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Biomedical subjects

A W Snyder

Publications and source records attributed to A W Snyder.

At least 19 recordsLinked to original sources
PubMed1990

Texture segregation with luminance and chromatic contrast.

Preattentive texture discrimination was investigated using low spatial frequency texture elements. The contrast between the texture elements and the background was either purely luminance or purely chromatic, or some combination of both these types of contrast. The threshold to discriminate correctly the location of a different textured region was obtained from each subject, as was each subject's threshold to detect the elements of the texture. Using the ratio of texture to element detection as a measure of the effectiveness of texture discrimination, little difference could be found between the perception of luminance or chromatic texture. However, there were large and significant variations among subjects with otherwise normal colour vision.

Adaptation, Ocular↗
PubMed1989

Stiles-Crawford effect in garter snake.

The optical geometry of the eye of the American garter snake suggests a highly pronounced Stiles-Crawford effect occurs at the level of its cone photoreceptors. However, it is demonstrated here that inner segments of the cones in this species have a high density of microdroplets, up to 0.1 microns in diameter, which increases the refractive index of the ellipsoid. Theoretical considerations involving geometrical optics show that these droplets, like conventional oil droplets, may be a specific adaptation of the garter snake to increase the sensitivity of its cones to oblique rays, thus reducing the potentially large Stiles-Crawford effect predicted for this vertebrate.

Animals↗
PubMed1987

Bi-partitioning and boundary detection in natural scenes.

We propose a strategy for early vision which tailors visual channels to the object-oriented characteristics of natural scenes. This strategy involves essentially two types of channel, one for encoding the locally dominant edges which form the boundaries of 'objects', and another for 'filling in' the regions within them. The selection of contrasts which characterize object boundaries rather than textural detail can be enhanced by making an estimate local of contrast, and setting a threshold accordingly. This procedure and other aspects of the model were first suggested by observations of insect visual cells.

Animals↗
PubMed1986

Optical image quality and the cone mosaic.

Contrary to the orthodox view that optical image quality should "match" the photoreceptor grain, anatomical data from the eyes of various animals suggest that the image quality is significantly superior to the potential resolution of the cone mosaic in most retinal regions. A new theory is presented to explain the existence of this relation and to better appreciate eye design. It predicts that photoreceptors are potentially visible through the natural optics.

Animals↗
PubMed1986

Why spatial frequency processing in the visual cortex?

The nature of redundancy in visual images is discussed and the methods of removing statistical redundancies are considered. It is demonstrated that local spatial frequency analysis is a highly successful method and it is thus argued that this determines the choice of representation of the visual image by the simple cells of the visual cortex.

Humans↗
PubMed1985

Irregularity and aliasing: solution?

Although irregularity in a sampling array reduces the Moire effects caused by undersampling, it makes interpolation more sensitive to noise. The advantages of irregularity are considered with this in mind.

Animals↗
PubMed1985

Cone mosaic observed directly through natural pupil of live vertebrate.

Using a simple ophthalmoscopic technique, the cone mosaic of a live garter snake was clearly visible when viewed directly through its natural pupil, providing the first definitive cone map in a vertebrate eye. The optical MTF was also measured in natural conditions by projecting gratings in the snakes' field of view. Marrying these two results gives, unequivocally, the interrelation of optical image quality and the photoreceptor mosaic.

Animals↗
PubMed1979

Human psychophysics: functional interpretation for contrast sensitivity versus spatial frequency curve.

The hypothesis that neural processing in the human visual pathways compensates for both optical degradation as well as noise contamination at the photoreceptor level is introduced and shown to be consistent with the high frequency portion of the contrast sensitivity function for threshold detection of sinusoidal gratings in addition to the suprathreshold phenomenon of matching sinusoidal gratings of different spatial frequencies. This offers a unifying interpretation for why, at threshold conditions, the high spatial frequency portion of the image is blurred as severely by the nervous system as it is by the optics (e.g. Campbell and Green, 1965) while in extreme suprathreshold conditions the nervous system effectively deblurs the image (e.g. Georgeson and Sullivan, 1975; Kulikowski, 1976). These conclusions do not necessitate a highly specific form of visual processing such as Fourier channeling.

Differential Threshold↗
PubMed1978

Light propagation in twisted anisotropic media: application to photoreceptors.

The propagation of light through a slowly twisting anisotropic medium is described by a coupled-mode theory; expressions are derived for the electric field for the case of a birefringent dichroic medium with a constant rate of twist. The method provides a simple and intuitive means for determing the effect of twisting on the linear birefringence and dichroic absorption of the medium, particularly when the light is initially linearly polarized. The theory is well suited to the analysis of light absorption in twisting insect photoreceptors, such as found in bees and ants. We provide full expressions and useful approximations for polarization sensitivity and the initial direction of polarization to give maximum absorption for several types of photoreceptors.

Animals↗
PubMed1977

Photoreceptor diameter and spacing for highest resolving power.

The maximum center-to-center angular spacing delta phi of photoreceptors tolerated for reconstructing the highest spatial frequency passed by a diffraction-limited pupil of diameter D is delta phi - lambda/D square root 3 when the photoreceptors are packed in a hexagonal array, where lambda is the wavelength in vacuum. This spacing gives the maximum signal-to-photon noise ratio when the inner segments touch. The mean luminance required for an eye to achieve its highest resolving power is independent of eye size, provided the retina is designed to sample the highest spatial frequency passed by the diffraction-limited optics.

Humans↗