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At least 397 records · Page 22Linked to original sources
PubMed2004

Maintaining spatial body alignment on a rotating platform by means of active counter-circling: role of vestibular and podokinesthetic afferents.

We investigated the behaviour of vision-deprived human subjects who try to maintain their horizontal alignment in space on a rotating platform by stepping about their own axis in counter-direction ('podomotor counter-rotation'), and we ask which of two alternative hypotheses best explains this behaviour. (1) The feedback hypothesis assumes that the podomotor counter-rotation is controlled by negative feedback of vestibular signals from the horizontal canals. (2) The reconstruction hypothesis holds that the vestibular cue first is combined with leg proprioceptive afferents signalling the individual's rotation on the platform ('podokinesthetic cue') in a way that reconstructs the platform's motion in space for internal representation; a negative (direction-inverted) copy of this representation then would drive the counter-rotation. Subjects were exposed to three different velocity profiles of platform rotation: VC, constant velocity rotation with sudden onset and offset; VS, sinusoidal rotation; VN, pseudorandom noise sequences. The subjects' response (i.show $132#e., their active self-rotation on the platform) to the onset and offset of VC rotations was reminiscent of a first-order lead system. Specifically, after rotation onset subjects immediately began to step on the platform in opposite direction; initially, the velocity of this response matched that of platform rotation, leading to a fairly good stabilisation of subjects' alignment in space. However, this response declined exponentially; consequently, subjects began to increasingly rotate in space along with the platform, ultimately stepping in place on the platform. After rotation offset, subjects immediately began to step around on the now stationary platform so as to continue their previous rotation in space; this response again declined exponentially until subjects became gradually stable again with respect to space. Within subjects, the time constant (tau) of these responses was similar for onset and offset. Across subjects it exhibited a conspicuous variability, ranging from 7 s to virtually infinity. The responses to VS and VN rotations were closely correlated to what could be predicted for each individual from his tau during VC on the assumption of a first-order lead system. We conclude that the mechanism stabilising body orientation basically is linear (no prediction with sinusoidal rotation, no extrapolation of constant velocity rotation). A comparison of the experimental results with simulations of the feedback hypothesis and of the reconstruction hypothesis suggests that the reconstruction hypothesis is a more likely description of the underlying processing of the vestibular and podokinesthetic cues.

Adult↗
PubMed2002

Mode-I fracture in a nonlinear lattice with viscoelastic forces.

We study mode-I fracture in a viscoelastic lattice model with a nonlinear force law, with a focus on the velocity and linear stability of the steady-state propagating solution. This study is a continuation both of the study of the piecewise-linear model in mode I, and the study of more general nonlinear force laws in mode-III fracture. At small driving, there is a strong dependency of the velocity curve on the dissipation and a strong sensitivity to the smoothness of the force law at large dissipation. At large driving we calculate, via a linear stability analysis, the critical velocity for the onset of instability as a function of the smoothness, the dissipation and the ratio of lattice spacing to critical extension. This critical velocity is seen to be very sensitive to these parameters. We confirm our calculations via direct numerical simulations of the initial value problem.

Journal Article↗
PubMed1984

Transvascular fluid shift and thoracic duct lymph: analysis of lymph formation in the rat.

To analyze the effect of changes in interstitial fluid on lymph production, intravascular infusions of saline were given to splenectomized rats under pentobarbital anesthesia at 3 different rates (2, 3, and 4 ml/100 g of body weight) over 10 min. Change of blood volume was continuously monitored and, simultaneously, thoracic duct lymph was collected during and after the infusion. Equilibrium was attained approximately 40 min after the infusion; regardless of the infusion rate, 10% of the infused volume was incorporated into the vascular space and 90% was filtered into the extravascular space. Thus, the amount of transvascular fluid shift showed a linear relationship with the infused volume. However, the drainage from thoracic duct lymph amounted to 5.9%, 11.4%, and 17.8% of the infused saline volume when given at the rate of 2, 3, and 4 ml/100 g, respectively. The relation of lymph flow and infused volume could be regarded as a nonlinear system. By means of a simulation study, this relation was found to be attributed to the nonlinearity of conductance for fluid movement from tissue to lymph duct, which was only one sixth of that determined for the capillary membrane. The drainage of lymph protein following the infusion was only about 50% of that expected from the interstitial protein concentration in even distribution of the infused saline. These characteristics of interstitial fluid space play an important role in absorbing water and, hence, buffering changes in circulating blood volume after volume loading.

Animals↗
PubMed2000

Rotation in clinostat results in apoptosis of osteoblastic ROS 17/2.8 cells.

Clinostat is an effective, ground-based tool which can be used to verify data from space flight, and to test hypotheses and experimental conditions for eventual space flights. Rotation in clinostat appears to mimic the microgravity environment by nulling the gravitational vector by continuous averaging. In the present study, we exposed osteoblast-like ROS 17/2.8 cells to a vector-averaged gravity environment in a clinostat and found that the cells undergo apoptotic death during the first 24 hr of clino-rotation. We suggest that apoptosis might be one of the mechanisms for reduced bone formation as observed in actual space flights.

Apoptosis↗
PubMed1999

Effects of spatial segmentation in the continuous model of excitation propagation in cardiac muscle.

INTRODUCTION: Spatial segmentation is essential for the numerical simulation of excitation propagation in cardiac muscle. METHODS AND RESULTS: This study evaluated the effects of spatial segmentation on action potential and on the velocity of propagation in a continuous one-dimensional model of cardiac muscle [intracellular and extracellular resistivities along (L) and transverse (T) to the muscle fibers: 402 omega(cm) (R(e), L), 3,620 omega(cm) (R(e), T), 48 omega(cm) (R(e), L), and 126 omega(cm) (R(e), T), J of Physiol 255:335-346, 1976) and either Luo-Rudy (L-R, Circ Res 68:1501-1526, 1991) or Beeler-Reuter (B-R, J Physiol 268:177-210, 1977) ionic currents. Related cable equations for active membrane are derived. Spatial segmentations of < 31.2 microm (L, L-R), < 11.5 microm (T, L-R), < 44.7 microm (L, B-R), and < 16.5 microm (T, B-R) were required for < 1% errors in the characteristic parameters of action potential. Similarly, spatial segmentations of < 54.5 microm (L, L-R), < 20.1 microm (T, L-R), < 84.3 microm (L, B-R), and < 31.2 microm (T, B-R) were required for < 1 % errors in the velocity of conduction. CONCLUSION: In general, spatial segmentations of < 26.9% and < 50.8% of the space constant of a fully activated membrane gave < 1.0% errors in the characteristic parameters of action potential and in the velocity of propagation, respectively, for both membranes. The action potential duration was relatively insensitive to the spatial segmentation. Our analysis suggests that lambda(full is a better criterion for the selection of spatial segmentation in numerical simulation than the space constant of the resting membrane.

Action Potentials↗
PubMed2006

Self-organizing path integration using a linked continuous attractor and competitive network: path integration of head direction.

A key issue is how networks in the brain learn to perform path integration, that is update a represented position using a velocity signal. Using head direction cells as an example, we show that a competitive network could self-organize to learn to respond to combinations of head direction and angular head rotation velocity. These combination cells can then be used to drive a continuous attractor network to the next head direction based on the incoming rotation signal. An associative synaptic modification rule with a short term memory trace enables preceding combination cell activity during training to be associated with the next position in the continuous attractor network. The network accounts for the presence of neurons found in the brain that respond to combinations of head direction and angular head rotation velocity. Analogous networks in the hippocampal system could self-organize to perform path integration of place and spatial view representations.

Action Potentials↗
PubMed1998

Finite element formulation of biphasic poroviscoelastic model for articular cartilage.

The purpose of the present study was to develop a computationally efficient finite element model that could be useful for parametric analysis of the biphasic poroviscoelastic (BPVE) behavior of articular cartilage under various loading conditions. The articular cartilage was modeled as the BPVE mixture of a porous, linear viscoelastic, and incompressible solid and an inviscid and incompressible fluid. A finite element (FE) formulation of the BPVE model was developed using two different algorithms, the continuous and discrete spectrum relaxation functions for the viscoelasticity of the solid matrix. These algorithms were applied to the creep and stress relaxation responses to the confined compression of articular cartilage, and a comparison of their performances was made. It was found that the discrete spectrum algorithm significantly saved CPU time and memory, as compared to the continuous spectrum algorithm. The consistency analysis for the present FE formulation was performed in comparison with the IMSL, a commercially available numerical software package. It was found that the present FE formulation yielded consistent results in predicting model behavior, whereas the IMSL subroutine produced inconsistent results in the velocity field, and thereby in the strain calculation.

Algorithms↗
PubMed1996

Scoring noncovalent protein-ligand interactions: a continuous differentiable function tuned to compute binding affinities.

Exploitation of protein structures for potential drug leads by molecular docking is critically dependent on methods for scoring putative protein-ligand interactions. An ideal function for scoring must exhibit predictive accuracy and high computational speed, and must be tolerant of variations in the relative protein-ligand molecular alignment and conformation. This paper describes the development of an empirically derived scoring function, based on the binding affinities of protein-ligand complexes coupled with their crystallographically determined structures. The function's primary terms involve hydrophobic and polar complementarity, with additional terms for entropic and solvation effects. The issue of alignment/conformation dependence was solved by constructing a continuous differentiable nonlinear function with the requirement that maxima in ligand conformation/alignment space corresponded closely to crystallographically determined structures. The expected error in the predicted affinity based on cross-validation was 1.0 log unit. The function is sufficiently fast and accurate to serve as the objective function of a molecular-docking search engine. The function is particularly well suited to the docking problem, since it has spatially narrow maxima that are broadly accessible via gradient descent.

Bacterial Proteins↗
PubMed2004

Intraluminal ultrasound intensity distribution and backscattered Doppler power.

Ultrasound (US) incident obliquely on a cylindrical vessel is redistributed in space when the propagation path includes walls with acoustic impedance different from that of the surrounding media. We investigated this using low-density polyethylene (PE) as the vessel wall material. Both simulations and experiments were carried out. Direct hydrophone measurements of the acoustic field were made within a half section of the PE tube, and the distribution of backscattered Doppler power along a scan line was obtained using a range-Doppler instrument. Both simulation and hydrophone results demonstrate lateral shadow regions within the lumen. In every one of various Doppler flow experiments conducted, the backscattered Doppler power, compensated for on-axis transducer behaviour, increased with depth. Simulation results for an incident continuous-wave (CW) plane wave show that it tends to be focused by the curvature of the PE tube walls. The wall interactions are, however, angle-dependent and so the behaviour of a focused US beam depends on the beam as well as the walls. This study demonstrates alterations in the spatial distribution of US within a cylindrical vessel as a result of known vessel wall properties. It also provides evidence that local intensity variations within the lumen affect the relative Doppler power backscattered from small sample volumes.

Blood Vessels↗
PubMed1997

Virtual reality for orthognathic surgery: the augmented reality environment concept.

PURPOSE: The objective of this study was to apply virtual reality technology to osteotomies of the facial skeleton. MATERIALS AND METHODS: Augmented reality can be considered a hybrid of virtual and real environment spaces, which are coregistered and simultaneously visualized. Using a see-through HMD (head-mounted display) and Interventional Video Tomography intraoperatively, partial visual immersion into a patient-related virtual data space augments the surgeon's perception as shown in an experimental study and clinical applications. RESULTS: Without limiting the surgical judgment, offering continuous observation of the operating field, the presented technology additionally provides visual access to invisible data of anatomy, physiology, and function and thus guarantees unencumbered and fluent surgery. CONCLUSION: Despite current shortcomings, augmented reality technology proved to be particularly well suited for use in osteotomies of the facial skeleton.

Anatomy, Cross-Sectional↗
PubMed2005

Schwarz meets Schwann: design and fabrication of biomorphic tissue engineering scaffolds.

Tissue engineering is a discipline at the leading edge of the field of computer assisted intervention. This multidisciplinary engineering science is based on the notion of design and fabrication of scaffolds- porous, three-dimensional "trellis-like" biomimetic structures that, on implantation, provide a viable environment to recuperate and regenerate damaged cells. Existing CAD-based approaches produce porous labyrinths with contra-naturam straight edges. The biomorphic geometry that mimics the secundam-naturam substrate would be one that is continuous through all space, partitioned into two not-necessarily-equal sub-spaces by a non-intersecting, two-sided surface. Minimal surfaces are ideal to describe such a space. We present results on the premier attempt in computer controlled fabrication and mechanical characterization of Triply Periodic Minimal Surfaces [TPMS]. This initiative is a significant step to link Schwann's 1838 cell theory with Schwarz's discovery of TPMS in 1865 to fabricate the previously elusive optimal biomorphic tissue analogs.

Biocompatible Materials↗
PubMed1998

Foil coverage of a crawl-space floor: measurements and modeling of radon entry.

The mitigative impact of covering the floor of a crawl space with a membrane has been studied under well-defined and controlled conditions. The measurements have been done with a homogeneous column of dry sand covered with a sheet of polyethylene foil. An air-filled volume on top of the column simulates a crawl space. The experiments mainly concern long-term measurements of the crawl-space radon concentration in combination with steady-state diffusive and combined advective and diffusive transport through the sand column and crawl space. The experimental data are analyzed with both simplified mass-balance models for radon entry into the crawl space and with a two-dimensional numerical model based on a finite-difference approach. In all experiments the influence of atmospheric pressure variations is clearly present. For most experiments the agreement between calculations, which make use of independently measured transport parameters for both sand and foil, and measurements is within 10%. However, the discrepancy is larger for experiments with continuous advective transport from the crawl space to the sand. With undamaged foil, the calculations overestimate the measurements by 20%. Reversely, with an opening in the center of the foil, the calculations underestimate the measurements by 20-40%. The results show that under controlled conditions radon transport from a foil-covered sand column into a crawl-space can be described within 40% on basis of separately measured parameters.

Atmospheric Pressure↗
PubMed1986

Simulation of the metabolism and enterohepatic circulation of endogenous chenodeoxycholic acid in man using a physiological pharmacokinetic model.

The metabolism and enterohepatic circulation of chenodeoxycholic acid (CDC), a major primary bile acid in man, has been stimulated using a multicompartmental physiological pharmacokinetic model which was previously reported and used to simulate the metabolism of cholic acid. The model features compartments and linear transfer coefficients. Compartments, which are defined as the pools of single chemical species in well defined anatomical volumes, are aggregated into nine 'spaces' based on anatomical and physiological considerations (liver, gall-bladder, bile ducts, duodeno-jejunum, ileum, colon, portal blood, sinusoidal blood, and general circulation). Each space contains several compartments which correspond to the compounds present in that space, for example, the compound in question and its biotransformation products. For CDC (as for cholic acid in the previous simulation) each space contains three compartments corresponding to the unconjugated bile acid, its glycine amidate, and its taurine amidate. Transfer coefficients, which denote the fractional amount of the compartment's contents exiting per unit time, are categorized according to function: flow, for example gall-bladder contraction (which involves transfer of all substances contained in the space at the same fractional rate); biotransformation (which transfers the substrate from one compartment to another within the same space); or transport (which denotes movements between contiguous compartments, belonging to different spaces across a diffusion membrane or a cellular barrier). The model is made time-dependent by incorporating meals which trigger gall-bladder emptying and modify intestinal flow. The transfer coefficients in the cholic acid model were modified for the CDC model since there is indirect evidence that CDC amidates (probably chenodeoxycholylglycine) are absorbed from the duodeno-jejunum and the first pass hepatic clearance of CDC species differs from that of cholyl species. The model was then used with all existing experimental data to simulate CDC metabolism in healthy humans over a 24-h period during which three meals were ingested. Satisfactory agreement was obtained between simulated and experimental data indicating that this model continues to be useful for describing the metabolism of bile acids and may also be of value for describing the metabolism of drugs whose metabolism is similar to that of bile acids.

Bile Acids and Salts↗
PubMed2002

A Monte Carlo EM approach for partially observable diffusion processes: theory and applications to neural networks.

We present a Monte Carlo approach for training partially observable diffusion processes. We apply the approach to diffusion networks, a stochastic version of continuous recurrent neural networks. The approach is aimed at learning probability distributions of continuous paths, not just expected values. Interestingly, the relevant activation statistics used by the learning rule presented here are inner products in the Hilbert space of square integrable functions. These inner products can be computed using Hebbian operations and do not require backpropagation of error signals. Moreover, standard kernel methods could potentially be applied to compute such inner products. We propose that the main reason that recurrent neural networks have not worked well in engineering applications (e.g., speech recognition) is that they implicitly rely on a very simplistic likelihood model. The diffusion network approach proposed here is much richer and may open new avenues for applications of recurrent neural networks. We present some analysis and simulations to support this view. Very encouraging results were obtained on a visual speech recognition task in which neural networks outperformed hidden Markov models.

Algorithms↗
PubMed2005

Intracranial pressure dynamics assessed by noninvasive ultrasound during 30 days of bed rest.

INTRODUCTION: Intracranial pressure (ICP) may be an important contributor to symptoms of space adaptation syndrome during the initial days of microgravity exposure. The temporary nature of these symptoms suggests that some physiologic adaptation or compensation occurs. Fluid shifts similar to those in microgravity can be simulated on Earth using head-down tilt (HDT) bed rest. This study was performed to calibrate a new noninvasive ICP instrument and to investigate ICP adaptation during 30 d of HDT bed rest. METHODS: A noninvasive ultrasound technique that measures small skull expansions with fluctuations in ICP was used to measure cranial oscillations before and near the end of 30-d HDT bed rest in eight healthy, male volunteers. Pulse phase-locked loop (PPLL) output voltage and arterial BP were continuously monitored and correlated. RESULTS: The amplitude of intracranial distance pulsation decreased during 30-d bed rest. Prior to bed rest, the PPLL amplitude was 25 +/- 9 mV and this amplitude was reduced by 60% to 9 +/- 4 mV (a value consistent with that of upright posture) at the end of HDT bed rest (p = 0.01). DISCUSSION: PPLL measurements of skull pulsations are acutely posture dependent, being significantly higher in supine and HDT as compared with upright posture. A cephalad fluid shift is probably the responsible mechanism. Our results indicate that there are adaptations to intracranial pooling of blood and tissue fluid during bed rest that reduce skull pulsation amplitudes to values similar to those obtained in normal upright posture. Detailed studies of the time course of cranial vessel and bone adaptations may provide insights into the potential adaptative mechanisms.

Aerospace Medicine↗
PubMed1991

Histopathological and ultrastructural changes in simulated large colonic torsion and reperfusion in ponies.

This investigation examines the histological and ultrastructural lesions of the colonic mucosa during terminal experimental infarction and subsequent reperfusion. Four ponies were anaesthetised and subjected to surgical torsion of the colon. Biopsies were collected at hourly intervals for 3 h, at which point the torsions were corrected. Circulation was re-established for 2 h and the bowel was re-biopsied at hourly intervals. The ponies were killed while under anaesthesia. During the 3 h experimental infarction, the bowel became macroscopically thickened and dark purple. Histologically, the mucosa degenerated from Grade 0 to Grade 3. Ultrastructurally, there was progressive micro-vascular distension with erythrodiapedesis and damage to the interstitial cells. Spaces developed between the bases and sides of the columnar epithelial cells and sloughing followed subsequently. During the 2 h reperfusion interval, the mucosa continued to degenerate rapidly to a Grade 5, and was characterised by extensive interstitial damage, oedema, cellular swelling, necrosis and mitochondrial damage. The results showed that the experimentally infarcted colonic mucosa degenerated sequentially. Following circulatory reestablishment, continued rapid mucosal degeneration characteristic of reperfusion injury occurred. Reperfusion injury is probably responsible, at least in part, for the often poor outcome of infarcted bowel in horses following surgical correction.

Animals↗
PubMed1999

Fluoroscopy as an imaging means for computer-assisted surgical navigation.

OBJECTIVE: Intraoperative fluoroscopy is a valuable tool for visualizing underlying bone and surgical tool positions in orthopedic procedures. Disadvantages of this technology include the need for continued radiation exposure for visual control, and cumbersome means of alignment. The purpose of this article was to highlight a new concept for a computer-assisted freehand navigation system that uses single intraoperatively acquired fluoroscopic images as a basis for real-time navigation of surgical tools. MATERIALS AND METHODS: Optoelectronic markers are placed on surgical tools, a patient reference, and the fluoroscope to track their position in space. Projection properties of the fluoroscope are acquired through an initial precalibration procedure using a tracked radiopaque phantom grid. Corrections are applied to compensate for both the fluoroscope's image intensifier distortions and the mechanical bending of the C-arm frame. This enables real-time simulation of surgical tool positions simultaneously in several single-shot fluoroscopic images. In addition, through optoelectronically tracked digitization of a target viewpoint, the fluoroscope can be numerically aligned at precise angles relative to the patient without any X-ray exposure. RESULTS: This article shows the feasibility of this technology through its use in cadaver trials to perform the difficult task of distal locking of femoral nails.

Bone Nails↗
PubMed1991

Numerical tests of a method for simulating electrical potentials on the cortical surface.

A mathematical imaging method for simulating cortical surface potentials was introduced at recent neurosciences meetings [1a], [1b], [2] and was applied to elucidate the neural origins of evoked responses in normal volunteers and certain patient populations. This method consists of the solution of an inward harmonic continuation problem and its effect is to simulate data that has not been attenuated and smeared by the skull. This cortical imaging technique (CIT) is validated by applying it to artificially derived data. Pairs of dipolar sources with different depths and separations are introduced into a spherical conducting medium simulating the head. Scalp potential maps are constructed by interpolating the simulated data between 28 "scalp" electrode positions. Noise is added to the data to approximate the variability in measured potentials that would be observed in practice. CIT is used in each case to construct potential maps on layers concentric to and within the layer representing the scalp. In several instances when the dipole pair is deep and closely spaced, the sources cannot be separated by the scalp topographical maps but are easily separated by the "cortical" topographical maps. CIT is also applied to scalp-recorded potentials evoked by bilateral median nerve stimulation and pattern-reversal visual stimulation.

Brain Mapping↗