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H R Wilson

Publications and source records attributed to H R Wilson.

231 records · Page 13Linked to original sources
PubMedDate not supplied

Spatial-frequency adaptation: evidence for a multiple-channel model of short-wavelength-sensitive-cone spatial vision.

The frequency selective effects of spatial adaptation were measured with vertically-oriented, cosine stimuli upon an intense long-wavelength (yellow) field, which isolated the short-wavelength-sensitive (S) cones. Consistent with isolated-S-cone spatial threshold and masking results, the adaptation measurements demonstrate S-cones input to multiple, orientation selective, spatial frequency mechanisms. Moreover, the adaptation measurements show the minimum number of S-cone mechanisms is three. The frequency tuning of each mechanism was derived from the S-cone threshold and masking results. Two of the tuning curves are bandpass with peak sensitivities in a vicinity of 0.7 and 1.4 c/deg, respectively. These two closely resemble tuning curves derived from results with luminance-modulated stimuli. Confined to the range of frequencies examined (0.25-2.83 c/deg), the third tuning curve is lowpass with a high-frequency cutoff of roughly 2.0 c/deg. However, subsequent measurements of orientation selectivity demonstrate the third mechanism to have bandpass frequency tuning as well.

Adaptation, Ocular↗
PubMedDate not supplied

A model for motion coherence and transparency.

A recent model for two-dimensional motion processing in MT has demonstrated that perceived direction can be accurately predicted by combining Fourier and non-Fourier component motion signals using a vector sum computation. The vector sum direction is computed by a neural network that weights Fourier and non-Fourier components by the cosine of the component direction relative to that of each pattern unit, after which competitive inhibition extracts the signals of the most active units. It is shown here that a minor modification of the connectivity in this network suffices to predict transitions from motion coherence to transparency under a wide range of circumstances. It is only necessary that the cosine weighting function and competitive inhibition be limited to directions within +/- 120 deg of each pattern unit's preferred direction. This network responds by signaling one pattern direction for coherent motion but two distinct directions for transparent motion. Based on this, neural networks with properties of MT and MST neurons can automatically signal motion coherence or transparency. In addition, the model accurately predicts motion repulsion under transparency conditions.

Animals↗
PubMedDate not supplied

A neural model of foveal light adaptation and afterimage formation.

Psychophysical research has documented the existence of three processes in light adaptation: a fast subtractive process, a divisive process that is fast at light onset and slower at light offset, and a very slow subtractive process (Hayhoe et al., 1987). In the neural model developed here, the fast subtractive process is identified with horizontal cell feedback onto cones and the divisive process with amacrine cell feedback onto bipolar cells. The very slow subtractive process is identified with the modulatory feedback circuit from amacrines via interplexiform cells to horizontal cells. A nonlinear dynamical model is developed incorporating these aspects of retinal circuitry along with both ON- and OFF-center M and P pathways. This model is shown to account for many aspects of foveal light adaptation, including negative afterimage formation, and to explain a number of the physiological differences between M and P ganglion cells, including their differing contrast-response functions.

Adaptation, Physiological↗
PubMedDate not supplied

Effectiveness of cast-joined Ni-Cr-Be structures.

This study tested the load transfer effectiveness of cast-joined structures under flexural loading conditions. Bars of Rexillium III alloy (Ni-Cr-Be) were tested under four-point bending conditions in an Instron testing machine. Six specimens were prepared for each of five interlocking designs. After wax elimination of the investment mold, new metal was cast into the central interlock area. Strain gauges were bonded across the interfaces between the as-cast and secondary cast structures on the bottom surface, and the specimens were loaded to failure in a four-point-bending fixture. Failure was assumed at a strain level of 0.1%, which corresponds to the tensile failure strain of feldspathic porcelain. Three as-cast bars were tested as controls. The average failure load of an interlock design used by Weiss and Munyon was significantly higher (P less than 0.05) than that of the remaining four designs but was significantly lower (P less than 0.05) than that of the solid bar. At the critical strain level of 0.1%, the load-transfer effectiveness of the Weiss and Munyon design was less than 22%. The results suggest that the cast-joining technique may increase the risk of failure in clinical situations where high flexural stresses exist.

Dental Alloys↗
PubMedDate not supplied

Prolonged progestin (quingestanol) therapy of menopausal diabetic women.

The reduction of high serum LH levels toward or to normal in diabetic women in the menopausal range recorded during the first year of treatment with a synthetic progestin, 17-alpha-ethynyl-19-nortestosterone acetate, 3-cyclopentylenol ether (quingestanol acetate), was maintained during the second and third years of daily ingestion of this steroid. Normal LH and normal or high FSH levels prior to therapy were not affected. Other endocrine indices, including serum PBI and T4, plasma 11(OH) corticosteroids, serum growth hormone titers, insulin responses to oral glucose, and urinary excretion of 17-ketosteroids, Porter-Silber chromogens, and 11-desoxycortisol metabolites, estrogens, and creatinine remained relatively unchanged during quingestanol therapy. At the 36th month of treatment, a small increase in fasting blood glucose levels with greater hypoglycemia after oral carbohydrate was noted, but this probably reflected the natural history of treated diabetes mellitus. The decrease in urinary steroid responses to partial blockade of adrenal 11-beta hydroxylase by metyrapone observed in the 12th month of quingestanol therapy was not evident at the end of 24 and 36 months of treatment. Quingestanol therapy was associated with maintenance of the increase in serum sodium from low-normal to mid-normal concentrations noted during the first year of treatment. Serum chloride increases were less frequent. Other serum electrolytes, solutes, proteins and other nitrogenous constituents, lipids, and enzymes and formed blood elements generally fluctuated within the pre-therapy ranges. Body weight, blood pressure, pulse rate, electrocardiograms, and perception of vibrations were about the same prior to and at the completion of the three-year course of therapy. The steroid was well tolerated.

Adult↗