Biomedical subjects
Publications and source records attributed to D M Levi.
There have been a number of reports on the effects of early occlusion on the visual system of cats, monkeys, and humans. In the present paper, a patient with amblyopia resulting from prolonged early occlusion in the treatment of strabismus is described, and implications for management of early strabismus are discussed.
A stratified probability sample of the Houston, Texas population was interviewed to assess public opinion of national health care and public awareness of vision care and its role in a national health care system. Over 80% of the population surveyed are in favor of a national health care bill, with a large percentage of the population favoring inclusion of vision care. Demographic trends are reported as they relate to various questions in the survey, and comparisons are made to a similar questionnaire one year earlier.
The VERs of 12 strabismic and anisometropic amblyopes were studied using unpatterned stimuli varied with respect to intensity, wavelength and frequency, and also patterned stimuli. While the unpatterned stimulus conditions did not differentiate between amblyopic and non-amblyopic eyes, VERs elicited by small checkerboard patterned stimuli were significantly reduced in amplitude in amblyopic eyes (p less than 0.01); however, there was no relationship between the reduction in visual acuity and that of the VER amplitude. VER latencies were not significantly different in amblyopic and non-amblyopic eyes across all stimulus conditions.
Six hysterical amblyopes and six controls, matched for age and sex, were evaluated as to acuity, color vision, visual fields, dark adaptometry and visual evoked responses (VER). Although the hysterical amblyopes all showed reduced acuity and constricted visual fields, no statistically significant differences were found for the dark adaptation final thresholds when compared to the control group. VERs to stimuli which were varied with respect to brightness, color, frequency and pattern, did not statistically differentiate hysterical amblyopes from the control group. The results appear to indicate the intactness of the visual pathways up to and including cortical area 17 in hysterical amblyopia.
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We report our experience with 98 patients who received primary multivisceral transplantations. Three eras can be distinguished based on the evolution of technique, immunosuppression, and monitoring: August 1994 to December 1997 (first era); January 1998 to December 2000 (second era); and January 2001 to present (third era). Sixteen patients were transplanted during the first era, 18 during the second era, and 64 during the third era. Fifty-three patients are alive with a median follow-up of 37.5 months (range: 1 to 116 months). The leading cause of mortality was infection (n = 17), followed by rejection (n = 6). Seven patients required retransplantation and five of them subsequently died. The estimated 3-year survival was 25% +/- 11% for era 1; 44% +/- 12% for era 2; and 58% +/- 7% for era 3. Additionally, 45.3% (29/64) of patients in the third era never developed rejection versus 23.5% (8/34) of patients in the first two eras combined. The percentage of patients who developed a moderate or severe rejection was significantly less in the third era compared with the first two eras combined, 31.6% (20/64) versus 67.6% (23/34). A comparison of the hazard rate of developing severe rejection showed a protective effect of the multivisceral graft (P = .0001). In conclusion, multivisceral transplantation is indicated for patients with short bowel syndrome and extended abdominal catastrophies. Evolution in surgical techniques, immunosuppression, and monitoring have improved patient survival, which is now similar to that of other complex solid organ transplants.
INTRODUCTION: In a prospective protocol we studied whether serum citrulline level within 30 days of an acute rejection was predictive of the episode. METHODS: An acute rejection episode was defined as the date of occurrence of any biopsy-proven rejection in which treatment was initiated until two successive biopsies showed no further rejection. We compared the mean citrulline level based on values determined within 30 days of the start of an acute rejection episode with the mean citrulline level measured on the same patient during a rejection-free period. Serum citrulline measurements were available immediately prior to the occurrence of rejection for 22 patients who experienced 37 episodes. RESULTS: For the 12 episodes of mild rejection, the mean serum citrulline level +/- SE (standard error) was 15.0 + 2.3 micromol/L prior to rejection and 18.8 +/- 2.4 micromol/L during the rejection-free periods. A paired t test of the mean differences was not significant (P = 17). For the 25 episodes of moderate or severe rejection, the mean serum citrulline level was 12.4 +/- 1.1 micromol/L before rejection and 18.8 +/- 2.0 micromol/L during the rejection-free periods. A paired t test of the mean difference was statistically significant (P = .002). CONCLUSIONS: Although further study of citrulline as a marker for the early detection of acute rejection episodes is needed, our hope is that its use will help to prevent some of these early episodes from evolving into full-blown moderate or severe grades of rejection.
BACKGROUND: Alemtuzumab (Campath-1H [C1H]) is a humanized monoclonal antibody directed against the CD 52 antigen that is present on the surface of T cells, B cells, natural killer cells and monocytes. We studied its application in intestinal transplantation. METHODS: This is a retrospective review of adult patients who underwent intestinal transplantation between December 1994 and May 2005. Group 1: non-C1H group (n = 39); group 2: C1H group (n = 37). C1H was administered as an induction immunosuppression in four doses (0.3 mg/kg), or in two doses (30 mg/kg). Tacrolimus levels were maintained at low level (5-10 ng/dL). No maintenance steroids were given. RESULTS: One-year survival of group 1 and group 2 patients were 57% and 70%, respectively. This difference is not statistically significant. Of 37 patients in group 2, 21 are alive. The incidence of rejection was lower in group 2 (P < .005). Average current tacrolimus level is 6.97 +/- 3.98 ng/dL. Seventeen patients (81%) are steroid free, and 15 (71%) are maintained solely on tacrolimus. There was no graft versus host disease in group 2. CONCLUSIONS: Our preliminary data suggest that C1H can provide effective immunosuppression for intestinal transplantation. Incidence of rejection was less with this regimen using low maintenance tacrolimus and minimal steroids.
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Objects falling across the physiological blind spot appear "complete" despite the absence of photoreceptors. Completion of objects may occur across the blind spot because (1) the blind spot is filled in with the background (the associative explanation); (2) the opposite sides of the blind spot may be contiguously represented in the cortex (i.e. the blind spot is simply sewn up-the retinotopic explanation); or (3) the blind spot may be sewn up, with compensatory expansion occurring around the blind spot (the compensation explanation). These theories would predict no size distortions regardless of object size; constant size distortions regardless of object size; and distortions that depend on the size of the object, respectively. To evaluate these explanations, we measured size distortions at the blind spot. We measured length distortions at the blind spot using a criterion-free two-alternative forced-choice method with feedback. Observers compared the lengths of test bars presented across the blind spot with lengths of reference bars presented at the corresponding location in the fellow eye. Test bar lengths ranged from 7-14 deg. Reference bar lengths were in the range of +/- 3 deg of test bar length. From the observers' responses the perceived length of each bar at the blind spot was estimated. Estimates of the precision of length discrimination at the blind spot were also obtained. Our results were consistent with the associative explanation. In all seven observers, length distortions at the blind spot were smaller than 1 deg (< 20% of the vertical height of the blind spot) for all bar lengths tested. For bars that were presented across the blind spot, the precision with which observers could discriminate length was comparable to that of normal periphery (Weber fraction approximately 20%). Both the veridicality and precision of perceived length are preserved around the blind spot.
A psychophysical analog to cortical receptive-field end-stopping has been demonstrated previously in spatial filters tuned to a wide range of spatial frequencies (Yu & Levi, 1997a). The current study investigated tuning characteristics in psychophysical spatial filter end-stopping. When a D6 (the sixth derivative of a Gaussian) target is masked by a center mask (placed in the putative spatial filter center), two end-zone masks (placed in the filter end-zones) reduce thresholds. This "end-stopping" effect (the reduction of masking induced by end-zone masks) was measured at various spatial frequencies and orientations of end-zone masks. End-stopping reached its maximal strength when the spatial frequency and/or orientation of the end-zone masks matched the spatial frequency and/or orientation of the target and center mask, showing spatial-frequency tuning and orientation tuning. The bandwidths of spatial-frequency and orientation tuning functions decreased with increasing target spatial frequency. At larger orientation differences, however, end-zone masks induced a secondary facilitation effect, which was maximal when the spatial frequency of end-zone masks equated the target spatial frequency. This facilitation effect might be related to certain types of contour and texture perception, such as perceptual pop-out.
The human visual system is capable of detecting a vernier misalignment with extraordinary accuracy. Since this remarkable precision in spatial localization is better than can be naively predicted by simple optical or anatomical considerations it has been termed a hyperacuity. So far no single neurone model seems capable of accounting for hyperacuity, and the retinal image might require reconstitution in a finer grained from in the visual cortex. We report here an electrophysiological correlate of hyperacuity recorded from the human visual cortex. The amplitude of the visually evoked potentials (v.e.ps) elicited by the appearance of a vernier offset varied systematically with the magnitude of the offset. Extrapolation of the function relating v.e.p. amplitude and log offset to zero voltage resulted in an electrophysiological estimate of vernier acuity that was similar to the observer's psychophysical threshold.
The human visual system is capable of making spatial discriminations with extraordinary accuracy. In normal foveal vision, relative position, width or size can be judged with an accuracy much finer than the size or spacing of even the smallest foveal cones. This remarkable accuracy of spatial vision has been termed 'hyperacuity'. Almost a century ago Ewald Hering proposed that the accuracy of Vernier acuity could be accounted for by averaging of discrete samples along the length of the lines comprising the targets; however, the discovery that Vernier acuity of a few arc seconds could be achieved with dots has rendered the nature and role of sampling in spatial discrimination unclear. We have been investigating the sampling of spatial information in central and peripheral vision (the perifovea) of normal human observers and in observers with strabismic amblyopia. Our results, presented here, show that peripheral vision and central vision of strabismic amblyopes differ qualitatively in their sampling characteristics from those of the normal fovea. Both the periphery and the central visual field of strabismic amblyopes demonstrate marked positional uncertainty which can be reduced by averaging of spatial information from discrete samples.