Biomedical subjects

G Buchsbaum

Publications and source records attributed to G Buchsbaum.

10 recordsLinked to original sources

PubMed1991

Temporal differences between color pathways within the retina as a possible origin of subjective colors.

We propose a model of temporal signal processing within the retina based on temporal differences between the color pathways which may explain the phenomenon of subjective color. We quantify the model by inferring impulse response functions from physiological data, and predict the output of the different color pathways to temporally modulated achromatic signals which produce the sensation of color. Certain achromatic temporally modulated signals create imbalances between the color pathways which are analogous to those produced by stationary chromatic signals.

Color Perception↗

PubMed1989

Global spatiochromatic mechanism accounting for luminance variations in contrast sensitivity functions.

Psychophysical luminance and chromatic spatial contrast sensitivity functions (CSF's) exhibit a number of variations that depend on the average luminance of the stimulus grating. The contrast sensitivity at each spatial frequency and the maximum resolvable spatial frequency decrease with decreasing average luminance. Most prominently, luminance CSF's have peaks that shift toward lower frequencies, broaden, and disappear as luminance decreases. Chromatic CSF's are flat at low frequencies and do not exhibit luminance-related variations in shape. In this paper, we account for the luminance-dependent variations of CSF's by incorporating them into an earlier model that accounts for the basic high-luminance shapes of psychophysical luminance and chromatic spatial CSF's. These variations are modeled by increasing the spatial extent of a global center mechanism while decreasing the inhibitory effect of a global surround on that center. The results support the hypothesis that the global center mechanism changes size as a means of maintaining the dynamic range and controlling the signal-to-noise ratio as luminance varies.

Biomechanical Phenomena↗

PubMed1988

Definitive irradiation in carcinoma of the vagina: long-term evaluation of results.

A retrospective analysis of 165 patients with histologically confirmed carcinoma of the vagina is reported. Actuarial disease-free 10-year survival was: Stage 0 (16 patients)--94%, Stage I (50 patients)--75%, Stage IIA (49 patients)--55%, Stage IIB (26 patients)--43%, Stage III (16 patients)--32%, Stage IV (8 patients)--0%. All but one of the in situ lesions were controlled with intracavitary therapy. Of the patients with Stage I disease, 86% showed no evidence of vaginal or pelvic recurrence. Most of them received interstitial or intracavitary therapy or both; the addition of external beam irradiation did not significantly increase survival or tumor control. In Stage IIA (paravaginal extension) 61% of the tumors were controlled with a combination of brachytherapy and external beam irradiation. Ten of 16 Stage III tumors were controlled in the pelvis. Two of the patients with Stage IV disease had no recurrence in the pelvis with relatively high doses of irradiation. The total incidence of distant metastases was 16% in Stage I, 30.6% in Stage IIA, 46.1% in Stage IIB, 62% in Stage III, and 50% in Stage IV. The dose of irradiation delivered to the primary tumor or the parametrial extension was critical in achieving successful results. The incidence of grade 2-3 complications (12%) is correlated with the stage of the tumor and type of treatment given. More effective irradiation techniques including the optimization of dose distribution by judicious combination of external irradiation and interstitial brachytherapy will be necessary to enhance loco-regional tumor control. The high incidence of distant metastases underscores the need for earlier diagnosis and effective systemic cytotoxic agents if survival is to be significantly improved in these patients.

Brachytherapy↗

PubMed1988

Inference of global spatiochromatic mechanisms from contrast sensitivity functions.

A single spatiochromatic signal can describe both chromatic and luminance cosine gratings by varying the spatial phase of that signal. This signal representation permits chromatic and luminance spatial contrast sensitivity functions to be described as linear combinations of two global spatiochromatic mechanisms. If the spatiochromatic mechanisms are assumed to have Gaussian spatial profiles, we obtain one mechanism with broad spatial frequency tuning and one with narrow tuning. We find that the relative spatial shapes and extents of these mechanisms are in agreement with the center-surround organization of receptive fields of single retinal and geniculate cells. Furthermore, the chromatic responses of the derived mechanisms are in accord with the color-opponent nature of spatially antagonistic receptive fields.

Biomechanical Phenomena↗

PubMed1988

Quantitative studies of color constancy.

In order to study color constancy, the color appearance of the center of a center-surround paradigm was measured by using multiple-alternative forced-response matching. The center was presented with (1) no surround, (2) an adjacent chromatic surround, or (3) a chromatic surround separated from the center by an achromatic gap. The center and the surrounds were presented under various simulated illuminants ranging from illuminant A to illuminant D75. We found that when no surround is present, color constancy fails; however, when surrounds are present, some degree of color constancy is displayed. We also found that color constancy is poor when chromatic induction is minimal. In addition, it was determined that, if the ratios of R, G, and B of the center to R, G, and B of the surround remain constant as the illuminant changes, color constancy results. (R, G, and B correspond to the outputs of the retinal color mechanisms).

Color↗

PubMed1985

The possible role of the cochlear frequency-position map in auditory signal coding.

The frequency-position map in the cochlea is considered as part of a signal decomposition mechanism whose purpose is redundancy reduction and information compression of auditory stimuli. A logarithmic-type frequency distribution is analytically derived from a heuristic model of the autocorrelation function of auditory signals and confronted with empirical data from the cat cochlea. It is argued that the logarithmic organization of frequency along the cochlea is designed to extract the maximum amount of information with the minimum number of hair cells.

Animals↗

PubMed1979

Optimum probabilistic processing in colour perception. I. Colour discrimination.

A comprehensive account of wavelength discrimination and colour saturation discrimination is given in terms of optimum probabilistic signal detection. The theory is a logical deduction from statistical estimation theory of the visual estimate of the spectral parameters of the stimulus. In place of geometrical concepts associated with colour-space geometry, stimulus discriminability is determined by optimum decision rules given by likelihood ratio tests on statistics that are postulated for the trichromatic responses. The classical line element theory and its formulations are deduced to be discriminability measures between signals. The different mathematical forms of classical theory are shown to correspond to different statistical constraints.

Color Perception↗

PubMed1979

Optimum probabilistic processing in colour perception. II. Colour vision as template matching.

A statistical approach to account for psychophysical phenomena in human colour vision is presented. The central visual processor is viewed as an optimum recognizer of stochastic patterns supplied by the periphery. The processor makes an optimum estimate of the spectral parameters of the stimulus, given the wavelength filter characteristics of the periphery, the stochastic nature of the information and an internal template to which the external stimulus is matched. The estimate is constrained in ways inferred from empirical phenomena. Subjective brightness of monochromatic stimuli and related constant brightness manifolds in the colour space constitute the constraint for brightness estimation. Results analogous and in accord with those of earlier line element theories are obtained. The Bezold-Brücke hue shift constitutes the basic constraint for hue estimation. The hue estimate involves interrelation between the fields in the experiment. Similarities and differences both in basic conceptions and results introduced by the template matching notions are discussed.

Color Perception↗

PubMed1978

Compatibility between psychophysical and physiological measurements of aural combination tones.

Neural studies of combination-tone (CT) responses in the eighth nerve and antroventral cochlear nucleus of anesthetized cats give evidence of stimuluslike intracochlear CT's with a similar amplitude spectrum as inferred from human psychophysics. An unsolved problem raised in these studies is the gross discrepency between the phase of CT's measured psychophysically and neurally. It is unknown whether incompatibility lies in the cochleas or the criteria for measuring CT's in the different experiments. New psychophysical experiments were developed to clarify this issue. Secondary CT's (SCT) were generated by the interaction between a primary cubic CT (CTT) and a third tone in the stimulus. The SCT measured by cancellation exhibits similar properties to those of a CT generated by the comparable two-tone stimulus. The SCT is eliminated when the CCT is cancelled. These findings support the view that CT's exist in the cochlea as spatially distinct, stimuluslike excitations, and that CT excitations are eliminated by psychophysical cancellation. The SCT phase provides a measure of the CCT phase without requiring direct cancellation of the CCT. Phase measurements by the new indirect and older direct methods imply that the CT phase may be constant with changing sound level of the primary stimulus as found in the neural studies; these measurements also reveal nonlinear phase effects not found in neural studies. The new data suggest that the CT phase discrepancy may be caused by a real difference between the nonlinear mechanisms in the alert humans and anesthetized cats, and provide new constraints for clarifying this issue through further study.

Acoustic Stimulation↗

PubMedDate not supplied

Rheology of the vitreous body. Part I: Viscoelasticity of human vitreous.

The rheological properties of the vitreous body of the eye are believed to be a function of composition and to differ among species, as well as to vary regionally within the vitreous. These properties are essential to the mechanical functioning of the eye. Although there are gross, qualitative data on vitreous rheology available in the literature, quantitative rheological measurements on human vitreous and on eyes of other species are sparse and incomplete. The aim of the research reported in this series of papers is to study the rheological behavior of human, bovine, and porcine vitreous, to measure the macromolecular and electrolyte content of these samples, and to correlate and compare these values for the different species as a function of location in the vitreous. In this paper, the rheological model used to correlate viscoelasticity of the vitreous is presented, and a detailed description of the rheological instrumentation and methods of analysis used is given. Data on the rheological properties of human vitreous, as a function of location within the eye, are presented. The results show that in the human eye there are significant differences in a number of the viscoelastic parameters as a function of location within the vitreous body.

Elasticity↗