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Biomedical subjects

Donald I A MacLeod

Publications and source records attributed to Donald I A MacLeod.

11 recordsLinked to original sources
PubMed2006

Contingent aftereffects distinguish conscious and preconscious color processing.

The brain can process input without perception, but what distinguishes conscious from preconscious processing? Using aftereffects induced by quickly alternating images, we show that cortical mechanisms track color much faster than perception, responding well to color alternations that are too rapid to be perceptible. The more restricted frequency response of the conscious perception of color suggests that extra integrative steps give conscious color perception a time course substantially slower than that of early cortical mechanisms.

Color↗
PubMed2005

Flexibility of spatial averaging in visual perception.

The classical receptive field (RF) concept-the idea that a visual neuron responds to fixed parts and properties of a stimulus-has been challenged by a series of recent physiological results. Here, we extend these findings to human vision, demonstrating that the extent of spatial averaging in contrast perception is also flexible, depending strongly on stimulus contrast and uniformity. At low contrast, spatial averaging is greatest (about 11 min of arc) within uniform regions such as edges, as expected if the relevant neurons have orientation-selective RFs. At high contrast, spatial averaging is minimal. These results can be understood if the visual system is balancing a trade-off between noise reduction, which favours large areas of averaging, and detail preservation, which favours minimal averaging. Two distinct populations of neurons with hard-wired RFs could account for our results, as could the more intriguing possibility of dynamic, contrast-dependent RFs.

Contrast Sensitivity↗
PubMed2005

The extended Maxwellian view (BIGMAX): a high-intensity, high-saturation color display for clinical diagnosis and vision research.

We describe a device that can display very high intensity (up to 400,000 cd/m2), high-resolution visual stimuli. The device is inexpensive, is easily controlled by a conventional computer and video card, and can be calibrated for use in vision research or clinical applications. The display is capable of presenting highly saturated, near spectral colors. Unlike Maxwellian view optical systems, our display can be viewed binocularly and does not require exacting head restraint. We describe the construction, give a design example, and describe our calibration procedure. Furthermore, we report measurements of the color gamut, spatial resolution, temporal characteristics, and the dynamic range of light intensity.

Computer Graphics↗
PubMed2004

Adaptation from invisible flicker.

Human ability to resolve temporal variation, or flicker, in the luminance (brightness) or chromaticity (color) of an image declines with increasing frequency and is limited, within the central visual field, to a critical flicker frequency of approximately 50 and 25 Hz, respectively. Much remains unknown about the neural filtering that underlies this frequency-dependent attenuation of flicker sensitivity, most notably the number of filtering stages involved and their neural loci. Here we use the process of flicker adaptation, by which an observer's flicker sensitivity is attenuated after prolonged exposure to flickering lights, as a functional landmark. We show that flicker adaptation is more sensitive to high temporal frequencies than is conscious perception and that prolonged exposure to invisible flicker of either luminance or chromaticity, at frequencies above the respective critical flicker frequency, can compromise our visual sensitivity. This suggests that multiple filtering stages, distributed across retinal and cortical loci that straddle the locus for flicker adaptation, are involved in the neural filtering of high temporal frequencies by the human visual system.

Adaptation, Psychological↗
PubMed2003

The origin of the oblique effect examined with pattern adaptation and masking.

The decreased visibility of obliquely oriented patterns as compared to horizontal or vertical ones is termed the oblique effect. The origin of the oblique effect in the chain of visual processing was examined by comparing the potency of oblique adapting gratings to the potency of horizontal ones. Oblique gratings (which were less visible but of equal physical contrast) were as powerful or more powerful than horizontal gratings as adapting stimuli. Obliquely oriented stimuli also produced a slightly stronger tilt aftereffect than stimuli near the cardinal axes. These results suggest that the diminished neural representation of oblique stimuli arises in the human cortex, rather than from impairments of sensitivity or resolution in the initial geniculo-cortical projection.

Adaptation, Ocular↗
PubMed2003

Colorimetry for CRT displays.

We analyze the sources of error in specifying color in CRT displays. These include errors inherent in the use of the color matching functions of the CIE 1931 standard observer when only colorimetric, not radiometric, calibrations are available. We provide transformation coefficients that prove to correct the deficiencies of this observer very well. We consider four different candidate sets of cone sensitivities. Some of these differ substantially; variation among candidate cone sensitivities exceeds the variation among phosphors. Finally, the effects of the recognized forms of observer variation on the visual responses (cone excitations or cone contrasts) generated by CRT stimuli are investigated and quantitatively specified. Cone pigment polymorphism gives rise to variation of a few per cent in relative excitation by the different phosphors--a variation larger than the errors ensuing from the adoption of the CIE standard observer, though smaller than the differences between some candidate cone sensitivities. Macular pigmentation has a larger influence, affecting mainly responses to the blue phosphor. The estimated combined effect of all sources of observer variation is comparable in magnitude with the largest differences between competing cone sensitivity estimates but is not enough to disrupt very seriously the relation between the L and M cone weights and the isoluminance settings of individual observers. It is also comparable with typical instrumental colorimetric errors, but we discuss these only briefly.

Colorimetry↗
PubMed2003

Surface segmentation based on the luminance and color statistics of natural scenes.

The luminance and color of surfaces in natural scenes are relatively independent under certain linear transformations, with the luminance of a surface providing little information about the color of that surface, and vice versa. However, differences in luminance between two locations in a natural scene remain strongly associated with differences in color. We used the statistics of the spatiochromatic structure of natural scenes as the priors for a Bayesian model that decides whether or not two points within an image fall on the same surface. This model provides a biologically plausible algorithm for surface segmentation that models observer segmentations well.

Color↗
PubMed2002

Influence of scene statistics on colour constancy.

The light reflected from an object depends not only on the surface properties of this object but also on the illuminant. The same is true for the excitations of the photoreceptors, which serve as the basis for the perceived colour. However, our visual system has the ability to perceive constant surface colours despite changes in illumination. The average chromaticity of the retinal image of a scene depends on the illumination, and thus might be used by the visual system to estimate the illumination and to modulate the correction that subserves colour constancy. But this measure is not sufficient: a reddish scene under white light can produce the same mean stimulation as a neutral scene in red light. Higher order scene statistics-for example, the correlation between redness and luminance within the image-allow these cases to be distinguished. Here we report that the human visual system does exploit such a statistic when estimating the illuminant, and gives it a weight that is statistically appropriate for the natural environment.

Adolescent↗
PubMed2002

Visual function before and after the removal of bilateral congenital cataracts in adulthood.

Subject Peter Doyle (PD) had congenital bilateral cataracts removed at the age of 43. Pre-operatively PD's visual acuity was 20/80, with a resolution limit around 15 cpd, and he experienced monocular diplopia with high contrast stimuli. Post-operatively PD's visual acuity improved to approximately 20/40, with a resolution limit around 25 cpd. Using a variety of pre- and post-operative tests we have documented a wide range of neural adaptations to his limited and distorted visual input, and have found a limited amount of post-operative adaptation to his newly improved visual input. These results show that the human visual system is capable of significant adaptation to the particular optical input that is experienced.

Adaptation, Physiological↗
PubMed2002

Color from invisible patterns.

Human pattern resolution is limited by optical blurring as well as neural filtering by a cascade of retinal and cortical sites with progressively lower resolution limits. Curiously, pattern structure can influence perceived color: a high-contrast, monochromatic (single wavelength) pattern appears desaturated (closer to white) relative to a uniform field of the same wavelength. Here we show that this desaturation is evident even when the pattern's frequency is too high for conscious perception, implicating a nonlinear process--namely light adaptation--at the level of single cone photoreceptors. We propose a neural mechanism in which fast, involuntary eye movements serve to shift control over perception between two competing cone populations, each operating at different levels of adaptation.

Color Perception↗