Biomedical subjects

H C Howland

Publications and source records attributed to H C Howland.

At least 73 records · Page 4Linked to original sources

PubMed1987

Vision of the Humboldt penguin (Spheniscus humboldti) in air and water.

Refractive states measured by retinoscopy and photorefraction indicate that the eyes of the Humboldt penguin, Spheniscus humboldti, are approximately emmetropic in air and water. Extensive myopia in air, as predicted by earlier authors and by a recent anatomical study, is non-existent. Photorefractive measurements of the refractive state, in water, of the Humboldt penguin indicate that it can accommodate sufficiently to make up the loss of the refractive power of the cornea. The cornea of the Humboldt penguin is flattened relative to the overall size of the eye. In all these respects (corneal flattening, and accommodation in air and water) the eyes of Humboldt penguins are like those of gentoo, (Pygoscelis papua), rockhopper (Eudyptes crestatus), Magellanic (Spheniscus magellanicus), and king penguins (Aptenodytes patagonica).

Animals↗

PubMed1987

Corneal accommodation in chick and pigeon.

We have investigated the role of changes in corneal radius of curvature in effecting accommodation in the bird's eye. It was found that in natural accommodation (measured by IR photoretinoscopy) changes of corneal radius of curvature (measured by IR photokeratometry) play an important role in both the chick and the pigeon. In the adult pigeon the cornea is indeed responsible for the largest part of natural accommodation (up to approx. 9 D). In this animal the corneal diameter (as seen from the optical axis of the eye) decreases in accommodation which can be taken to explain the change of corneal radius of curvature. In the chicken, corneal accommodation is combined with other mechanisms (total accommodative range 15-17 D, corneal accommodation about 8 D). The chicken's cornea is aspherical within the pupil area leading to large measurement variation in photokeratometry if the Purkinje images are not symmetrical to the pupillary axis.

Adaptation, Physiological↗

PubMed1987

The optics of comparative ophthalmoscopy.

One factor peculiar to the practice of comparative ophthalmoscopy is the very large variation in ocular size of the animals examined, a factor which is ignored in current textbook treatments of the subject. We have computed values of lateral magnification, axial magnification, angular field of view and linear field of view for 19 species of terrestrial vertebrates. The dimensional value of a 1 diopter change in direct ophthalmoscopic focus was also determined. The anterior focal length of the eye in air and the vitreal refractive index were the intrinsic optical parameters of the animal's eye necessary for these calculations. Where these values were not available from the literature, the vitreal refractive index was assumed to be 1.336 and the anterior focal length was estimated as two-thirds of the axial length using a regression equation we derived from data in the literature. The angular field of view in ophthalmoscopy was shown to be invariant in the emmetropic eye and equal to the angular subtense of the ophthalmoscopic beam. The lateral field of view and retinal depth corresponding to a 1 diopter change in direct ophthalmoscopic focus varied directly with the anterior focal length of the eye. The remaining parameters of lateral and axial magnification varied inversely with the anterior focal length of the animal's eyes. These findings provide a basis for evaluating the relative size and significance of ophthalmoscopically viewed features in terrestrial vertebrate eyes.

Animals↗

PubMed1987

Accommodation in infants as measured by photorefraction.

We developed a video camera photorefractive procedure that allows on-line monitoring of accommodation from a distance of 1-2m, and have used it to study accommodation in twenty-nine 2- to 10-month-old human infants. We presented square wave gratings and random checkerboard targets at 25, 33, 50, and 100 cm from the infant. All infants altered their accommodation in the appropriate direction for changing target distances. We also presented infants with a slowly rotating circular striped target. We did not observe any systematic change in accommodation of the astigmatic infants to the rotating striped target, nor any significant change in their accommodation to stationary targets as a function of target type. Measured speeds of accommodation in infants were at least 4.6 D/sec, comparable to that found for adults.

Accommodation, Ocular↗

PubMed1987

Simple pointspread retinoscope suitable for vision screening.

A simple pointspread retinoscope and method of retinoscopy are described which use optical principles similar to those of photoretinoscopy or static photographic skiascopy. In this instrument the degree of defocus of an eye is estimated from the disappearance of the bright fundal reflex or "crescent" in the eye of the subject when the distance between the light source and the eye of the observer is increased systematically. The instrument is particularly suitable for large scale screening efforts because it is easy to use and can be constructed from inexpensive materials. The pointspread retinoscope was calibrated against an artificial eye of known defocus. Additionally, results from pointspread retinoscopy were compared against meridional photorefractive data from a sample of infants and young children, and a significant correlation was found to exist between measurements made by the two methods.

Child↗

PubMed1987

A photorefractive characterization of focusing ability of infants and young children.

The authors have used isotropic and orthogonal photorefraction to examine the focusing ability of 360 normal uncyclopleged infants and young children (ages, 2 mo to 5 yr). Photorefractive reflexes were recorded on 400 ASA color transparency film at 1.5 m, and the subject viewed the photographer's face located directly above the camera. The photographs of reflexes were projected at X 20 on a computer-driven digitizing tablet and converted to the ophthalmic prescriptions that would correct the subject for the camera distance. Using these prescriptions the authors established 95% confidence limits as a function of age for each of four different tests: net sphere of most defocused eye; cylinder of most astigmatic eye; difference in spheres between two eyes; and difference in cylinder between two eyes. Spheric defocus was found to decrease uniformly as a function of age, as did the cylinder of the most astigmatic eye. However, the symmetry of focus of the eyes, as judged by spheric and cylindric differences, was established within the first year of life. By relaxing the limits of each individual test, a test battery was constructed that was failed by 5% of the subject population, with the individual tests contributing in approximately equal proportions to the failure rate. The use of this test battery for detecting focusing conditions in infancy and early childhood that can lead to amblyopia is discussed.

Child, Preschool↗

PubMed1986

On the gekko pupil and Scheiner's disc.

The four pinhole apertures of the constricted gekko pupil are an adaptation for decreasing the depth of field of the eye, while decreasing the total light flux to the retina. This may be useful for distance estimation at high light levels.

Animals↗

PubMed1986

Natural accommodation in the growing chicken.

A new technique, Infrared Photoretinoscopy, has been employed for recording natural accommodation in the chicken. The illumination of the pupil by the fundus reflection of infrared light provided by high output light emitting diodes (I.R. LED's) was monitored on a video screen. The defocus of the eye could be calculated by evaluating the fraction of the pupil which was illuminated. It was found that: in the chick the full range of accommodation (about 17D) is present in the first day after hatching, accommodation acts completely independently in both eyes, the "near pupillary response" is weaker in younger chicks, the pupil constriction in response to light starts at higher luminance in the younger chicks the developmental decrease of the f/number is not sufficient to explain the change of the pupil reaction to light. Problems resulting from the use of drugs in order to measure the refractive state using normal retinoscopy are discussed.

Accommodation, Ocular↗

PubMed1986

Studies of otoconial development in a "giant-crystal" strain of chicks using scanning electron microscopy, polarized light microscopy, and X-ray crystallography.

Otolith formation was studied in a mutant strain of low-fertility Delaware chicks which exhibit an otolithic defect. In all chicks of this strain, otoliths were present as a fused crystal mass which contained abnormally large (giant) otoconia. Studies of the formation of such otoliths during embryonic development revealed that from the very earliest stages the otoconia were much larger than normal, and in the saccular and utricular otoliths formed a fused mass. These results are interpreted as supporting a hypothesis of the de novo formation of giant otoconia in this giant-crystal strain as opposed to the recrystallization hypothesis proposed for other, dissimilar mutant mammals and birds which also produce giant otoconia.

Animals↗

PubMed1985

Studies of otoconia in the developing chick by polarized light microscopy.

Using polarized light microscopy we were able to observe the mineralization patterns of embryonic and neonatal chick otoconia. We compared preparations of freshly dissected material spread under mineral oil to material that had been treated with various fixatives and dehydration agents. We found that the standard fixation agent, glutaraldehyde, and some immersion oils etched embryonic chick otoconia but that fixation with 70% acetone or 70% alcohol followed by dehydration to 100% acetone or 100% alcohol left the otoconia intact. The size and shape of freshly dissected chick otoconia observed with polarized light microscopy were similar to those of acetone-fixed, critical-point-dried material examined by SEM. Embryonic forms of otoconia were found to have a fluted pattern that was different in morphology from otoconia found in hatched chicks and adults. Embryonic chick otoconia did not exhibit the multifaceted surface morphology seen in embryonic rat otoconia. Comparisons of the same fields of otoconia under phase contrast microscopy and polarized light microscopy indicated that the freshly dissected otoconia of embryos exhibit little or no unmineralized (non-birefringent) material but that glutaraldehyde-fixed otoconia exhibited unmineralized areas where etching had occurred. Size frequency distributions of freshly dissected embryonic and mature otoconia in five ages of embryos and hatched chicks were consistent with a hypothesized developmental sequence of otoconia.

Animals↗

PubMed1985

Photokeratometric and photorefractive measurements of astigmatism in infants and young children.

We have measured corneal and total astigmatism on 161 eyes of 90 normal infants and young children aged 2 months to 5 years using the techniques of photorefraction and photokeratometry. Both the mean corneal and mean total astigmatism were found to decrease with increasing age. We found large and significant correlations between corneal and total astigmatism in the 0-1, 1-2 and 2-3 year-old infants and children. Previous studies have shown that photorefractive measurements of astigmatism compare well with retinoscopic measurements of the same infants. As part of this study we measured the corneal astigmatism of 19 adults (38 eyes) by photokeratometry and with a clinical keratometer and found that the two measures were well correlated (r = 0.820). We conclude that the major component of total astigmatism seen in uncyclopleged infants of ages 0-3 years is corneal in nature.

Adult↗

PubMed1985

Optics of photoretinoscopy: results from ray tracing.

Recent photographic studies have generated renewed interest in the optics of the retinoscopic (skiascopic) image. In photoretinoscopy the subject's eye is illuminated by a point source of light and the fundal image of this light at the plane of the subject's pupil is observed or photographed from a position near the source. The image so obtained is a function of the dioptric defocus of the eye relative to the camera, D, the distance of the source and camera from the eye, A, the radius of the subject's pupil, R, and the distance of the point source from the edge of the camera aperture, E. The fraction of the subject's pupil which is not filled with light is termed the "Dark Fraction" (DF) and may be computed from a simple ray tracing model of the eye. It is given in terms of the above parameters by the equation DF = E/(2ARD). The validity of this equation was tested with a photoretinoscope and artificial eye. The photoretinoscope, which is a novel modification of an isotropic photorefractor, is also described.

Humans↗

PubMed1984

Penguin vision in air and water.

Refractive states measured by retinoscopy and photorefraction indicate that rockhopper (Eudyptes crestatus), Magellanic (Spheniscus magellanicus) and gentoo (Pygoscelis papua) penguins are approximately emmetropic in air and water. Extensive myopia in air, as predicted by early authors, is nonexistent. Photorefractive measurements of refractive state in water indicate that rockhopper, gentoo, Magellanic and king (Aptenodytes patagonica) penguins can accommodate sufficiently to make up for the loss of refractive power of the cornea. Corneas of rockhopper and Megellanic penguins are flattened relative to the overall size of the eye. This feature minimizes the optical effect of submergence.

Accommodation, Ocular↗

PubMed1984

Centrifugation of 2 g does not affect otoconial mineralization in chick embryos.

Chick embryos were grown on a rotating centrifuge for periods of 2 wk to study the effects of increased gravity on the relative growth of their otoliths and femurs. Embryos, 20-21 days of age, centrifuged at 2 g showed no difference in utricular otoconial CaCO3 weights and femur CaPO4 weights compared with stationary control embryos of the same age. In addition, control embryos rotated at the center of the centrifuge had otoconial CaCO3 weights and femur CaPO4 weights not significantly different from stationary controls or from embryos rotated at 2 g. Accordingly these data do not support the hypothesis that the weight of the embryonic otoconial membrane is involved in the feedback regulation of its growth.

Animals↗

PubMed1984

Objective technique for the determination of monochromatic aberrations of the human eye.

The subjective crossed-cylinder aberroscope method of Howland and Howland [J. Opt. Soc. Am. 67, 1508 (1977)] has been modified by the addition of a beam splitter and a camera to permit direct photographic recording of the distorted retinal image of the aberroscope grid. The ocular aberration can then be deduced from direct measurements of the grid distortion. Preliminary results on 11 subjects confirm earlier findings that comalike, third-order aberrations are more important than spherical or other fourth-order aberrations in degrading the retinal image and for the average subject, the diffraction-limited pupil size is approximately 3 mm. This new objective method for measuring wave aberration yields significantly less variance in population estimates of the coefficients of the wave-aberration polynomial than that of the previous subjective method.

Astigmatism↗

PubMed1984

Photorefractive measurements of astigmatism in infants and young children.

We have photorefracted 312 infants and young children in Tompkins county in New York state and characterized the magnitude and axes of their astigmatisms. The population was solicited by letter from birth announcements or photographed at nursery schools, and was composed of children free of ocular problems. As had been previously reported, we found that the prevalence of astigmatisms of greater than 1 diopter decreased with increasing age. Roughly 7 of 10 infants aged 1 year of younger had one eye with at least 1 diopter of astigmatism or more, while only 1 out of 10 children of age four or more had an eye with 1 or more diopters of astigmatism. A longitudinal study of 26 infants also showed a decrease in the amount of astigmatism over the first three years of life. In infants up to 2 years of age from Tompkins County, the ratio of against-the-rule:oblique:with-the-rule astigmatisms was 15:9:1 in the most astigmatic eye. Generally the axes of oblique astigmatism showed bilateral symmetry. Photorefraction of a group of 159 infants from the city of Seattle showed a similar prevalence of astigmatism, but a slightly different ratio of kinds of astigmatism.

Astigmatism↗

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

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↗