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Biomedical subjects

H C Howland

Publications and source records attributed to H C Howland.

At least 91 records · Page 5Linked to original sources
PubMed1983

Photorefractive studies of normal and handicapped infants and children.

We have photorefracted 360 infants and children of ages 2 months to 5 years from and around Tompkins County, New York, who were free of any obvious ocular pathology. From these studies we have established 95% confidence limits for largest cylinder, largest sphere, and spherical and cylindrical differences between eyes as a function of age. Using this normal data we have begun to photorefract handicapped children from special childrens' centers in and around Tompkins County. Our preliminary data indicate a higher prevalance (greater than 4 X ) of refractive anomalies in handicapped children, as compared to normal children, even when allowance for ocular pathology is made.

Child, Preschool↗
PubMed1983

Optics of photorefraction: orthogonal and isotropic methods.

Analysis of the optics of photorefractively computed ray tracing shows that, for short camera-to-subject distances, the function relating image size to defocus of the eye is not symmetrical for errors of focus in front of and behind the camera. This asymmetry is exploited in the new method of isotropic photorefraction, in which the supplementary cylinder lenses of the original orthogonal photorefractors are replaced by defocusing of the camera lens itself. By comparing photographs taken with the camera focused in front of and behind the subject, the sign of the eyes' defocus (myopic or hyperopic relative to the camera) can be determined. The axis of any astigmatism is readily apparent as the direction in which the photorefractive images are elongated. The method is well adapted for the refractive screening of infants and young children.

Astigmatism↗
PubMed1980

The optics of static photographic skiascopy. Comments on a paper by K. Kaakinen: A simple method for screening of children with strabismus, anisometropia or ametropia by simultaneous photography of the corneal and the fundus reflexes.

The appearance of reflexes in static, photographic skiascopy is a function of myopic or hyperopic focus of the subject relative to the camera, camera to subject distance, subject pupil size, distance of flash relative to optic axis and camera aperture stop. This paper specifies this function.

Cornea↗
PubMed1978

Infant astigmatism measured by photorefraction.

Photorefraction of a sample of 93 infants of ages 1 day to 12 months showed that 63 percent of the subjects had astigmatism of 0.75 diopter or greater, and 12 percent greater than 2 diopters. Seventy percent of these astigmatisms were in the horizontal-vertical meridians. By comparison, only 8 percent of a sample of 26 adults tested by the same method showed astigmatism (all 0.75 to 1 diopter). The high incidence of infant astigmatism has implications for critical periods in human visual development and for infant acuity.

Adult↗
PubMed1976

Subjective measurement of high-order aberrations of the eye.

We used an apparatus similar to Tscherning's aberroscope, and analyzed subjects' drawings to obtain the wave aberration surfaces of 55 eyes. This analysis permitted a Taylor series representation of the wave aberrations to terms of the fourth order. The results revealed a wide variety in type and severity of high-order aberrations in which "cylindrical" aberrations were prominent and cases of classical spherical aberrations were rare. We computed the monochromatic modulation transfer function curves for the range of observations. The overall findings suggest a more prominent role for monochromatic high-order aberrations in degrading the visual image than has hitherto been assumed.

Humans↗
PubMedDate not supplied

Infant eyes: optics and accommodation.

Infant eyes grow rapidly in the first year of life. They exhibit considerable amounts of astigmatism, most frequently in the horizontal/vertical meridians with the horizontal meridian focused myopically relative to the vertical (i.e. 'against-the-rule"). By photokeratometry it is shown that most of this astigmatism in young infants is corneal in nature. The ability to accurately focus a visual target at varying distances exists to some extent from birth, but improves rapidly in the first 4 to 6 months of life until the net spherical error of focus is less than 0.5 diopters for targets 1.5 meters distant. At this stage, the astigmatism of the eye accounts for more than half of the defocus of the worst focused meridian. The persistence of astigmatism beyond the development of accurate focusing raises questions as to its conceivable functional significance. Because the mechanism of accommodation in adults may involve one or more of several methods for detecting the sign of defocus, including astigmatic blur, a possible role of astigmatism in infant focusing mechanisms is described.

Accommodation, Ocular↗