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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↗
PubMed1981

Electrostatic control of enzyme reactions: effect of ionic strength on the pKa of an essential acidic group on glucose oxidase.

The dissociation constant of an essential acidic group on the reduced form of glucose oxidase from Aspergillus niger (K4) has been found to be extremely sensitive to ionic strength. Increasing the ionic strength from 0.025 to 0.225 causes a decrease in pK4,obsd of 0.9 pH unit, from 8.2 to 7.3. Analysis of the ionic strength dependence of pK4,obsd, making the assumption that the enzyme is a homogeneously charged impenetrable sphere [Edsall, J. T., & Wyman, J. (1958) Biophysical Chemistry, Vol. 1, pp 282-289, 512-514, Academic Press, New York], predicts that the intrinsic pKa of the acidic group is 6.7 and that the charge on the protein is -78. The enzyme was titrated from its isoelectric point (pH 4.05) to pH 7.7, the pH at which the ionic strength dependence was determined. It was found to have an actual charge at that pH of -77, in remarkable agreement with the theoretical prediction. Thus, glucose oxidase exerts electrostatic control on pK4,obsd as though it were a uniformly charged sphere. The group responsible for pK4,obsd has not been identified. However, its measured delta H degrees obsd of 8.0 kcal mol-1 and delta S degrees obsd of -6.1 cal mol-1 K-1, together with its pKa of 6.7, are consistent with the group being a histidine residue.

Aspergillus niger↗
PubMed1980

Mortality surveillance in collaborative trials.

The Hypertension Detection and Follow-Up Program (HDFP) carried out two pilot surveillances covering the enumerated population to test procedures to be used in assessing the ability of the program to influence life expectancy in the total population. A rigorously sequenced pilot survey of 2,611 households was conducted and carefully monitored through two mailings, telephone contacts, home visits, and communication with "contact" persons. The response rates at each stage varied among the 13 centers. Overall, there was a 42.7% yield from the first mailing; 42.5% of those receiving a second mailing was completed; 78.0% for telephone 61.3% for the home visits and 55.2% from the "contact" persons. Overall, 97.4% of all persons had vital status ascertained. The second phase relaxed the rigorous sequential survey requirements and reduced the reporting requirements from every ten days to monthly. Overall, 93.3% were successfully ascertained. Reduced survey structure, slightly increased mobility (from 12% to 13%), increased workload from 200 to 400 households per center, and a longer time interval between initial enumeration and the mortality ascertainment are among the reasons for performance decline.

Adult↗
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↗