Gravitaxis Research Articles

Cross-diffusion-driven hydrodynamic instabilities in a double-layer system: General classification and nonlinear simulations.

Cross diffusion, whereby a flux of a given species entrains the diffusive transport of another species, can trigger buoyancy-driven hydrodynamic instabilities at the interface of initially stable stratifications. Starting from a simple three-component case, we introduce a theoretical framework to classify cross-diffusion-induced hydrodynamic phenomena in two-layer stratifications under the action of the gravitational field. A cross-diffusion-convection (CDC) model is derived by coupling the fickian diffusion formalism to Stokes equations. In order to isolate the effect of cross-diffusion in the convective destabilization of a double-layer system, we impose a starting concentration jump of one species in the bottom layer while the other one is homogeneously distributed over the spatial domain. This initial configuration avoids the concurrence of classic Rayleigh-Taylor or differential-diffusion convective instabilities, and it also allows us to activate selectively the cross-diffusion feedback by which the heterogeneously distributed species influences the diffusive transport of the other species. We identify two types of hydrodynamic modes [the negative cross-diffusion-driven convection (NCC) and the positive cross-diffusion-driven convection (PCC)], corresponding to the sign of this operational cross-diffusion term. By studying the space-time density profiles along the gravitational axis we obtain analytical conditions for the onset of convection in terms of two important parameters only: the operational cross-diffusivity and the buoyancy ratio, giving the relative contribution of the two species to the global density. The general classification of the NCC and PCC scenarios in such parameter space is supported by numerical simulations of the fully nonlinear CDC problem. The resulting convective patterns compare favorably with recent experimental results found in microemulsion systems.

Scaling and coordination deficits during dynamic object manipulation in Parkinson's disease

The ability to reach for and dynamically manipulate objects in a dexterous fashion requires scaling and coordination of arm, hand, and fingertip forces during reach and grasp components of this behavior. The neural substrates underlying dynamic object manipulation are not well understood. Insight into the role of basal ganglia-thalamocortical circuits in object manipulation can come from the study of patients with Parkinson's disease (PD). We hypothesized that scaling and coordination aspects of motor control are differentially affected by this disorder. We asked 20 PD patients and 23 age-matched control subjects to reach for, grasp, and lift virtual objects along prescribed paths. The movements were subdivided into two types, intensive (scaling) and coordinative, by detecting their underlying self-similarity. PD patients off medication were significantly impaired relative to control subjects for both aspects of movement. Intensive deficits, reduced peak speed and aperture, were seen during the reach. Coordinative deficits were observed during the reach, namely, the relative position along the trajectory at which peak speed and aperture were achieved, and during the lift, when objects tilted with respect to the gravitational axis. These results suggest that basal ganglia-thalamocortical circuits may play an important role in fine motor coordination. Dopaminergic therapy significantly improved intensive but not coordinative aspects of movements. These findings are consistent with a framework in which tonic levels of dopamine in the dorsal striatum encode the energetic cost of a movement, thereby improving intensive or scaling aspects of movement. However, repletion of brain dopamine levels does not restore finely coordinated movement.

Neuroanatomy of space, body, and posture perception in patients with right hemisphere stroke

To specify the neuroanatomical correlates of biases in the representations of the gravitational vertical (subjective vertical [SV]) and body axis (subjective straight ahead [SSA]), as well as postural difficulties, in patients with hemispheric stroke. The analysis focused on right hemisphere lesions in 21 neglect patients and 21 non-neglect patients (using MRIcro software) and related performance in 2 experimental tasks (SV and SSA) and a clinical balance assessment. Voxel-based lesion-symptom mapping was used to highlight brain areas in which lesions best explained the severity of task biases (p < 0.01). The bias in the representation of body orientation was found to be strongly related to lesions of the anterior parietal cortex and the middle part of the superior temporal gyrus. The SV errors were associated with more widespread lesions of the posterior parietal and temporal cortices. Imbalance was preferentially associated with lesions of the posterior insula and the adjacent temporoparietal cortex. This study evidenced a cortical dissociation for body-centered and gravitational representations biases, which may reflect the differential involvement of these brain regions in spatial information processing. The lesions involved in representation biases (especially of the SV) and postural difficulties overlapped to some extent in the temporoparietal, superior temporal, and posterior insular regions of the cortex.

Preferential concentration and settling of heavy particles in homogeneous turbulence

Voronoï diagrams are used to analyze one-way coupling direct numerical simulation data of heavy particles settling in homogeneous turbulence. Preferential concentration and clustering of the inertial particles are analyzed for an extended range of particle Stokes and Rouse numbers. Influence of preferential concentration on the settling velocity enhancement is addressed from statistics of particle and flow field quantities conditioned on the local concentration. While gravity is found to have almost no influence on the global characteristics of preferential concentration, the conditional statistics bring out a refined preferential sampling of the flow field resulting from the gravitational effects. This preferential sampling shows that beside the descending fluid velocity contribution, the settling velocity is further increased by the descending fluid acceleration. This effect cannot be detected from global estimations of the particle concentration field. A 2D analysis of the Voronoï cells is also presented to investigate their shape and orientation. It is found that clusters can be represented as 2D elongated manifolds. Their shape is shown to be similar in zero and non-zero gravity fields while Voronoï cells tend to be more elongated for Stokes numbers around unity. Under the gravity effects, they tend to be preferentially oriented perpendicularly to the gravitational axis.

Software for subjective visual vertical assessment: an observational cross-sectional study

Spatial orientation in relation to the gravitational axis is significantly important for the maintenance of the posture, gait and for most of the human's motor activities. The subjective visual vertical exam evaluates the individual's perception of vertical orientation. ObjectivesThe aims of this study were (1) to develop a virtual system to evaluate the subjective visual vertical exam, (2) to provide a simple tool to clinical practice and (3) to assess the subjective visual vertical values of h ealthy subjects using the new software. Study Design: observational cross-sectional study. MethodsThirty healthy volunteers performed the subjective visual vertical exam in both static and dynamic conditions. The exam consisted in adjusting a virtual line in the vertical position using the computer mouse. For the static condition, the virtual line was projected in a white background. For the dynamic condition, black circles rotated in clockwise or counterclockwise directions. Six measurements were taken and the mean deviations in relation to the real vertical calculated. ResultsThe mean values of subjective visual vertical measurements were: static −0.372°; ± 1.21; dynamic clockwise 1.53° ± 1.80 and dynamic counterclockwise −1.11° ± 2.46. ConclusionThis software showed to be practical and accurate to be used in clinical routines.

Sobre «dependencia» y otros cuentos. Reflexiones en torno a la Ley de Promoción de la Autonomía Personal y Atención a las Personas en Situación de Dependencia

The text analyzes the Law for the Promotion of Personal Autonomy and Care for People in Situations of Dependency from a specific analytical angle: the commitment to focus criticism on the sustainability of life, thereby abandoning the markets as the gravitational axis of understanding of reality and political proposals. Set against the traditional notion of citizenship, we propose to construct a new notion, that of citizenship, which allows us to restore our position as subjects of a society which puts the sustainability of life in the centre and which is organized around the needs of people. We question the idea that it is possible to reconcile significant social advances with the maintenance of neo-liberal economic policies, a reconciliation which seems to ratify different laws which have been passed or are planned. The law of «dependency» is configured as a response to a situation of social emergency, resulting from the care crisis, but not as a tool to reverse historical injustices in the distribution of work and the denial of social rights: the right to have a dignified life in which functional diversity is respected and valued and the right to choose whether we wish to care or not.

Regional lung volume changes during high-frequency oscillatory ventilation*

To investigate regional lung volume changes occurring during an inflation-deflation maneuver using high-frequency oscillatory ventilation. Prospective animal trial. Animal research laboratory. Six Yorkshire swine. Electrical impedance tomography was used to quantify regional ventilation during high-frequency oscillatory ventilation. The electrical impedance tomography-derived center of ventilation was used to describe the distribution of regional ventilation, whereas spectral analysis was used to describe regional ventilation-induced impedance changes. Lung injury was induced using surfactant lavage. Animals were transitioned to high-frequency oscillatory ventilation and a slow inflation-deflation maneuver was performed by changing mean airway pressure by 5 cm H2O every 15 mins to a maximum mean airway pressure of 40 cm H2O. The induction of lung injury was associated with a significant shift of the center of ventilation toward nondependent areas and an increase in shunt fraction (p < .001). During the following inflation-deflation maneuver using high-frequency oscillatory ventilation, inflation was associated with a shift of the center of ventilation from nondependent to dependent areas. Center of ventilation was significantly correlated with the shunt fraction (p < .001). Analyzing different lung layers along the gravitational axis separately, nondependent lung areas showed significantly decreased regional ventilation-induced impedance changes at higher pressures, suggesting overdistension, whereas dependent lung areas showed increased impedance changes, suggesting recruitment. The reverse was observed during deflation (all p < .05). The center of ventilation during high-frequency oscillatory ventilation correlated with oxygenating efficiency as measured by the shunt fraction. Lung recruitment during high-frequency oscillatory ventilation produced a significant shift of regional ventilation toward dependent areas of the lung and led to overdistension of nondependent areas.

Egocentric and allocentric reference frames for catching a falling object

When programming movement, one must account for gravitational acceleration. This is particularly important when catching a falling object because the task requires a precise estimate of time-to-contact. Knowledge of gravity's effects is intimately linked to our definition of 'up' and 'down'. Both directions can be described in an allocentric reference frame, based on visual and/or gravitational cues, or in an egocentric reference frame in which the body axis is taken as vertical. To test which frame humans use to predict gravity's effect, we asked participants to intercept virtual balls approaching from above or below with artificially controlled acceleration that could be congruent or not with gravity. To dissociate between these frames, subjects were seated upright (trunk parallel to gravity) or lying down (body axis orthogonal to the gravitational axis). We report data in line with the use of an allocentric reference frame and discuss its relevance depending on available gravity-related cues.

Differences in regional pulmonary pressure–impedance curves before and after lung injury assessed with a novel algorithm

Global pressure–volume (PV) curves are an adjunct measure to describe lung characteristics in patients with acute respiratory distress syndrome (ARDS). There is convincing evidence that high peak inspiratory pressures (PIP) cause barotrauma, while optimized positive end-expiratory pressure (PEEP) helps avoid mechanical injury to the lungs by preventing repeated alveolar opening and closing. The optimal values of PIP and PEEP are deduced from the shape of the PV curve by the identification of so-called lower and upper inflection points. However, it has been demonstrated using electrical impedance tomography (EIT) that the inflection points vary across the lung. This study employs a simple curve-fitting technique to automatically define inflection points on both pressure–volume (PV) and pressure–impedance (PI) curves to asses the differences between global PV and regional PI estimates in animals before and after induced lung injury. The results demonstrate a clear increase in lower inflection point (LIP) along the gravitational axis both before and after lung injury. Moreover, it is clear from comparison of the local EIT-derived LIPs with those derived from global PV curves that a ventilation strategy based on the PV curve alone may leave dependent areas of the lung collapsed. EIT-based PI curve analysis may help choosing an optimal ventilation strategy.

Factors Influencing Manual Discrimination of Orientations in 5-Month-Old Infants

Factors affecting manual discrimination of spatial orientations and orientation preferences in 5-month-old infants have been investigated by using a familiarisation/reaction to novelty procedure. In the first experiment we explored whether the 'vertical preference' observed by Gentaz and Streri (2004 Journal of Cognitive Neuroscience 16 1-7) and Kerzerho et al (2005 Neuro Report 16 1833-1837) is an intrinsic preference or whether it is due to familiarisation. In the second experiment we examined whether the magnitude of angular deviation to the vertical influences the direction of preference. Results showed that when both gravitational vertical and body axes were aligned, the intrinsic 'vertical preference' exists when the angular difference between the two orientations was 10 degrees. When the angular deviation from the vertical was greater than 10 degrees, the novel orientation was preferred. This modification of orientation preference in the manual discrimination of orientation by 5-month-old infants is discussed in relation to the magnitude of the angular deviation to the gravitational vertical and the spatial reference cues available in each condition.

Adaptation of Orientation Vectors of Otolith-Related Central Vestibular Neurons to Gravity

Behavioral experiments indicate that central pathways that process otolith-ocular and perceptual information have adaptive capabilities. Because polarization vectors of otolith afferents are directly related to the electro-mechanical properties of the hair cell bundle, it is unlikely that they change their direction of excitation. This indicates that the adaptation must take place in central pathways. Here we demonstrate for the first time that otolith polarization vectors of canal-otolith convergent neurons in the vestibular nuclei have adaptive capability. A total of 10 vestibular-only and vestibular-plus-saccade neurons were recorded extracellularly in two monkeys before and after they were in side-down positions for 2 h. The spatial characteristics of the otolith input were determined from the response vector orientation (RVO), which is the projection of the otolith polarization vector, onto the head horizontal plane. The RVOs had no specific orientation before animals were in side-down positions but moved toward the gravitational axis after the animals were tilted for extended periods. Vector reorientations varied from 0 to 109 degrees and were linearly related to the original deviation of the RVOs from gravity in the position of adaptation. Such reorientation of central polarization vectors could provide the basis for changes in perception and eye movements related to prolonged head tilts relative to gravity or in microgravity.

Perception of hand motion direction uses a gravitational reference

We studied possible frames of reference for kinesthetic perception of imposed hand motion direction in the frontal plane in ten young adult subjects with no history of neuromuscular disease. In one experiment, subjects were instructed to set unseen hand motion imposed by a motorized linear slide device parallel to the trunk-fixed longitudinal axis, seven visually specified axes and vertical (gravitational axis) while in a standard erect head/trunk posture and with head/trunk orientation varied. The visually specified axes were presented on a head-mounted display that also blocked vision of the external environment. In a second experiment using the same device, subjects set unseen hand motion parallel to vertical and to subjective oblique directions of 45 degrees clockwise (cw) and counter clockwise (ccw) from vertical in erect and varied head/trunk postures. Errors for setting hand motion to vertical and to verbally specified oblique axes (45 degrees cw and ccw from vertical) were lower than to the trunk longitudinal axis and visually specified axes. There were clear oblique effects in setting hand motion to visually specified axes and to subjective oblique (45 degrees cw and ccw) axes. When head and trunk orientation were varied, variable errors were higher for all axes, but remained lowest for vertical and subjective oblique axes. Moreover, errors for setting hand motion to all axes depended on head/trunk orientation. Overall, these results show that kinesthetic perception of imposed hand motion uses a subjective gravitational frame of reference that varies somewhat with head/trunk orientation.

Effect of head position on vestibular evoked myogenic potentials with toneburst stimuli

Conclusion. The present study demonstrated the robustness of VEMP testing with toneburst stimuli, since it is hardly affected by head position, i.e. base or tonic excitation levels of the saccule and inferior vestibular nerve. However, the small but highly significant difference found in latency should not be neglected: the gravitational axis in the upright position may have some special effect on tonic excitation of the saccule. Objectives. To evaluate the effect of head positions on vestibular evoked myogenic potentials (VEMPs) with toneburst stimuli. Materials and methods. VEMPs were recorded with short tonebursts of 500 Hz in 14 normal subjects in 5 head positions (upright, nose up, ear up, nose down, and ear down). The three parameters analyzed were: 1) latency of p13, 2) latency of n23, and 3) corrected amplitude of p13-n23 (CA p13-n23). Results. One-way repeated measures ANOVA showed significant effects on both p13 (p=0.0245) and n23 (p<0.0001) latencies, but not on CA p13-n23. Bonferroni's post hoc test demonstrated that there were significant differences in n23 latency between the upright position and all other head positions leaning on the bed.

Repeatability of the Subjective Visual Vertical and Horizontal

The Subjective Visual Vertical [SVV] is the angle between the gravitational vertical axis and the position of a linear visual marker adjusted vertically by a subject. The Subjective Visual Horizontal [SVH] is similarly defined. Both SVV and SVH are reported to represent function of otoliths and vertical semicircular canals. Current practice of measuring SVV and SVH uses an average of multiple readings to obtain a mean value. This prospective study investigates repeatability of these tests in controls and patients with peripheral vestibular pathology. Twelve replicate readings of SVV and SVH were recorded in 17 controls, 9 patients with unilateral peripheral vestibular dysfunction [uPVD] and 10 patients with definite Meniere's disease [MD]. Control subjects repeated the measurements on day 8 and day 30 and patient groups repeated the measurements one hour later, due to limitations of compliance. Mean SVV/SVH and indicators of repeatability were computed and compared between patients and controls. Repeatabi...

Imbalances in regional lung ventilation: a validation study on electrical impedance tomography.

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right-left ventilation imbalances (bias = 0% and limits of agreement = -10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r(2) > or = 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Magnetic Orientation of Amino Acid Crystals.

The effect of a magnetic field on the orientation of diamagnetic, non-aromatic amino acids crystals was studied. Amino acids, such as L-alanine or glycine, were crystallized from supersaturated solutions (pure and containing other amino acids as additives) under a vertical or horizontal high magnetic field. We prepared an optical microscope system for in situ observation, which could be used under a high magnetic field. According to the observed results of crystallization with the microscope, the c-axes of small nucleated L-alanine crystals moving in the solution were parallel to the direction of the magnetic field. In the case of glycine, the crystals grew so that the ac-planes were parallel and c-axes were at angles of 55° to the direction of magnetic field. This behavior in orientation is explained based on small magnetic anisotropies of diamagnetic crystals.Even if the direction of the magnetic field to the gravitational axis was changed, the directions of crystal axes to the magnetic field remain unchanged. Magnetic orientation of DAST (4-N, N-dimethylamino-4′-N′-methyl-stilbazoliumtosylate) crystal, one of the nonlinear optical materials, was also studied for comparison. As similar to the above case, DAST crystals obtained in 1T were oriented so that their a-axes were parallel to the magnetic field. The orientation ratios of the growing crystals were determined at several intensities of magnetic field.

Internal reference frames for representation and storage of visual information: the role of gravity.

Experimental studies of visual mechanisms suggest that the CNS represents image information with respect to preferred horizontal and vertical axes, as shown by a phenomenon known as the “oblique effect”. In the current study we used this effect to evaluate the influence of gravity on the representation and storage of visual orientation information. Subjects performed a psychophysical task in which a visually-presented stimulus line was aligned with the remembered orientation of a reference stimulus line presented moments before. The experiments were made on 5 cosmonauts during orbital space flight and additionally on 13 subjects in conditions of normal gravity with a tilting chair. Data were analyzed with respect to response variability and timing. On earth, these measurements for this task show a distinct preference for horizontally and vertically oriented stimuli when the body and gravitational axes were aligned. This preference was markedly decreased or disappeared when the body axis was tilted with respect to gravity; this effect was not connected with ocular counter-rolling nor could we find a preference of any other intermediate axis between the gravity and body aligned axes. On the other hand, the preference for vertical and horizontal axes was maintained for tests performed in microgravity over the course of a 6 month flight, starting from flight day 6. We concluded that subjects normally process visual orientation information in a multi-modal reference frame that combines both proprioceptive and gravitational cues when both are available, but that a proprioceptive reference frame is sufficient for this task in the absence of gravity after a short period of adaptation. Some of the results from this study have been previously published in a preliminary report [7].

Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect.

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).

No apparent effect of nitric oxide on the local matching of pulmonary perfusion and ventilation in awake sheep.

The respiratory tissue in the lung receives nitric oxide (NO) from two sources; NO produced in upper airways, and NO produced in lung parenchyma. It has been hypothesized that optimal local matching of ventilation and perfusion (which is necessary for effective gas exchange) is ensured because well-ventilated lung tissue has a higher concentration of NO and thereby higher blood flow owing to the vasodilatory effect of NO. To test this hypothesis, we simultaneously measured the distributions of local (regions of approximately 1.5 cm3) blood flow (radioactive microspheres) and local ventilation (fluorescent aerosol) in five tracheostomized, awake and standing sheep. Tracers for perfusion and ventilation were administered (1) at baseline, (2) during endogenous NO production blockage (L-NAME 25 mg kg-1) and administration of NO free air, and (3) when the sheep received exogenous NO ( approximately 30 p.p.m.), but having its endogenous NO production blocked. The intrapulmonary distribution of ventilation was similar in all three situations. Within horizontal levels of the lung, distribution of perfusion was not affected by variable access to NO, but along the gravitational axis perfusion was more evenly distributed when the sheep had no access to NO. Exogenous NO tended to restore the baseline vertical profile. These changes in vertical distribution of perfusion can be explained by the effect of variable NO concentrations on pulmonary arterial pressure and cardiac output. Variable access to NO had no effect on arterial blood gases. We conclude that NO is important for the vertical distribution of pulmonary perfusion, but has no apparent effect on the local matching of ventilation and perfusion within horizontal layers of the lung.

直線加速度刺激負荷による重力軸認知の影響

直線加速度刺激負荷による重力軸認知の影響