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Biomedical subjects

A B Watson

Publications and source records attributed to A B Watson.

At least 19 recordsLinked to original sources
PubMed1992

Transfer of contrast sensitivity in linear visual networks.

Contrast sensitivity is a useful measure of the ability of an observer to distinguish contrast signals from noise. Although usually applied to human observers, contrast sensitivity can also be defined operationally for individual visual neurons. In a model linear neuron consisting of a filter and noise source, this operational measure is a function of filter gain, noise power spectrum, signal duration, and a performance criterion. This definition allows one to relate the sensitivities of linear neurons at different levels in the visual pathway. Mathematical formulae describing these relationships are derived, and the general model is applied to the specific problem of relating the sensitivities of parvocellular LGN neurons and cortical simple cells in the primate.

Animals↗
PubMed1991

Comparison of transvaginal and transabdominal ultrasonography in ectopic pregnancy.

The authors retrospectively reviewed 45 consecutive cases of proven ectopic pregnancy for which both transvaginal (TV) and transabdominal (TA) ultrasonography had been performed to compare the diagnostic efficacy of the two imaging techniques. The criteria for a diagnosis of ectopic gestation included an extrauterine gestational sac containing a fetus or a fetal pole, or an empty extrauterine sac. Solid or complex adnexal masses with evidence of hemoperitoneum were considered suggestive but nondiagnostic. TV ultrasonography was superior to TA ultrasonography in 22 cases (49%) and inferior in 3 (7%). In the remaining 20 cases (44%) the two methods yielded similar information. For cases in which TV ultrasonography was superior, this method provided clear evidence of ectopic pregnancy in 11 cases in which TA ultrasonography demonstrated nonspecific masses or normal adnexa; in the other 11 cases both methods led to the correct diagnosis, but TV ultrasonography provided additional useful information. The authors conclude that TV ultrasonography has a definite role in improving the diagnosis of ectopic pregnancy.

Abdomen↗
PubMed1990

Optimal displacement in apparent motion and quadrature models of motion sensing.

A grating appears to move if it is displaced by some amount between two brief presentations, or between multiple successive presentations. A number of recent experiments have examined the influence of displacement size upon either the sensitivity to motion, or upon the induced motion aftereffect. Several recent motion models are based upon quadrature filters that respond in opposite quadrants in the spatiotemporal frequency plane. Predictions of the quadrature model are derived for both two-frame and multiframe displays. Quadrature models generally predict an optimal displacement of 1/4 cycle for two-frame displays, but in the multiframe case the prediction depends entirely on the frame rate.

Humans↗
PubMed1990

Effect of contrast on the perceived direction of a moving plaid.

We performed a series of experiments examining the effect of contrast on the perception of moving plaids. This was done to test the hypothesis put forth by Adelson and Movshon (1982) that the human visual system determines the direction of a moving plaid in a two-staged process: decomposition into component motion followed by application of the intersection of constraints rule. Although there is recent evidence that the first tenet of their hypothesis is correct, i.e. that plaid motion is initially decomposed into the motion of the individual grating components (Movshon, Adelson, Gizzi & Newsome, 1986; Welch, 1989), the nature of the second-stage combination rule has not as yet been established. We found that when the gratings within the plaid are of different contrast, the perceived direction is not predicted by the intersection of constraints rule. There is a strong (up to 20 deg) bias in the direction of the higher-contrast grating. A revised model, which incorporates a contrast-dependent weighting of perceived grating speed as observed for 1-D patterns (Thompson, 1982), can quantitatively predict most of our results. We discuss our results in the context of various models of human visual motion processing and of physiological responses of neurons in the primate visual system.

Adolescent↗
PubMed1990

Gain, noise, and contrast sensitivity of linear visual neurons.

Contrast sensitivity is a measure of the ability of an observer to detect contrast signals of particular spatial and temporal frequencies. A formal definition of contrast sensitivity that can be applied to individual linear visual neurons is derived. A neuron is modeled by a contrast transfer function and its modulus, contrast gain, and by a noise power spectrum. The distributions of neural responses to signal and blank presentations are derived, and from these, a definition of contrast sensitivity is obtained. This formal definition may be used to relate the sensitivities of various populations of neurons, and to relate the sensitivities of neurons to that of the behaving animal.

Animals↗
PubMed1990

Perceptual-components architecture for digital video.

A perceptual-components architecture for digital video partitions the image stream into signal components in a manner analogues to that used in the human visual system. These components consist of achromatic and opponent color channels, divided into static and motion channels, further divided into bands of particular spatial frequency and orientation. Bits are allocated to an individual band in accord with visual sensitivity to that band and in accord with the properties of visual masking. This architecture is argued to have desirable features such as efficiency, error tolerance, scalability, device independence, and extensibility.

Color Perception↗
PubMed1990

The method of constant stimuli is inefficient.

Simpson (1988) has argued that the method of constant stimuli is as efficient as adaptive methods of threshold estimation, and has supported this claim with simulations. We show that Simpson's simulations are not a reasonable model of the experimental process, and that more plausible simulations confirm that adaptive methods are much more efficient than the method of constant stimuli.

Attention↗
PubMed1990

Transvaginal sonographic evaluation of the retrodisplaced uterus.

To determine the clinical value of transvaginal sonography in the assessment of retrodisplaced uteri, the authors reviewed, retrospectively, 500 consecutive transvaginal (TV) and transabdominal (TA) sonograms. Of the 494 patients examined, 27 had a retrodisplaced uterus. Transvaginal sonography was superior to TA sonography in 25 patients, providing improved visualization of the endometrial canal, myometrium, adnexa and cul-de-sac. In 7 of these 25 patients, the findings that suggested the diagnosis were only seen on TV scanning. These findings included intrauterine pregnancy (three patients), fluid collection in the cul-de-sac (two patients), fluid collection in the endometrial canal (one patient) and an embedded intrauterine contraceptive device (one patient). The two techniques yielded the same information in two other patients. In no patient was TA sonography more informative than TV sonography. The authors, therefore, conclude that TV sonography is the procedure of choice in assessing a retrodisplaced uterus and that additional examination with TV sonography is advisable whenever a retrodisplaced uterus is suspected.

Female↗
PubMed1989

A hexagonal orthogonal-oriented pyramid as a model of image representation in visual cortex.

Retinal ganglion cells represent the visual image with a spatial code, in which each cell conveys information about a small region in the image. In contrast, cells of primary visual cortex employ a hybrid space-frequency code in which each cell conveys information about a region that is local in space, spatial frequency, and orientation. Despite the presumable importance of this transformation, we lack any comprehensive notion of how it occurs. Here we describe a mathematical model for this transformation. The hexagonal orthogonal-oriented quadrature pyramid (HOP) transform, which operates on a hexagonal input lattice, employs basis functions that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The basis functions, which are generated from seven basic types through a recursive process, form an image code of the pyramid type. The seven basis functions, six bandpass and one low-pass, occupy a point and a hexagon of six nearest neighbors on a hexagonal sample lattice. The six bandpass basis functions consist of three with even symmetry, and three with odd symmetry. The three even kernels are rotations of 0, 60, and 120 degrees of a common kernel; likewise for the three odd kernels. At the lowest level, the inputs are image samples. At each higher level, the input lattice is provided by the low-pass coefficients computed at the previous level. At each level, the output is subsampled in such a way as to yield a new hexagonal lattice with a spacing square root 7 larger than the previous level, so that the number of coefficients is reduced by a factor of seven at each level. In the biological model, the input lattice is the retinal ganglion cell array. The resulting scheme provides a compact, efficient code of the image and generates receptive fields that resemble those of the primary visual cortex.

Computer Simulation↗
PubMed1988

Outcome following standardized 185 MBq dose 131I therapy for Graves' disease.

The clinical outcome of 199 patients with Graves' disease treated with standardized 185MBq 131I therapy doses has been analysed. Most patients were controlled with antithyroid drugs prior to the 131I therapy, and also received antithyroid drugs for several months following 131I. The median follow-up period was 5.5 years. The single 185MBq 131I dose successfully treated 72.4% of patients. The 1, 2 and 5 year hypothyroid figures were 15.5%, 19.3% and 27.3%, respectively. Previous thyroidectomy was associated with an increased hypothyroid rate. Retreatment was required by 25.6%, with 3.5% requiring more than two 131I doses. Discriminant analysis of pretreatment variables suggests that patients with large goitres or severe disease (serum T3 greater than 10nmol/l) should be treated with higher doses of 131I.

Adolescent↗
PubMed1987

Estimation of local spatial scale.

The concept of local scale asserts that for a given class of psychophysical measurements, performance at any two visual field locations is equated by magnifying the targets by the local scale associated with each location. Local scale has been hypothesized to be equal to cortical magnification or alternatively to the linear density of receptors or ganglion cells. Here, we show that it is possible to estimate local scale without prior knowledge about the scale or its physiological basis.

Fovea Centralis↗
PubMed1987

Efficiency of a model human image code.

Hypothetical schemes for neural representation of visual information can be expressed as explicit image codes. We may test whether a given code is sufficient, in the sense of retaining all the information that the human perceives, and necessary, in the sense of retaining only that information. The latter is a test of efficiency. Here, we explore a code modeled on the simple cells of the primate striate cortex. The Cortex transform maps a digital image into a set of subimages (layers) that are bandpass in spatial frequency and orientation. The layers are sampled so as to minimize the number of samples and still avoid aliasing. Samples are quantized in a manner that exploits the bandpass contrast-masking properties of human vision. The entropy of the samples is computed to provide a lower bound on the code size. Finally, the image is reconstructed from the code. We devise psychophysical methods for comparing the original and reconstructed images to evaluate the sufficiency of the code. When each resolution is coded at the threshold for detection artifacts, the image-code size is about 1 bit/pixel.

Humans↗
PubMed1986

Apparent motion occurs only between similar spatial frequencies.

Whether apparent motion occurs only between targets of similar spatial frequency was examined. A "crossed phi" method was used, in which two potentially different targets exchange position. If the targets differ in a feature that controls apparent motion then they will appear to move so as to exchange positions; if they differ in an irrelevant feature, no apparent exchange will occur. It was found that apparent motion occurs only between similar spatial frequencies. This result is interpreted in the light of recent models of human visual motion sensing, in which motion estimates are computed separately within a number of spatial frequency bands.

Humans↗
PubMed1986

Preventative effect of etretinate therapy on multiple actinic keratoses.

Multiple actinic keratoses (MAK) are premalignant lesions that may produce a significant treatment problem not always controllable by destructive therapy. Etretinate therapy has been shown to produce both prophylactic and therapeutic effects on premalignant epithelial tumors of mice. Results are presented on an 8-month double blind crossover trial (4 months placebo/4 months etretinate, randomly assigned) in 15 patients with severe MAK. Fourteen patients improved significantly on etretinate, whereas five of six patients who had placebo in the first 4-month period became worse as measured by the number of lesions and diameter of larger lesions. However, eight of nine patients who had placebo in the second 4-month period (ie, following etretinate therapy) showed no significant increase in the number of actinic keratoses, suggesting that etretinate may prevent the appearance of new lesions during, and for some time after, its administration. Moderate dose-dependent mucocutaneous side effects were frequent. It is proposed that an annual 3-month course of etretinate may help control severe MAK patients, minimizing the need for destructive therapies.

Adult↗
PubMed1978

The surgical diathermy: principles of operation and safe use.

An understanding of the principles of operation and hazards of the surgical diathermy is necessary for the safety of the anaesthetized patient. The surgical diathermy performs its function by the application of high density radio frequency current which can be used to cut or coagulate tissue. Its improper use can result in electrical burns and even electrocution. The principles underlying its safe use are outlined, and detailed recommendations are made to ensure the patient's safety.

Disposable Equipment↗