Search PubMed⌕ Search

Biomedical subjects

M J Morgan

Publications and source records attributed to M J Morgan.

At least 73 records · Page 4Linked to original sources
PubMed1997

Discrimination of the spatial derivatives of horizontal binocular disparity.

Observers discriminated the relative disparity, disparity gradient, and disparity curvature of surfaces defined by horizontal binocular disparity in random-dot stereograms. In experiment 1, thresholds for discriminating the depth of sinusoidal corrugations were very similar for different corrugation frequencies, despite large differences in disparity gradient and disparity curvature. Thus observers used a relative disparity cue in preference to a slant or curvature cue. Experiment 2 isolated the spatial derivatives of disparity by jittering the other available cues, using surfaces with square-wave, triangle-wave, and parabolic-wave profiles. Weber fractions were 4%-10% for relative disparity, 6%-12% for disparity gradient, and 15%-30% for disparity curvature. Experiment 3 confirmed this result for larger surfaces. The study supports the view that human stereoscopic vision aims to represent the local scene relative to the observer, at the expense of computing intrinsic properties of objects, such as curvature.

Depth Perception↗
PubMed1997

Contrast discrimination function: spatial cuing effects.

The effects of spatial cuing were measured for discrimination between an increment and a decrement on a target's pedestal contrast. Discrimination thresholds measured in the absence of a spatial cue were always higher than corresponding thresholds measured in the presence of a spatial cue, except when pedestal contrast was near zero. Uncued discrimination thresholds rose monotonically with pedestal contrast; cued discrimination thresholds formed a dipper function of pedestal contrast. A spatial-uncertainty model incorporating a nonlinear transducer produced similar results.

Contrast Sensitivity↗
PubMed1996

Slim defines a novel family of LIM-proteins expressed in skeletal muscle.

We have assembled the complete protein sequence of the skeletal muscle LIM-protein SLIM by aligning overlapping cDNA sequences. These cDNA sequences were identified from our own sequencing and from BLASTn searches of non-redundant cDNA databases. The predicted SLIM protein sequence included four LIM-domains and a novel single zinc finger domain located in the N-terminal region. Similar sequences to SLIM were identified and termed SLIM2 and SLIM3. The SLIM3 cDNA sequence was identified subsequently as a partial sequence of the of the LIM-protein DRAL. The number and spacing of the LIM domains was common to all three protein sequences. The mRNA for each protein was detected in human masseter muscle RNA by Northern analysis. We suggest that these proteins belong to a novel family of LIM proteins that are expressed in human skeletal muscle.

Amino Acid Sequence↗
PubMed1996

Apparent speed of type I symmetrical plaids.

The apparent speed of plaids made up of two gratings having the same spatial frequency and the same speed was evaluated (symmetrical type I). The plaids were moving vertically as defined by the intersection-of-constraints (IOC) rule with a mean duration of 300 msec. The comparison-stimulus was a horizontal line moving vertically. The main goal of the study was to test the effect of the spatial frequency of the Distortion Product (DP). Here the DP velocity is identical to the IOC velocity. The main effect is that reducing the DP spatial frequency from 4 to 1 c/deg decreases apparent speed. A smaller effect is due to the speed of the components: when this speed becomes relatively smaller than the IOC speed, apparent speed of the plaid decreases. Finally, the DP temporal frequency seems to determine the upper limit (about 16 Hz) beyond which the plaid appears as a non-rigid moving and flickering pattern.

Differential Threshold↗
PubMed1995

Mechanisms for dynamic stereomotion respond selectively to horizontal velocity components.

When dynamic visual noise such as the 'snow' on a detuned television receiver is inspected with a delay between the signals in the two eyes, the noise appears to rotate in depth around a vertical axis. We propose that this dynamic noise stereophenomenon arises because channels tuned jointly to depth and motion respond selectively to horizontal motion components in the noise. We used spatially filtered stimuli to reduce the strength of vertically oriented Fourier components in the noise, and found that this reduced the strength of the stereophenomenon. Reducing the strength of horizontally oriented components had a lesser effect. Our evidence is consistent with the conclusion that stereo-motion is predominantly detected by oriented receptive fields with an aspect ratio similar to those of cortical simple cells.

Depth Perception↗
PubMed1995

Stereoscopic depth perception at high velocities.

The view of the world from different perspectives provided by the two eyes is used by the human visual system to compute the relative distances and solid shapes of objects. However, the traditional theory of binocular disparity takes little account of the fact that a moving target will stimulate many different sets of disparate points in the two eyes with a range of temporal delays. Here we show that stereoacuity for periodic grating is not degraded by velocities of up to 640 degrees s-1 provided that they do not move at a greater rate than 30 cycles s-1. The minimum detectable spatial phase difference between the eyes was equivalent to a spatial phase difference of about 5 degrees and an interocular temporal delay as small as 450 microseconds. We suggest that stereopsis for moving targets is accomplished by neurons having a spatial-temporal phase shift in their receptive fields between the eyes.

Depth Perception↗
PubMed1995

Structure and organization of the human glucose phosphate isomerase gene (GPI).

Two overlapping yeast artificial chromosome clones containing the human glucose 6-phosphate isomerase gene (GPI) have been isolated. PCR and direct sequencing were used to determine the exon/intron structure of the gene. The gene spans in excess of 40 kb and consists of 18 exons ranging in size from 44 to 153 bp. All splice sites conform to the GT/AG rule.

Animals↗
PubMed1995

The developmental regulation of a novel muscle LIM-protein.

Using a cDNA clone derived from a human muscle library we have identified a novel and highly conserved 2.3kb homologue which is highly expressed in skeletal muscle. The partial sequence contains at least three LIM domains and shows greatest homology with the group of LIM-proteins associated with the cytoskeleton and focal adhesion plaques which include zyxin and paxillin. This homologue is maximally expressed in differentiated ovine primary muscle cultures. It is also expressed in the ovine fetus from at least 50 days of gestation and is increasingly upregulated from 120 days of gestation to 8 weeks after birth after which it declines. This period corresponds to the period of greatest muscle fibre hypertrophy and suggests a role for this homologue in either the elaboration of muscle fibre matrix anchorage or the regulation of muscle fibre hypertrophy itself.

Adolescent↗
PubMed1995

Sequence characterization of ENU-induced mutants of glucose phosphate isomerase in mouse.

Four of five mutations producing GPI1 null lethal phenotypes in the homozygous state, which were previously identified from the offspring of male mice, spermatogonially treated with N-ethyl N-nitrosourea (ENU), have been characterized at the nucleotide level by reverse transcription of RNA from heterozygotes for mutant and wild-type alleles and cycle sequencing with cDNA-derived primers. In three of the mutations studied, a single nucleotide substitution, altering the predicted amino acid on translation, was observed in the mutant allele. In Gpi1-sam1H amino acid residue 277, TCA Ser (wild type), is altered to CCA Pro, and in Gpi1-sbm3H and Gpi1-sbm4H amino acid residue 510 Asp GAC (wild type) is altered to GGC Gly. These ENU-induced mutations occur at A-T base pairs in agreement with the current view of the mechanism of action for this mutagen. These changes also occur at residues implicated as being important in the catalytic functioning of the enzyme, from crystallographic studies, and may explain the loss of enzyme function. The fourth identified mutation, Gpi1-sbm2H, is a deletion of amino acid residues Arg134 to Leu162 inclusive, which may arise from incorrect splicing of mRNA; a fifth mutation has remained undetermined.

Animals↗
PubMed1995

Sequence characterization of alleles Gpi1-Sa and Gpi1-Sb at the glucose phosphate isomerase structural locus.

The sequences of alleles Gpi1-sa and Gpi1-sb at the glucose phosphate isomerase structural locus have been determined from cDNA of the mouse inbred strains 101/H Gpi1-sa and C3H/HeH Gpi1-sb by RT PCR and direct sequencing of the amplified products. Four individual nucleotide differences were observed between the two alleles. The difference at amino acid residue 94, (Gpi1-sa GAT Asp, Gpi1-sb AAT Asn) may account for the differing electrophoretic migration, isoelectric point, and thermostability of the two alleles. Two of the other observed differences in the coding region (amino acid residue 12 Leu, Gpi1-sa CTC, Gpi1-sb CTG and amino acid residue 17 Arg, Gpi1-sa CGC, Gpi1-sb CGT) are silent and do not affect the predicted amino acid residues on translation. The fourth observed difference is located within the 3' noncoding sequences of the cDNA. The change at amino acid residue 94 is associated with the presence of a Hinf1 restriction site in Gpi1-sb, which is absent in Gpi1-sa, and may be a useful method for determining this marker.

Alleles↗
PubMed1995

Characterization of cDNAs coding for glucose phosphate isomerase and phosphoglycerate kinase in Chinese hamster ovary cell line CHO-K1 and identification of defects in R1.1.7, a glycolysis-deficient variant of CHO-K1.

Full-length cDNAs for glucose phosphate isomerase (GPI) and phosphoglycerate kinase (PGK) of the Chinese hamster ovary cell line CHO-K1 have been characterized using RT-PCR and cycle sequencing of the PCR-amplified templates. Mutations in both genes have been identified in a glycolysis-deficient Chinese hamster ovary cell line, R1.1.7, derived from CHO-K1 cells.

Animals↗
PubMed1995

Force and moment distributions among osseointegrated dental implants.

Distributions of force, bending moment and torque are determined by structural analysis for an osseointegrated dental implant system. The system is a dental prosthesis rigidly connected to bone by implants. Since the implants have the lowest flexural rigidity of the structural components, they are considered to be the only elastic components of the system. In the analysis, the number and position of the implants are variable and the magnitude, direction and location of the applied load are arbitrary. The distributions found by force and moment balances are in the form of simple algebraic equations, a form which is useful for clinicians in determining the number and location of implants so that forces and moments are shared equitably. One immediate result of the structural analysis is that the bending moment due to the vertical component of the applied load--a moment which has previously been neglected--can produce stresses in the implant which are an order of magnitude larger than the direct axial stresses.

Biomechanical Phenomena↗
PubMed1995

Contrast detection facilitation by spatially separated targets and inducers.

We measured contrast detection thresholds for a small (3.6 x 3.6 arc min) square target in the presence and absence of spatially identical pedestal stimuli, and of a spatially non-overlapping inducing line (3.6 x 23 arc min). Results for the pedestal stimuli replicated the classical "dipper function", thresholds being reduced by near-threshold pedestals and increased at higher pedestal contrasts. An inducer without a pedestal also decreased detection thresholds. When the inducer and pedestal were combined, their effects were additive. Thus the inducer facilitated target detection when the pedestal was absent but raised detection thresholds when the pedestal contrast was sufficient by itself to lower threshold. Inducers of opposite polarity to the target did not consistently decrease target thresholds, even when they were clearly visible, arguing against spatial uncertainty as the explanation of the inducer effect. The inducer effect was independent of the length of the inducer except with small (< 3.6 arc min) stimuli, and was abolished by increasing target-inducer separation beyond about 10 arc min.

Contrast Sensitivity↗
PubMed1995

Detection of orientationally multimodal textures.

Oriented textures were produced with the use of probability density functions modulated sinusoidally over orientation. Orientational contrast sensitivity functions (OCSFs) for a task involving the discrimination of these patterns from orientationally-random textures were found for several human observers. An inverse Fourier transform of this OCSF yielded a weighting function, or filter, defined over orientation. The weighting function is broad, with a half-height full-width of 34 deg. This orientational filter was able to predict human performance in further discrimination tasks employing a variety of probability density functions over orientation.

Contrast Sensitivity↗
PubMed1995

Cerebral glucose utilization in polysubstance abuse.

Regional cerebral glucose metabolism in subjects with histories of polysubstance abuse was compared to that in control subjects who were drawn from the same community. The substance abuse group showed lower absolute metabolic rates for glucose in lateral occipital gyrus and higher normalized metabolic rates in temporal and frontal areas, including orbitofrontal cortex. It is suggested that some patterns of brain function associated with polysubstance abuse may represent consequences of drug exposure, or they could reflect pre-existing differences that may be relevant to the etiology and maintenance of polysubstance abuse.

Adult↗
PubMed1995

The orthogonal orientation shift and spatial filtering.

A line abutting two tilted flanks is apparently shifted towards the orientation orthogonal to the flanks and at the same time is reduced in its apparent length. It has been suggested that both effects are caused by band-pass spatial filtering, followed by location of the end points of the line at the peaks in the filtered image. Here implications of the filtering explanation of these effects are explored further. In the first experiment, it was predicted that orientation thresholds (as opposed to biases) would be increased for short line lengths, and would be further increased by abutting bars. The predictions were confirmed. It was shown in experiment 2 that the orientation shift was reduced by a small (4 min arc) gap between target lines and orthogonal flanks. In experiment 3 the threshold elevations and the orientation shift produced by orthogonal and tilted flanks were compared. Last, in experiment 4, the threshold elevations and orientation shift produced by orthogonal and tilted flanks, at different retinal eccentricities varying from 0 to 3.2 deg were compared, and the prediction that the magnitude of the orientation shift would decrease with line length and increase with eccentricity was confirmed. The connection is explored between the orientation shift and the Zöllner illusion, and demonstrations are presented of the Zöllner effect in which target and inducing lines are of opposite contrast on a gray background. It is concluded that the Judd and Zöllner illusions do not depend upon a single mechanism.

Differential Threshold↗