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Biomedical subjects

J A Solomon

Publications and source records attributed to J A Solomon.

At least 19 recordsLinked to original sources
PubMed1999

Transducer model produces facilitation from opposite-sign flanks.

Small spots, lines and Gabor patterns can be easier to detect when they are superimposed upon similar spots, lines and Gabor patterns. Traditionally, such facilitation has been understood to be a consequence of nonlinear contrast transduction. Facilitation has also been reported to arise from non-overlapping patterns with opposite sign. We point out that this result does not preclude the traditional explanation for superimposed targets. Moreover, we find that facilitation from opposite-sign flanks is weaker than facilitation from same-sign flanks. Simulations with a transducer model produce opposite-sign facilitation.

Contrast Sensitivity↗
PubMed1999

Dichoptically cancelled motion.

We sought to determine whether or not motion-from-texture mechanisms have access to monocular input. Adopting a strategy used by Kolb and Braun (1995. Nature, 377, 336-338), we created drifting textures that were invisible to purely binocular processes. Monocular signals readily conveyed motions defined by local orientation and flicker. However, when left- and right-eye signals were displayed simultaneously, only flicker motion was visible. We conclude that motion-from-texture mechanisms do not have access to monocular input. Further evidence suggests that motion from texture involves attentional tracking.

Discrimination, Psychological↗
PubMed1997

Model of visual contrast gain control and pattern masking.

We have implemented a model of contrast gain and control in human vision that incorporates a number of key features, including a contrast sensitivity function, multiple oriented bandpass channels, accelerating nonlinearities, and a devisive inhibitory gain control pool. The parameters of this model have been optimized through a fit to the recent data that describe masking of a Gabor function by cosine and Gabor masks [J. M. Foley, "Human luminance pattern mechanisms: masking experiments require a new model," J. Opt. Soc. Am. A 11, 1710 (1994)]. The model achieves a good fit to the data. We also demonstrate how the concept of recruitment may accommodate a variant of this model in which excitatory and inhibitory paths have a common accelerating nonlinearity, but which include multiple channels tuned to different levels of contrast.

Computer Simulation↗
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

GI manifestations of Ehlers-Danlos syndrome.

Ehlers-Danlos syndrome (EDS) is an inherited disorder of connective tissue that is distinguished not only by the triad of skin hyperextensibility, articular hypermobility, and tissue fragility but also by its heterogeneity on clinical, genetic, and biochemical grounds. The phenotypical variance that characterizes this syndrome often makes its recognition difficult, and failure to recognize the disease despite a classic course is not uncommon. Diagnosis is paramount, however, so life-threatening associations can be searched for and unique principles of management can be instituted. Patients are prone to GI catastrophes such as perforation and massive bleeding, which can be compounded by grave surgical and vascular complications. A thorough knowledge of the GI manifestations of EDS and their management is mandatory to prevent unnecessary morbidity and mortality.

Ehlers-Danlos Syndrome↗
PubMed1995

1st- and 2nd-order motion and texture resolution in central and peripheral vision.

STIMULI. The 1st-order stimuli are moving sine gratings. The 2nd-order stimuli are fields of static visual texture, whose contrasts are modulated by moving sine gratings. Neither the spatial slant (orientation) nor the direction of motion of these 2nd-order (microbalanced) stimuli can be detected by a Fourier analysis; they are invisible to Reichardt and motion-energy detectors. METHOD. For these dynamic stimuli, when presented both centrally and in an annular window extending from 8 to 10 deg in eccentricity, we measured the highest spatial frequency for which discrimination between +/- 45 deg texture slants and discrimination between opposite directions of motion were each possible. RESULTS. For sufficiently low spatial frequencies, slant and direction can be discriminated in both central and peripheral vision, for both 1st- and for 2nd-order stimuli. For both 1st- and 2nd-order stimuli, at both retinal locations, slant discrimination is possible at higher spatial frequencies than direction discrimination. For both 1st- and 2nd-order stimuli, motion resolution decreases 2-3 times more rapidly with eccentricity than does texture resolution. CONCLUSIONS. (1) 1st- and 2nd-order motion scale similarly with eccentricity. (2) 1st- and 2nd-order texture scale similarly with eccentricity. (3) The central/peripheral resolution fall-off is 2-3 times greater for motion than for texture.

Contrast Sensitivity↗
PubMed1994

The visual filter mediating letter identification.

We hear periodic sounds, or tones, by means of parallel auditory filters, each tuned to a band of temporal frequency, and we see periodic patterns, or gratings, by means of parallel visual filters, each tuned to a band of spatial frequency. Beyond helping us to see gratings, do these visual filters participate in everyday tasks such as reading and object recognition? After all, grafting visibility only requires the distinguishing of pattern from blank, whereas object recognition, for example letter identification, requires classification by the observer into one of many learned categories. Here we make use of results from hearing research, applying to vision a noise-masking paradigm that reveals the filter(s) mediating any threshold task. We find that letter-identification and grating-detection filters are identical, showing that the recognition of these objects at one size is mediated and constrained by a single visual filter, or 'channel'.

Humans↗
PubMed1994

Full-wave and half-wave processes in second-order motion and texture.

A theory of human second-order motion perception is proposed and further applied to the discrimination of texture slant. The computational algorithms for deriving the direction of left-right motion from a sequence of images are equivalent to the algorithms for deriving the direction of slant (e.g. from top left to bottom right or top right to bottom left) in a single 2D image. There is a broad range of phenomena for which Fourier analysis of the image plus a few simple rules gives a good account of human perception. The problem with this first-order analysis is that there exists a broad class of 'microbalanced' stimuli in which the motion or slant is completely obvious to human subjects but is invisible to first-order analysis. Microbalanced stimuli require second-order analysis which consists of non-linear preprocessing (spatiotemporal filtering followed by rectification of the input signal) before standard motion or slant analysis. To determine whether the second-order rectification is half-wave or full-wave, we construct two special microbalanced stimulus types: 'half-wave stimuli' whose motion (or texture slant) is interpretable by a half-wave rectifying system but not by full-wave or a first-order (Fourier) analysis and 'full-wave stimuli' which are interpretable only after full-wave rectification. Such experiments show that second-order texture-slant perception utilizes both half-wave and full-wave processes, second-order motion-direction discrimination depends predominantly on full-wave rectification and second-order spatial interactions such as lateral contrast-contrast inhibition and second-order Mach bands are exclusively full-wave.

Algorithms↗
PubMed1994

Full-wave and half-wave rectification in second-order motion perception.

UNLABELLED: Microbalanced stimuli are dynamic displays which do not stimulate motion mechanisms that apply standard (Fourier-energy or autocorrelational) motion analysis directly to the visual signal. In order to extract motion information from microbalanced stimuli, Chubb and Sperling [(1988) Journal of the Optical Society of America, 5, 1986-2006] proposed that the human visual system performs a rectifying transformation on the visual signal prior to standard motion analysis. The current research employs two novel types of microbalanced stimuli: half-wave stimuli preserve motion information following half-wave rectification (with a threshold) but lose motion information following full-wave rectification; full-wave stimuli preserve motion information following full-wave rectification but lose motion information following half-wave rectification. Additionally, Fourier stimuli, ordinary square-wave gratings, were used to stimulate standard motion mechanisms. Psychometric functions (direction discrimination vs stimulus contrast) were obtained for each type of stimulus when presented alone, and when masked by each of the other stimuli (presented as moving masks and also as nonmoving, counterphase-flickering masks). RESULTS: given sufficient contrast, all three types of stimulus convey motion. However, only one-third of the population can perceive the motion of the half-wave stimulus. Observers are able to process the motion information contained in the Fourier stimulus slightly more efficiently than the information in the full-wave stimulus but are much less efficient in processing half-wave motion information. Moving masks are more effective than counterphase masks at hampering direction discrimination, indicating that some of the masking effect is interference between motion mechanisms, and some occurs at earlier stages. When either full-wave and Fourier or half-wave and Fourier gratings are presented simultaneously, there is a wide range of relative contrasts within which the motion directions of both gratings are easily determinable. Conversely, when half-wave and full-wave gratings are combined, the direction of only one of these gratings can be determined with high accuracy. CONCLUSIONS: the results indicate that three motion computations are carried out, any two in parallel: one standard ("first order") and two non-Fourier ("second-order") computations that employ full-wave and half-wave rectification.

Contrast Sensitivity↗
PubMed1994

Healthcare reform and practice choices: a survey of osteopathic medical students.

Medical students are major stakeholders in the changing world of healthcare. Initiatives to change the specialty makeup and geographic distribution of the physician workforce, changes in the organization of healthcare delivery systems, and financing systems for healthcare will profoundly alter their practice environment while they are in school and just beginning to make career decisions. The decisions of osteopathic medical students, who currently make up 30% of the profession, will shape the profession's response to the new initiatives and determine its place in emerging healthcare systems. The authors surveyed first- and second-year osteopathic medical students to ascertain their perceived knowledge, opinions, and intentions as they relate to healthcare reform. They discovered that increasing numbers of students intend to practice in primary care fields. The student respondents support some of the major changes proposed by reformers in greater numbers than physicians as a whole, and have different priorities that they would like to see addressed in reform. This study reveals links between respondents' intentions to practice primary care and support for specific reform items, but the results show no link between anticipated indebtedness and respondents' intentions to enter primary care or specialty fields. The authors conclude that healthcare reform, although still in debate, has already exerted an influence on the decision-making processes of medical students.

Attitude↗
PubMed1993

The lateral inhibition of perceived contrast is indifferent to on-center/off-center segregation, but specific to orientation.

UNLABELLED: When a central test patch C, composed of an isotropic spatial texture, is surrounded by a texture field S, the perceived contrast of C depends substantially on the contrast of the surround S. When C is surrounded by a high contrast texture with a similar spatial frequency content, it appears to have less contrast than when it is surrounded by a uniform field. Here, we employ two novel textures: T+ which is designed to selectively stimulate only the on-center system, and T-, the off-center system. When C and S are of type T+ and T-, the reduction of C's apparent contrast does not vary with the combination of T+, T-. This demonstrates that the reduction of C's apparent contrast is mediated by a mechanism whose neural locus is central to the interaction between on-center and off-center visual systems. We further demonstrate orientation specificity: the reduction of grating C's apparent contrast by a surround grating S, of the same spatial frequency is greatest when C and S have equal orientation. Using dynamically phase-shifting sinusoidal gratings of 3.3, 10 and 20 c/deg, we measured reduction of apparent contrast using different contrast-combinations of C and S. RESULTS: (1) S gratings, both parallel and perpendicular to C, cause a reduction in C's apparent contrast relative to a uniform surround. (2) In all of the viewing conditions, the reduction of apparent contrast induced by the parallel surrounds was at least as great as that induced by the perpendicular surrounds. Often it was much greater. (3) Orientation specificity increases with increasing spatial frequency and with decreasing stimulus contrast.

Contrast Sensitivity↗
PubMed1992

Lymphocyte and lymphocyte subset numbers in blood and in bronchoalveolar lavage and pleural fluid in various forms of human pulmonary tuberculosis at presentation and during recovery.

BACKGROUND: Lymphocytes have a central role in human defences against mycobacteria. A study was designed to assess the relation between lymphocyte responses and clinical pattern of disease, nutrition and recovery during treatment in patients with tuberculosis. METHODS: Lymphocyte numbers and subsets (on the basis of CD3, CD4, and CD8 monoclonal antibodies) were measured in peripheral blood and, where appropriate, bronchoalveolar lavage or pleural fluid of patients with different forms of pulmonary tuberculosis. Eleven had localised pulmonary tuberculosis, 18 miliary tuberculosis and seven a tuberculous pleural effusion. RESULTS: CD4 lymphocytes were found in greatly increased numbers in pleural fluid and were relatively depleted in the blood. Lymphocyte numbers in bronchoalveolar lavage fluid varied widely in localised pulmonary and miliary tuberculosis but were highest in lavage fluid from patients with miliary tuberculosis. This was due to an increase in CD8 lymphocytes, which were also increased in the blood. Lymphocyte numbers bore no relation to nutrition, symptom duration, or radiographic profusion scores. In miliary tuberculosis the time taken for the chest radiograph to clear (mean (SD) 17.6 (7.8) weeks) correlated with lymphocyte numbers in lavage fluid, especially CD8 cells (r = 0.74), but not with the patients' age or nutrition. After 8 weeks' treatment, total and CD4 lymphocyte numbers in lavage fluid showed a substantial increase. CONCLUSION: The association of CD8 cells with delayed recovery is compatible with suppression of the antimycobacterial action of macrophages. The switch to predominance of CD4 cells in lavage fluid during successful treatment supports the view that they may have a role in eliminating mycobacteria.

Adult↗
PubMed1992

CP-0127, a novel potent bradykinin antagonist, increases survival in rat and rabbit models of endotoxin shock.

The bradykinin antagonist dimer CP-0127 was found to be a potent and selective inhibitor of the depressor response to bradykinin in the anaesthetized rat and rabbit. When given as a single dose s.c. (3.6 mumol/kg), the depressor response to bradykinin was blocked for the duration of the experiment (4 hours). In anaesthetized control rats, LPS from E. coli produced a profound and immediate hypotensive response, while in rats infused with CP-0127, the response to LPS was almost totally reversed. In addition, CP-0127 given as a single subcutaneous dose (3.6 mumol/kg) to rats 1 hour before LPS challenge produced a 93% survival rate, compared to 14% in control animals. Finally, a survival rate of 86% was achieved in rabbits infused with CP-0127 at 0.36 nmol/kg/min i.v., compared to 45.5% in saline-infused control animals given LPS (500 micrograms/kg i.v.). The results of these experiments provide evidence for a significant role for the kallikrein-kinin system in these models of endotoxic shock, and indicate the therapeutic potential of a bradykinin antagonist such as CP-0127 for treating this disorder in man.

Animals↗
PubMed1989

Texture interactions determine perceived contrast.

For a patch of random visual texture embedded in a surrounding background of similar texture, we demonstrate that the perceived contrast of the texture patch depends substantially on the contrast of the background. When the texture patch is surrounded by high-contrast texture, the bright points of the texture patch appear dimmer, and simultaneously, its dark points appear less dark than when it is surrounded by a uniform background. The induced reduction of apparent contrast is greatly diminished when (i) the texture patch and background are filtered into nonoverlapping spatial frequency bands or (ii) the texture patch and background are presented to different eyes. Our results are unanticipated by all current theories of lightness perception and point to a perceptual mechanism for contrast gain control occurring at an early cortical or precortical neural locus.

Contrast Sensitivity↗