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Biomedical subjects

M J Morgan

Publications and source records attributed to M J Morgan.

At least 91 records · Page 5Linked to original sources
PubMed1995

"The analogy between stereo depth and brightness": a reexamination.

A forced-choice experiment is reported, the results of which demonstrate that the cyclopean analogues of the Hermann grid, the embedded squares, and Mach bands can be readily observed. It is concluded that the earlier failure by Brookes and Stevens to observe these cyclopean analogues does not prove that spatial filtering of horizontal disparity is fundamentally different from the spatial filtering of the luminance image.

Choice Behavior↗
PubMed1994

Identification of a novel tandemly repeated sequence present in an intron of the glucose phosphate isomerase (GPI) gene in mouse and man.

Glucose phosphate isomerase (GPI, glucose 6-phosphate ketol-isomerase, EC 5.3.1.9) is a housekeeping gene expressed in all tissues and organisms that utilize glycolysis and gluconeogenesis. Deficiency in humans leads to a rare form of nonspherocytic hemolytic anemia. We have isolated a 3.2-kb mouse cDNA containing glucose phosphate isomerase coding sequence and a 2.1-kb intronic sequence and a large proportion of the human gene (approaching 55 kb) in four phage lambda recombinants. A 4-kb intronic fragment from the human gene showing homology to the mouse intronic sequence has been isolated and sequenced. The fragment contains approximately 1.5 kb of sequence that is composed of 30 repeat units of a novel 50-bp tandemly repeated units. The mouse intronic sequence contains 18 similar units. The human consensus sequence differs from the mouse consensus sequence at only 7 positions out of 50 (positions 16, 26, 27, 42, 43, 47, and 48). A probe containing the repeat element detects polymorphisms, specific to glucose phosphate isomerase, in human DNA. The repeat element does not appear to be present at any other loci in human DNA. The conservation of this intronic repeat element extends to pig and Chinese hamster.

Animals↗
PubMed1994

Motion displacement thresholds for compound stimuli predicted by the displacement of centroids.

Minimum displacement thresholds were measured using a compound motion stimulus, in which a single bar split into oppositely moving components, having different luminances and absolute displacements. The results fit a simple model in which observers detect the movement of the centroid of the combined luminance pattern of the two components. This was so even when the two components were separated by more than the static spatial resolution limit of 30 sec arc.

Humans↗
PubMed1994

What direction of motion do we see if luminance but not colour contrast is reversed during displacement? Psychophysical evidence for a signed-colour input to motion detection.

To investigate the effects of colour upon motion detection, directional discrimination by human observers was determined using two-frame kinematograms in which the two classes of element composing the pattern could differ either in luminance alone (achromatic condition), or in both colour and luminance (chromatic condition). The elements in the second frame could either have the same colour/luminance as corresponding elements in the first frame, or they could be changed (swapped) in colour and/or luminance. The angular size of the elements was varied by changing the viewing distance. Changing colour between frames disrupted motion detection when the angular size of elements was large (0.9 deg) but not when they were small (0.225 deg), replicating a previous result. Detection of motion with chromatic patterns was generally superior to that with achromatic patterns, particularly with large element size. Luminance swap combined with colour swap produced the "reverse phi" phenomenon: however, when luminance was swapped with colour staying the same between frames, forward motion was seen, suggesting that forward motion based on colour dominated over reversed motion based on luminance. We conclude that signed chromatic information has an input to motion detection at low but less so at high spatial frequencies. Information across colour and luminance is combined in a final common pathway for motion detection, resulting either in enhancement if they are in agreement, or in disruption if they conflict.

Color Perception↗
PubMed1994

Motion discrimination in two-frame sequences with differing spatial frequency content.

We measured the upper threshold for directional motion discrimination (Dmax) in two-frame random binary luminance patterns (random dot kinematograms) in which either one or both frames was spatially low-pass filtered by convolution with a Gaussian filter. When both frames were low-pass filtered, Dmax increased as a function of the standard deviation of the Gaussian blurring function, in agreement with previous findings. However, when only one of the two frames was blurred, Dmax showed little change with blurring space constants below about 20 min arc, and at larger space constants motion discrimination became impossible. We take this as evidence against the proposal that Dmax is preferentially determined by motion signals from high spatial frequencies; and as evidence for the alternative that Dmax depends upon the mean spatial interval between features in the pattern after a single stage of spatial frequency pre-filtering. The breakdown in motion discrimination for space constants above about 20 min arc can be predicted from the computed effects of blurring upon the correlation between features (zero-bounded regions) in the broad-band and spatially filtered patterns. At values of blur where motion discrimination began to collapse there was a temporal order asymmetry; discrimination was easier when the low-pass pattern preceded the broadband pattern than when the broadband pattern appeared first. We propose that the temporally sustained high spatial frequency signal in the broadband pattern is delayed relative to the more transient low frequency signal; or alternatively, that the inhibitory surround of the spatial prefilter is switched in after a delay relative to the excitatory centre. The processing-delay interpretation was tested and confirmed in a second experiment by manipulating the frame duration.

Discrimination, Psychological↗
PubMed1994

The need for wider dissemination of CPR skills: are schools the answer?

The value of instructing members of the public in CPR is now widely recognised, but community training schemes which rely largely on volunteers may fail to reach their targets. CPR training for lay people is often a once only activity and it has been shown that, without revision, skills deteriorate rapidly. By teaching CPR in secondary schools all social classes and ethnic groups could be reached, and retention of skills improved by regular revision. Health education has shown that it may be beneficial to use older pupils as instruction assistants.

Cardiopulmonary Resuscitation↗
PubMed1994

Morphine analgesia in the formalin test: evidence for forebrain and midbrain sites of action.

A mapping study was performed to determine where in the rat brain morphine acts to produce analgesia in the formalin test, which is an animal model of prolonged pain associated with tissue injury. A single dose (5 nmol) of morphine was bilaterally microinjected into a wide range of brain areas throughout the midbrain and forebrain. Strong analgesia was elicited from the posterior hypothalamic area, the periaqueductal gray and ventral tegmental area. Other sites from which analgesia was elicited were the nucleus accumbens and a few sites in the retrorubral field and caudate-putamen. Analgesia from the periaqueductal gray or nucleus accumbens was accompanied by decreased locomotor activity and catalepsy, whereas analgesia from the posterior hypothalamic area or ventral tegmentum was accompanied by a noticeable increase in locomotor activity and rearing. Morphine into various thalamic nuclei had no effect. These results indicate that the primary sites of action of morphine in the formalin test are probably the posterior hypothalamic area and periaqueductal gray, with an additional contribution from regions innervated by tegmental dopamine cells.

Animals↗
PubMed1993

The effects of adaptation and masking on incremental thresholds for contrast.

Using a temporal two-alternative forced-choice procedure, we measured thresholds for detecting increments in contrast of a 2 c/deg vertical grating at a wide range of pedestal contrasts, (1) before and after adapting to a grating of the same orientation and spatial frequency, and (2) in the presence of superimposed masks that varied in either orientation or spatial frequency. The adapting grating and all masks were of fixed 40% contrast. The results show that prior adaptation and concurrent masking have qualitatively similar effects on incremental thresholds; both raise threshold at low pedestal contrasts and leave them unaltered at higher contrasts. But masks have greater effects than adaptors, the effect of an orthogonal mask, or one two octaves higher in spatial frequency, being about the same as a parallel adaptor of the same spatial frequency as the pedestal grating. The results are explained by a model of Ross and Speed [(1991) Proceedings of the Royal Society of London B, 246, 61-69] that assumes that masks and adaptors both reposition the transducer function of contrast sensitive mechanisms and that masks, but not adaptors, also stimulate the detecting mechanism.

Adaptation, Ocular↗
PubMed1993

Stereo and motion disparities interfere with positional averaging.

If required to judge the spatial separation between features that are sub-parts of larger objects, observers are biased in the direction of reporting the separation between the centroids of the objects. We predicted that this bias would be overcome by separating the target features from the larger objects, either by stereoscopic disparity or by relative motion. We measured the observer's perceptual biases for the "cluster illusion" with the target dots specified by: (1) contrast alone, (2) contrast and disparity, (3) disparity alone, (4) contrast and motion, and (5) motion alone. Disparity had the effect predicted, but the effects of motion were less clear.

Contrast Sensitivity↗
PubMed1993

Comparison of DNA fingerprinting and serotyping for identification of avian Pasteurella multocida isolates.

The DNA fingerprint profiles and serotypes of 63 avian Pasteurella multocida field isolates, 13 attenuated vaccine isolates (propagated from vaccines manufactured by five companies), and 16 somatic reference strains were compared. DNA fingerprinting established the relationship of isolates that could not be distinguished by serotyping. Of the 76 isolates, 28 DNA fingerprint profiles and 12 somatic types were recognized. One isolate was nonreactive with 16 reference somatic and 5 reference capsule-type antisera. Thirty-one field isolates and seven vaccine isolates were identified as capsule type A. Twenty-nine field isolates and six vaccine isolates were nonencapsulated. Three field isolates were capsule type F. Isolates of capsule types B, D, and E were not found. One field isolate, identified as somatic type 7, had a DNA fingerprint identical to that of the somatic reference type 6 profile. Twelve field isolates had profiles identical to the somatic reference type 3 strain profile; 11 of these were identified as somatic type 3, 4, and 1 was identified as somatic type 3. The DNA fingerprint profiles of 50 field isolates and 13 attenuated vaccine isolates did not match profiles of the 16 somatic type reference strains. Twenty-five DNA fingerprint profiles were recognized from 30 of these field isolates. The DNA fingerprint profiles of 20 field isolates and 13 attenuated vaccine isolates were identical. Three somatic types (3; 3,4; and 4,16) were identified from the field isolates, and two somatic types (3 and 3,4) were identified from the attenuated vaccine isolates. DNA fingerprinting is useful for accurate identification and epidemiologic study of P. multocida isolates.

Animals↗
PubMed1993

Sensitivity to subjective effects of cocaine in drug abusers: relationship to cerebral ventricle size.

OBJECTIVE: The purpose of this study was to assess the functional significance of ventricle-brain ratio (VBR) in terms of how it might affect sensitivity to cocaine, an indirect dopamine agonist. METHOD: Relationships between VBR and subjective responses to acute intravenous cocaine hydrochloride were examined in 20 male polydrug abusers. Tests were performed in conjunction with positron emission tomography scans to measure cerebral glucose metabolism. RESULTS: Subjective measures of effects of cocaine, including self-report ratings of intensity of the drug effect, scores on the morphine-benzedrine scale of the Addiction Research Center Inventory, and several items on visual analogue scales, correlated negatively with VBR. VBR also differed significantly among subjects who were grouped according to scores on items ("rush" and "crash") of the Cocaine-Sensitive Scale (larger VBR in subjects with weaker responses). VBR was not correlated with cocaine-induced changes in cerebral metabolic rates for glucose. CONCLUSIONS: Relative insensitivity to the subjective effects of cocaine in polydrug abusers with ventricle enlargement suggests that ventriculomegaly may reflect changes in periventricular brain regions that mediate these effects of cocaine.

Adult↗
PubMed1993

Novel developmentally regulated exon identified in the rat fast skeletal muscle troponin T gene.

In theory, the rat fast skeletal muscle troponin T gene can generate 64 different isoforms. Here we report the identification of a novel alternative exon (exon y) that increases the potential isoform variation to 128. The inclusion of exon y in fast skeletal muscle troponin T mRNA occurs in perinatal, but not adult, skeletal muscle. Exon y is located between exons 8 and 9. This is the first time that a developmentally regulated exon located amongst a set of alternatively spliced exons has been described. Exon y is included in two mRNA isoforms. The proteins that these mRNAs would encode have molecular masses greater than that of the largest fast skeletal muscle troponin T isoform lacking exon y. These two proteins correlate well in both size and pattern of expression with the two fast skeletal muscle troponin T isoforms expressed in perinatal skeletal muscle. These results indicate that there is coordinated regulation of the splicing of exon y with other alternative exons.

Alternative Splicing↗