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Biomedical subjects

S P Tripathy

Publications and source records attributed to S P Tripathy.

At least 19 recordsLinked to original sources
PubMed2004

An extensive indoor 222Rn/220Rn monitoring in Shillong, India.

The behaviour of ubiquitous radon (222Rn), thoron (220Rn) and their progeny in the indoor atmosphere generally reflects a complex interplay between a number of processes, the most important of which are radioactive alpha-decay, ventilation, attachment to aerosols and deposition on surfaces. The present work involved a long-term (1997-2000) passive monitoring of 222Rn and 220Rn in the indoor environment of Shillong, Meghalaya. The north-east region of India being a zone of high seismicity, the indoor radon and thoron map of the region will provide a better insight and a valuable database for any study related to radon and thoron anomalies.

Air Pollutants, Radioactive↗
PubMed2004

Hardness ratios of different neutron spectra.

Extensive data have been gathered in the past on the response of different detectors, based on the registration of neutron-induced fissions in bismuth, gold, tantalum and thorium by the spark-replica counter and the thin film breakdown counter. These detectors make it possible to exploit the excellent characteristics of the fission reactions for the measurements of high-energy neutrons. Most of the investigations have been carried out at the quasi-monoenergetic neutron beam facility at The Svedberg Laboratory-TSL of the Uppsala University in cooperation with the Khlopin Radium Institute (KRI). The responses of different fission detectors in the neutron energy range 35-180 MeV have been evaluated: a region where the predictive power of available nuclear reaction models and codes is not reliable yet. For neutron energy >200 MeV, the fission-detector responses have been derived from the data of the proton fission cross sections. By using the ratio of the responses of these detectors, a simple and accurate way to evaluate the spectrum hardness can be obtained, thus providing a tool to obtain spectral information needed for neutron dosimetry without the need to know the entire spectrum. Extensive data have been already obtained for the high-energy neutron spectrum from the CERN concrete facility. In the present paper, the measured values of the response ratios for different fissile detectors exposed at the CERN facility are compared with those calculated for the spectra from the same facility and from different altitudes in the atmosphere, respectively.

Environmental Exposure↗
PubMed2000

Absence of coreceptor switch with disease progression in human immunodeficiency virus infections in India.

The envelope glycoprotein of the human immunodeficiency virus (HIV) utilizes CD4 as a receptor and CCR5 and/or CXCR4 as coreceptor to gain entry into the cell. The CCR5-tropic viruses, observed early in infection, could be important in transmission and the CXCR4-tropic viruses, observed late, may play an important role in disease progression. Viruses from 40 HIV-positive, asymptomatic or symptomatic individuals in India were isolated. Of 40 isolates 39 used CCR5. Thirty-three isolates were subtype C, 3 isolates were subtype A, and 4 isolates were HIV-2. Only 1 HIV-2 isolate, from a symptomatic individual, was dualtropic. Therefore, a majority of isolates from India belonged to subtype C and all the isolates utilized CCR5 exclusively irrespective of HIV disease status.

Adult↗
PubMed1999

Looking behind a pathological blind spot in human retina.

Recent work suggests that dichoptic lateral interactions occur in the region of the visual field of one eye that corresponds to the physiological blind spot in the other eye (Tripathy, S. P., & Levi, D. M. (1994). The two-dimensional shape of spatial interaction zones in the parafovea. Vision Research, 34, 1127-1138.) Here we ask whether dichoptic lateral interactions occur in the region of the visual field of one eye that corresponds to a pathological blind spot, a retinal coloboma in the other eye. To address this question we had the observer report the orientation of a letter 'T' presented within this region in the presence of flanking 'T's presented to the other eye around the coloboma. A large drop in performance was seen due to the flanks, showing the existence of dichoptic lateral interactions in this monocular region. The presence of these dichoptic interactions in a region lacking direct retinal afferents from one eye is consistent with the proposition that long-range horizontal connections of the primary visual cortex mediate these interactions.

Coloboma↗
PubMed1999

Detecting collinear dots in noise.

We estimated the sensitivity for detecting a row of collinear target elements (usually dots) by measuring the maximum density of randomly positioned noise elements that allowed 75% correct detection of the orientation of alignment (binary choice: horizontal versus vertical) of the target elements. We varied the number of target elements, their mode of generation, and their accuracy of positioning. As reported previously (Moulden (1994) Higher-order processing in the visual system. Ciba Foundation Symposium 184. Chichester: Wiley), target detection improved rapidly until the number of target elements reached about seven, and then improved more slowly beyond this point. However, this break was reduced (and often removed entirely) when the target array was formed by repositioning pre-existing noise elements lying close to the target location, rather than by superimposition of additional target elements onto the noise array. This almost linear slope of improvement, coupled with the observation that target detection was disrupted more by random jitter of target elements at right angles to their axis of alignment than by jittering along this axis, argues against a two-stage process of perceptual grouping (Moulden, 1994) and supports instead an explanation based on the operation of a single mechanism. This single mechanism explanation is further supported by the observation that intrinsic positional uncertainty (estimated from the results of jitter experiments) was independent of target element number. Additional experiments showed that target detection is facilitated by aperiodic noise dots that fall close to the target axis. The results are discussed in relation to alternative explanations of perceptual grouping.

Gestalt Theory↗
PubMed1998

Insights into motion perception by observer modeling.

The statistical efficiency of human observers performing a simplified version of the motion detection task of Salzman and Newsome [Science 264, 231 (1994)] is high but not perfect. This reduced efficiency may be caused by noise internal to the observers or by the observers' using strategies that are different from that used by an ideal machine. We therefore investigated which of three simple models best accounts for the observers' performance. The models compared were a motion detector that uses the proportion of dots in the first frame that move coherently (as would an ideal machine), a model that bases its decision on the number of dots that move, and a model that differentially weights motions that occur at different locations in the visual field (for instance, differentially weights the point of fixation and the periphery). We compared these models by explicitly modeling the human observers' performance. We recorded the exact stimulus configuration on each trial together with the observer's response, and, for the different models, we found the parameters that best predicted the observer's performance in a least-squares sense. We then used N-fold cross validation to compare the models and hence the associated hypotheses. Our results show that the performance of observers is based on the proportion, not the absolute number, of dots that are moving and that there was no evidence of any differential spatial weighting. Whereas this method of modeling the observers' response is demonstrated only for one simple psychophysical paradigm, it is general and can be applied to any psychophysical framework in which the entire stimulus can be recorded.

Algorithms↗
PubMed1997

Correspondence noise and signal pooling in the detection of coherent visual motion.

In the random dot kinematograms used to analyze the detection of coherent motion in the middle temporal visual area (MT) and in psychophysical experiments the exact way that dots are paired between successive presentations is not known by the observer. We show how to calculate the limit to coherence threshold caused by this uncertainty, which we call "correspondence noise." We compare ideal thresholds limited only by this noise with those of human observers when dot density, ratio of dot numbers in two fields, area of stimulus, number of fields, and method of generation of the coherent dots are varied. The observed thresholds vary in the same way as the ideal thresholds over wide ranges, but they are much higher. We think this difference is because the ideal detector takes advantage of the high precision with which dots are placed in the kinematograms, whereas the neural motion system can only operate with low precision. When kinematograms are generated with decreased precision of dot placement, the ideal detector no longer has this advantage, and the gap between ideal and actual performance is greatly reduced. Because the signals that result from objects moving in the real world are scattered over broad ranges of direction and velocity, high precision is not needed, and it is advantageous for the motion system to pool information over broad ranges. Other mismatches between kinematograms and the neural motion system, and internal noise, may also elevate human thresholds relative to the ideal detector. The importance of external noise suggests that the neurons of MT form a vast array of optimal filters, each matched to a different combination of parameters in the multidimensional space required to define motion in patches of the visual field.

Adult↗
PubMed1997

Increasing trend of HIV seroprevalence among pulmonary tuberculosis patients in Pune, India.

A total of 4618 tuberculosis patients attending the TB clinic at the Sassoon General Hospitals, Pune between 1991 and 1996 were screened for anti-HIV antibodies. Of these 694 were found reactive in enzyme immuno assay (EIA) and 624 were further confirmed by a second test, either rapid EIA or Western blot. HIV-1 reactivity was predominant among tuberculosis patients with HIV-2 reactivity appearing only in 1995. HIV-2 seroreactivity accounted for 0.54 and 1.02 per cent of all HIV reactive samples in 1995 and 1996. HIV-1 and HIV-2 dual reactivity accounted for 1.63 and 2.04 per cent of all infections in 1995 and 1996. The overall seroprevalence of HIV among newly diagnosed tuberculosis patients rose from 3.2 per cent in 1991 to 20.1 per cent in 1996.

HIV Seroprevalence↗
PubMed1997

Randomised controlled clinical trial of short course chemotherapy in abdominal tuberculosis: a five-year report.

OBJECTIVES: To assess and compare the efficacy of a 6-month short-course chemotherapy regimen (SCC) with that of a 12-month standard regimen in the treatment of abdominal tuberculosis. DESIGN AND SUBJECTS: A total of 193 adult patients with evidence of abdominal tuberculosis were randomly allocated to one of two daily regimens: 1) a 6-month SCC regimen with rifampicin, isoniazid and pyrazinamide for 2 months followed by rifampicin with isoniazid for another 4 months (6R series) and 2) and 12-month standard regimen of ethambutol and isoniazid with streptomycin supplemented for 2 weeks (12E series). Surgery was undertaken only for patients suspected to have obstruction or perforation of the intestine. RESULTS: A total of 163 (85 6R, 78 12E) patients were available for efficacy analysis after exclusion of 30 patients for various reasons. At the end of treatment clinical status was normal in 84 (99%) in 6R patients and in 73 (94%) in 12E patients. Of these, 147 patients completed follow-up for 5 years; none had relapsed requiring treatment for abdominal tuberculosis. CONCLUSION: A 6-month SCC regimen has been found to be as effective as the standard 12-month regimen in the treatment of all forms of abdominal tuberculosis.

Adolescent↗
PubMed1996

Localization of a peripheral patch: the role of blur and spatial frequency.

Peripheral vision serves to direct our attention and fixation to objects of interest. This requires that the visual system be capable of accurately localizing peripherally presented targets having different spatial structures. The question we address is "to what extent does stimulus spatial structure influence the precision of peripheral localization?" To address this issue, we measured the precision of spatial localization (with reference to a foveal target) for a single Gaussian or Gabor patch briefly presented in the periphery. For both stimuli, we find that when the standard deviation of the stimulus envelope (SD) is less than 1/5 the stimulus eccentricity, localization thresholds are independent of SD and are approximately 1/50 of eccentricity. For larger values of SD, localization thresholds increase linearly with increasing SD, and are approximately 1/5 of SD. The results hold over a range of eccentricities (from 2.5 to 10 deg) and stimulus contrasts (from near detection threshold to 80%). In addition, for Gabor patches, the results are independent of frequency, phase and orientation of the carrier.

Humans↗
PubMed1996

Two-dot alignment across the physiological blind spot.

Three competing hypotheses have been proposed for the cortical representation of the blind spot. These are: (i) the regions surrounding the blind spot maintain their spatial values; (ii) the opposite sides of the blind spot are represented adjacently at the cortex, so that the blind spot is "sewn-up"; and (iii) the blind spot is sewn-up with compensation occurring in the immediate surround of the blind spot, so that spatial values are distorted only in the immediate surround of the blind spot. To distinguish between these hypotheses we used a two-dot alignment task, with the two dots straddling the blind spot at varying dot separations. Thresholds in the two-dot alignment task are limited by the cortical separation of the two dots. When thresholds for alignment across the blind spot are compared with thresholds over intact retina at the same eccentricity, the three hypotheses predict: (i) no change in thresholds; (ii) a lowering of thresholds; and (iii) a lowering of thresholds but only at separations slightly greater than the diameter of the blind spot. Thresholds across the blind spot were closely similar to thresholds across intact retina. The results do not support a sewing-up (with or without compensation) of the blind spot. Rather, our results are consistent with a preservation of spatial values around the blind spot.

Fixation, Ocular↗
PubMed1995

On the filling in of the visual blind spot: some rules of thumb.

In monocular viewing there is a region in the peripheral visual field that is blind owing to the absence of photoreceptors at the site where the optic nerve exits the eye. This region, like certain other blind spots, nonetheless appears filled in. Several novel demonstrations of filling in at the blind spot have recently been reported. Here the implications of many of these effects are critically reevaluated. Specifically, it is argued that many blind-spot phenomena taken to support early filling in (eg pop out and alteration in apparent motion) are actually consistent with the thesis that the visual blind spot is treated by early perceptual processing as a region of reduced or absent information. In support of this, it is shown that many perceptual effects observed in blindspot completion are similar in detail to the amodally perceived completion of partly occluded objects viewed somewhat peripherally. The goals were to point out striking similarities between blind-spot completion and the amodal completion of occluded parts of surfaces, and to provide a common theoretical framework for understanding these phenomena in the context of surface segregation and perceptual interpolation.

Humans↗
PubMed1995

The human immunodeficiency virus epidemic in India. Current magnitude and future projections.

Since the first case of the human immunodeficiency virus (HIV) infection was documented in India in 1986, there has been an explosive increase in HIV transmission. In this review we describe the chronology of the HIV epidemic in India, preliminary information about the clinical presentation of the acquired immunodeficiency syndrome (AIDS), the biological and behavioral aspects of HIV transmission in the Indian setting, and projections regarding the future of the HIV epidemic in India. Using recent data obtained by searching the computerized literature and published abstracts, conference proceedings, and publications from the government of India, we show that India is experiencing a major epidemic of HIV transmission in high-risk populations of commercial sex workers, sexually transmitted disease clinic patients, intravenous drug users, and commercial blood donors. There is also evidence of recent spread of the epidemic from these high-risk groups to other risk groups in India, and from urban centers to rural populations. It is estimated that 1.6 million people are currently infected with HIV in India. At the present rate of transmission, India will have the largest number of HIV-infected individuals of any country in the world by the end of this decade, with more than 5 million infected individuals. Because limited data are available, comprehensive and well-designed epidemiologic surveys are urgently needed to adequately characterize the HIV epidemic in India and to help implement targeted and effective educational and prevention-oriented programs.

Acquired Immunodeficiency Syndrome↗
PubMed1994

Long-range dichoptic interactions in the human visual cortex in the region corresponding to the blind spot.

The region of the visual field of one eye that corresponds to the blind spot of the contralateral eye is believed to be monocular. We measured dichoptic contour interaction in this region of the visual field in humans by having observers report the orientation of a test letter "T" presented to this region, in the presence of flanking T's presented around the blind spot of the fellow eye. A large drop in performance was seen because of the flanks, showing clearly the existence of dichoptic contour interaction in this "monocular" region of the visual field. This suggests that the cortical representation of the region of the visual field that corresponds to the contralateral eye's blind spot is not strictly monocular. The absence of direct retinal afferents from one eye to this region of cortex suggests the involvement of horizontal cortical connections in the contour interaction phenomenon. Our estimates of the extent of contour interaction in mm of striate cortex are comparable to the reported lengths of the long-range horizontal connections in the striate cortex of monkeys. Our results are consistent with the proposition that long-range horizontal connections of the striate cortex may mediate the contour interaction phenomenon.

Form Perception↗
PubMed1994

Epidemic of herpes zoster following HIV epidemic in Manipur, India.

Since 1989, injecting drug use (IDU) related HIV infection has affected thousands of young adults in Manipur, a north eastern state of India bordering Myanmar following a similar kind of epidemic in adjoining countries like Thailand and Myanmar. During a clinical surveillance of a group of HIV positive IDUs for a natural history study at Manipur, herpes zoster (HZ) emerged as the most specific early HIV related illness (positive predictive value of 100%) in patients belonging to the age group of 12-45 years. Data collected from the dermatology departments of the two main hospitals of the state revealed that there had been an epidemic of HZ since 1990 (rate of 1990 being 11.3/1000 compared to 6.5/1000 in 1989, P value < 0.0001) among males of 12-45 years. The epidemic of HZ has been attributed to the preceding epidemic of IDU related HIV in the same age and gender group occurring 1 year earlier. HZ should be recognised as a marker condition similar to tuberculosis indicating the necessity of screening for HIV in regions where the dual problem of IDU and HIV exist in young adults.

AIDS-Related Opportunistic Infections↗
PubMed1994

The effect of similarity and duration on spatial interaction in peripheral vision.

Spatial interactions are extensive in the peripheral visual field, extending up to about half the retinal eccentricity of the target (Toet and Levi, Vision Res. 32, 1349-1357, 1992). In the present study it is shown that the degree and extent of peripheral spatial interaction depends in large measure on the similarity between test and flanking stimuli. The stimulus consisted of a test T surrounded by four distracting flanking Ts, each randomly oriented. The task was to determine the orientation of the test T. The test and flanking Ts differed in contrast polarity, shape, depth, color, eye of origin, or contrast. When the target and flanks differed in contrast polarity, depth, or shape, performance improved markedly for all observers. A color difference enhanced the performance of most but not all observers. Eye-of-origin had no effect, that is, spatial interaction was identical when the target and flanks were presented to the same eye, or to opposite eyes. The role of stimulus duration in spatial interaction was examined in two additional experiments. In the first, the stimulus viewing duration was increased in order to allow the observer time to serially search for the test T. In the second experiment, a postmask was presented at the location of the test T. The results of these experiments showed that the influence of similarity was independent of stimulus duration and the postmask, and suggest that serial search does not play an important role in the spatial interaction effects reported here. The extent of spatial interaction is correlated with the ability to do parallel search.

Color Perception↗
PubMed1994

Health consequences of vasectomy in India.

Vasectomy acceptance has been declining in India during the past 20 years. Even if the risk of prostate cancer is marginally higher in vasectomized men, this risk in India has to be assessed against the immediate safety and other possible long-term benefits of this procedure. The Indian Council of Medical Research has launched a case-control study, which is unlikely to be time-consuming or beyond the available means and resources, in order to obtain definitive data on this problem. Meanwhile, vasectomy remains an excellent procedure for the couple seeking permanent methods of contraception, and specific efforts to promote its acceptance must continue in India.

Case-Control Studies↗