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Biomedical subjects

M J Morgan

Publications and source records attributed to M J Morgan.

At least 145 records · Page 8Linked to original sources
PubMed1987

Opponent model for line interval discrimination: interval and vernier performance compared.

Vernier and spatial interval thresholds were determined as a function of contrast, using thin bars with a Gaussian luminance profile. The previously reported contrast dependence of vernier acuity was confirmed, but contrast had a much smaller effect upon interval acuity. A subsidiary experiment showed that the relative contrast of the two bars in an interval discrimination could be randomly perturbed over trials without substantially reducing acuity. A model of interval discrimination is developed, including an opponent stage between hypothetical spatial coincidence detectors.

Discrimination, Psychological↗
PubMed1987

The role of endogenous opioids in footshock-induced hyperthermia.

Either opioid or nonopioid forms of stress-induced analgesia can be elicited depending on the intensity or duration of the stressor. Several forms of stress have also been shown to cause hyperthermia in the rat. The present study investigated whether opioid and nonopioid forms of such stress-induced hyperthermia can be elicited in the rat as a function of footshock current intensity. Rats were given footshock after pretreatment with saline or naltrexone, or chronic morphinization. Footshock produced hyperthermia, the degree of which was found to be a function of current intensity. While the peak rise in temperature was not affected by naltrexone or chronic morphine administration, the rate of return to baseline temperature was slowed by these treatments. Thus, the endogenous opioid system appears to be involved in the return to normal body temperature following footshock, but not in the footshock-induced rise in temperature.

Animals↗
PubMed1986

Opioid and non-opioid stress analgesia from cold water swim: importance of stress severity.

We have previously reported that stress severity plays an important role in determining the neurochemical basis of stress-induced analgesia from inescapable footshock. Increasing severity (duration or intensity of continuous footshock) causes a shift in mediation of the resultant analgesia from opioid to non-opioid. In this study, we find that stress severity plays a similar role in analgesia from cold water swim. More severe swims (longer duration or lower water temperature) produce stress analgesia insensitive to the opiate antagonist, naltrexone, whereas less severe swims produce analgesia significantly attenuated by this drug.

Analgesia↗
PubMed1986

The Münsterberg figure and twisted cords.

The Münsterberg tilt effect can be explained by the effects of band-pass filtering in the visual system, supplemented by the principle that peaks and troughs in the filtered image are represented separately at the stage of orientation encoding. Circularly symmetric band-pass filtering of the Münsterberg figure reveals that both peaks and troughs along the mortar are oriented along lines that are slightly tilted with respect to the horizontal. The result is a Fraser twisted cord, which underlies the Münsterberg effect. We use the principle of the twisted cord to derive a new version of the Münsterberg effect.

Filtration↗
PubMed1986

Positional acuity without monocular cues.

The accuracy with which human observers can determine the spatial location of a shape boundary was measured by vernier alignment. The vernier targets were presented as random-dot stereograms, with varying amounts of camouflage in the monocular image. Camouflage decreased vernier acuity, but when the camouflage was broken by stereoscopic disparity, acuity was improved. In the limiting case when the shape boundaries were defined by disparity information alone, vernier thresholds (75% correct, binary forced-choice) were in the region of 40 s visual angle. This is poor acuity in comparison to vernier thresholds with monocular contour, but if the limited resolution acuity for stereopsis is taken into account, cyclopean and monocular positional acuities can be considered quite similar in relation to their respective resolution limits.

Cues↗
PubMed1986

The detection of spatial discontinuities: interactions between contrast and spatial contiguity.

Thresholds were measured for the detection of spatial discontinuities (notches and bumps) along luminance boundaries. At high contrasts of the boundary, thresholds expressed in terms of the spatial notch/bump height fell well inside the hyperacuity range. Expressed as luminance increment thresholds between adjacent photoreceptors, the same thresholds were similar to those previously reported by Hartridge and by Hecht and Mintz for the detection of a single line. The ability of observers to detect differences in the height of a boundary on either side of a mean luminance gap was also investigated, and the effect of the gap was found to depend upon stimulus contrast. At high contrasts the introduction of a gap increased thresholds, but at the lowest contrasts, thresholds were unaffected by a gap. The role of different spatial frequency and orientational mechanisms in vernier acuity is discussed.

Contrast Sensitivity↗
PubMed1985

Analgesic, locomotor and lethal effects of morphine in the mouse: strain comparisons.

We have compared the analgesic, locomotor stimulatory and lethal effects of morphine in two strains of mice, C57BL/6BY and CXBK. The CXBK strain is known to be deficient in central opioid binding sites and to be less sensitive than the C57 strain to certain effects of morphine and endogenous opioids. We found that the CXBK strain was less sensitive than the C57s to the analgesic and locomotor effects of morphine, but did not significantly differ in regard to morphine's lethal effect. The strain differences in sensitivity to the analgesic and locomotor effects were not uniform. The CXBK strain was much less sensitive than the C57 strain to the analgesic effect but only moderately less sensitive to the locomotor stimulatory effect. These differences may relate to previously demonstrated strain differences in the amounts of mu 1 and mu 2 opioid binding in central nervous system areas thought to mediate these behaviors.

Analgesia↗
PubMed1985

A theory of the primitive spatial code in human vision.

MIRAGE, a theory for the primitive coding of the (1D) spatial distribution of luminance changes by the human visual system is developed from a theoretical examination of the practical problems associated with the characterization of such changes. The main novel feature of the theory is that the multiplicity of spatial filters in human vision is assumed to exist principally to transmit a broad bandwidth signal of considerable redundancy: the filters are not assumed to be marked with their centre frequency or bandwidth, and are not analyzed independently. The theory is largely independent of the particular filter transfer function form. MIRAGE is applied to a range of one-dimensional luminance patterns, and demonstrates several well-known brightness illusions, and a structured grouping principle. It is finally shown to be supported by a wide range of psychophysical data.

Filtration↗
PubMed1985

Positional acuity with chromatic stimuli.

Theoretical reasons are presented for expecting a high precision spatial acuity task (vernier acuity) to be more difficult with an equiluminous stimulus than with stimuli containing luminance cues. This prediction was verified in Experiment 1. In a second experiment, it was shown that the result of the first experiment could not be explained by reduced detectability of the equiluminous target bar. We explain these results by the conflicting demands of chromatic and spatial differencing within a single mechanism, and propose that this also explains the similarities between long and medium wavelength cones in their spectral sensitivities.

Color Perception↗
PubMed1985

Vernier acuity predicted from changes in the light distribution of the retinal image.

If two thin bars of different luminance are placed side by side, their joint spatial position in a Vernier alignment task is determined simply by their relative luminances. The threshold luminance contrast difference required to produce a just detectable change in spatial position corresponds to a spatial shift of 5-20 arcsec in the centroid of the retinal light distribution, depending upon contrast relative to the background. This technique may be used to measure acuity with a display that has a spatial resolution considerably worse than the Vernier offset threshold. We have also extended the centroid technique to components that differ both in wavelength and luminance. Colour was found to make no essential difference to the task. Taking into account the spread of light in the retinal image, the manifest contrast thresholds are equivalent to threshold intensity increments between adjacent foveal receptors of less than 1% comparable to the values reported by Hecht and Mintz for dark line detection.

Contrast Sensitivity↗
PubMed1985

Spatial and spatial-frequency primitives in spatial-interval discrimination.

Thresholds for spatial-interval discrimination were determined under conditions designed to introduce randomness in the spatial-frequency content of the stimuli from trial to trial. Neither a random scaling of the display nor the addition of flanking bars at varying distances affected the observer's ability to judge the relative interval between pairs of bars, provided that the flanking bars were clearly resolved from the targets. Performance deteriorated only if the flanking bars were too close (less than about 2 arc min) or if the display was optically blurred. We conclude that the results pose difficulties for spatial-frequency theories of interval discrimination.

Discrimination, Psychological↗
PubMed1984

Spatial frequency interference effects and interpolation in vernier acuity.

Discrete spatial sampling of sinusoidal gratings has a detrimental effect upon vernier acuity for such stimuli if the sampling rate is less than 20 c/deg. We have examined the possibility that interpolation failure is due to masking by spatial frequency components near to the signal frequency. In an explicit masking paradigm, vernier acuity for a sine wave grating was found to be adversely affected by the presence of a masking grating falling within a critical band of frequencies near the test target. In the sampled stimulus, removal of sampling replicas similar in frequency to the test improved acuity, but the extent of the residual interference by high frequency components could not be predicted from the masking data. The high frequency interference effect depended on fixed phase relations between frequency components, creating narrow bright bars in the spatial luminance profile, and was greatly reduced by phase randomizing the sampling replicas. Various explanations of this nonlinear behaviour are discussed, including the Marr-Hildreth theory of edge detection.

Form Perception↗
PubMed1984

Spatial filters and the localization of luminance changes in human vision.

Relative location acuities similar to vernier acuity were measured for a variety of different luminance profiles and contrasts. The results are interpreted in terms of the extraction of zero-bounded centroids in the responses of a limited range of bandpass spatial filters, with centre frequencies between 3.5 and 30 c/deg. The process by which the outputs of the filters are combined is examined, and MIRAGE, a specific proposal, tested and found to be supported. Analysis of the results obtained at lower contrasts suggests that the major source of noise lies not in the amplification of the filters, but is due to the location of the centroids being distorted by spurious noise zero-crossings. The pattern of noise intrusion is readily explained in terms of MIRAGE.

Form Perception↗
PubMed1984

Intensity-response nonlinearities and the theory of edge localization.

If an edge having a particular blur profile is exchanged for an edge at the same location but having a different blur profile, apparent movement between the two edges may be seen. The spatial extent of this movement was measured by a cancellation technique. Implications for the theory of edge location are discussed, including the possibility that spatial filtering is preceded by a nonlinear transduction stage.

Filtration↗
PubMed1983

Behavioural techniques in the management of aerophagia in patients with hiatus hernia.

Aerophagia has been shown to occur in association with hiatus hernia, and it has been suggested that it may have an aetiological role. Two behavioural methods are described, which are designed to reduce the rate of swallowing. They were applied to a group of 12 patients with hiatus hernia, in whom aerophagia had been demonstrated. Patients were on the whole successful in reducing their rate of spontaneous swallowing. An association was observed between symptomatic improvement and successful reduction of swallowing rate. This was still apparent on follow-up at nine months. Symptomatic improvement, however, on the whole appeared to be short-lived. The possible clinical significance of aerophagia in hiatus hernia is discussed.

Aerophagy↗
PubMed1983

The use of different cues in vernier acuity.

The roles of the various cues in the traditional vernier target are examined. We conclude that there are at least two mechanisms by which vernier acuities of the order of 5 sec arc may be obtained. The two cues are the overall slope of the target, and the relative positional differences. By using vernier targets that are degraded in two different ways, we can demonstrate each mechanism.

Humans↗