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Biomedical subjects

T Shipley

Publications and source records attributed to T Shipley.

At least 19 recordsLinked to original sources
PubMed1990

Neurological implications of aniseikonic tilts and stereoscopic completions through homogeneous visual space.

In 1965, it was first reported that BROKEN visual contours complete themselves in textured stereovision across empty or homogeneous space and that they do so strongly following directly the optics of crossed versus uncrossed disparities. The importance was then also noted of this completion effect for any neurophysiological model of stereovision. We now extend these measurements with several additional targets and to an analogous aniseikonic target, and confirm that the completion range has a maximum of about 5 degrees. There is much dependence upon target eccentricity and disparity sign, and some on size. That aniseikonic tilts may also be generated over this same range confirms the fact that global neurointegrative processes are of critical importance in all facets of stereovision, and in texture vision in general.

Aniseikonia↗
PubMed1988

Disparity acceleration effects in stereoscopy and their importance for theoretical neurology.

Stereoscopic perceptions have a special significance for brain modeling, because they require a special form of cortical integration for their appearance. We have newly observed, along ridges of very high stereoscopic disparity accelerations, narrow bands of exaggerated visual depth. These observations signal the importance of local excitation accelerations for future models of brain functions. Some key parameters of disparity acceleration bands are identified and their effects measured.

Cerebral Cortex↗
PubMed1987

Field processes in stereovision. A description of stereopsis appropriate to ophthalmology and visual perception.

There is, as yet, no satisfactory theory of stereopsis, despite the fact that our overt knowledge of "solid seeing" is now about 150 years old, and that contributions to our understanding come today from many fields: ophthalmology, psychology, psychophysics, neurophysiology, computer modelling, and optical-TV display technology. We review herein, and demonstrate for the reader whenever possible, certain key perceptual properties of the stereoscopic event of which any general theory must take account: vector stereoscopy and the neural grid, depth in empty visual fields, the relationship between stereoscopic and cognitive contours, stereoscopic contour formation in the presence of blur (thus, at low levels of central visual acuity), the phenomenon of cortical locking and of neural grid evocation in the presence of either peripheral or central rivalry, certain unusual ranges of figural mismatch and the concept of the horopter in relation to modern single cell electroneurophysiology in animals and to the constancy of visual directions. Some comments are also made on the concept of disparity processing by single cortical neurons, together with a short discussion of the implications of certain views of the genetics of stereovision for the perception of novel random texture sine-wave stereograms. We conclude that any theory pertinent to ophthalmology and visual science must combine the global concepts of cortical integration, the neural lock and the neural grid, herein introduced, with the more classical concepts of particulate or local binocular cortical correspondence. Certain preliminary steps in this direction are presented.

Cognition↗
PubMed1984

Interhemispherical comparisons in the processing of contour and random texture sinewave stereograms.

Contour and random texture stereograms were developed, using continuously variable, vertically oriented, sinewave horizontal disparities and variable texture densities. The contours were both computer printed and generated, and presented on a dual-beam oscilloscope; the textures were generated on a UNIVAC-to-Calcomp plotter, photographed, and then presented as slides, via rear-view polarized screens, in both static and dynamic modes. By means of fixation control, in normal subjects, the images in the right and left visual fields (thus: left and right visual cortices) were studied either separately or together. Parameters such as apparent depth, rate of depth-phi-motion, target density (matching and mismatching), depth ripple rate, Panum's horizontal fusion-disparity limits, and imposed monocular vertical prismatic imbalance, were studied for the separate hemispheres. In all but a few instances, the results show comparable, thus symmetrical, performances for right and left visual cortices. In those few instances where we could say that clear inter-cortical differences were found, they were found with both contour and texture targets. Furthermore, the density range of the targets (from 0.5 down to 0.005) was chosen so as to cover the phenomenal and physical range from true textures, at the high density end, to single disparate dots, at the low disparity end. But no sharp flex points were found, for any of several parameters, when moving from textured to dot targets. Although generally, observer and hemispherical variance were greater with the higher densities, the curves (with density) were ogival or S-shaped in form and never discontinuous. These results are discussed in the context of two previous findings in the literature. We cannot support the claim that there is somehow a difference in the way in which the visual cortex processes localized dot or contour targets from the way in which it processes pattern or texture targets. Secondly, the literature tends increasingly to support the contention that right occipital injuries hinder the processing of texture stereograms but not that of dot or contour stereograms. Since we could find only a scattered enhancement of right hemispherical prowess in normal vision, with both sorts of stereograms, this suggests that, - should these effects be reliably found in such patients, - they would have a different and non-congenital basis.

Brain↗
PubMed1976

Color perimetry.

Seven subjects were studied to determine the reproducibility of color isopters utilizing a Tubingen perimeter with targets equated for radiant energy and separate for heterochromatic flicker luminance. Achromatic threshold recognition of targets for equal luminance gave smaller isopters with longer wavelengths (red). Color recognition thresholds, on the other hand, showed large blue, midzone red and green, and small yellow isopters. The target recognition and color recognition thresholds for equal energy targets gave smaller red isopters. The data support Traquair's contention that all color isopters would be equivalent if hue, saturation, and intensity were equated. Clinically, the detection of subtle peripheral and central field defects might reside in the use of appropriately selected equally bright-colored targets.

Color Perception↗