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Biomedical subjects

M C Campbell

Publications and source records attributed to M C Campbell.

26 records · Page 2Linked to original sources
PubMed1990

Effect of illuminance on the directions of chromostereopsis and transverse chromatic aberration observed with natural pupils.

The direction of chromostereopsis observed with a natural pupil and the direction of the monocularly perceived disparity between coloured targets (F and C lines - 486 and 656 nm) have been measured in a sample of 30 subjects at 10 and 1000 lx. At both illumination levels approximately equal numbers of subjects perceived positive and negative chromostereopsis. When the ambient illumination was increased, a reversal in the direction of the chromostereopsis occurred for 16 subjects. For six of them a change from a positive to a negative chromostereopsis was observed, a pattern not reported in previous studies. In most cases at both high and low illuminance, there was an absence of perception of a monocular disparity (transverse chromatic aberration), although chromostereopsis was reported, indicating that binocular information can be obtained from sub-threshold monocular disparities. At low illuminance, the lack of correlation between the direction of chromostereopsis and transverse chromatic aberration (TCA) may indicate that there may be a supplementary binocular factor in chromostereopsis.

Adult↗
PubMed1989

Unexpectedly large enhancement of a severely constricted field with reversed Galilean telescopes.

A 25-year-old female presented with severely constricted visual fields secondary to stroke associated with mitral valve prolapse. Repeatably measured central fields of less than 4 degrees diameter showed an unexpected enlargement up to 20 to 40 degrees diameter, when fitted with reversed full field 1.3x and 1.7x Galilean telescopes. Although initial tests indicated no psychogenic component, fields of approximately 20 degrees diameter were eventually obtained through an imitation telescope made with 2 plano lenses. This technique may be useful in determining whether there is a psychogenic component to inexplicable field results in other patients.

Adult↗
PubMed1989

Optical constancy of the chick lens during pre- and post-hatching ocular development.

Embryonic and post-embryonic development of the ocular lens is associated with the continual production of new secondary lens fibers by the mitotic activity of equatorial epithelial cells. This continual development affects lens size and shape and refractive index distribution. Study of embryonic lens optical function has been largely ignored. The optical characteristics of the developing chick lens, including paraxial and eccentric focal lengths, were measured during the embryonic period of development and up to 15 days after hatching. Measurements were made with an automated scanning laser system in which the video image of a helium-neon laser beam refracted by an excised lens in solution is digitized. Focal length is measured for beams moving in small steps on either side of that center. Measurements were made on excised lenses as well as with the lens in situ within the anterior segment of the eye. The results, collected from a study of a total of 80 lenses, indicates that embryo lenses at 6-7 days of incubation have long and very variable focal lengths. At the tenth embryo day, focal length drops by more than one-half and focal variations, between lenses and for different beam positions within a single lens, is reduced. Further measures for 14- and 17-day embryo lenses, as well as for lenses from hatchling and 5-, 10- and 15-day-old chicks, indicate that there is little change in focal length, either paraxially or for eccentric beam positions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Variation of dark focus of accommodation with laser speckle exposure duration.

Dark focus of accommodation (DFA) was measured in 10 subjects using a computer-aided He-Ne Badal laser optometer having high temporal and amplitude resolution. Accommodative responses were tracked during 5 min sessions for 10 speckle exposure durations ranging from 100 to 1000 ms. DFA-speckle exposure functions fell into four distinct groups. This identifies the laser speckle exposure time as a possible contaminant in measures of DFA made by laser optometers if individual variability across exposure times is ignored. Group averaging of accommodative responses for laser exposure times obfuscates the reported individual variability.

Accommodation, Ocular↗
PubMed1984

Measurement of refractive index in an intact crystalline lens.

A knowledge of the refractive index distribution within the crystalline lens is necessary to define the optical properties of the eye. This report describes the first nondestructive method for measuring the refractive index of intact crystalline lenses based on the work of Chu (1977) [Electron Lett. 13, 736-738] and Barrell and Pask (1978) [Opt. Commun. 27, 230-234] for optical fibre preforms. The method is simpler and more accurate than previous methods and shows that the rat crystalline lens has a smooth and almost parabolic distribution of refractive index.

Animals↗