PubMed Health⌕ Search

Biomedical subjects

N Rubin

Publications and source records attributed to N Rubin.

At least 19 recordsLinked to original sources
PubMed2001

Segmentation in structure from motion: modeling and psychophysics.

Much work has been done on the question of how the visual system extracts the three-dimensional (3D) structure and motion of an object from two-dimensional (2D) motion information, a problem known as 'Structure from Motion', or SFM. Much less is known, however, about the human ability to recover structure and motion when the optic flow field arises from multiple objects, although observations of this ability date as early as Ullman's well-known two-cylinders stimulus [The interpretation of visual motion (1979)]. In the presence of multiple objects, the SFM problem is further aggravated by the need to solve the segmentation problem, i.e. deciding which motion signal belongs to which object. Here, we present a model for how the human visual system solves the combined SFM and segmentation problems, which we term SSFM, concurrently. The model is based on computation of a simple scalar property of the optic flow field known as def, which was previously shown to be used by human observers in SFM. The def values of many triplets of moving dots are computed, and the identification of multiple objects the image is based on detecting multiple peaks in the histogram of def values. In five experiments, we show that human SSFM performance is consistent with the predictions of the model. We compare the predictions of our model to those of other theoretical approaches, in particular those that use a rigidity hypothesis, and discuss the validity of each approach as a model for human SSFM.

Discrimination, Psychological↗
PubMed2001

The role of junctions in surface completion and contour matching.

It has been suggested that contour junctions may be used as cues for occlusion. Ecologically, T-junctions and L-junctions are concurrent with situations of occlusion: they arise when the bounding contour of the occluding surface intersects with that of the occluded surface. However, there are other image properties that can be used as cues for occlusion. Here the role of junctions is directly compared with other occlusion cues--specifically, relatability and surface-similarity--in the emergence of amodal completion and illusory contour perception. Stimuli have been constructed that differ only in the junction structure, with the other occlusion cues kept unchanged. L-junctions and T-junctions were eliminated from the image or manipulated so as to be locally inconsistent with the (still valid) global occlusion interpretation. Although the other occlusion cues of relatability and surface similarity still existed in the image, subjects reported not perceiving illusory contours or amodal completion in junction-manipulated images. Junction manipulation also affected the perceived stereoscopic depth and motion of image regions, depending on whether they were perceived to amodally complete with a disjoint region in the image. These results are interpreted in terms of the role of junctions in the processes of surface completion and contour matching. It is proposed that junctions, being a local cue for occlusion, are used to launch completion processes. Other, more global occlusion cues, such as relatability, play a part at a later stage, once completion processes have been launched.

Cues↗
PubMed1997

Abrupt learning and retinal size specificity in illusory-contour perception.

BACKGROUND: In behavioral studies of learning, a distinction is commonly made between gradual and abrupt improvements in performance. The learning of perceptual and motor skills is often characterized by gradual, incremental improvement, and is found not to generalize over stimulus manipulations such as changes in the size or location of the retinal image. In contrast, marked improvement in performance can occur suddenly - a phenomenon which has been termed 'insight'. Consequently, the brain mechanisms subserving the two types of learning are commonly thought of as distinct. Here, we examine learning of a perceptual task in which improvement appears to exhibit characteristics of both gradual and abrupt learning. RESULTS: We describe experiments on illusory-contour perception in which the observers underwent an abrupt, dramatic improvement in performance, resembling an incident of insight. At the same time, however, the phenomenon showed a degree of stimulus-specificity that was previously thought to characterize incremental, gradual learning. The improvement was triggered only by specific visual stimuli, whereas other, quite similar, stimuli were found to be ineffective for training; the learning did not generalize to a new retinal image size, and re-training was necessary for different-sized images. CONCLUSIONS: The juxtaposition of abrupt and stimulus-specific learning that we observed suggests that the distinction between the two forms of learning needs to be revised. Rather than postulating two distinct mechanisms, incremental and insightful learning need to be addressed within a single framework. In particular, the findings suggest that learning may involve interactions between multiple levels of representations of the stimulus.

Adolescent↗
PubMed1997

Pharmacokinetics and pharmacodynamics of remifentanil in persons with renal failure compared with healthy volunteers.

BACKGROUND: Remifentanil is an opioid analgesic for use in anesthesia. An ester linkage renders it susceptible to rapid metabolism by blood and tissue esterases. Thus it was hypothesized that remifentanil elimination would be independent of renal function. Because its principal metabolite (GR90291) is eliminated renally, it would depend on renal function. This study was designed to evaluate the pharmacokinetics and pharmacodynamics of remifentanil and its metabolite in persons with and without renal failure. METHODS: Two groups of volunteers received two-stage infusions of remifentanil: low dose with 0.0125 microg x kg(-1) x min(-1) for 1 h followed by 0.025 microg x kg(-1) x min(-1) for 3 h; and high dose with 0.025 microg x kg(-1) x min(-1) for 1 h followed by 0.05 microg x kg(-1) x min(-1) for 3 h. Blood samples were collected for analysis of remifentanil and GR90291 concentrations. The pharmacokinetics of remifentanil were fit using a one-compartment pharmacokinetic model. Remifentanil's effect was determined intermittently using minute ventilation during a hypercapnic (7.5% CO2) challenge. RESULTS: Fifteen patients with renal failure and eight control participants were enrolled. The clearance and volume of distribution of remifentanil were not different between those with renal failure and the controls. Patients with renal failure showed a marked reduction in the elimination of GR90291; the half-life of the metabolite increased from 1.5 h in the controls to more than 26 h in patients with renal failure. The steady-state concentration of GR90291 is likely to be more than 25 times higher in persons with renal failure. There were no obvious differences in opioid effects on minute ventilation in the controls and in patients with renal failure. CONCLUSIONS: The pharmacokinetics and pharmacodynamics of remifentanil were not altered in patients with renal disease, but the elimination of its principal metabolite, GR90291, was markedly reduced. Based on simulations, the concentration of GR90291 at the end of a 12-h remifentanil infusion of 2 microg x kg(-1) x min(-1) is not likely to produce significant opioid effects.

Analgesics, Opioid↗
PubMed1997

Condom use and first-time urinary tract infection.

We evaluated the effects of condom use, lubricated condom use, and spermicide use on risk of acquiring first urinary tract infection in a case-control study of sexually active college women ages 18-39 years. Cases (N = 144) were women with first urinary tract infection that was confirmed by culture recruited at the student health service; controls (N = 286) were women without a history of urinary tract infection who were randomly sampled from all women enrolled at the university. Participants completed a self-administered questionnaire regarding type and frequency of condom use during the previous 2 weeks. Condoms and spermicides usually were used in combination with each other or oral contraceptives. After adjusting for frequency of vaginal intercourse, using unlubricated condoms compared with using no birth control method strongly increased the risk of first urinary tract infection (odds ratio = 29.1; 95% confidence interval = 3.1-1,335). Using a lubricated condom (with or without spermicide in the lubricant) or a spermicidal cream or gel with an unlubricated condom was associated with two- to eightfold risk of first urinary tract infection. Unlubricated condom use was strongly associated with risk of first urinary tract infection, but this effect was largely neutralized by using a spermicidal cream or gel with the unlubricated condom or by using a lubricated condom.

Adolescent↗
PubMed1996

Enhanced perception of illusory contours in the lower versus upper visual hemifields.

The visual world consciously perceived is very different from the spatial array of photo-receptor activation present on our retinae; it is composed of segregated surfaces, organized into distinct objects. An important component of this organizational process, the segmentation of an image into figures and background, is shown to be performed much better in the lower visual field. This finding is demonstrated by the performance in two tasks that involve the perception of illusory contours. This asymmetry indicates a neural specialization that may be related to the anatomical discontinuity along the representation of the horizontal meridian in extrastriate visual cortex.

Fixation, Ocular↗
PubMed1996

The cost-effectiveness of placing urinary tract infection treatment over the counter.

We use cost-effectiveness analysis to estimate the economic and health implications of approving the over-the-counter sale of oral antibiotics for treatment of urinary tract infection (UTI). We consider two alternatives for over-the-counter availability and examine the reduced economic cost from avoided doctors' visits, along with the potential additional costs associated with higher over-the-counter treatment prices or increased use due to greater access. We also consider important noneconomic costs such as reduced symptom days, restricted activity days, and growing resistance to antibiotics. Our analysis covers a 20-year time horizon to capture the long-term effects, from a societal perspective, of a policy decision to place UTI treatment over the counter today. In addition, we present sensitivity analyses to test the effects of the assumptions in our model. We estimate that the economic costs of placing UTI treatment over the counter outweigh the benefits, unless there is extensive patient education and a mechanism for allowing patients to properly self-diagnose to reduce markedly the number of doctors' visits, mistreated symptoms, and threat of resistance to antibiotics. Only if doctors' visits were reduced to 64.6% of current levels would the economic benefits of over-the-counter treatment begin to be realized. UTI is one of the most common complaints among women and accounts for a large number of doctor visits each year. However, the costs of over-the-counter distribution of UTI treatment, particularly those due to the risk of decreasing the time until standard treatments become ineffective due to bacterial resistance, outweigh the short-term gains of decreased symptom days and increased access to treatment.

Adolescent↗
PubMed1996

The difficult patient: creation of a curriculum by third-year family practice residents.

BACKGROUND AND OBJECTIVES: Family practice residents tend to perceive psychosocial problems as less important than other factual aspects of their curriculum. To address this, we developed a problem-based learning (PBL) approach to the biopsychosocial model of medical care. METHODS: Third-year residents presented current problematic patients at a PBL advanced psychiatry conference. Data were collected on topics generated, resident attendance, and conference evaluation. RESULTS: The residents generated a topic list that closely matched a prior faculty-generated list. Some topics were discussed on multiple occasions; others not on the faculty list were also generated. Topics omitted were mental health in the physician's home, crisis prevention/intervention, and troubled marriages. In 1992-1995, resident attendance ranged from 56%-79%. A minority (0-5 residents) consistently attended fewer than 50% of the conferences. Residents evaluated the conference from "good" to "very good." CONCLUSIONS: By employing a PBL format and allowing residents to present current patients, conferences were better attended and covered almost all topics previously identified as important. PBL promoted efficient use of residents' time since residents were only required to research the literature about group knowledge deficits. This provided good training for the residents and excellent continuing medical education for faculty.

Attitude of Health Personnel↗
PubMed1995

Illusory contours activate specific regions in human visual cortex: evidence from functional magnetic resonance imaging.

The neural basis for perceptual grouping operations in the human visual system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functional magnetic resonance imaging to investigate these processes noninvasively. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned inducers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial slices were acquired during baseline, visual stimulation, and poststimulation periods. Activated regions were identified by a multistage statistical analysis of the activation for each volume element sampled and were compared across conditions. Specific brain regions were activated in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These unique regions were found primarily in the right hemisphere for all four subjects and demonstrate that specific brain regions are activated during the kind of perceptual grouping operations involved in illusory contour perception.

Brain Mapping↗
PubMed1995

Restricted ability to recover three-dimensional global motion from one-dimensional motion signals: psychophysical observations.

We tested human ability to recover the 3D structure and motion information from time-varying images where only 1D motion cues were available. Under these conditions, observers exhibit poor performance in discriminating between two perpendicular axes of rotation, or discriminating between rigid and non-rigid 3D motion. This behavior of the visual system is to be contrasted with the good depth from motion performance exhibited when 2D motion cues are given in the image, as was found previously in numerous studies, and also in the work presented here. In a related paper, we suggest a theoretical framework in which to understand this differential performance on the basis of the two types of motion cues (1D vs 2D). Our findings are consistent with those of previous studies of frontoparallel motion, where it was shown that in many cases, the 1D cues alone were not integrated by the visual system into the correct global motion percept. This accumulating evidence suggests that oriented (1D) motion detectors alone cannot account for observed human performance of global motion perception, and that the role of units such as point or endpoint detectors should be studied further.

Cues↗
PubMed1995

Restricted ability to recover three-dimensional global motion from one-dimensional local signals: theoretical observations.

Recovering 3D information from a 2D time-varying image is a vital task which human observers face daily. Numerous models exist which compute global 3D structure and motion on the basis of 2D local motion measurements of point-like elements. On the other hand, both experimental and computational research of early visual motion mechanisms emphasize the role of oriented (1D) detectors. Therefore, it is important to find out whether indeed 1D motion signals can serve as primary cues for 3D global motion computation. We have addressed this question by combining mathematical results and perceptual observations. We show that given the 2D-projected 1D instantaneous velocity field, it is mathematically impossible to discriminate rigid rotations from non-rigid transformations and/or to recover the rotation parameters. We relate this fact to existing results in cases where localized (point-like) cues are present, and to our own experiments on human performance in global motion perception when only 1D cues are given. Taken together, the data suggest a necessary role for localized information in early motion mechanisms and call for further physiological and psychophysical research in that direction.

Cues↗
PubMed1994

Incomplete processing of progastrin expressed by human colon cancer cells: role of noncarboxyamidated gastrins.

Gastrin is mitogenic for several colon cancers. To assess a possible autocrine role of gastrin in colon cancers, we examined human colon cancer cell lines for expression of gastrin mRNA and various forms of gastrin. Gastrin mRNA was not detected in the majority of colon cancer cell lines by Northern hybridization but was detected in all human colon cancer lines by the sensitive method of reverse transcriptase-polymerase chain reaction (PCR). Gastrin mRNA was quantitated by the competitive PCR method. The majority of cell lines expressed very low levels of gastrin mRNA (< 1-5 copies/cell); only one cell line expressed > 20 copies/cell. The mature carboxyamidated form of gastrin was not detected in any of the cell lines by radioimmunoassay or immunocytochemistry. Results suggested that either gastrin mRNA expressed by colon cancer cells was altered (mutated) or posttranslational processing of progastrin was incomplete. Gastrin cDNA from all the colon cancer cell lines had an identical sequence to the published sequence of human gastrin cDNA. Specific antibodies against precursor forms of gastrin were used, and significant concentrations of nonamidated (glycine-extended) and prepro forms of gastrin were measured in tumor extracts of representative colon cancer cell lines. The presence of precursor forms of gastrin suggested a lack of one or more of the processing enzymes and/or cofactors. Significant concentrations of the processing enzyme (peptidylglycine alpha-amidating monooxygenase) were detected in colon cancer cells by immunocytochemistry. Therefore, lack of other cofactors or enzymes may be contributing to incomplete processing of precursor forms of gastrin, which merits further investigation. Since low levels of gastrin mRNA were expressed by the majority of human colon cancer cell lines and progastrin was incompletely processed, it seems unlikely that gastrin can function as a viable autocrine growth factor for colon cancer cells. High concentrations of glycine-extended gastrin-17 (GG) (> 10(-6) M) were mitogenic for a gastrin-responsive human colon cancer (DLD-1) cell line in vitro. It remains to be seen if GG or other precursor forms of gastrin are similarly mitogenic in vivo, which may then lend credibility to a possible autocrine role of gastrinlike peptides in colon cancers.

Base Sequence↗
PubMed1993

Insulinlike growth factors and binding proteins in colon cancer.

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

Amino Acid Sequence↗
PubMed1993

Isolating the effect of one-dimensional motion signals on the perceived direction of moving two-dimensional objects.

A considerable body of evidence suggests the existence of a two-stage mechanism for the detection of global motion. In the first stage the motion of elongated contours is extracted and then at the second stage these one-dimensional (1D) motion signals are combined. What is the nature of the computation carried out in combining the 1D motion signals towards forming a global motion percept? We devised a set of stimuli that differentiate between different possible computations. In particular, they distinguish between a velocity-space construction (such as intersection of constraints) and a linear computation such as vector averaging. In addition, these stimuli do not contain two-dimensional (2D) motion signals such as line intersections, that allow unambiguous determination of global velocity. Stimuli were presented in uncrossed disparity relative to the aperture through which they were presented, to reduce the effect of line terminator motion. We found that subjects are unable to detect the veridical global direction of motion for these stimuli. Instead, they perceive the stimulus pattern to be moving in a direction which reflects the average of its 1D motion components. Our results suggest that the visual system is not equipped with a mechanism implementing a velocity-space computation of global motion.

Humans↗
PubMed1991

Broca's aphasia following damage to Wernicke's area. For or against traditional aphasiology?

Classic aphasiology has been challenged by studies that have employed cranial computed tomography to test predicted anatomic-behavioral correlations. We treated a patient who developed a classic Broca's aphasia but whose computed tomographic scan revealed damage to Wernicke's area, thus seeming to contradict the principles of traditional aphasiology. However, subsequent information obtained by magnetic resonance imaging, intracarotid amobarbital (Amytal) testing, and electrophysiologic studies, including cortical stimulation, demonstrated that the brain-behavior correlations in this patient can be understood in terms of the formulations of traditional aphasiology.

Adult↗
PubMed1989

Electron-transfer flavoprotein from anaerobic Ascaris suum mitochondria and its role in NADH-dependent 2-methyl branched-chain enoyl-CoA reduction.

Electron-transfer flavoprotein was purified to apparent homogeneity from mitochondria of the parasitic nematode, Ascaris suum. The native molecular weight of the enzyme was 70,000, as estimated by gel filtration, and it migrated as two bands with apparent subunit molecular weights of 37,000 and 31,500 during sodium dodecylsulfate polyacrylamide gel electrophoresis. The enzyme exhibited an absorption coefficient for the bound FAD of 13.5 mM-1.cm-1 at 436 nm and a protein/flavin (270 nm/436 nm) ratio of 5.6. While the ascarid enzyme is similar to its mammalian counterpart, physiologically it functions in the reverse direction, shuttling reducing power from the electron-transport chain to a soluble 2-methyl branched-chain enoyl CoA reductase. Indeed, when A. suum submitochondrial particles were incubated with NADH, 2-methylcrotonyl-CoA and purified A. suum 2-methyl branched-chain enoyl-CoA reductase, 2-methylbutyryl-CoA formation was proportional to the amount of electron-transfer flavoprotein added.

Acyl Coenzyme A↗