by fourth order aberrations. (3) The increase in corneal WFV is consistent with reported decreases in visual function (contrast sensitivity and low contrast visual acuity), particularly for large pupil diameters in combination with large surgically induced changes in refractive error.

Cornea↗
PubMed1996

Accuracy and precision of the Tomey ViVA infrared photorefractor.

PURPOSE: The Fortune Optical (Tomey ViVA) VRB-100 video refractor was tested to determine its accuracy and precision in measuring manifest refractions of human eyes with and without cycloplegia. The specific issues addressed included its accuracy in measuring spherical and cylindrical refractive errors and its precision in refracting near emmetropia. METHODS: To determine its ability to measure moderate to high (> 4.00 D) myopia, we compared the VIVA's refractions to those taken by a Canon Autorefractor R1 and a retinoscopist. A spherical lens series from -7.00 to + 7.00 D at 1.00 D intervals, or -5.00 to + 5.00 at 0.50 D intervals, was placed over a subject's eye, which was then covered by an infrared (IR) filter, refracted, and analyzed to determine the VIVA's ability to measure spheres. Subjects with refractive errors of -2.00 to + 2.00 DS (diopters sphere) and 0 to 1.00 DC (diopters cylinder) were refracted 7 to 15 times during 1 sitting to determine the VIVA's precision. The instrument's accuracy in measuring cylinders was tested by placing + 3.00 to -3.00 D cylinders (at 0.50 D intervals) over the eyes of subjects at 0 degree, 45 degrees, 60 degrees, 75 degrees, 90 degrees, 105 degrees, 120 degrees, and 135 degrees. RESULTS: The VIVA measured spheres of +/- 3.00 D with a root mean squared (rms) error of 0.5 +/- 0.1 D. Beyond this power, its accuracy progressively worsened. In some subjects, irregular intensity profiles compromised the VIVA's accuracy even with low spherical refractive errors. The VIVA was very precise in measuring spheres from + 2.00 to -2.00 D and cylinders from 0 to 1.00 D. Although the ViVA adequately measured all cylinder powers at 0 degree and 90 degrees, the accuracy of cylindrical power measurement decreased with obliquity; only cylinders < or = 1.00 D magnitude were accurately measured at 45 degrees and 135 degrees. CONCLUSIONS: We conclude that, although the ViVA offers many attractive features for vision screening, it is seriously limited by its inability to property detect and measure oblique astigmatic errors.

Humans↗
PubMed1996

The crossed-cylinder aberroscope: an alternative method of calculation of the aberrations.

The distorted retinal grid image in the cross-cylinder aberroscope is conventionally analysed using a method based upon orthogonal polynomials. This method restricts the amount of data that can be extracted from the grid image and requires the real grid that is placed between the cross-cylinders to be pre-distorted, with the amount of pre-distortion depending upon the vertex distance. We present an alternative method based upon the minimization of least squares that does not have these restrictions and show that it gives essentially the same results as the original orthogonal polynomial method. Furthermore, the minimization of least squares method also provides a measure of 'goodness' of fit (e.g. the minimum of the sum of the squares of the deviations.

Humans↗
PubMed1995

The mechanism of lenticular accommodation in chicks.

In the chick eye, accommodation for near objects is brought about by changes in the focal length of the lens and by changes in the corneal radius of curvature. Several different mechanisms of lenticular accommodation have been proposed for the avian eye. These include a role for the ciliary muscle, a role for the iris muscle, and a role for changes in intraocular pressure. We have studied accommodation in the chick eye using electrical stimulation of the Edinger-Westphal nucleus, electric-field stimulation of enucleated eyes, in vitro measurement of changes in back vertex distance of the lens, and histology. We present evidence showing that, in the chick eye, lenticular accommodation is induced primarily by a contraction of the muscle fibers at the peripheral edge of the iris. During accommodation, the peripheral muscle fibers of the iris contract to apply a force through the ciliary processes to the anterior equatorial surface of the lens. This increases the focal power of the lens. When accommodation is relaxed, the lens is returned to its unaccommodated state by the elasticity of the pectinate ligament and the ciliary body. Contractions of the posterior ciliary muscle and changes in intraocular pressure, forces that have previously been proposed to play major roles in lenticular accommodation, are shown to be of secondary importance only.

Accommodation, Ocular↗
PubMed1995

Correlations between familial refractive error and children's non-cycloplegic refractions.

We examined the spherical equivalent refractions of 237 subjects who had been seen four or more times in a longitudinal study of refractive development in normal children. We employed both photorefraction and autorefraction using a Canon R1 autorefractor. We performed an analysis of variance (F-test for Lack of Fit) to determine the significance of a linear regression in fitting these refractions against three different measures of familial refractive status. One measure included only the number of myopic parents, a second took into account the number of hyperopic parents, and a third included the refractive states of extended genetic relatives. We found no significant correlation between photorefractive data and familial refractions; however, we did find significant, albeit weak, correlations between all measures of familial refractive status and infrared autorefractions of their children. A linear relationship between autorefractive data and overall familial refractive state was found to be most significant. Linear regression of children's autorefractions against a measure of parental myopia and hyperopia was less significant, while a linear model fitting only a measure of parental myopia was least significant, in addition to showing other non-linear trends. We attribute the failure to find a significant correlation between parent and offspring refractions using children's photorefractive data to vignetting by the apparatus and to the short distance of the fixation target. We believe the significant correlation patterns found with the autorefractor reflect the inheritance patterns of parental and familial refractive states.

Adolescent↗
PubMed1995

Differences in eye growth and the response to visual deprivation in different strains of chicken.

Several laboratories studying visual deprivation myopia in the domestic chick report varying degrees of axial elongation and myopia induced by similar visual deprivation techniques. In this study we tested the hypothesis that in different strains of chick the eyes respond differently to visual deprivation. We compared under identical conditions two strains of White Leghorn chick commonly used in ocular development research--the Cornell-K strain (K) and Washington H & N Strain (H/N). The normal development of the eye was found to vary significantly between these strains of White Leghorn chicks. The K strain normally develops flatter corneas, thicker lenses, and larger eyes than the H/N strain. The response to visual deprivation also varies significantly between strains. For example, we find that 2 weeks of visual deprivation in the K strain results in less elongation of the vitreous chamber and flattening of the cornea yielding lower levels of induced myopia compared to the H/N strain. Our results show that while visual experience clearly affects normal ocular development in both strains of chick, the nature of the effect depends upon not only the type and duration of the experience but the genetics of the subject population as well.

Age Factors↗
PubMed1995

Constant light produces severe corneal flattening and hyperopia in chickens.

In this study we report on the effects of constant light (CL) on the refractive development and ocular morphology of White Leghorn chicks (Cornell K-strain). Refractive state and corneal curvature were measured by IR photoretinoscopy and IR keratometry respectively. The axial lengths of the ocular components were measured by A-scan ultrasonography. We find that constant light produces significant hyperopia compared to controls in as few as 10 days (7.4 vs 4.0 D). This is apparently the result of flatter than normal corneal curvature (radius of curvature: 3.22 vs 3.08 mm) as vitreous chamber depth is significantly deeper in CL eyes than controls at that age (5.6 vs 5.1 mm). In contrast to other reports, if CL rearing is continued for longer periods the hyperopia progresses, even though vitreous chamber depth continues to increase. After 11 weeks of CL severe hyperopia was observed (18.2 vs 2.8 D). Long term CL is also found to produce shallow anterior chambers, corneal thickening, lenticular thinning and cataracts, and damage to the retina, pigment epithelium, and choroid.

Animals↗
PubMed1995

Raptors lack lower-field myopia.

The presence of lower-field myopia (described in chickens, pigeons, quail and amphibians) allows these animals to keep the ground in focus while performing other visual tasks. A relationship has also been reported between the eye height and the degree of myopia observed. All of the animals reported in the literature to date are ground-foraging species. Using infrared neutralizing video retinoscopy and static photoretinoscopy we found a lower-field myopia to be absent in the barn owl (Tyto alba), Swainson's hawk (Buteo swainsonii), Cooper's hawk (Accipiter cooperi) and American kestrel (Falco sparverius). These findings suggest that the presence or absence of a lower-field myopia is a function of the visual ecology of the animal.

Animals↗
PubMed1995

In vitro changes in back vertex distance of chick and pigeon lenses: species differences and the effects of aging.

We have used a scanning laser technique to measure in vitro changes in back vertex distance of chick and pigeon lenses. Enucleated eyes were dissected, leaving the lens naturally suspended by the ciliary body and intraocular muscles. Ray tracing techniques were used to measure the resting back vertex distance of the lenses by passing a laser beam through the lens and scanning it across the pupillary aperture. The pupil diameter was measured videographically. The measurements were repeated while the intraocular muscles were pharmacologically stimulated with increasing concentrations of either nicotine or carbachol. Drug stimulation caused changes in pupil diameter and changes in the back vertex distance of the lenses. These experiments were conducted on the eyes of young chicks, young pigeons, and on the eyes of three pigeons older than 10 yr. The lenses from the eyes of the old pigeons had the greatest resting back vertex distance, followed by those of the young pigeons and finally those of the young chicks. Lenses from the eyes of young chicks and young pigeons underwent similar drug-induced changes in back vertex distance, but the lenses from old pigeon eyes showed an almost complete absence of such changes. Further, we demonstrated that, just as in the chick eye, lenticular changes in pigeon eyes are due to a contraction of the iris muscle. This is evident because after the iris has been removed the lens undergoes no change in back vertex distance during stimulation. We conclude with a discussion of the lenticular accommodative ability of the pigeon eye with reference to the recently reported accommodative mechanism of the chick eye and a comparison of chick and pigeon iris morphology.

Accommodation, Ocular↗
PubMed1995

Myopia.

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Accommodation, Ocular↗
PubMed1995

How accurately can videokeratographic systems measure surface elevation?

INTRODUCTION: Surface topography, as opposed to dioptric topography, defines the corneal surface in simple terms without assumptions. Accordingly, it is important to know how well surface topography can be measured with current videokeratometric machines. PURPOSE: The purpose of this paper is to quantify the accuracy with which the TMS-1 Corneal Modeling System can measure the surface topography of calibrated spherical, elliptical, and bicurve surfaces. METHODS: The Computed Anatomy TMS-1 videokeratometer was used to measure three spherical, three elliptical, and two bicurve surfaces with known characteristics. Surface characteristics were either back-calculated from the dioptric files or directly obtained from the TMS-1 elevation file for each of 6400 points (256 points in each of 25 rings). The accuracy with which each method determined the true surface was quantified by calculating the root mean squared error (RMSE) of the 6400 measured surface elevations from the known surface elevation at each sampling point. RESULTS: (1) For spherical and elliptical surfaces, back-calculation of surface elevation from the dioptric file can be made with RMSE of 5 mu or less. (2) For spheres but not elliptical surfaces the TMS-1 elevation file defines the surface with RMSE 5 mu or less. (3) The surface area measured by placido-based videokeratometers varies with surface curvature. (4) RMSE in measured surface elevation increase as the distance from the videokeratometric axis increases. (5) For bicurves, the dioptric maps are smoothed by the TMS-1 over abrupt transitions and for large transitions never recover. Additionally, our back-calculation methods further smooth abrupt transitions, making the RMSE of the bicurve surface that is back-calculated from the dioptric file larger than the RMSE of the surface generated from the TMS-1 elevation file. CONCLUSIONS: Surface elevations can be back-calculated from dioptric files with RMSE of 5 microns or less for spheres and elliptical surfaces as long as there are no areas of abrupt transition. If areas of abrupt transition exist, the TMS-1 elevation file provides more accurate surface profile data.

Cornea↗
PubMed1995

The anatomy of the ciliary region of the chicken eye.

PURPOSE: To describe the detailed anatomy of the ciliary region of the chicken eye. METHODS: Fifty-two eyes from White Leghorn chickens were examined in the course of this study. Descriptions are based on specimens examined using microdissection and bright field microscopy of sections embedded in paraffin or epon. Microdissection was assisted through the use of an iodine-based stain. RESULTS: The ciliary region of the chicken eye is asymmetric through the horizontal plane, with the distance from the limbus to the equator of the eye being greatest temporally. This asymmetry is reflected in the relative development of the ciliary musculature. The nasal ciliary muscle fibers are the shortest of any of the quadrants, and the nasal quadrant lacks a well-developed scleral venous sinus. The ciliary musculature is approximately 2.5 mm in extent (temporally) and is composed of two regional groups (anterior and posterior) within which five distinct arrangements of muscle fibers can be recognized. The majority of fibers insert on fibrous elements associated with the inner or outer walls of the scleral venous sinus, which, in turn, are continuous with the inner stromal elements of the cornea. CONCLUSIONS: The ciliary musculature of the chicken eye is composed of two major muscle groups within which five arrangements of muscle fibers have been identified. The anatomy of the ciliary muscle is consistent with the recently proposed functions of altering the corneal curvature for corneal accommodation and moving the ciliary body anteriorly as a part of the lenticular accommodative mechanism. The ciliary muscle also may serve in the regulation of aqueous dynamics within the eye.

Accommodation, Ocular↗
PubMed1994

The mechanism of corneal accommodation in chicks.

Corneal accommodation can account for up to 9 D of accommodation in a freely behaving chick. We have explored the possibility that changes in corneal curvature are due to changes in intraocular pressure (IOP) during accommodation. In an in vitro preparation we demonstrate that increasing the pressure will tend to flatten the cornea. We have used electrical stimulation of the Edinger-Westphal (EW) nucleus to further test the pressure hypothesis in vivo by recording IOP changes in the eye during EW-stimulated accommodation and by artificially modulating the IOP to assess the effects on corneal curvature. During EW stimulation there is an increase in IOP on the order of 1-3 mmHg which tends to flatten the curvature of the cornea, thus eliminating changes in IOP as a possible mechanism of corneal accommodation. Slit-lamp observations of accommodative changes at the corneo-scleral margin and electrical stimulation of dissected eyes in vitro indicate that corneal accommodation is mediated by a contraction of the ciliary muscles, which exerts a pull on the inner lamella of the cornea, flattening the peripheral cornea and increasing the curvature of the central cornea. Histological examination of the ciliary region of the eye confirms the appropriate positioning of the ciliary muscles. We conclude that corneal accommodation in the chick eye is accomplished by a ciliary muscle-mediated mechanism.

Accommodation, Ocular↗
PubMed1993

Magnification and visual acuity in refractive surgery.

In comparisons of retinal image size within the same eye before and after refractive surgery, a change in the plane of correction from the spectacle to the cornea induces a change in retinal magnification. Comparing retinal image size between eyes of different individuals, a change in the plane of correction as well as the type of ametropia (axial or refractive) interacts to change the retinal magnification. Consequently, comparing acuity before and after refractive surgery without considering the effects of retinal magnification can be misleading. Magnification effects can be large, accounting for a visual acuity increase of 1 line or more. Here we model the magnification induced by refractive surgery in various reference eyes and discuss implications in the context of current clinical trials.

Humans↗
PubMed1993

Chromatic aberration and accommodation: their role in emmetropization in the chick.

The roles of chromatic aberration and accommodation as cues to emmetropization in the chick were investigated. Myopia was induced monocularly by lid suture for a period of 1-2 weeks from hatching, after which eyes were reopened and the recovery process followed. Monochromatic light (ML) rearing conditions and ciliary nerve section surgery were used to eliminate chromatic aberration and accommodative activity respectively. Control animals were reared in white light (WL). When accommodation was left intact, chickens reared under monochromatic light were able to recover normally. However, ciliary nerve section produced hyperopia, deepening of the anterior chamber and a tendency towards axial lens thinning, irrespective of the light conditions used. Hyperopic refractive errors peaked at 4 weeks (mean refractive errors: +5.7 D, +4.21 D for ML, WL groups respectively, 4 weeks), with the ML group still exhibiting significant hyperopia at 7 weeks. Ciliary nerve section did not prevent the myopic response to lid suture (mean refractive errors: -22.65 D; -25 D for ML, WL groups respectively, 1 week) nor the elimination of myopia when eyes were reopened. These data indicate that neither accommodation nor chromatic aberration are fundamental to the gross operation of the emmetropization process although they may be essential for the fine tuning of refraction.

Accommodation, Ocular↗
PubMed1993

Visual optics and retinal cone topography in the common marmoset (Callithrix jacchus).

The common marmoset (Callithrix jacchus) is a small, diurnal, New World monkey amenable to vision research. In this paper we describe the visual optics and cone photoreceptor topography of the normal adult marmoset. Paraxial optical ray-tracing shows that the marmoset eye is well represented as a scaled-down version of the human eye. The density of foveal and perifoveal cone photoreceptors in the marmoset is as high, and in peripheral retina higher, than those reported in humans and macaques. The foveal acuity predicted by the Nyquist limits set by the cone mosaic (30 c/deg) is in agreement with behavioral measures of visual acuity. Foveal depth of focus is remarkably small (< 0.2 D) for an eye of this size (axial length about 11 mm). Estimates of the amplitude of accommodation using infrared photorefraction indicate that the marmoset is capable of more than 20 D of accommodation.

Animals↗