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D I MacLeod

Publications and source records attributed to D I MacLeod.

At least 55 records · Page 3Linked to original sources
PubMed1980

Blue-sensitive cones do not contribute to luminance.

By using violet backgrounds we selectively altered blue-cone sensitivity but found no change in flicker photometric sensitivity. This indicates that blue cones do not contribute to luminance as defined by flicker photometry.

Color Perception↗
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.

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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↗
PubMed1974

Rod origin of prolonged afterimages.

Afterimages fade against any unchanging background but generally reappear if the background changes suddenly. Under some conditions, however, a change of background color fails to revive a faded afterimage. This happens only if the interchanged backgrounds equally stimulate the rod receptors. It follows that afterimages seen under these conditions are generated by rods.

Afterimage↗
PubMed1974

The bleaching and regeneration of rhodopsin in the cat.

1. The processes of bleaching and regeneration were monitored by retinal densitometry in living cats.2. Neither bleaching nor regeneration of rhodopsin can be described by the simple kinetic equation (Alpern, 1971) found valid for man.3. After a strong 1 min bleach, the retina contains more unbleached rhodopsin than expected on the basis of the initial bleaching rate.4. During the first 9 min after a 1 min bleach, cats regenerate rhodopsin only slowly; density changes during this period are dominated by formation and decay of metarhodopsin III. Subsequently, rhodopsin regeneration accelerates to a rate of 50%/11 min.5. No such delay precedes recovery from a prolonged (20 min) bleach.

Animals↗
PubMed1973

Background configuration and rod threshold.

1. This paper investigates the variation in rod threshold when a small test flash is seen against backgrounds of different sizes. Over a substantial range of luminances above absolute threshold, the test flash is less easily seen against small backgrounds than large. This confirms earlier results.2. If an annular surround is added to a small circular background, threshold is reduced when background and annulus are equiluminous (uniform field), but rises rapidly as the annulus is made brighter or dimmer than the background. This cannot be explained by the threshold-elevating effects of light scattered on to the background from the surround, for threshold rises with annulus luminance faster than it does on uniform fields of equal luminance.3. If the surround is not a complete annulus but a windmill-shaped cross, threshold is higher than on a uniform field, no matter what the windmill luminance. Thus it is not the addition of light per se to the surround which reduces threshold.4. This conclusion is reinforced by the results of another experiment. The test flash is seen on a large uniform field. When superimposed on this field, a thin ring, light or dark, which causes only a small change in mean luminance, produces an appreciable rise in threshold.5. The addition of an equiluminous red surround to a small red background so as to create a uniform field causes a marked drop in test flash threshold, but a scotopically equal blue surround, that creates a uniform field for rods, does not alter the threshold. Since the test flash is seen only by rods it follows that signals from cones can alter rod threshold.6. Known or probable behaviour of retinal mechanisms cannot account for our results. All the operations which elevate threshold above its level on a large uniform field produce contours in the vicinity of the test flash. This we take as evidence that signals from the test stimulus are suppressed or reduced by other signals present only when the background is locally non-uniform.

Color↗