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A Reeves

Publications and source records attributed to A Reeves.

16 recordsLinked to original sources
PubMed1992

Electroretinograms (ERGs) and visual-evoked potentials (VEPs) elicited by pattern displacement.

The relation between the amplitude of visual responses to a checkerboard stimulus and the degree of lateral displacement of the checks was examined across different check sizes with simultaneously recorded electroretinograms (ERGs) and visual-evoked potentials (VEPs). The amplitudes of both the b-wave and the after-potential of the ERG increase linearly with pattern displacement. However, the major components of the VEP (N70 and P100) were smaller than expected from linearity for both small checks with small displacements (thresholding) and for large checks with large displacements (saturation). These results suggest that the ERG is proportional to the number of receptors stimulated, but the VEP reflects neural processes influenced by the spatial structure of the stimulus.

Adult↗
PubMed1992

How visual imagery interferes with vision.

Mental visual imagery interferes with vision: the Perky (1910) effect. Is the effect optical, sensory, perceptual, attentional, or just a response bias? Acuity was measured (in undergraduates and graduates) using target lines, with and without images (of lines). Optics (fixation, pupil size, accommodation), response bias, global attention (effort; diversion of attention to imagery), perceptual assimilation (target incorporation by imagery) and perceptual masking (of target by imagery) all fail to explain the effect. Foveally, local attention plays a limited role, as the Perky effect in divided attention is half that in focused attention, but this interaction vanishes with extrafoveal targets. Images produce primarily sensory interference, mimicking a reduction in target energy.

Adult↗
PubMed1992

Effect of cefodizime on parameters of cell-mediated immunity in vitro.

A positive effect of cefodizime (CAS 69739-16-8), a new aminothiazolyl cephalosporin, on a number of immunological variables, and in particular phagocytosis, was demonstrated in several test systems. The aim of the present investigation was to establish whether clinically relevant concentrations of cefodizime affect cell-mediated immune variables. Peripheral lymphocytes from healthy subjects were isolated and incubated with cefodizime in increasing concentrations from 0 to 200 mg/l. The effects of cefodizime on membrane-bound antigenic determinants of the lymphocytes were determined in the rosette inhibition test, and its effects on the proliferative capacity of lymphocytes after stimulation with phytohaemagglutinin, concanavalin A and pokeweed mitogen were determined in the lymphocyte transformation test. Cefodizime inhibited rosette formation in a concentration dependent manner. A direct inhibitory effect on proliferation was not, however, demonstrated in the lymphocyte transformation test. Indeed, stimulation of mitogen-induced lymphocyte transformation, particularly of concanavalin A-sensitive cells was observed at concentrations higher than 100 mg/l. The findings in healthy volunteers were reproduced in samples from three female patients with impaired host-defence. These results may suggest a positive effect of cefodizime on the proliferative capacity of the cellular immune system. However, no conclusions can be drawn on the clinical relevance of these findings until the results of in vivo investigations are available.

Animals↗
PubMed1991

Simultaneous color constancy: paper with diverse Munsell values.

Arend and Reeves [J. Opt. Soc. Am. A 3, 1743 (1986)] described measurements of color constancy in computer simulations of arrays of colored papers of equal Munsell value under 4000-, 6500-, and 10,000-K daylight illuminants. We report an extension of those experiments to chromatic arrays spanning a wide range of Munsell values. The computer-simulated scene included a standard array of Munsell papers under 6500-K illumination and a test array, an identical array of the same papers under 4000 or 10,000 K. Observers adjusted a patch in the test array in order to match the corresponding patch in the standard array by one of two criteria. They either matched hue and saturation or they made surface-color matches, in which the test patch was made to "look as if it were cut from the same pice of paper as the standard patch." The test and the standard patches were surrounded by a single color (annulus display) or by many colors (Mondrian display). The data agreed with those of our previous equal-value experiment. The paper matches were often approximately color constant. The hue-saturation matches were in the correct direction for constancy but were always closer to a chromaticity match (no constancy) than to the chromaticity required for hue-saturation constancy.

Adaptation, Ocular↗
PubMed1991

Effect of luminance on suprathreshold contrast perception.

Perceived contrast was measured under natural viewing conditions with the use of contrast-matching and magnitude-estimation paradigms and found to be independent of luminance over a range of luminances from 37.5 down to 8 cd/m2. However, this contrast constancy broke down when the dimmer target was below 8 cd/m2. The perceived contrast of the dimmer target then fell below that expected from contrast constancy. The extended range of contrast constancy previously reported [J. Physiol. 252, 627 (1975); Vision Res. 16, 1419 (1976)] has been thought to imply neural mechanisms with unlimited constancy, but these researchers permitted differential adaptation to the brighter and dimmer targets, which were seen haploscopically (by different eyes). As our natural-viewing procedure ensured that both bright and dim targets were presented to retinal areas in a roughly constant state of adaptation, our failure to find extended contrast constancy implies an important limitation on the neural processing of contrast.

Adaptation, Ocular↗
PubMed1991

Spatial linearity of the pattern electroretinogram.

We modeled the spatial-frequency sensitivity of the human pattern-reversal electroretinogram (PERG) with the linear, two-parameter, spatially bandpass model of Kelly [J. Opt. Soc. Am. A 2, 810 (1985)]. In the model temporal linearity or linearity with luminance is not assumed, but linearity with contrast is assumed. Measurements relating PERG amplitudes to stimulus element size were taken from 13 earlier reports. Stimuli were two-dimensional Fourier analyzed. The bandpass model fitted well and thus supported linearity (spatial superposition) and suggested that large PERG's to large checks (low-pass data) reflect mainly responses to higher-spatial-frequency stimulus components.

Electroretinography↗
PubMed1990

Perceived lightness, but not brightness, of achromatic surfaces depends on perceived depth information.

Three experiments were conducted in an attempt to replicate and clarify Gilchrist's (1977, 1980) experiments on the effects of depth information on judgments of achromatic surface color. Gilchrist found that coplanarity, and not retinal adjacency, was the dominant factor in determining achromatic color matches. Because such matches can be made on the basis of either brightness or lightness, we obtained judgments of both qualities. Stereopsis was added to enhance the perceived depth effect of Gilchrist's display, which was otherwise simulated closely on a high-resolution CRT. The results for lightness followed the same pattern as those of Gilchrist, but were smaller in magnitude. This discrepancy may reflect reduced extraneous lighting effects in our displays. Our results therefore agree with related studies in suggesting that lightness matches are based on relationships among coplanar surfaces. Brightness matches, however, were not influenced by perceived depth.

Adult↗
PubMed1987

Visual imagery selectively reduces vernier acuity.

Mental imagery interferes with perception. This, an example of the 'Perky effect', was studied for vernier acuity. Mean accuracy for reporting the offset of vertical line targets declined from 80% to 65% when subjects were requested to imagine vertical lines near fixation. Images of horizontal lines or of a grey mist in the fixation region lowered accuracy to a similar extent. However, accuracy was barely affected when the image was requested 1.5 deg or more from the target. The Perky effect remained strong for at least 4 s after an instruction to 'clear' the image away. The results were not due to imagery-induced changes in fixation, pupil diameter, or accommodation, or (at least primarily) to central attentional or decisional factors. Rather, imagery produces a local, pattern-insensitive, and relatively long-lasting reduction in visual sensitivity. The sensitivity loss may be mimicked by a 0.24 log unit reduction in target energy.

Fixation, Ocular↗
PubMed1987

Field additivity of Stiles's Pi-4 color mechanism.

Stiles's middle-wave color mechanism Pi-4 is field additive for mixtures of 410-, 480-, 530-, 622-, and 670-nm adaptation fields, when studied with a 1-deg, 200-msec, 500-nm foveal test flash at 10 times absolute threshold.

Adaptation, Physiological↗
PubMed1979

Comparison of interwave latencies of brain stem auditory evoked responses in narcoleptics, primary insomniacs and normal controls.

A study of brain stem auditory evoked responses (BAER) was carried out in 10 narcoleptics, 10 primary insomniacs and 10 normal controls to determine if a neurophysiologic abnormality could be detected in these primary sleep disorders. The mean interpeak conduction times of Wave I-III, III-V and Iv were compared between the following groups: normal controls awake and in monitored sleep; narcoleptics awake and in monitored sleep, normal controls awake and narcoleptics awake; normal controls awake and insomniacs awake; narcoleptics awake and insomniacs awake; narcoleptics with cataplexy (n = 6) awake and narcoleptics without cataplexy (n = 4) awake. No significant differences were found which suggests that these sleep disorders represent dysfunctions which do not involve brain stem structures subserving the BAER.

Adult↗
PubMed1979

Absence of parasympathetic denervation of the iris in alcoholics.

Forty patients with a history of alcohol abuse were treated with a 2% solution of methacholine to determine the prevalence of iridic parasympathetic denervation. Three (8%) of the alcoholics showed iris constriction of 1.0 mm or more, compared with three (8%) control subjects. In contrast, damage to the iridic parasympathetic nerve supply occurs in approximately 80% of patients with diabetes mellitus as determined by the same pharmacological technique.

Alcoholism↗
PubMed1978

Division of the corpus callosum for uncontrollable epilepsy.

Cerebral commissurotomy, the "split-brain" procedure, has been employed for the control of intractable seizures, in conjunction with moderate doses of anticonvulsant drugs. The results have been encouraging in several small series. The use of microsurgical techniques and the restriction of surgery to one commissure, the corpus callosum, has reduced morbidity without apparent change in result. The eight patients in our first series who underwent the prescribed division of several forebrain commissures are compared to the four patients in our second series who underwent division of the corpus callosum alone. The technique of callosotomy is described.

Adult↗
PubMed1977

Cerebral commissurotomy for control of intractable seizures.

Cerebral commissurotomy or the "split-brain" procedure may be a valuable adjunct to anticonvulsants for the control of seizures in people whose epilepsy cannot be relieved by anticonvulsants alone, and who are not candidate for the standard methods of surgery. Corpus callosotomy, a revised form of the usual division of many commissures, is a safer operation and appears to be equally effective. The complex clinical aspects of cure and treatment are emphasized.

Adolescent↗
PubMedDate not supplied

Leishmania infecting man and wild animals in Saudi Arabia. 7. Partial protection of mice against Leishmania major by prior infection with L. arabica.

A survey of inbred mouse strains showed that strain C3H/he was the most comparable to man in respect of its susceptibility to Leishmania major and the subsequent healing of lesions produced by this organism. L. arabica proved to have a lower virulence than L. major and prior inoculation with the former resulted in a decrease of the lesion sizes following subsequent L. major challenge. Moreover, L. major lesions that did develop in mice previously inoculated with L. arabica generally healed faster.

Animals↗