Search PubMed⌕ Search

Biomedical subjects

D I MacLeod

Publications and source records attributed to D I MacLeod.

10 recordsLinked to original sources
PubMed1992

A visual nonlinearity fed by single cones.

An intensive nonlinearity in the visual system can produce distortion products, or difference frequency gratings, when observers view two high contrast, high spatial frequency interference fringes of slightly different frequency or orientation added together at the retina. These distortion products are visible even when the two fringes imaged on the retina are above the resolution limit. Our experiments take advantage of this nonlinearity to measure the spatial filtering in the visual system following the formation of the retinal image, but preceding the site of the nonlinearity. The point spread function corresponding to this spatial filter is so small that it can be entirely explained by light integration within the apertures of foveal and parafoveal cones. The small size of this point spread function implies that (1) laser interferometry avoids contrast losses inherent in the eye's optics at spatial frequencies as high as 130 c/deg, (2) retinal scatter causes negligible image degradation in the fovea and parafoveal retina, (3) eye movements have little or no effect on contrast sensitivity to the distortion product and (4) that there is no neural spatial summation in the visual system prior to the site of the nonlinearity. Distortion products could also be observed when a bright interference fringe was briefly flashed on the fovea and a test interference fringe was viewed through the resulting afterimage. Measurements of the point spread function at stages in the visual system that precede the generation of this distortion product were similar to those obtained with simultaneous presentation of the two fringes, implying that the aftereffect of light adaptation is extremely local, no larger than the dimensions of single cones.

Adaptation, Ocular↗
PubMed1979

Chromaticity diagram showing cone excitation by stimuli of equal luminance.

In a space where Cartesian coordinates represent the excitations of the three cone types involved in color vision, a plane of constant luminance provides a chromaticity diagram in which excitation of each cone type (at constant luminance) is represented by a linear scale (horizontal or vertical), and in which the center-of-gravity rule applies with weights proportional to luminance.

Color Perception↗
PubMed1978

Visual sensitivity.

Explore the source record for details and available documents.

Adaptation, Ocular↗
PubMed1978

A displaced Stiles-Crawford effect associated with an eccentric pupil.

Receptors tend to be oriented toward a point near the center of the pupil in the normal eye. We report psychophysical studies of receptor orientation and directional sensitivity in a subject whose right pupil is displaced nearly 3 mm nasally as a result of injury sustained 25 years ago. The Stiles-Crawford effect was measured for foveal cones and for parafoveal cones and rods. Greatest sensitivity was found in all cases at a point close to the center of the natural pupil, indicating that the receptors in this eye are trained toward the abnormally situated pupil. At large angles of incidence, foveal cones exhibited a clear asymptote of sensitivity 0.83 log units below the sensitivity for axially incident light. Parafoveal cones were more directionally sensitive, with a suggestion of an asymptote for oblique incidence about 1.2 log units below the sensitivity for axial incidence. Rods showed a sensitivity pattern decentered like that of the cones, with a greatest observed sensitivity loss of 0.28 log units. Best acuity for cones was observed for entrance pupils close to the optical axis of the eye, remote from the pupillary region for best sensitivity.

Child, Preschool↗
PubMed1977

Rod photoreceptors detect rapid flicker.

It is widely believed that human rods cannot detect rapid flicker. With rod-isolation techniques, however, light-adapted rods detect flicker frequencies as high as 28 hertz, and the function relating rod critical flicker frequency to stimulus intensity contains two distinct branches. Human rod vision may, therefore, depend on two independent mechanisms.

Dark Adaptation↗
PubMed1976

The dark adaptation curve of rods measured by their after-image.

1. The common dark adaptation curve exhibits two branches; the course of the rod branch cannot normally be measured at early times since it lies above the observed cone thresholds. In this paper we measure it. 2. this is done by observing the negative after-image against a uniform background critically adjusted in luminance. 3. adjacent to the bleached area to be studied is a second area more strongly bleached. If the background intensity is below threshold for the less bleached area it will not be seen there; but if the background is above that threshold, this area will be seen brighter than the other. 4. the dark adapted threshold on the less bleached area is therefore the background luminance which just permits the two areas to be distinguished in the after-image. 5. after 5 min cones have quite recovered, and thus have no after-image to contaminate the rod image. 6. the rod curve measured by after-image is traced over 5 units of log threshold: it is an exponential with half life of 4.5 min, and coincides with the time course of regeneration of rhodopsin in man.

Afterimage↗