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At least 1,063 records · Page 59Linked to original sources
PubMed2002

Clinical value of T-wave alternans assessment.

Microvolt-level T-wave alternans (TWA) is a new arrhythmia risk marker to assess subtle changes in repolarization that has been introduced for arrhythmia risk stratification. Recent experimental studies have demonstrated that it reflects a heartrate dependent increased spatial dispersion of repolarization associated with unidirectional conduction block, and reentry that may result in the occurrence of ventricular fibrillation. Clinical studies have convincingly demonstrated that TWA is closely related to arrhythmia induction in the electrophysiology (EP) laboratory as well as to the occurrence of spontaneous ventricular tachyarrhythmias in patients undergoing EP study. Subsequent studies showed that TWA-assessed noninvasively-is predictive of future arrhythmic events in patients with implanted ICDs as well as for ventricular tachyarrhythmias in patients with congestive heart failure without a prior history of arrhythmias. There is still controversy, however, about the predictive value of TWA in patients following acute myocardial infarction (MI). Several studies which differ in patient selection, pharmacologic treatment of the patients, and endpoint definitions, have reported conflicting results. Therefore, studies with a large number of unselected patients after acute MI on optimal treatment according to contemporary therapeutic guidelines as well as of patients with reduced left ventricular ejection fraction following MI are needed to define its role with regard to identifying patients who may benefit from primary preventive ICD therapy. Future research should also focus on evaluation of alternative methods to increase heart rate (i.e., pharmacological stimulation) in an attempt to reduce the proportion of incomplete tests in patients with insufficient increase in heart rate during exercise testing.

Anti-Arrhythmia Agents↗
PubMed1997

The influence of stress conditions on the wear of UHMWPE for total joint replacements.

In vitro studies of the effect of contact stress on the wear of ultra high molecular weight polyethylene (UHMWPE) in orthopaedic applications have produced contradictory results which predict both increased and decreased wear with increasing contact stress. In vivo studies of functioning hip prostheses have reported that 22 mm femoral heads generate lower linear and volumetric wear rates than 32 mm femoral heads. The effect of decreasing the head size will increase the contact stress but decrease the sliding distance per motion cycle. The present study consists of wear experiments under a range of contact stress magnitudes and application conditions in order to simulate the wear processes occurring in vivo. The results from these tests indicated that the wear factor actually decreases with increasing contact stress if the stress was not varied with time. If a time dependent or spatially varying stress was applied, the wear factor can increase greatly when compared to similar magnitude constant contact stress. This effect may be due to the complex relationship between the rate of wear particle generation and the rate at which the particles are released from the interface. The results of these wear experiments are discussed in terms of the influence of the stress conditions upon potential wear processes in total hip and knee prostheses.

Journal Article↗
PubMed2003

Attention, spatial representation, and visual neglect: simulating emergent attention and spatial memory in the selective attention for identification model (SAIM).

The selective attention for identification model (SAIM) is presented. This uses a spatial window to select visual information for recognition, binding parts to objects and generating translation-invariant recognition. The model provides a qualitative account of both normal and disordered attention. Simulations of normal attention demonstrate 2-object costs and effects of object familiarity on selection, global precedence, spatial cueing, and inhibition of return. When lesioned, SAIM demonstrated either view- or object-centered neglect or spatial extinction, depending on the type and extent of lesion. The model provides a framework to unify (a) object- and space-based theories of normal selection, (b) dissociations within the syndrome of unilateral neglect, and (c) attentional and representational accounts of neglect.

Attention↗
PubMed2004

Automatic keypress activation in skilled typing.

The assumption that letters automatically activate corresponding keypresses in skilled typing was investigated. Participants responded to the color of letters (congruent condition: responding finger was the one usually used to type the letter). Participants skilled in typing showed a congruency effect: unskilled participants did not (Experiment 1). The automatic activation included characteristics of the movement usually performed to type the letters (Experiment 2). Responding with crossed hands on an external response device (Experiment 3) provided evidence for effector-dependent representations only, whereas responding on a keyhoard (Experiment 4) resulted in evidence for effector-dependent and spatial representations. Thus, motoric skill proficiency is accompanied by automatic activation processes that probably contribute to high performance levels.

Automatism↗
PubMed1988

Real-time imaging of evoked activity in local circuits of the salamander olfactory bulb.

The encoding of olfactory information in the central nervous system (CNS) depends on spatially distributed patterns of activity generated simultaneously in many neuronal circuits. Optical neurophysiological recording permits analysis of neural activity non-invasively and with high spatial and temporal resolution. Here, a video method for imaging voltage-sensitive dye fluorescence in vivo is used to map neuronal activity in local circuits of the salamander olfactory bulb. The method permits the imaging of simultaneous ensemble transmembrane activity in real time. After electrical stimulation of the olfactory nerve, activity spreads centripetally from the sites of synaptic input to generate nonhomogeneous response patterns that are presumably mediated by local circuits within the bulbar layers. The results also show the overlapping temporal sequences of activation of cell groups in each layer. The method thus provides high resolution, sequential video images of the spatial and temporal progression of transmembrane events in neuronal circuits after afferent stimulation and offers the opportunity for studying ensemble events in other brain regions.

Animals↗
PubMed1989

Independent hemispheric attentional systems mediate visual search in split-brain patients.

The primate visual system is adept at identifying objects embedded within complex displays that contain a variety of potentially distracting elements. Theories of visual perception postulate that this ability depends on spatial selective attention, a mechanism analogous to a spotlight or zoom lens, which concentrates high-level processing resources on restricted portions of the visual field. Previous studies in which attention was pre-cued to specific locations in the visual field have shown that the spotlight has a single, unified focus, even in the disconnected hemispheres of patients who have undergone surgical transection of the corpus callosum. Here we demonstrate that an independent focus of attention is deployed by each of the surgically separated hemispheres in a visual search task, such that bilateral stimulus arrays can be scanned at a faster rate by 'split-brain' subjects than by normal control subjects. The attentional system used for visual search therefore seems to be functionally and anatomically distinct from the system that mediates voluntary orienting of attention.

Attention↗
PubMed2003

Cell-specific mitotic defect and dyserythropoiesis associated with erythroid band 3 deficiency.

Most eukaryotic cell types use a common program to regulate the process of cell division. During mitosis, successful partitioning of the genetic material depends on spatially coordinated chromosome movement and cell cleavage. Here we characterize a zebrafish mutant, retsina (ret), that exhibits an erythroid-specific defect in cell division with marked dyserythropoiesis similar to human congenital dyserythropoietic anemia. Erythroblasts from ret fish show binuclearity and undergo apoptosis due to a failure in the completion of chromosome segregation and cytokinesis. Through positional cloning, we show that the ret mutation is in a gene (slc4a1) encoding the anion exchanger 1 (also called band 3 and AE1), an erythroid-specific cytoskeletal protein. We further show an association between deficiency in Slc4a1 and mitotic defects in the mouse. Rescue experiments in ret zebrafish embryos expressing transgenic slc4a1 with a variety of mutations show that the requirement for band 3 in normal erythroid mitosis is mediated through its protein 4.1R-binding domains. Our report establishes an evolutionarily conserved role for band 3 in erythroid-specific cell division and illustrates the concept of cell-specific adaptation for mitosis.

Amino Acid Sequence↗
PubMed2001

Differential activation of hippocampus and amygdala following spatial learning under stress.

We examined the activation of memory-related processes in the hippocampus and the amygdala following spatial learning under stress, in the rat. Animals were trained in a water maze in a massed spatial task under two stress conditions (cold and warm water). In the dorsal CA1, training was accompanied by increased phosphorylation of ERK2 only in animals that have acquired the task (irrespective of whether they were trained in cold or warm water). In the amygdala, significant activation of ERK2 was found only in animals that learned the task well under high levels of stress. Hence, the results suggest that the amygdala and the hippocampus are differentially activated following spatial learning, depending on the level of stress involved.

Amygdala↗
PubMed1999

Near vision, lags of accommodation and myopia.

It is shown that, with some simplifying assumptions about the nature of the accommodation response/stimulus curve, geometrical optics predicts that lags in accommodation will result in ocular modulation transfer for any linear size of grating component in an object being independent of object distance. Thus bringing an object nearer to the eye should not improve the contrast of the retinal image of any of its details. However, when the effects of diffraction and aberration are considered, together with the angular spatial frequency dependence of the retinal contrast threshold, the expected advantages of short observation distances for the detection of fine detail emerge. It is shown that a reduced slope for the accommodation response/stimulus curve leads to increased lag and reduced modulation transfer: the possible role of such effects in the development of some types of myopia is discussed.

Accommodation, Ocular↗
PubMed2006

Electronic transportation through asymmetrically substituted oligo(phenylene ethynylene)s: studied by first principles nonequilibrium Green's function formalism.

Theoretical investigations of a series of asymmetrically substituted conducting molecular wires [oligo(phenylene ethynylene)s] have been carried out using density functional theory and nonequilibrium Green's function formalism. To get the molecular rectification, the electron-donating group (-NH2) and the electron-withdrawing group (-NO2) are placed on the different positions of the molecular wire. The dependences of spatial distribution and lowest unoccupied molecular orbital (LUMO) energy level on the applied voltage have been found playing dominating but opposite roles in controlling the rectification behavior. In the tested bias range, since the shift LUMO energy level is more important, the electrons transfer more easily from donor to acceptor through the molecular junction in general.

Journal Article↗
PubMed1996

Evidence of a global oblique effect in human extrafoveal vision.

Analysis of recently published human contrast-sensitivity data obtained along the cardinal and major oblique visual-field meridians of a single subject has demonstrated a consistently greater sensitivity at a given eccentricity to horizontally oriented as compared with obliquely oriented gratings. This difference was evident not only at foveal but also at several eccentric loci over a range of low to medium spatial frequencies. This observation is to be distinguished in extrafoveal fixation from the well-documented oblique effect, which describes the variation in sensitivity with orientation at a single visual-field locus. With periodic stimuli which were well localised in space and frequency, and had comparable spatial-summation properties, a spatial-frequency dependency of what could be termed the global oblique effect could be demonstrated along isoeccentric contours centred on the fovea (eccentricity 0 deg) out to an eccentricity of at least 40 deg.

Contrast Sensitivity↗
PubMed2005

Effects of hand orientation and delay on the verbal judgment of haptically perceived orientation.

We examined the haptic perception of orientations of a single bar throughout the horizontal plane using a verbal response: participants were to assign a number of minutes to the orientation of a bar defined with respect to the stimulus table. Performance was found to be systematically biased. Deviations were consistent with, yet much smaller than, those resulting from haptic motor matching tasks. The size and direction of the deviations were found to correlate with hand orientation, and not to depend on spatial location per se, suggesting a role for hand-centred reference frames in biasing performance. Delaying the response by 10 s led to a small improvement only of right-hand perceptions, indicating different hemispheric involvement in processes involved in retaining and/or recoding of haptic orientation information. Also the haptic oblique effect was found with the current verbal response. Importantly, it was affected neither by hand orientation nor by delay, suggesting that the oblique effect is independent of the aforementioned deviations in orientation perception.

Adult↗
PubMed2006

Anisotropy of cell adhesive microenvironment governs cell internal organization and orientation of polarity.

Control of the establishment of cell polarity is an essential function in tissue morphogenesis and renewal that depends on spatial cues provided by the extracellular environment. The molecular role of cell-cell or cell-extracellular matrix (ECM) contacts on the establishment of cell polarity has been well characterized. It has been hypothesized that the geometry of the cell adhesive microenvironment was directing cell surface polarization and internal organization. To define how the extracellular environment affects cell polarity, we analyzed the organization of individual cells plated on defined micropatterned substrates imposing cells to spread on various combinations of adhesive and nonadhesive areas. The reproducible normalization effect on overall cell compartmentalization enabled quantification of the spatial organization of the actin network and associated proteins, the spatial distribution of microtubules, and the positioning of nucleus, centrosome, and Golgi apparatus. By using specific micropatterns and statistical analysis of cell compartment positions, we demonstrated that ECM geometry determines the orientation of cell polarity axes. The nucleus-centrosome orientations were reproducibly directed toward cell adhesive edges. The anisotropy of the cell cortex in response to the adhesive conditions did not affect the centrosome positioning at the cell centroid. Based on the quantification of microtubule plus end distribution we propose a working model that accounts for that observation. We conclude that, in addition to molecular composition and mechanical properties, ECM geometry plays a key role in developmental processes.

Anisotropy↗
PubMed2002

Migration of nerve growth cones requires detergent-resistant membranes in a spatially defined and substrate-dependent manner.

Motility of nerve growth cones (GCs) is regulated by region-specific activities of cell adhesion molecules (CAMs). CAM activities could be modified by their localization to detergent-resistant membranes (DRMs), specialized microdomains enriched in signaling molecules. This paper deals with a question of whether DRMs are involved in GC migration stimulated by three CAMs; L1, N-cadherin (Ncad), and beta1 integrin. We demonstrate that L1 and Ncad are present in DRMs, whereas beta1 integrin is exclusively detected in non-DRMs of neurons and that localization of L1 and Ncad to DRMs is developmentally regulated. GC migration mediated by L1 and Ncad but not by beta1 integrin is inhibited after DRM disruption by micro-scale chromophore-assisted laser inactivation (micro-CALI) of GM1 gangliosides or by pharmacological treatments that deplete cellular cholesterol or sphingolipids, essential components for DRMs. Characteristic morphology of GCs induced by L1 and Ncad is also affected by micro-CALI-mediated DRM disruption. Micro-CALI within the peripheral domain of GCs, or even within smaller areas such as the filopodia and the lamellipodia, is sufficient to impair their migration. However, micro-CALI within the central domain does not affect GC migration. These results demonstrate the region-specific involvement of DRMs in CAM-dependent GC behavior.

3T3 Cells↗
PubMed1968

The electrical activity of embryonic chick heart cells isolated in tissue culture singly or in interconnected cell sheets.

Embryonic chick heart cells were cultured on a plastic surface in sparse sheets of 2-50 cells mutually in contact, or isolated as single cells. Conditions are described which permitted conjoint cells to be impaled with recording microelectrodes with 75 % success, and isolated single cells with 8 % success. It is proposed that cells in electrical contact with neighbors are protected from irreversible damage by the penetrating electrode, by a flow of ions or other substances from connected cells across low-impedance intercellular junctions. Action potentials recorded from conjoint and isolated single cells were similar in form and amplitude. The height or shape of the action potential thus appears not to depend upon spatial relationships of one cell to another. As the external potassium concentration was increased from 1.3 mM to 6 mM, cells became hyperpolarized while the afterhyperpolarization was reduced. At higher potassium levels, the afterhyperpolarization disappeared, the slope of the slow diastolic depolarization decreased, and resting potential fell along a linear curve with a slope of 61 mv per 10-fold increase in potassium. In pacemaker cells the diastolic depolarization consists of two phases: (a) recovery from the afterpotential of the previous action potential and (b) the pacemaker potential. These phases are separated by a point of inflection, and represent manifestations of different mechanisms. Evidence is presented that it is the point of inflection (PBA) rather than the point of maximal diastolic potential, that should be taken as the resting potential.

Action Potentials↗
PubMed2000

Coevolution of Contrary Choices in Host-Parasitoid Systems.

We investigate patch selection strategies of hosts and parasitoids in heterogeneous environments. Previous theoretical work showed that when host traits vary among patches, coevolved populations of hosts and parasitoids make congruent choices (i.e., hosts and parasitoids preferentially select the same patches) and exhibit direct density dependence in the distribution of percent parasitism. However, host-parasitoid systems in the field show a range of patterns in percent parasitism, while behavioral studies indicate that hosts and parasitoids can exhibit contrary choices (i.e., hosts avoid patches favored by the parasitoid). We extend previous theory by permitting life-history traits of the parasitoid as well as the host to vary among patches. Our analysis implies that in coevolutionarily stable populations, hosts preferentially select patches that intrinsically support higher host equilibrium numbers (i.e., the equilibrium number achieved by hosts when both populations are confined to a single patch) and that parasitoids preferentially select patches that intrinsically support higher parasitoid equilibrium numbers (i.e., the equilibrium number achieved by the parasitoids when both populations are confined to a patch). Using this result, we show how variation in life-history traits among patches leads to contrary or congruent choices or leads to direct density dependence, inverse density dependence, or density independence in the distribution of percent parasitism. In addition, we determine when populations playing the coevolutionarily stable strategies are ecologically stable. Our analysis shows that heterogeneous environments containing patches where the intrinsic rate of growth of the host and the survivorship rate of the parasitoid are low result in the coevolved populations exhibiting contrary choices and, as a result, promote ecological stability.

coevolution↗
PubMed1984

Analysis of x-ray computed tomography images using the noise power spectrum and autocorrelation function.

A discrete representation of the reconstruction process, consistent with the method of data collection, has been used to derive expressions for the noise power spectrum, autocorrelation function and noise equivalent quanta (NEQ) of a computed tomography (CT) image. These parameters have been expressed in terms of basic scanning factors such as tube current, exposure time, slice width and number of detectors. Each of these factors affects the overall magnitude of the noise power spectrum, but the spatial frequency dependence is also determined by the type of reconstruction filter used in the computer algorithm. The noise power spectrum has been calculated for scanners employing either a ramp or Hanning weighted ramp filter. Predictions made from this theoretical analysis have been compared with experimental measurements made on various CT scanners. Measurements were made of the modulation transfer function (MTF) by techniques which permitted us to deduce the contributions of the algorithmic and non-algorithmic components to the overall MTF. NEQ values have been calculated for a number of CT scanners.

Humans↗