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PubMed1994

Influence of obstacles on lipid lateral diffusion: computer simulation of FRAP experiments and application to proteoliposomes and biomembranes.

Fluorescence Recovery After Photobleaching experiments were simulated using a computer approach in which a membrane lipid leaflet was mimicked using a triangular lattice obstructed with randomly distributed immobile and non-overlapping circular obstacles. Influence of the radius r and area fraction c of these obstacles and of the radius R of the observation area on the relative diffusion coefficient D* (Eq. (1)) and mobile fraction M was analyzed. A phenomenological equation relating D* to r and c was established. Fitting this equation to the FRAP data we obtained with the probe NBD-PC embedded in bacteriorhodopsin/egg-PC multilayers suggests that this transmembrane protein rigidifies the surrounding lipid phase over a distance of about 18 A (approximately equal to two lipid layers) from the protein surface. In contrast, analysis of published diffusion constants obtained for lipids in the presence of gramicidin suggests that in terms of lateral diffusion, this relatively small polypeptide does not significantly affect the surrounding lipid phase. With respect to the mobile fraction M, and for point obstacles above the percolation threshold, an increase in R led to a decrease in M which can be associated with the existence of closed domains whose average size and diffusion properties can be determined. Adaptation of this model to the re-interpretation of the FRAP data obtained by Yechiel and Edidin (J Cell Biol (1987) 115:755-760) for the plasma membrane of human fibroblasts consistently leads to the suggestion that the lateral organization of this membrane would be of the confined type, with closed lipid domains of approximately equal to 0.5 microns 2 in area.

Bacteriorhodopsins↗
PubMed1996

A biologically inspired neural net for trajectory formation and obstacle avoidance.

In this paper we present a biologically inspired two-layered neural network for trajectory formation and obstacle avoidance. The two topographically ordered neural maps consist of analog neurons having continuous dynamics. The first layer, the sensory map, receives sensory information and builds up an activity pattern which contains the optimal solution (i.e. shortest path without collisions) for any given set of current position, target positions and obstacle positions. Targets and obstacles are allowed to move, in which case the activity pattern in the sensory map will change accordingly. The time evolution of the neural activity in the second layer, the motor map, results in a moving cluster of activity, which can be interpreted as a population vector. Through the feedforward connections between the two layers, input of the sensory map directs the movement of the cluster along the optimal path from the current position of the cluster to the target position. The smooth trajectory is the result of the intrinsic dynamics of the network only. No supervisor is required. The output of the motor map can be used for direct control of an autonomous system in a cluttered environment or for control of the actuators of a biological limb or robot manipulator. The system is able to reach a target even in the presence of an external perturbation. Computer simulations of a point robot and a multi-joint manipulator illustrate the theory.

Avoidance Learning↗
PubMed1985

Collision with and avoidance of obstacles by blind cave fish Anoptichthys jordani (Characidae).

Blind Mexican cave fish (Anoptichthys jordani) were released into unknown surroundings and their swimming tracks were recorded. During the first 24 h in a new tank, i.e., in unfamiliar environments, the average swimming velocity of the fish was higher than during the remaining time in the tank. Collision with obstacles was recorded by an electrical contact detection method. Single frame analysis of video-recordings revealed that collision can be correlated with tail movements when the fish is close to an obstacle, whereas avoidance correlates with gliding during approach to an obstacle. These behavior patterns are discussed in the context of the flow field hypothesis and inhibition of the lateral line organ during movement.

Animals↗
PubMed2004

Cerebellar damage produces context-dependent deficits in control of leg dynamics during obstacle avoidance.

It has been suggested that the cerebellum is an important contributor to CNS prediction and control of intersegmental dynamics during voluntary multijoint reaching movements. Leg movements subserve different behavioral goals, e.g., locomotion versus voluntary stepping, which may or may not be under similar dynamic control. The objective was to determine whether cerebellar leg hypermetria (excessive foot elevation) during obstacle avoidance in locomotion and voluntary stepping could be attributed to a particular deficit in appropriately controlling intersegmental dynamics. We compared the performance of eight individuals with cerebellar damage to eight healthy controls as they walked or voluntarily stepped in place over a small obstacle. Joint kinematics and dynamics were calculated during swing phase for both movement contexts. The kinematic analysis showed that hypermetria occurred during both walking and stepping and was associated with excessive knee flexion. When present, the amplitude of hypermetria was greater during stepping compared to walking. During stepping, subjects with cerebellar damage produced excessive knee flexor muscle torques and consequently overcompensated for interaction and gravitational torques normally used to decelerate the limb. During walking, the torque pattern was very similar to that of control subjects walking over a taller obstacle, and therefore might be a voluntary compensatory strategy to avoid tripping. Our results show that the extent of kinematic and dynamic abnormalities associated with cerebellar leg hypermetria is context-specific, with more fundamental abnormalities of leg dynamics being apparent during stepping as opposed to walking.

Adult↗
PubMed2004

Older women strongly prefer stride lengthening to shortening in avoiding obstacles.

In the present study the obstacle avoidance strategy during treadmill walking was investigated in ten young (aged 19-32) and ten older females (aged 65-78). Minimisation of displacement of the foot from its original landing position has been proposed to be the main criterion for the selection of alternate foot placement. Each participant performed 60 obstacle avoidance trials. Foot-obstacle configurations were varied in order to obtain both lengthening and shortening avoidance reactions. For each trial it was calculated how much lengthening and how much shortening of the stride was required minimally for successful avoidance. The difference between required lengthening and required shortening was expressed as a percentage of the control stride length and was used as a measure of minimal displacement. The behavior of young females was in agreement with the minimal displacement criterion. The older females, however, exhibited a strong preference for stride lengthening, even in situations in which stride shortening would be highly favorable. The explanation for the long step strategy preference of the older females is discussed in terms of age-related changes in decision-making, differences between young and older persons in the unobstructed gait pattern, and safety considerations.

Adult↗
PubMed1999

Utility of MRI in detecting obstacles to reduction in developmental dysplasia of the hip: comparison with two-directional arthrography and correlation with intraoperative findings.

We studied the utility of magnetic resonance imaging (MRI) in detecting obstacles to reduction in developmental dysplasia of the hip (DDH) by comparing MRI findings with two-directional arthrograms and intraoperative findings. In 36 patients there were 38 DDHs; 23 complete dislocations and 15 residual subluxations. Coronal and transverse sections of T1-weighted images were used. Interpositions in the acetabulum and the anterior, superior, and posterior portions of the limbus, which were intracapsular obstacles to reduction, were evaluated. MRI and arthrography were useful for assessing the shape of the anterior portion of the limbus. In complete dislocations, MRI findings proved more valuable to detect deformities of the posterior portion of the limbus. Four of ten limbus excisions showed intermediate intensity signals on MRI. The histology was characterized by reparative granulation tissue with new capillary formation and organized thrombi. MRI proved useful for detecting obstacles to reduction, as well as for assessing morphological and histological abnormalities.

Arthrography↗
PubMed2004

Obstacle avoidance during human walking: effects of biomechanical constraints on performance.

OBJECTIVE: To determine whether fixation of the ankle joint, the knee joint, or both increasingly affects the performance of a newly learned task, that is, stepping over an obstacle. DESIGN: Randomized trial. SETTING: Research laboratory of a university hospital in Switzerland. PARTICIPANTS: Eighteen healthy, young volunteers. INTERVENTION: Subjects walked on a treadmill and, with reduced vision, stepped with the right leg over a randomly approaching obstacle. They adapted to the task during the 2 runs. In the third run, fixating orthoses of the ankle-foot (AFO), knee (KO), or both (KAFO) were attached to the left leg. MAIN OUTCOME MEASURE: The "performance" consisted of leg muscle activity, joint movements, swing phase duration, and the clearance between the foot and the obstacle. The changes within runs (adaptation) and between runs (eg, transfer) were evaluated. RESULTS: The attached orthoses caused a reduced transfer of performance in the KAFO and KO between runs 2 and 3. No differences in the rate of adaptation were observed among the 3 groups during the third run. CONCLUSIONS: A movement restriction of the supporting leg worsened the performance of the contralateral leg in a locomotor task. Performance was more affected by knee-joint fixation than by ankle-joint fixation alone and, consequently, the need for relearning is greater.

Adaptation, Physiological↗
PubMed2004

Age-related reduction in sagittal plane center of mass motion during obstacle crossing.

Accidental falls are a leading cause of injury and death in the growing elderly population. Traumatic falls are frequent, costly, and debilitating. Control of balance during locomotion is critical for safe ambulation, but relatively little is known about the natural effect of aging on dynamic balance control. Samples of healthy young (n = 13) and elderly (n = 13) subjects were compared in the interactive measures of center of mass (COM) and center of pressure (COP) during level walking and obstacle crossing conditions. Obstacle heights were normalized to individual body height (2.5%, 5%, 10%, and 15%). Temporal-distance (T-D) variables of gait were also compared. Statistical analyses were conducted using a two-way ANOVA for subject group and obstacle height. T-D parameters were not significantly different between groups; nor were frontal plane COM and COP parameters. Significant age differences did exist for antero-posterior (A/P) motion of the COM (decreased motion in the elderly), and its relationship with the COP (reduced separation between the two variables in the elderly). Anterior COM velocities were also significantly lower in the elderly group. The results confirm the ability of healthy elderly adults to maintain dynamic balance control in the frontal plane during locomotion. Reduced A/P distances between the COM and COP indicate a conservative reduction of the mechanical load on joints of the supporting limb. This conservative strategy may be related to a reduction in muscle strength as it occurs in the natural aging process.

Accidental Falls↗
PubMed2005

Strategies to overcome obstacles in the facilitation of critical thinking in nursing education.

UNLABELLED: This paper seeks to describe strategies that can be used to overcome obstacles in the facilitation of critical thinking in nursing education. A qualitative, explorative, descriptive and contextual design was used to conduct the research in which fourth year basic comprehensive students and nurse educators volunteered to take part in the study by signing an informed consent. The participants were purposively selected. Focus group interviews were used to collect data from both groups. Tesch's descriptive method of open coding described in (Creswell, J. 1994. RESEARCH DESIGN: Qualitative and quantitative approach. Sage, London.) was used to analyse data. (Democratic Nursing Organisation of South Africa, 1998. Ethical standards for nurse researchers. Denosa, Pretoria) ethical standards for research were observed to maintain the standard and quality of the research. (Lincoln, Y.S., Guba, E.G. 1985. Naturalistic inquiry. Sage, London.) framework was used to ensure trustworthiness of the study. The following obstacles were identified and recontextualised within the existing literature to be able to describe the strategies to overcome the identified obstacles to the facilitation of critical thinking of students: the educators' lack of knowledge; use of teaching and assessment methods that do not facilitate critical thinking of learners; the negative attitudes of educators and their resistance to change; inappropriate selection process and poor educational background that did not facilitate critical thinking of students; inadequate socialisation, cultural and instructional language incompetence. Findings indicated that there is a need for nurse educators to model critical thinking in all aspects of nursing education. It is recommended that there be a whole paradigm shift in nursing education from the traditional teacher-centred methods to a more learner-centred approach that will facilitate critical thinking of student nurses.

Attitude of Health Personnel↗
PubMed2001

Growth of branched actin networks against obstacles.

A method for simulating the growth of branched actin networks against obstacles has been developed. The method is based on simple stochastic events, including addition or removal of monomers at filament ends, capping of filament ends, nucleation of branches from existing filaments, and detachment of branches; the network structure for several different models of the branching process has also been studied. The models differ with regard to their inclusion of effects such as preferred branch orientations, filament uncapping at the obstacle, and preferential branching at filament ends. The actin ultrastructure near the membrane in lamellipodia is reasonably well produced if preferential branching in the direction of the obstacle or barbed-end uncapping effects are included. Uncapping effects cause the structures to have a few very long filaments that are similar to those seen in pathogen-induced "actin tails." The dependence of the growth velocity, branch spacing, and network density on the rate parameters for the various processes is quite different among the branching models. An analytic theory of the growth velocity and branch spacing of the network is described. Experiments are suggested that could distinguish among some of the branching models.

Actin Depolymerizing Factors↗
PubMed1994

Anomalous diffusion due to obstacles: a Monte Carlo study.

In normal lateral diffusion, the mean-square displacement of the diffusing species is proportional to time. But in disordered systems anomalous diffusion may occur, in which the mean-square displacement is proportional to some other power of time. In the presence of moderate concentrations of obstacles, diffusion is anomalous over short distances and normal over long distances. Monte Carlo calculations are used to characterize anomalous diffusion for obstacle concentrations between zero and the percolation threshold. As the obstacle concentration approaches the percolation threshold, diffusion becomes more anomalous over longer distances; the anomalous diffusion exponent and the crossover length both increase. The crossover length and time show whether anomalous diffusion can be observed in a given experiment.

Biophysical Phenomena↗
PubMedDate not supplied

Platelet adhesion onto the wall of a flow chamber with an obstacle.

In the present study, the data of the initial adhesion of platelets onto the wall of a flow chamber with an obstacle in steady human blood flows were obtained. The flowfields and the distribution of stress-related factors were simulated numerically by a finite volume method and the fluid dynamic effect on the platelet adhesion is discussed. In addition to the wall shear effect, the normal stress effect was also taken into account. A parameter Vn/[Vt] was devised to assess the combined effect of both shear and normal forces in platelet adhesion. It was found that the peak adhesion occurred next to, but not on, the impingement point on the obstacle where the value of Vn/[Vt] was negative. In these regions, direct impact played a major role in platelet adhesion. On the other hand, near the separation point before the obstacle where Vn/[Vt] was insignificant, the mechanism was believed to be different from that in the direct impact region. Denser adhesion there might be caused by the accumulation and frequent collision of particles due to flow retardation and/or detour of the flow path. Interestingly, relatively low adhesion was found inside the recirculation regions. These results show that the normal stress effect (impingement) should be considered in platelet adhesion in addition to the shear effect.

Blood Flow Velocity↗
PubMed1999

Altered kinetic strategy for the control of swing limb elevation over obstacles in unilateral below-knee amputee gait.

Our goal was to document the kinetic strategies for obstacle avoidance in below-knee amputees. Kinematic data were collected as unilateral below-knee traumatic amputees stepped over obstacles of various heights in the walking path. Inverse dynamics were employed to calculate power profiles and work during the limb-elevation and limb-lowering phases. Limb elevation was achieved by employing a different strategy of intra-limb interaction for elevation of the prosthetic limb than for the sound limb, which was similar to that seen in healthy adult non-amputees. As obstacle height increased, prosthetic side knee flexion was increased by modulating the work done at the hip, and not the knee, as seen on the sound side. Although the strength of the muscles about the residual knee was preserved, the range of motion of that knee had previously been found to be somewhat limited. Perhaps more importantly, potential instability of the interface between the stump and the prosthetic socket, and associated discomfort at the stump could explain the altered limb-elevation strategy. Interestingly, the limb-lowering strategy seen in the sound limb and in non-amputees already features modulation of rotational and translational work at the hip, so an alternate strategy was not required. Thus, following a major insult to the sensory and neuromuscular system, the CNS is able to update the internal model of the locomotor apparatus as the individual uses the new limb in a variety of movements, and modify control strategies as appropriate.

Adaptation, Physiological↗
PubMed2004

Reaching between obstacles in spatial neglect and visual extinction.

The aim of the present studies was to investigate whether 'perception' and 'visually guided action' could be dissociated with regard to two different aspects of the neglect syndrome. In the first study we tested a group of patients with neglect in two tasks, both within the same experimental setting. One task was to bisect a space between two objects, while the other required subjects to reach between the same pair of objects en route to a target area, so that the objects became potential obstacles to the reach. In the second study we tested a patient with visual extinction to double simultaneous stimulation, using a similar reaching task. Our aim was to determine whether visual awareness of obstacles in the workspace was necessary for successful navigation. In both studies we found evidence that reaching responses took normal account of the presence and location of obstacles on the left side, despite the tendency to neglect such left-sided information in more explicit perceptual tasks. We interpret both sets of results within a theoretical framework that identifies on-line visuomotor control with the occipito-parietal 'dorsal stream' (along with associated premotor and subcortical structures), and visual perception with the occipito-temporal 'ventral stream', plus associated temporo-parietal areas.

Aged↗
PubMed2003

Swing phase kinetics and kinematics of knee replacement patients during obstacle avoidance.

Proper knee joint function is essential for safe and effective mobility within a complex environment. Following knee joint replacement, joint structure and function are altered, often requiring individuals to adjust normal movement patterns in order to adapt to these changes. Such adaptations may either improve function or lead to movement patterns that may potentially be unsafe for individuals. To investigate this issue, a group of individuals who had undergone knee replacement was examined while performing an obstacle avoidance task. Their performance was compared with that of healthy age-matched controls. Participants walked along a 10 m walkway a total 48 times. Trials were divided equally between unobstructed walking and clearing a 6 or 18 cm obstacle during both right and left limb lead. Lead limb kinetic and kinematic variables were examined and revealed that members of the surgical group exhibited decreased active surgical knee flexion and diminished surgical knee flexor work. In order to maintain toe clearance at control levels, patients were observed to increase hip hiking and hip flexor work, in addition to laterally displacing the surgical toe during swing over the obstacle. Despite allowing individuals to maintain adequate toe clearance, these compensatory strategies may lead to an increased instability and pose a threat to safety in this population. In addition, an increased demand placed on the hip joint of the surgical limb may be undesirable in this population. Research aimed at determining how to best maximize surgical knee function must continue if these potential negative sequela are to be minimized.

Adaptation, Physiological↗
PubMed2001

An analytical solution to the problem of the orientation of rigid particles by planar obstacles. Application to membrane systems and to the calculation of dipolar couplings in protein NMR spectroscopy.

Nonspherical particles or molecules experience an ordering effect in the presence of obstacles due to the restrictions they place on the orientation of those molecules that are in their proximity. Obstacles may be the limits of a membrane in which the molecule is embedded, oriented mesoscopic systems such as bicelles, or membrane fragments used to induce weak protein alignment in a magnetic field. The overall shape of most proteins can be described to a good approximation by an ellipsoidal particle. Here we describe and solve analytically the problem of the orientation of ellipsoidal particles by planar obstacles. Simple expressions are derived for the orientational distribution function and the order parameter. These expressions allow the analytical calculation of the residual dipolar couplings for a protein of known three-dimensional structure oriented by steric effects. The results are in good agreement with experiment and with the results of previously described simulations. However, they are obtained analytically in a fraction of the time and therefore open the possibility to include the optimization of the overall shape in the determination of three-dimensional structures using residual dipolar coupling constraints. The equations derived are general and can also be applied to problems of a completely different nature. In particular, previous equations describing the orientation of particles embedded in membranes are verified and generalized here.

Membrane Proteins↗
PubMed2005

Obstacles for rural American Indians seeking alcohol, drug, or mental health treatment.

The purpose of this study was to identify factors associated with 4 clusters of obstacles (self-reliance, privacy issues, quality of care, and communication and trust) to mental health and substance abuse treatment in 3 treatment sectors for residents of 3 reservations in the United States. Participants (N=3,084) disclosed whether they had sought treatment for emotional, drug, or alcohol problems in the past year and, if so, whether they had faced obstacles in obtaining care from Indian Health Services, tribal services, and other public or private systems. Correlates of these obstacles included negative social support, instrumental social support, utility of counselors, utility of family doctors, treatment sector, treatment type, diagnosis of an anxiety disorder, and tribe.

Adolescent↗
PubMed2004

Automatic avoidance of obstacles is a dorsal stream function: evidence from optic ataxia.

When we reach out to pick something up, our arm is directed to the target by visuomotor networks in the cortical dorsal stream. However, our reach trajectories are influenced also by nontarget objects, which might be construed as potential obstacles. We tested two patients with bilateral dorsal-stream (parietal lesions, both of whom were impaired at pointing to visual stimuli (optic ataxia). We asked them to reach between two cylinders, which varied in location from trial to trial. We found that the patients' reaches remained invariant with changes in obstacle location. In a control task when they were asked to point midway between the two objects, however, their responses shifted in an orderly fashion. We conclude that the dorsal stream provides the visual guidance we automatically build into our movements to avoid potential obstacles, as well as that required to ensure arrival at the target.

Adult↗