Biomedical subjects

D R Simmons

Publications and source records attributed to D R Simmons.

14 recordsLinked to original sources

PubMed1999

On the apparent collapse of stereopsis in random-dot-stereograms at isoluminance.

We have investigated the apparent collapse of stereopsis obtained with random-dot-stereograms at isoluminance. Contrast thresholds for both depth and form discrimination of targets in random-dot- and figural stereograms were measured at a number of disparitics, using both isoluminant and isochromatic stimuli. All contrast thresholds for stereoscopic tasks were normalised to contrast thresholds for detecting the appropriate stimulus. We found that at isoluminance contrast thresholds for depth judgements were not higher for random-dot compared to figural stereograms, even when normalised to the same thresholds obtained with isochromatic stimuli. On the other hand contrast thresholds for three-dimensional form judgements were much higher than those for depth judgements in isoluminant, compared to isochromatic random-dot-stereograms. This specific impairment of stereoscopic form (as opposed to depth) processing at isoluminance was confirmed in a further experiment in which subjects were required to judge the presence and orientation of depth corrugations in a disparity-modulated random-dot-stereogram.

Color Perception↗

PubMed1998

On the binocular summation of chromatic contrast.

The binocular summation of chromatic contrast was investigated under a variety of stimulus conditions. Binocular and monocular contrast detection thresholds were measured using 0.5 cpd Gabor patches. It was found that, using stimuli which contained combinations of chromatic and luminance contrast, binocular detection could take place independently in luminance-contrast- and chromatic-contrast-sensitive mechanisms. It was also found that, with chromatic stimuli, levels of binocular summation were above those expected from probability summation between the eyes, and thus showed evidence for binocular neural summation within chromatic detection mechanisms. The implications of these results for (a) the binocularity of chromatic detection mechanisms, and (b) the suggested link between stereopsis and binocular neural summation, are discussed.

Color Perception↗

PubMed1998

The minimum contrast requirements for stereopsis.

A number of researchers have compared the contrast requirements for stereopsis with those for detection of the stereoscopic stimulus, but they have generally failed to allow for the fact that stereopsis requires a detectable stimulus in both eyes at the same time. It is argued that the most appropriate detection threshold for this comparison is that for simultaneous monocular detection (SMD) of the stereoscopic half images. Experiments in which this comparison threshold has been used are summarised and the hypothesis generated that, on using stimuli that are localised in both space and spatial frequency (e.g. Gabor patches or differences of Gaussians), a range of disparities can always be found over which contrast thresholds for depth identification are less than or equal to this SMD threshold (the SMD hypothesis). It is argued that the success of this hypothesis in describing data obtained with these stimuli is consistent with the notions of labelled lines for disparity sign and the size--disparity correlation. Last, experiments are reported in which contrast thresholds for stereoscopic depth identification (front/back) were measured with interocular differences in contrast. The data obtained are consistent with the presence of both inhibitory and excitatory interactions between the eyes when unequal monocular contrasts are presented. The implications of these results and the SMD hypothesis for theories of stereopsis and binocular function are discussed.

Contrast Sensitivity↗

PubMed1998

The missing-fundamental illusion at isoluminance.

The missing-fundamental illusion describes how a square wave with its fundamental Fourier component removed appears as a square wave. This illusion is normally explained with reference to the bandpass nature of the luminance-contrast-sensitivity function, together with a 'default-to-square-wave' rule. Since the chromatic-contrast-sensitivity function is low-pass, we should not expect a missing-fundamental illusion at isoluminance. Using a simultaneous-detection-and-identification paradigm to eliminate contrast as a cue to discrimination, we nevertheless found that chromatic missing fundamentals and square waves could not be separately identified at detection threshold: just under twice the contrast required to detect the stimuli was needed to identify them. To test whether this was due to insufficiently narrow chromatic-channel bandwidths, we measured detection and identification thresholds for chromatic F and 3F sine-wave gratings. In this case identification was possible almost at detection threshold, suggesting that channel bandwidth limitations were not the critical factor. It is suggested that the weak missing-fundamental illusion observed at isoluminance probably reflects the operation of mechanisms similar to those that are responsible for the chromatic Craik-Cornsweet illusion.

Color Perception↗

PubMed1997

Abnormal intraoperative cholangiography. Treatment options and long-term follow-up.

OBJECTIVE: To determine the long-term outcome in patients with filling defects on intraoperative cholangiography. DESIGN: Case series; retrospective review. SETTING: Community teaching hospital. PATIENTS: All patients (n = 872) undergoing cholecystectomy from July 1993 through June 1995. Of 281 intraoperative cholangiograms performed, 89 had abnormal findings. Defects were classified as stone (n = 47), unsure (n = 29), and artifact (n = 13). Medical records were reviewed for immediate and long-term follow-up results. INTERVENTION: Need for common bile duct exploration (CBDE) or endoscopic retrograde cholangiopancreatography (ERCP). OUTCOME: Morbidity and interventions required 1 to 3 years after surgery. RESULTS: Of the 47 patients with suspected stones, 24 underwent successful operative bile duct clearance. One patient required irrigation. Of the 22 patients who left the operating room with unresolved stones, only 2 ERCPs were required. Of the 29 patients with unsure filling defects, operative clearance was successful in 1; irrigation achieved clearance in 4. Only 1 of the 24 patients who left the operating room with unsure filling defects required subsequent ERCP. CONCLUSIONS: Observation of common bile duct defects of 4 mm or smaller is an appropriate clinical alternative. Defects of 5 mm or larger represent a gray area, although few 5- to 8-mm stones will cause subsequent symptoms. In our experience, if stone extraction is clinically important, especially if the patient has jaundice, open CBDE is more effective than transcystic laparoscopic CBDE.

Cholangiography↗

PubMed1997

On the independence of chromatic and achromatic stereopsis mechanisms.

The extent to which the processing of stereoscopic depth information can take place separately in colour-contrast-sensitive and luminance-contrast-sensitive mechanisms has been investigated. Contrast thresholds for stereoscopic depth identification (front/back) were measured using 0.5 c/deg Gabor patches. The stimuli possessed different amounts of colour and luminance contrast ranging from isoluminance (red/green) to isochrominance (yellow/black) through intermediate values. Two models for combining chromatic and achromatic stereopsis information were tested. The first (single-pathway) model assumed colour and luminance contrast summation within a single luminance-contrast-sensitive mechanism before stereoscopic judgement. The second (dual-pathway) model assumed probability summation between independent chromatic and achromatic stereopsis mechanisms. The latter model provided the better fit to the data. In providing evidence in favour of an independent chromatic stereopsis mechanism, it was shown that luminance artifacts were unlikely to be the cause of maintained stereopsis at isoluminance. The possible neural substrates of chromatic stereopsis are discussed.

Color Perception↗

PubMed1996

Stereoacuity and colour contrast.

We have measured the contrast dependence of stereoacuity using both horizontally and vertically oriented, isoluminant (red-green) and isochromatic (yellow-black), 0.5 c/deg Gabor patches. For comparison, contrasts were computed in multiples of detection threshold, where detection threshold was defined as the contrast required for the stimulus to be simultaneously detectable in each eye. Disparity thresholds (1/stereoacuity) for vertical chromatic Gabors were higher than those for vertical luminance Gabors by a factor of between 4 and 9 depending on contrast, and declined less steeply with contrast. Disparity thresholds for horizontal chromatic Gabors were very high (130-210 min arc) compared with horizontal luminance Gabors (by a factor of between 9 and 17) and were only measurable at contrasts above 10 times simultaneous monocular detection threshold. These results support the view that chromatic stereoscopic processing is less precise than luminance stereoscopic processing, and that there is a special deficit in the processing of disparity with horizontally oriented chromatic stimuli. The implications of these results for the role of colour vision in stereopsis are discussed.

Color Perception↗

PubMed1995

Galanin is contained in GABAergic neurons in the rat spinal dorsal horn.

In order to determine which types of neuron in laminae I-III of the rat spinal dorsal horn contain the peptide galanin, pre-embedding immunocytochemistry with antiserum to galanin was combined with post-embedding detection of GABA- and glycine-like immunoreactivities. Sixty-eight galanin immunoreactive neurons in laminae I-III selected from four rats were examined, and in each case semi-thin sections through the cell body were tested with a monoclonal antibody to GABA and an antiserum to glycine. All of the 68 galanin-immunoreactive neurons tested were GABA-immunoreactive, while only one of them (in lamina III) was glycine-immunoreactive. This suggests that galanin is contained in inhibitory interneurons, and that (like enkephalin, neuropeptide Y and thyrotropin-releasing hormone) it is mainly restricted to GABAergic neurons which do not use glycine as a co-transmitter.

Animals↗

PubMed1995

Differences between stereopsis with isoluminant and isochromatic stimuli.

Contrast thresholds for stereoscopic depth identification (crossed or uncrossed) were measured as a function of disparity by use of isoluminant (red-green) and isochromatic (yellow-black) 0.5 cycles/deg Gabor patches. For the purposes of comparison, stimulus contrasts were scaled by their respective detection thresholds. The Gabor patches could be either vertically or horizontally oriented. It was found that the disparity dependence of the depth-identification contrast thresholds was similar for both chromatic and luminance patterns if the stimuli were vertically oriented, with the overall level of performance worse for the chromatic patterns by a factor of approximately 2 (6 dB). With horizontal patterns this difference was much larger, by a factor of approximately 7 (17 dB). These results suggest first that stereopsis in the absence of luminance cues is supported by a less-contrast-sensitive linear mechanism than that which supports stereopsis in the presence of luminance cues and second that the corresponding nonlinear chromatic stereo mechanism is either nonexistent or very weak. The implications of these data for previous studies of stereopsis at isoluminance is discussed.

Color Perception↗

PubMed1994

Contrast thresholds for stereoscopic depth identification with isoluminant and isochromatic stimuli.

Contrast thresholds for stereoscopic depth identification (crossed or uncrossed) were measured as a function of disparity using isoluminant (red-green) and isochromatic (yellow-black) 0.5 c/deg Gabor patches. For the purposes of comparison stimulus contrasts were scaled by their respective detection thresholds. The detection thresholds employed were computed from the monocular detection thresholds of the stereo half-images, based on the assumption that simultaneous detection of these half-images in each eye was a sufficient condition for stereopsis. It was found that the disparity tuning of both chromatic and luminance mechanisms was similar, with a performance peak for a binocular phase disparity of 50-120 deg. However, more contrast was required, relative to detection threshold, for the chromatic patterns to evoke a sensation of stereoscopic depth. These results suggest that stereopsis in the absence of luminance cues is supported by a less-contrast-sensitive analogue of the system that supports stereopsis in the presence of luminance cues. The results are also consistent with there being a lower density of disparity-selective mechanisms in the chromatic pathway. The implications of these data for previous studies of stereopsis at isoluminance is discussed.

Calibration↗

PubMed1994

Viewpoint--invariance of contour-curvature discrimination over extended performance levels.

A traditional approach to the problem of human object recognition is to assume that the visual system represents an object in terms of invariant quantities. This study considers a generalization of this approach to the problem of visually recognizing differences in shape, specifically between contour markings on a planar surface, as the position of the surface varies in space. For a given 'shape cue', perceived differences in contour may be quantified by threshold values--Weber fractions--at a particular criterion level of performance. A necessary theoretical condition for the Weber fraction to be constant with the relative viewpoint of the observer is that the cue should be a relative invariant under the natural spatial transformations of the image. Some data are reviewed showing how statistically efficient some cues are in explaining the observed discriminability of symmetric curved contours at a fixed criterion performance level of 75%. A new, fuller analysis is presented showing that the efficiency of the most efficient cue satisfying the invariance condition is maintained over a wide range of criterion performance levels, from 53% to 92% correct, corresponding to discrimination index values of 0.1 to 2.0. Over these levels, Weber's law was found empirically to hold almost exactly.

Animals↗

PubMed1993

The cue for contour-curvature discrimination.

Seven potential geometric cues for contour-curvature discrimination were tested: curvature, turning-angle, arc-length, arc-length-divided-by-chord-length, maximum-deviation (sag), mean-deviation and area. Three experiments were performed, each requiring the discrimination of two simultaneously presented, 1-sec-duration, curved-line stimuli, whose chord-lengths ranged from 12 to 48 arcmin visual angle and whose curvatures ranged from 0 to 0.13 arcmin-1. Experiments 1 and 2 determined for each cue the smallest detectable increment (the increment threshold) as a function of cue value, for a set of spatial transformations of the stimulus (one- and two-dimensional scalings) equivalent to changes in viewing distance and direction. In accordance with statistical estimation theory, the "best" cue was defined as the most efficient one, that is, the one which best accounted for the variance in the data. As a control, Expt 3 compared increment-threshold functions for circular and elliptical arcs of constant chord-length and circular arcs of constant arc-length. Over all three experiments, only sag and its linear approximation, mean-deviation, accounted well for the variance in the data; sag provided the best predictor, and its increment-threshold function satisfied Weber's law over almost all of the stimulus range. Additionally, sag has a special theoretical property (shared only with mean-deviation and area): the relationship it defines (in proportional terms) between curved contours in an image of an object or a scene is constant and independent of viewing distance and direction.

Adult↗

PubMed1993

Evidence for the corelease of dynorphin and glutamate from rat hippocampal mossy fiber terminals.

Hippocampal mossy fiber (MF) nerve endings may be isolated in a subcellular fraction (P3) that releases both prodynorphin-derived peptides and glutamate (Glu) in a calcium-dependent manner when depolarized. However, this isolation procedure does not yield a pure preparation of MF synaptosomes. The present study evaluates the proportion of dynorphin (Dyn) and Glu that is released from synaptosomes in the P3 fraction that are of MF origin. We have addressed this issue by determining the degree to which a selective lesion of the dentate granule cell/MF system in vivo concomitantly reduces the exocytosis of Dyn and Glu from the P3 subcellular fraction. Unilateral injections of colchicine into the dentate gyrus resulted in a substantial and selective degeneration of the granule cell/MF pathway in the rat hippocampal formation. The overall integrated density of the Timm-stained band, which corresponds to the position of the MF terminal field, was estimated to be reduced by 75%. After this extensive loss of MF boutons, the K(+)-evoked release of Dyn and Glu from the P3 fraction was reduced by 95 and 51%, respectively. The loss of Timm staining and evoked Dyn release indicate that colchicine effectively eliminated MF synaptosomes from the P3 fraction. Those subcellular entities that were not destroyed by colchicine comprised approximately 50% of the protein and evoked Glu release measured by using the P3 fraction. In addition, the present results demonstrate that the inhibitory potency of the kappa opioid agonist U-50,488H was not altered by the elimination of MF boutons from this synaptosomal preparation. This finding indicates that U-50, 488H is capable of suppressing Glu exocytosis from both MF and non-MF synaptosomes. These results are consistent with the hypothesis that Dyn peptides and Glu are co-released from hippocampal MF terminals.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗

PubMed1986

Quantitative levels of C-reactive protein in cerebrospinal fluid in patients with bacterial meningitis and other conditions.

We measured levels of C-reactive protein (CRP) in the cerebrospinal fluid in 145 children, using a solid-phase radioimmunoassay. The CRP levels in 49 patients with culture-proved bacterial meningitis ranged from 0 to 51,000 ng/ml (median 1460 ng/ml). In 33 patients with aseptic meningitis, values were much lower range 0 to 438 ng/ml; (median 17 ng/ml). In patients with CSF pleocytosis (greater than 10 WBC/microliter), CRP greater than 100 ng/ml was 95% accurate in identifying those with bacterial meningitis. However, a few patients with bacterial meningitis and little or no CSF pleocytosis had low levels of CRP at admission. Among the 63 patients with nonmeningitic conditions, those with bacterial infections frequently (10 of 13 had CRP greater than 100 ng/ml, whereas CRP elevations were infrequent (seven (18%) of 40) in patients with viral infections and other conditions. CRP diffuses into the CSF as readily as other proteins, but in bacterial meningitis the CSF/serum ratio of CRP was lower than that of albumin and IgG. The measurement of CRP in CSF is potentially a very useful diagnostic tool, but certain inherent limitations must be recognized, because some patients may fail to mount a prompt inflammatory response.

Adolescent↗