Search PubMed⌕ Search

Biomedical subjects

M J Morgan

Publications and source records attributed to M J Morgan.

At least 55 records · Page 3Linked to original sources
PubMed1999

Concurrent measurement of perceived speed and speed discrimination threshold using the method of single stimuli.

Velocity matching using the method of Constant Stimuli shows that perceived velocity varies with contrast [Thompson, P. (1982). Perceived rate of movement depends upon contrast. Vision Research, 22, 377-380]. Random contrast jitter would therefore be expected to increase the slopes of psychometric functions, and thus the velocity discrimination threshold. However, McKee, S., Silverman, G., and Nakayama, K. [(1986) Precise velocity discrimination despite random variation in temporal frequency. Vision Research, 26, 609-620] found no effect of contrast jitter on thresholds, using the method of single stimuli. To determine whether this apparent discrepancy is due to the difference in methodology, or to the different ranges of temporal frequencies used in the two studies, we used the method of single stimuli to measure psychometric functions at three different velocities (0.5, 2.0 and 4.0 degrees/s). We found that contrast jitter increased thresholds at low but not at high velocities. Separate analysis of the psychometric functions at each contrast level showed that increases in contrast increased perceived velocity at low standard speeds (0.5 degree/s) but not at high. We conclude that the effect of contrast on perceived speed is real, and not a methodological artefact, but that it is found only at low temporal frequencies.

Contrast Sensitivity↗
PubMed1999

Contrast facilitation in motion detection: evidence for a Reichardt detector in human vision.

We measured contrast thresholds for detecting the direction of movement of a grating in a two-frame sequence with a quadrature shift between frames. The threshold contrast for the first frame was determined for a range of contrasts of the second frame. As the contrast of the second frame was increased, the threshold contrast of the first frame initially fell, and then remained constant at a value that was approximately 0.3 log units below the threshold contrast when the two frames had the same contrast (the yoked threshold). We show that this motion facilitation effect is a specific prediction of a Reichardt detector, with additive noise before the multiplication site. When the order of the two frames was reversed, the motion facilitation effect was found for contrast of the second frame, but at sufficiently high contrasts of the first frame, masking was found. The temporal order asymmetry is not found at near yoked-threshold contrasts of the first frame. The Motion Facilitation Effect was smaller than the contrast facilitation in a classical contrast discrimination experiment.

Contrast Sensitivity↗
PubMed1999

The Questionnaire of Smoking Urges is sensitive to abstinence and exposure to smoking-related cues.

The Questionnaire of Smoking Urges (QSU) was designed to measure cravings elicited by abstinence from cigarette smoking. In the present study, the sensitivity of the QSU both to brief periods of abstinence and to exposure to smoking-related cues was investigated. A progressive ratio (PR) operant procedure was also employed, in which behaviour was maintained either by primary reinforcement (puffs on a cigarette) or by secondary reinforcement (tokens exchanged later for puffs on a cigarette) in separate experiments. Dependent smokers, who were not abstinent or who were abstinent for either 2 or 4 h, were tested in both a cue-rich and a no-cue environment. The results indicated that the QSU was sensitive both to brief periods of abstinence and to exposure to smoking-related cues, but the effect of smoking-related cues on QSU scores was maximal in non-abstinent smokers. In contrast, neither abstinence nor exposure to smoking-related cues significantly influenced the number of reinforcers earned, or the number of responses made, in either PR task. However, there was a significant interaction between gender and degree of abstinence on PR performance under primary reinforcement, and correlations between PR and QSU measures that were dependent on a complex interaction between the presence/absence of smoking-related cues and the type of reinforcer used. These results suggest that the QSU is sensitive to subtle influences on subjective urges to smoke in dependent smokers and, although PR measures are relatively insensitive to these influences, there is some evidence that PR performance is related to subjective measures of the urge to smoke.

Carbon Monoxide↗
PubMed1999

Conservative full-mouth resin renewal.

The treatment of this patient involved the coordination of periodontal, orthodontic, restorative, and aesthetic considerations. It was unique because it involved only resin as the primary restorative material, which allowed for conservative preparations and restorations. In the posterior, the use of direct and indirect resins resulted in the removal of little or no healthy tooth structure. In the anterior, the use of direct resin veneers required minimal removal of enamel, while still achieving proper function and aesthetics. Resin restorations in this particular case allowed for an acceptable clinical result and a highly satisfied patient.

Adult↗
PubMed1998

The identification and expression of heme oxygenase-2 alternative transcripts in the mouse.

Murine heme oxygenase-2 (HO-2) cDNA sequences were determined through the assembly of mouse expressed sequence tag (EST) sequences using the rat HO-2 sequence as a template. The sequence analysis revealed two mRNA isoforms, probably arising through alternative splicing, which differed in their 5'-untranslated region (UTR), and were named HO-2a and HO-2b. One EST sequence included an extended 3'-UTR and suggested there may be a choice of poly-adenylation (poly-A) signal sequence. Reverse transcriptase polymerase chain reaction (PCR) suggested that HO-2a mRNA may be specifically expressed in the testis, while HO-2b mRNA was present in all tissues analysed. Furthermore, HO-2a and HO-2b transcripts were both found to include the extended 3'-UTR, but these transcripts were detected only in the testis. Northern analysis of a greater range of tissues confirmed the testis-specific expression of HO-2a mRNA and suggested that the transcripts which included the extended 3'-UTR were a small minority of the HO-2 mRNA population. These alternative murine HO-2 transcripts suggest that mechanisms such as mRNA transport, translational efficiency or mRNA turnover may be implicated in the regulation of HO-2 gene expression, most notably in the testis.

5' Untranslated Regions↗
PubMed1998

Recreational use of "ecstasy" (MDMA) is associated with elevated impulsivity.

Recent preclinical evidence suggests that repeated exposure to 3, 4-methylenedioxy-methamphetamine (MDMA; "ecstasy") produces long-term reductions in serotonin (5-HT) levels. 5-HT has been implicated in the regulation of mood, anxiety, aggression, impulsivity, and cognition. Accordingly, in the first of two separate studies, these variables were investigated in three groups: (1) MDMA group--recreational ecstasy users (who also used other illicit substances); (2) polydrug controls--who had never taken ecstasy, but otherwise had drug histories and personal characteristics similar to the ecstasy users; and (3) nondrug controls--who had never used illicit drugs, but had similar personal characteristics. All participants completed mood (Likert) scales, personality questionnaires (which included the impulsiveness, venturesomeness and empathy questionnaire-IVE), spatial span and "Tower of London" (TOL) tests, and a behavioural measure of impulsivity, the matching familiar figures test (MFF20). There were no group differences in mood, anxiety, anger/hostility, and cognitive performance, but the MDMA group committed significantly more errors in the MFF20. Subsequently, a larger sample of participants were administered mood (the General Health Questionnaire or GHQ) and personality (IVE) questionnaires before the administration of a TOL test, followed by the MFF20, and a second TOL test. Although there were no group differences in TOL performance, ecstasy users were again found to commit more errors in the MFF20 than polydrug users. Furthermore, the GHQ and IVE scores of the ecstasy users in the second study indicated, respectively, that they were more psychologically disturbed and impulsive than nondrug controls. The combined data from the two studies indicated that ecstasy users exhibited elevated impulsivity on both self-report and behavioral measures and that those who had taken the most ecstasy had the most elevated trait impulsiveness scores. These findings are consistent with previous evidence that elevated levels of impulsivity in humans are associated with reduced levels of serotonergic function.

Adult↗
PubMed1998

Visual search for a tilted target: tests of spatial uncertainty models.

We report that spatial cueing of a parafoveal target in the presence of distractors enhances orientational acuity for that target. When no distractors were present, orientation thresholds were in the range 1-4 degrees. For long exposure times, distractors increased threshold by the amount predicted from a winner-takes-all spatial uncertainty model. For short (100-msec) exposures followed by a random dot mask, the rise in threshold with distractors was considerably greater than that predicted from spatial uncertainty. For brief exposures the effect of distractors was greater when the target and distractors were spatially crowded rather than widely spaced. Adding a tilt to the distractors in the opposite direction to the target increased thresholds still further. Cueing the target with a spatial pointer decreased the effect of distractors, even when they were crowded. We suggest that when attention cannot be appropriately focused, discrimination is carried out by a relatively coarse texture analyser, which averages over several elements, and that focused attention permits the analysis of the target over a smaller area of space.

Attention↗
PubMed1997

How the human visual system encodes the orientation of a texture, and why it makes mistakes.

Human observers are exquisitely sensitive to tilt in the orientation of a line. We can detect rotations away from the vertical of 0.5 degrees. It has been suggested [1,2] that this accuracy is a result of the orientation-selectivity of simple cells in the primary visual cortex (V1), many of which have receptive fields with an elliptical shape [3]. However, it is possible to sense the tilt of many stimuli that are unlikely to have their tilt directly encoded by such cells. For example, a garment such as a tie with diagonal stripes would predominantly stimulate cells in V1 tuned to an orientation of the stripes; yet we could tell whether or not the garment as a whole was tilted from the vertical. The perception of oriented textures is subject to systematic errors, however. A striking example is the Fraser 'twisted cord' illusion (Fig. 1) in which we see the global orientation of the horizontal texture-defined lines as being tilted in the direction of its locally tilted segments. If the component segments are at a larger angle (30 degrees) to the global orientation, on the other hand, the perceived shift is in the opposite direction. We have measured these effects psychophysically, and we propose a model in which second-order orientation units receive excitation from V1 units of similar orientation, but inhibition from V1 units of dissimilar orientation. Our model correctly predicts that making the textures different in average brightness from the background will reduce the illusions.

Humans↗
PubMed1997

Motion deblurring in human vision.

Under normal viewing conditions we are little conscious of blur in moving objects, despite the persistence of vision. Moving objects look more blurred in brief than in long exposures, suggesting an active mechanism for suppressing motion blur. To see whether blur suppression would improve visual discrimination of objects, we measured blur discrimination thresholds for moving Gaussian-blurred edges and bars. The observer's task was to decide which of two moving stimuli, presented successively, was the more blurred. It is known that for stationary objects the just-noticeable difference in blur increases with baseline blur; therefore, if motion increases blur, it would be expected to increase the just-noticeable difference in blur. An active deblurring mechanism, on the other hand, would be expected to counteract the detrimental effects of motion blur on discrimination performance. We found, however, that motion increased thresholds for blur discrimination, both for brief (40 ms) and for longer (150 ms) exposures. We conclude that motion deblurring is a subjective effect, which does not enhance visual discrimination performance. Moving objects appear sharp, not because of some special mechanism that removes blur, but because the visual system is unable to perform the discrimination necessary to decide whether the moving object is really sharp or not.

Discrimination, Psychological↗
PubMed1997

The spatial limit for motion detection in noise depends on element size, not on spatial frequency.

When a random spatial noise pattern is displaced for a short distance it seems to move coherently, but when the displacement exceeds a certain value, the direction of motion cannot be clearly perceived. We measured the displacement limit (Dmax) for a two-frame sequence and found that it depended on the size of the elements comprising the random pattern, even when low spatial frequencies were removed from the pattern by spatial band-pass filtering. Dmax depended strongly on contrast for the filtered patterns, but less so for the unaltered patterns. The data support a model for low level motion detection in which the maximum motion displacement that can be detected is determined by the mean separation of pattern elements, following a stage of low-pass spatial filtering, and in which the upper spatial displacement depends upon the pattern statistics, not upon the size of detectors in the visual system.

Filtration↗
PubMed1997

The orientational resolution of human texture perception.

A major determinant of human texture segregation and discrimination is the orientational content of the stimuli used. We have investigated the ability of observers to resolve features defined in the orientation domain in a variety of textures. It was found that features had to be separated by at least 13 deg for subjects to discriminate orientationally bimodal textures from same-variance unimodal textures. For larger separations, the determinant of performance was the magnitude of the central "dip" in the probability density functions determining the bimodal textures. Resolution performance can be modelled by assuming that a filtering process over orientation demodulates the central dip in the bimodal texture and that discrimination depends on criterion depth in the resulting function. Such modelling produces relatively broad estimates of the bandwidth ranging from about 10-20 deg. Performance was similar for both line and Gabor micropattern stimuli.

Discrimination, Psychological↗
PubMed1997

The aperture problem in stereopsis.

Stereoacuity was determined for gratings and gabor patches as their orientation was varied. Acuity was constant for 1 c/deg and 2 c/deg gratings over the range 0-80 deg when it was expressed as positional shift at right angles to the grating orientation. The same was true of an 8 c/deg elongated gabor patch. However, the threshold disparity at right angles to the major axis of an oriented gaussian patch rose as it was tilted away from the vertical. Also, thresholds for a circularly symmetrical gaussian patch rose steeply with the angle of the disparity away from the horizontal. Disparities of the centroids of 4 c/deg gabor patches could be detected equally well at angles of 0 and 70 deg, independently of the angle of the carrier grating. The data indicate a variety of rules for stereoscopic matching. Large-field gratings are matched by detecting orthogonal phase shifts, or alternatively phase shifts along the horizontal axis. Smaller patches of grating are matched by their centroids, independently of their angle. Small gaussian patches are difficult to match in any direction other than along the horizontal axis. The difference between gaussian patches and 4 c/deg gabor patches with the same envelope argues against the involvement of a second-order rectifying non-linearity preceding matching.

Depth Perception↗
PubMed1997

The combination of filters in early spatial vision: a retrospective analysis of the MIRAGE model.

Since the discovery of spatial-frequency-tuned channels in the visual system, most theories attempting to account for pattern encoding have assumed that the filters can be independently accessed and flexibly combined. We review here an alternative model, 'MIRAGE', in which the filters are inflexibly combined before pattern analysis. In the MIRAGE model the half-wave rectified outputs of all spatial-frequency channels are combined before locating spatial zero-bounded regions in the neural image, which serve as the spatial primitives for pattern analysis. We describe the evidence that led to this model, and review recent evidence on the rules of filter combination.

Humans↗
PubMed1997

Monochromatic microtomographic imaging of osteoporotic bone.

A monochromatic beam obtained from laboratory based instrumentation has been used for the first time to carry out a microtomographic x-ray scan of a small segment of osteoporotic bone. The resulting tomograph provides an accurate map of the x-ray linear attenuation coefficient at the characteristic energy for Cu K alpha x-rays. The bone segment was imaged in a variety of ways which are common to microtomography in order to compare the efficacy of different experimental configurations. A Si crystal monochromator was placed before the x-ray detector in order to reject radiation scattered by the sample. In addition, polychromatic scans were compared with monochromatic scans. The effects of multiple scatter can be seen upon comparison of scans with and without the second monochromator. The results indicate that beam-hardening and x-ray scattering can significantly alter an image, thereby distorting the interpretation of what is physically present in the specimen. An accurate, monochromatic tomographic scanning facility is being developed in an attempt to map meaningful physical constants for various materials.

Bone and Bones↗
PubMed1997

Cerebral glucose utilization is reduced in second test session.

Cerebral glucose utilization was higher during the first positron emission tomography (PET) session than during the second session, as assayed using the PET [18F]fluorodeoxyglucose method in male human volunteers. This difference was due largely to data from subjects with low-trait anxiety, since subjects with high anxiety showed similar metabolism in both PET sessions. High-anxiety subjects showed greater right/ left ratios of cerebral metabolism than low-anxiety subjects, particularly during the second PET session. These findings suggest that the level of anxiety may be an important variable to consider in PET studies using multiple sessions.

Adult↗