Search PubMed⌕ Search

Biomedical subjects

A Reeves

Publications and source records attributed to A Reeves.

At least 37 records · Page 2Linked to original sources
PubMed1996

Flicker brightness enhancement and visual nonlinearity.

The purpose of this study was to investigate the nonlinear mechanism underlying brightness enhancement, in which a flickering stimulus appears brighter than a steady stimulus of equal mean luminance. The flickering and matching stimuli were temporally alternated. Both were cosine windowed to minimize the potential effects of temporal transients. Subjects adjusted the amplitude of the matching stimulus to match it in brightness to the flickering stimulus. The temporal frequency, modulation, and waveform of the flickering stimulus were varied. With sinusoidal flicker, brightness enhancement increased with increasing modulation at all frequencies, peaking at about 16 Hz at full modulation. The results were modeled by a broad temporal filter followed by a single accelerating nonlinearity. The derived temporal sensitivity of the early filter inferred from brightness enhancement decreased more slowly at high frequencies than the filter(s) inferred from flicker modulation thresholds. With low frequency sawtooth flicker, brightness enhancement was phase-dependent at low, but not at high modulations, suggesting that multiple neural mechanisms may also be involved in addition to an early nonlinearity.

Female↗
PubMed1996

Expectancy of line segment orientation.

The question was asked whether briefly flashed line segments are easier to detect when presented at an expected, rather than an unexpected, orientation. Detection rates were measured in a two-interval forced choice (2IFC) paradigm that did not require the subject to identify the orientation of the line segment, only to detect its presence. The 2IFC paradigm was used to rule out bias or criterion effects. Subjects were led to expect lines in a particular or primary orientation by being presented lines with that orientation as cues before every trial, and by being tested with only that orientation during practice. Lines of the orthogonal, probe orientation replaced the primary on 25% of experimental trials. When the stimulus location was known in advance, lines of the primary orientation were detected more accurately than were probe lines, but when stimulus location was not known, detection rates were equal. Detection rates were also equal when subjects were informed of the probe at the end of the practice period, so that both orientations were expected; hence the subjects' expectations, not the probability of stimulus occurrence, are necessary for the effect to occur. Thus expecting a line of a particular orientation at a particular location facilities its detection.

Form Perception↗
PubMed1995

The short-term effect of nicotine chewing gum in patients with Parkinson's disease.

Because of the inverse association of cigarette smoking with the risk of Parkinson's disease, we performed a short-term, double-blind, randomized controlled trial of nicotine polacrilex resin gum in patients with this disease. Forty-eight subjects were randomly assigned to chew either nicotine gum or placebo gum three times at 2-h intervals, with evaluation of symptoms before and after the trial. The nicotine gum was reasonably well tolerated, but there were no substantial differences in Parkinson's disease symptoms between the two treatment groups.

Aged↗
PubMed1995

Field additivity of the middle-wavelength cone pathway under various test and field configurations.

The field additivity of the M-cone pathway was measured with psychometric functions at 10 times absolute threshold on monochromatic fields and their mixtures. Observers detected a 500 nm test on 530 or 610 nm fields, and a 530 nm test on 481 or 622 nm fields. For both sets of wavelengths, field additivity held with the 1 deg test, 10 deg field condition which defines II-4 and with the 3.6 min are test on a 8.6 min arc field used to isolate the M fundamental by Stockman [(1983) Ph. D. thesis, Trinity College, Cambridge University, Cambridge]. Sub-additivity occurred for a 1 deg test on a 1 deg field, a condition for Foster's "spectral sharpening" which may evince opponency.

Adult↗
PubMed1993

A review of the adverse events profile of cefpirome.

Cefpirome is a new fourth generation injectable cephalosporin antibiotic. Its tolerability profile was established in a programme of 16 clinical studies involving 3103 patients in Europe and the US. The overall incidence of clinical adverse events with cefpirome was 21.9% compared with 27.1% with comparators (ceftazidime, imipenem, ceftriaxone). Adverse events thought possibly related to treatment occurred in 12.5% of patients receiving cefpirome and 13.7% of recipients of comparator agents. Withdrawals from treatment due to adverse events occurred in 5.1 and 5.0% of patients receiving cefpirome and comparators, respectively. The commonest adverse events thought possibly related to treatment were gastrointestinal symptoms (mainly diarrhoea in 1.6 and 1.7%, respectively) and rash (1.4 and 1.4%, respectively). Comparison with data obtained from the literature for ceftazidime, ceftriaxone and the third generation cephalosporins in general shows that the adverse event profile of cefpirome is similar to that of other broad-spectrum injectable cephalosporins.

Cephalosporins↗
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↗
PubMed1986

Interocular sensitization to a rod-detected test.

Intense red light adaptation of one eye lowers the dark adapted ("absolute") threshold of a 661 nm, extrafoveal, 1.02 deg test flash in the other eye, by about 0.15 log units, for 10-15 min. This effect ("interocular sensitization") also occurs with an extrafoveal 491 nm test, but does not occur if the 661 nm test is foveal, or is made small and brief. Blue or green light adaptations, matched either photopically or scotopically to the red, do not produce interocular sensitization. Thus the conditions producing the effect include intense red light adaptation of one eye, and scotopically mediated detection in the other.

Adaptation, Ocular↗