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Biomedical subjects

M J Wright

Publications and source records attributed to M J Wright.

At least 91 records · Page 5Linked to original sources
PubMed1990

Covert orientation of visual attention in Parkinson's disease: an impairment in the maintenance of attention.

Covert orientation of attention was studied in a group of patients with idiopathic Parkinson's disease and compared to a matched control group using a cued reaction time task which measured disengagement, covert movement and engagement of attention. Parkinson patients had an increased response latency and disengaged from attended locations more readily than controls. This impairment in the maintenance of attention in Parkinson's disease was comparable to that produced in a previous study by pharmacological blockade of brain catecholamines in normal subjects. It is suggested that an impairment in the maintenance of oriented attention may underlie some of the cognitive deficits reported in Parkinson's disease.

Aged↗
PubMed1987

Spatiotemporal properties of grating motion detection in the center and the periphery of the visual field.

Sensitivity to motion was studied by using sinusoidal gratings of contrast 0.4 that were displaced sinusoidally over time. At low motion temporal frequencies, thresholds were proportional to velocity and were independent of spatial frequency. Sensitivity, defined as the reciprocal of the threshold displacement amplitude, declined exponentially with temporal frequencies above the peak. Peak motion sensitivities decreased with visual field eccentricity but could be equalized by applying a spatial scaling function. However, this left a small relative superiority in the periphery for high temporal frequencies of motion, and possible explanations for this residual temporal inhomogeneity are considered.

Fovea Centralis↗
PubMed1986

Matching velocity in central and peripheral vision.

The apparent velocity of peripheral, drifting sinusoidal gratings was measured as a function of eccentricity and viewing distance. Gratings appeared to move more slowly in the periphery. Apparent velocities in fovea and periphery could be matched by an appropriate spatial scaling of peripheral gratings. This scaling factor provides a psychophysical measure of the changing spatial grain of the visual system with eccentricity. Scaling factors were found to be the same for lower threshold of motion and for velocity matching with standard gratings of 2 and 6 Hz. The finding generalised over a range of standard temporal frequencies (less than 7 Hz) and spatial frequencies (1.2-9 c/deg). The psychophysically determined scaling factors were found to be proportional to the square root of macaque mean cortical receptive field area as a function of eccentricity. The data support a ratio strategy for encoding motion in which motion information is expressed relative to the changing spatial grain of the visual system. Locations for the apparent identity of physically identical grating motion fell along a straight line in space, prompting an explanation of these visual field effects in terms of the acquisition of environmental information from optic flow.

Form Perception↗
PubMed1986

Apparent velocity of motion aftereffects in central and peripheral vision.

Adapting to a drifting grating (temporal frequency 4 Hz, contrast 0.4) in the periphery gave rise to a motion aftereffect (MAE) when the grating was stopped. A standard unadapted foveal grating was matched to the apparent velocity of the MAE, and the matching velocity was approximately constant regardless of the visual field position and spatial frequency of the adapting grating. On the other hand, when the MAE was measured by nulling with real motion of the test grating, nulling velocity was found to increase with eccentricity. The nulling velocity was constant when scaled to compensate for changes in the spatial 'grain' of the visual field. Thus apparent velocity of MAE is constant across the visual field, but requires a greater velocity of real motion to cancel it in the periphery. This confirms that the mechanism underlying MAE is spatially-scaled with eccentricity, but temporally homogeneous. A further indication of temporal homogeneity is that when MAE is tracked, by matching or by nulling, the time course of temporal decay of the aftereffect is similar for central and for peripheral stimuli.

Adaptation, Ocular↗
PubMed1985

Invariant tuning of motion aftereffect.

Motion aftereffects (MAE) to drifting gratings were measured with a cancellation technique. For a constant adapting grating, MAE cancellation velocity increased linearly with visual field eccentricity. The slope of the function was dependent upon adapting temporal frequency. MAE velocity was independent of spatial frequency (2-16 c/deg) and approximately constant when expressed as an M-scaled velocity. The MAE was broadly tuned to temporal frequency and temporal tuning was independent of spatial frequency and eccentricity. Reducing contrast over the range 1.0-0.1 reduced the magnitude of the MAE at high temporal frequencies but had no influence at low and moderate temporal frequencies. The results support a ratio model of velocity coding whose spatial reference varies with eccentricity in approximate accordance with the human cortical magnification factor.

Adaptation, Ocular↗
PubMed1985

Lower thresholds of motion for gratings as a function of eccentricity and contrast.

We investigated the lower threshold for motion (LTM) of gratings as a function of position in the visual field, spatial frequency and contrast and we compared motion thresholds for sine wave and square wave luminance profiles. For contrasts below 0.05 the lower threshold for motion was raised; the increase in threshold being dependent upon spatial frequency. At contrast levels above 0.05, LTM was found to be a constant velocity at any given spatial location but increased with eccentricity of view. Raised thresholds for motion at eccentric locations could be compensated by increasing the size of eccentric gratings in proportion to M-1, where M is the cortical magnification factor, a procedure which standardises the cortical representation at differing eccentricities. Thus LTM could be expressed as a constant cortical velocity for grating contrasts above 0.05 at all stimulus locations investigated. We interpret our data as support for a ratio model of velocity coding.

Adult↗
PubMed1985

The relationship of displacement thresholds for oscillating gratings to cortical magnification, spatiotemporal frequency and contrast.

The threshold amplitude for detecting square-wave oscillatory motion of a sinusoidal grating (contrast 0.3) was approximately 14 sec arc for foveal viewing, and was independent of spatial frequency in the range 2-16 c/deg. Threshold displacement amplitude (TD) was greater for eccentric stimuli; but when these were scaled inversely with the cortical magnification factor (M) it was found that TD was equivalent to a constant displacement in cortically-scaled units (0.03 mm). As grating contrast was reduced, invariance of TD over spatial frequency disappeared and consequently for a contrast of 0.03 TD was minimum at 6-8 c/deg. TD was systematically elevated at low contrast and low spatial frequency. Sensitivity to sinusoidal displacements was maximum at 2-10 Hz; for temporal frequencies below 1 Hz, TD was proportional to velocity. The results provide evidence that oscillatory displacement thresholds in the hyperacuity range depend on motion-detecting mechanisms whose spatial dimensions are correlated with M-1.

Form Perception↗
PubMed1984

Scanning and transmission electron microscopy as tools for the study of phagocytosis of bacteria adherent to hard surfaces.

A technique for studying the phagocytosis of bacteria colonizing hard surfaces is described. Rabbit peritoneal macrophages were allowed to settle on the surface of high-molecular-weight polyethylene which had been previously colonized by Escherichia coli. To ascertain the presence of bacteria on the surface of the polyethylene and the degree of spreading of the attached macrophages, the preparations were observed by scanning electron microscopy. The ingestion of E. coli by the macrophages was studied by transmission electron microscopy on ultrathin sections of resin-embedded monolayers after their separation from the polyethylene surface. Numerous intracellular bacteria were located near the area of attachment of the macrophages to the substrata, suggesting that the phagocytosis of bacteria adherent to the surface of the plastic had taken place.

Animals↗
PubMed1984

Phagosomal membranes of Mycobacterium bovis BCG-immune alveolar macrophages are resistant to disruption by Mycobacterium tuberculosis H37Rv.

Data obtained in this study reaffirm that virulent Mycobacterium tuberculosis H37Rv has a potent phagosome-destroying capacity when ingested by normal alveolar macrophages. In contrast, Mycobacterium bovis BCG-immune alveolar macrophages are highly resistant to this virulence mechanism. BCG-immune sera incubated with BCG-immune alveolar macrophages did not increase resistance of BCG-immune alveolar macrophages as compared with the data obtained from experiments with normal sera. BCG-immune sera failed to confer resistance to normal alveolar macrophages against the phagosomal membrane-destroying H37Rv virulence mechanism.

Animals↗
PubMed1984

Disruption of phagosomal membranes of normal alveolar macrophages by the H37Rv strain of Mycobacterium tuberculosis. A correlate of virulence.

The H37Rv and H37Ra strains of Mycobacterium tuberculosis were incubated with normal rabbit alveolar macrophages and examined by electron microscopy at 5 to 6 and 18 to 24 h of incubation. At the 18- to 24-h incubation interval, 60 to 100% of the endocytosed organisms of the H37Rv strain disrupted the phagosomal membranes and appeared free in the cytoplasm of the macrophages. In contrast, the H37Ra strain lacked this putative virulence characteristic, and greater than 99% of the organisms appeared within intact phagosomes. Heating the organisms of the H37Rv strain abrogated to a large extent, but not completely, their capacity to disrupt phagosomal membranes. In the course of the interaction of organisms of the virulent H37Rv strain with phagosomal membranes of normal rabbit alveolar macrophages, adherence of the phagosomal membrane to the surface of the organisms was a prominent feature that was followed by fragmentation and apparent disintegration of the membrane. This potential virulence characteristic could explain why there is essentially no resistance expressed in the lung during the early postinfectious period of primary infection to M. tuberculosis.

Animals↗
PubMed1983

Spatiotemporal contrast sensitivity and visual field locus.

Contrast sensitivity, measured as a function of retinal eccentricity for stimuli differing in temporal and spatial frequency (0.25-9 c/deg; 0-16 Hz, 0-12 degrees eccentricity), was maximum at the fovea and declined linearly with eccentricity. The slope of the decrease depended upon spatial but not temporal frequency. Contrast sensitivity for drifting gratings was approximately twice that for sinusoidal counterphase gratings at all eccentricities. For central viewing log contrast sensitivity increased with grating length. The shape of this function was systematically related to spatial frequency but independent of temporal frequency, indicating that the visual field is homogeneous in sensitivity for change in contrast over time. The implications of these findings for mechanisms of threshold vision in fovea and periphery are discussed.

Form Perception↗
PubMed1983

The red eye.

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Blepharitis↗
PubMed1982

Contrast sensitivity and adaptation as a function of grating length.

For gratings of variable length (L), contrast sensitivity increased with square root L up to a critical value equal to the width of 7-14 grating cycles, for spatial frequencies throughout the range 4-32 c/deg. For multiple gratings the critical value was the same, but the increase in sensitivity with length was more gradual. Adaptation to both single and multiple gratings produced length-specific elevation of thresholds with a bandwidth greater than 2 X that of spatial frequency-specific adaptation. The findings are interpreted in terms of multiple, independent channels tuned to spatial frequency and orientation, with the encoding of length dependent on simultaneous activity in several such channels.

Adaptation, Ocular↗
PubMed1982

The effects of contrast and length of gratings on the visual evoked potential.

Visual evoked potentials (VEP's) were obtained to grating stimuli modulated in phase at 8 Hz. The amplitude of the VEP was linearly related to the length of a centrally-fixated grating up to a critical length value equivalent to 9-16 grating cycles, for a range of spatial frequencies; thereafter amplitude first decreased then increased with increasing length. VEP amplitude was proportional to log. contrast as well as to length, and the intercept of the regression line with the contrast axis coincided with the psychophysical threshold for gratings of different lengths. More sensitive measurements with low-contrast stimuli around threshold indicated that in this range VEP amplitude is linearly related to contrast, and there is no threshold nonlinearity. The effects of visual field location and adaptation on the VEP to gratings of different lengths and spatial frequencies were also studied. The relationship of these findings to psychophysical, single-cell and transient VEP data is discussed.

Adaptation, Ocular↗
PubMed1982

Use of chemotaxis chambers for studying in vitro bacterial colonization of biomaterials.

Blind-well chemotaxis chambers were used to study the in vitro bacterial adhesion and colonization of biomaterials. Staphylococcus aureus and Pseudomonas aeruginosa were selected for the bacterial inocula. Abundant growth on the surfaces of methyl methacrylate, polyethylene, stainless steel, and Vitallium was detected by using scanning electron microscopy after 24 h of incubation. The culture technique employed proved to be of value for the study of surface bacterial colonization of inert materials.

Anti-Bacterial Agents↗
PubMed1981

Tissue fixation and staining by osmium tetraoxide: a possible role for alkaloids.

In vitro studies on the reactions of osmium tetraoxide, OsO4, with isoquinoline, pyridine, quinuclidine, and a series of structurally related alkaloids suggest that these species, under the conditions of normal tissue staining and fixation, are potentially osmiophilic. The structure of the products and their reactions with unsaturated substrates are described. The relevance of these results to the process of tissue staining and fixation of plant tissues containing alkaloids by OsO4 is discussed.

Alkaloids↗