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D I MacLeod

Publications and source records attributed to D I MacLeod.

At least 19 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↗
PubMed1991

The temporal properties of the human short-wave photoreceptors and their associated pathways.

Flicker modulation sensitivity measurements made on high intensity orange steady backgrounds indicate that signals from short-wavelength sensitive cones (S-cones) have access to two pathways. At low S-cone adaptation levels the frequency response falls quickly with increasing frequency, but at higher adaptation levels it extends to much higher frequencies. At these higher S-cone adaptation levels, the following procedures can selectively expose either a process sensitive to low frequencies or one more sensitive to higher frequencies: (1) at high flicker frequencies, the S-cone signal can be nulled by a long-wavelength sensitive cone (L-cone) signal of suitable amplitude and phase, but at low frequencies a residual flicker persists; the modulation sensitivity for the residual flicker is lowpass in shape with a rapid decline in sensitivity with increasing flicker frequency; (2) sensitivity to flicker in the presence of a 17 Hz S- or L-cone mask is also lowpass with a similarly steep loss of high frequency sensitivity; yet (3) sensitivity to flicker during transient stimulation of the S-cones at 0.5 Hz is comparatively wideband (and slightly bandpass) in shape. The S-cone signal produced by the high frequency process is almost as well-maintained towards high frequencies as M- and L-cone signals. Furthermore, it is capable of participating in flicker photometric nulls with M- and L-cone signals. At low frequencies, however, when the low frequency S-cone signal is also present, satisfactory nulls can not be found. From these and phenomenological considerations, we identify the low and high frequency S-cone processes as S-cone inputs to the chromatic and luminance pathways, respectively. The phase adjustments needed to optimize flicker photometric nulls reveal that the S-cone input to the luminance pathway is actually inverted, but this is demonstrable only at relatively low frequencies: at medium or high frequencies the S-cone influence can be synergistic with that of the other cone types because of a delay in the transmission of S-cone signals.

Adaptation, Ocular↗
PubMed1991

Visual sensitivity to spatially sampled modulation in human observers.

Thresholds were measured for detecting spatial luminance modulation in regular lattices of visually discrete dots. Thresholds for modulation of a lattice are generally higher than the corresponding threshold for modulation of a continuous field, and the size of the threshold elevation, which depends on the spacing of the lattice elements, can be as large as a one log unit. The largest threshold elevations are seen when the sample spacing is 12 min arc or greater. These results are similar to those observed by Burr, Ross and Morrone [Vision Research, 25, 717-727 (1985)], who proposed an explanation based on a compressive point nonlinearity. Although their explanation is not consistent with the present data, the results may be explained in terms of nonlinear saturation of a spatially opponent stage early in the visual pathway. Theories based on response compression cannot explain the further observation that the threshold elevations due to spatial sampling are also dependent on modulation frequency: the greatest elevations occur with higher modulation frequencies. The idea that this is due to masking of the modulation frequency by the spatial frequencies in the sampling lattice is considered.

Contrast Sensitivity↗
PubMed1990

The spread of adaptation in human foveal and parafoveal cone vision.

We investigated the spread of bleaching adaptation for human cone vision in the central fovea and at an eccentricity of 5 deg in the nasal retina. Cone thresholds measured after adaptation to a grating bleach were compared to those measured after a uniform bleach. We conclude that the foveal and parafoveal cone systems show excellent localization of the effects of adaptation. For areas 2.5-5 min removed from the bleach, our measurement show only small sensitivity losses amounting to between 0.10 and 0.25 log unit elevation in threshold, after taking account of optical scatter.

Adaptation, Ocular↗
PubMed1989

Spatial organization of sensitivity regulation in rod vision.

To investigate whether scotopic sensitivity is set locally or in neural "pools", we have tested the spatial variation in sensitivity after bleaching with gratings using 3 different methods. One experiment circumvented the influence of involuntary eye movements by deliberately randomizing the horizontal position of a fine test line on the area bleached by the vertical gratings. The spatial variation of threshold across the bleached area is reflected in the width of the frequency-of-seeing curve. A clear difference between the probability-of-seeing curves following a grating bleach and a uniform bleach was seen only up to between 4.2 and 6.3 c/deg, suggesting that adaptation signals are pooled so as to almost obliterate the contrast in finer gratings than this. In a second experiment the lowest bleaching-grating contrast (for a space-averaged initial rhodopsin bleach of 10%) that produced a patterned afterimage stayed close to the scotopic threshold contrast for frequencies from 1 to 6.4 c/deg, but it rose above the contrast threshold at high spatial frequencies. This slight loss of sensitivity at the high frequencies is more evidence for pooling in adaptation. A third experiment assessed the sensitivity profile at the adapting site without any influence of later stages of neural integration. Bleaching and test gratings of slightly different spatial frequency were flashed successively. If the effect of bleaching is restricted to the bleached rods, the observer will effectively be looking at the test grating through a grid of sensitive and insensitive stripes in his own retina. The two gratings come in and out of register at the difference frequency, and a corresponding low-frequency grating should be visible even when the test and bleaching gratings are not themselves resolved by the later stages. We could not see the difference frequency unless the test and bleach gratings were themselves coarse enough to be resolvable in rod vision. This is very strong evidence against any model in which each rod has its own sensitivity-regulating mechanism, and instead supports (for these conditions) Rushton's view that adaptation is entirely the work of a neural pool. The estimated pool size is about 10 min arc of visual angle.

Adaptation, Ocular↗
PubMed1989

Rod flicker perception: scotopic duality, phase lags and destructive interference.

Rod vision has a duality of organization: at mesopic luminances rod signals have access to a slow, sensitive pathway (which we refer to, following Stiles, as pi 0) and a fast, insensitive pathway (pi' 0). The phase lag between the two rod signals increases with frequency until at 15-Hz the rod signals transmitted through the two pathways emerge out-of-phase, so that destructive interference produces a nulling of the apparent flicker. Relative to the cones, the phase lag of pi' 0 is roughly half that of pi 0. Thus at 15-Hz pi' 0 signals can be out-of-phase with cone signals, so that the signals from the slower pathway, pi 0, are actually in phase with cone signals. We have investigated the frequency response, adaptation behavior and phase characteristics of the two rod processes. The slower process, pi 0 is more sensitive than pi' 0, and dominates from absolute threshold up to low mesopic levels. The adaptation of pi 0 seems not to be associated with a change in time constant, but rather with simple response compression or sensitivity scaling. The time constant of pi' 0, however, does change with adaptation. There are large differences in the way that light adaptation changes the sensitivity of the two processes: signals from pi'0 may evade part of the postreceptoral sensitivity regulating mechanism normally associated with rod vision. The ability of signals from pi 0 and pi' 0 to reinforce or cancel each other, however, suggests that they are later reunited in a common pathway.

Dark Adaptation↗
PubMed1988

Reciprocity between luminance and dot density in the perception of brightness.

Thresholds were measured for detecting perturbations in a regular lattice of dots by modulating local dot density, local dot luminance, or some combination of the two. For high mean densities (dot spacing less than or equal to 15 min of arc), perturbations in local density increase the perceived brightnesses of the individually resolved elements in the more densely filled regions, and appear (at near threshold levels) as modulations of brightness rather than density. This illusory brightness modulation may be nulled by applying a real luminance modulation to make the lattice elements appear equally bright. Once this is done, thresholds for detecting any nonuniformity in the array are elevated compared to thresholds for detecting uncompensated density modulation. This result suggests that uncompensated density modulation is detected via the illusory brightness variations. This interpretation suggests that dot brightness is determined on the basis of the space average luminance of an area a substantial fraction of 1 deg in diameter. To test this hypothesis, thresholds were measured for detecting luminance modulation in a regular array of dots viewed against a comparatively dim background, where the modulation was applied to the dots themselves, to the background alone, or to both the dots and the background in either reinforcing or cancelling relative phase. For small, closely spaced dots, the threshold for modulation of luminance can be predicted on the basis of the amplitude of the Fourier component at the modulation frequency, regardless of whether it is carried by dots, the background, or both. The threshold is greatly elevated when modulation in the dots cancels the background modulation, so that there is contrast modulation of the dots, but no net energy at the fundamental frequency (zero amplitude of the Fourier component). For large, coarsely spaced dots, on the other hand, thresholds for conditions which contain energy at the fundamental modulation frequency are higher. The threshold increase is much greater when the modulation is applied to the dots than when it is applied to the background. This result suggests that the coarsely spaced dots are saturating the response of spatially opponent units. This hypothesis was confirmed by tests using backgrounds with the same luminance as the dots; threshold elevations selective for dots or background were abolished.

Form Perception↗
PubMed1988

Factors underlying individual differences in the color matches of normal observers.

We have used a factor analysis of the Stiles-Burch [Opt. Acta 6, 1 (1959)] 10 degrees field color matches to examine the basis of individual differences in the color matches made by observers with normal color vision. The differences in the matches are primarily due to interobserver variations in the macular-pigment density [with a standard deviation (sigma) of 0.12 at 460 nm]; the lens-pigment density (sigma = 0.18 at 400 nm); the spectral position of the long-wavelength-sensitive (sigma = 50.3 cm-1), medium-wavelength sensitive (sigma = 31.9 cm-1), and short-wavelength-sensitive (sigma = 45.3 cm-1) photopigments; the covarying densities of the three photopigments (sigma = 0.045); and the degree of rod intrusion. Variations in the different factors appear to be uncorrelated. Comparable estimates of the sources and range of interobserver differences in color matching were obtained from a similar analysis of the Stiles-Burch 2 degrees color matches [Opt. Acta 2, 168 (1955)].

Color Perception↗
PubMed1988

Direct psychophysical estimates of the cone-pigment absorption spectra.

The absorption spectra of the long- and medium-wavelength-sensitive cone photopigments were derived by determining the spectra that best accounted for either the individual differences in the Stiles-Burch 10 degrees color matches [Opt. Acta 6, 1 (1959)] or the changes in color matches at high light levels due to photopigment bleaching [Vision Res. 20, 23 (1980)]. The estimates were made by finding the best-fitting coefficients for an 11th-order polynomial function of wavelength, with no requirement that the resulting sensitivities be consistent with the color-matching functions. The estimates are independent of the scaling effects of any inert screening filters and therefore directly reflect the photopigment sensitivities. The spectra implied by the differences in the matches are similar to the absorption spectra of Smith et al. [Vision Res. 16, 1087 (1976)], which were used as initial estimates. However, the peak sensitivity of the required long-wavelength-sensitive pigment is shifted toward slightly longer wavelengths.

Color Perception↗
PubMed1987

Improvement in human vision under bright light: grain or gain?

1. The factor by which increment threshold changes with changing background intensity is less if the test flash is small than if it is large. This is commonly attributed to a reduction of the area over which visual signals are integrated as light adaptation increases. 2. We propose and test an alternative hypothesis that the change in slope is the result of purely local processes: if it is assumed that increasing the background intensity increases the exponent of the local response function, but does not alter the extent of spatial integration, then the threshold of the small test flash will rise more slowly than the threshold of the large test flash simply because the small test flash is of a higher intensity than the large and therefore evokes a correspondingly greater local response. 3. We measured small and large test field increment thresholds and dichoptic brightness matches as a function of background intensity. 4. The log-log slopes of the small and large field increment threshold functions differed by not more than about 20%, suggesting that even under the conventional interpretation of such data, the change of spatial integration is less than is usually supposed. 5. The intensity of a large (2.3 deg) suprathreshold test field matched to a standard in the other eye varies with increasing background intensity with the same shallow slope as the small test (2.6 min) threshold versus intensity function; this is in agreement with the predictions of the local non-linearity hypothesis and suggests that there is no substantial change in spatial integration during light adaptation.

Adaptation, Ocular↗
PubMed1987

Equiluminance: spatial and temporal factors and the contribution of blue-sensitive cones.

Equiluminance ratios for red/green, red/blue and green/blue sine-wave gratings were determined by using a minimum-motion heterochromatic matching technique that permitted reliable settings at temporal frequencies as low as 0.5 Hz. The red/green equiluminance ratio was influenced by temporal but not spatial frequency, the green/blue ratio was influenced by spatial but not temporal frequency, and the red/blue ratio was influenced by both. After bleaching of the blue-sensitive cones, there was no change in equiluminance ratios, indicating no contribution of the blue-sensitive cones to the luminance channel even at low temporal and spatial frequencies. The inhomogeneity of yellow pigmentation within the macular region was identified as the source of the spatial-frequency effect on the blue/green ratio.

Color↗
PubMed1986

The equivalent background of bleaching.

Stiles and Crawford proposed that a retinal region bleached by preexposure to intense light behaves as if it were illuminated by some steady veiling or background luminance. We test this notion by comparing the afterimage of a bleaching light with a steady (and retinally stabilized) light of adjustable intensity, in the manner of Barlow and Sparrock. With their matching procedure, and also with a new procedure, we find as they did that during the rod phase of recovery the afterimage does look like a stabilized field of an intensity which, presented as a background, brings visual sensitivity to the same level. It is as if the two conditions produce equal signals at some stage of the visual pathway. Liked Barlow and Sparrock we observe a rod-cone break in the afterimage matches. However, we argue that the appearance of the rod-cone break presents a paradox and we show a way to resolve it.

Afterimage↗
PubMed1985

Visual thresholds for shearing motion in monkey and man.

A reaction-time task was used to determine the visual motion thresholds in humans and in macaque monkeys for sinusoidally modulated shearing motion of a random dot display. It was found that humans and macaques were very similar in their spatial frequency sensitivity profiles for shearing motion. These profiles were of a U-shape for all human and monkey subjects tested. Temporal frequency, varied over a wide range, did not influence the shape of the spatial frequency sensitivity curve, but only the threshold amplitudes. The above results held both for single and multiple temporal cycles of shearing motion. Previous reports for the human, using these same shearing motion stimuli, indicated no increase in threshold at the lower spatial frequencies. The reason for this discrepancy is that thresholds in the previous studies were not determined at a low enough spatial frequency to see clearly this increase in thresholds. Because of the striking similarity of the data for man and macaque, it is suggested that similar neural mechanisms underly the shearing motion sensitivity of the two species.

Animals↗
PubMed1985

Sensitivity to shearing and compressive motion in random dots.

The sensitivity of the visual system to motion of differentially moving random dots was measured. Two kinds of one-dimensional motion were compared: standing-wave patterns where dot movement amplitude varied as a sinusoidal function of position along the axis of dot movement (longitudinal or compressional waves) and patterns of motion where dot movement amplitude varied as a sinusoidal function orthogonal to the axis of motion (transverse or shearing waves). Spatial frequency, temporal frequency, and orientation of the motion were varied. The major finding was a much larger threshold rise for shear than for compression when motion spatial frequency increased beyond 1 cycle deg-1. Control experiments ruled out the extraneous cues of local luminance or local dot density. No conspicuous low spatial-frequency rise in thresholds for any type of differential motion was seen at the lowest spatial frequencies tested, and no difference was seen between horizontal and vertical motion. The results suggest that at the motion threshold spatial integration is greatest in a direction orthogonal to the direction of motion, a view consistent with elongated receptive fields most sensitive to motion orthogonal to their major axis.

Humans↗
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↗