Semicircular canal contribution to the perception of roll tilt during gondola centrifugation

BACKGROUND: Spatial disorientation is an important problem in aviation. The mechanisms behind the sensation of roll tilt during coordinated turns are not well known. The present study aimed at elucidating what kind of semicircular canal information might cause tilts of the subjective horizontal during gondola centrifugation. METHODS: The subjective visual horizontal (SVH) was measured by means of an adjustable visual line in darkness. Subjects (n = 8) underwent four centrifuge runs (2 G, 5 min), sitting in different positions, i.e., heading forwards, backwards, centripetally, and centrifugally. The roll position of the gondola (60 degrees at 2 G) was controlled so that the subject was always upright with respect to the resultant gravitoinertial force vector. Thus, the semicircular-canal stimulus components in yaw, pitch, and roll were varied to some extent independently of each other. RESULTS: For the forward position the SVH was substantially tilted in a direction compensatory with respect to the inclination of the gondola. For the backward position there was also a tendency to a compensatory SVH tilt. In all subjects the magnitude of tilt was larger for the forward position than for the backward. The group means were +20.9 +/- 8.4 degrees and -6.9 +/- 10.5 degrees (positive sign designates a clockwise deviation of the SVH), p < 0.001, n = 8. There were no significant SVH tilts for the centripetal (+6.4 +/- 10.7 degrees) and centrifugal (+2.1 +/- 4.8 degrees) positions. The effects of deceleration of the centrifuge were very small for all positions. CONCLUSION: These findings suggest that the substantial SVH tilt after acceleration heading forwards is not directly related to any single component of semicircular canal stimulation but depends on the ability of the brain to expediently process complex stimulus patterns.     

Optokinetic stimulation in microgravity: torsional eye movements and subjective visual vertical

INTRODUCTION: Microgravity provides unique sensory inputs to the vestibular and oculomotor systems. We sought to determine the effects of long-term spaceflight on sensing of spatial orientation. METHODS: Two cosmonauts participated in experiments on human vestibulo-visual interactions during a long-term mission (178 d) in the MIR station in 1995. During circular optokinetic stimulation (OKS) the tonic torsional eye position (torsional beating field, TBF) and the subjective visual vertical (SVV) were recorded on several days of the space mission as well as pre- and post-flight. A reference data set was obtained from healthy subjects on Earth, in whom the TBF was measured in upright and in prone positions. RESULTS: Neither cosmonaut showed changes in the SVV or the TBF values during the first days in microgravity. On flight day 149, cosmonaut A showed an increase of both values, which continued to rise by 4- and 10-fold until the end of the flight (TBF: 8.1 degrees; SVV: 216.8 degrees). This cosmonaut reported that the increase was accompanied by a loss of spatial orientation. In contrast, cosmonaut B's values remained at pre-flight levels (TBF: 1.6 degrees; SVV: 4.4 degrees). Post-flight values of the TBF did not significantly differ from pre-flight values for either cosmonaut. Subjects showed an increase of the TBF by more than a factor of 2 in prone position (range -7.7 degrees to +10.2 degrees) compared with upright position (range -3.7 degrees to +3.4 degrees). CONCLUSIONS: Pre-flight, post-flight and during the first part of the flight, both cosmonauts exhibited values similar to those of normal subjects in an upright position. The increased TBF values of cosmonaut A from flight day 110 on were within the range of the normal subjects in prone (face-down) position, when the gravity vector cannot be used to stabilize the TBF against the rotating stimulus (the axis of rotation is parallel to the gravity vector). The increasing deviations of cosmonaut A's SVV values in-flight suggest the presence of an internal body reference system, which weakened throughout the flight and thus lost its stabilizing effect.     

便携式主观视觉垂直线知觉检测仪的研制与应用

目的　研制便携式主观视觉垂直线(SVV)知觉检测仪,提供耳石器功能检测设备。　方法　便携式SVV知觉检测仪由主机、受检者控制设备、充电设备构成。暗室条件下,受检者通过操纵主机上的光标移动到自认为的“重力垂直线”位置,根据光标与实际重力垂直线的偏差判断两侧耳石器功能是否对称。受试者为8名青年健康男性志愿者,在静坐和转椅90°/s轴心旋转时分别进行4次SVV检测,每次间隔1 d。　结果　4次检测结果之间没有显著差别(P>0 05),但90°/s转椅轴心旋转与静坐时的SVV数据间有显著差别(P<0 05),转椅轴心旋转时的SVV数据要高于静坐时的检测数据。　结论　便携式SVV知觉检测仪具有良好的重复性和有效性,可以用于人体耳石功能检测,具有良好的应用前景。     

The peripheral nervous system and the perception of verticality

Orientation of the body with respect to gravity is based on integration of visual, vestibular and somatosensory signals. Here, we investigated the subjective postural vertical (SPV) and visual vertical (SVV) in three patients with bilateral somatosensory deafferentation and a group of age-matched normal subjects. Our hypothesis was that the patients with bilateral somatosensory deafferentation may show tilt induced bias in the construction of their SPV, with a normal SVV. Patient 1 had a severe Guillain Barré syndrome and almost complete absence of peripheral sensation, the two other patients had a thoracic spinal injury with a sensory loss from T6-7 down. On initial testing, compared with normal subjects and the patients with spinal injury, Patient 1 had a significant bias in SPV towards the side of a preceding tilt in both directions. Several months later, after significant improvement of sensation, this tilt-induced bias in SPV had resolved completely. In addition, Patient 1 had a significantly enlarged "cone of verticality", which did not change following improvement in peripheral sensation, reflecting persisting disturbance in the perception of body verticality. In the two patients with spinal injury, bias towards the side of a preceding tilt was not significant. These findings confirm the importance of somatosensory input from the trunk to the perception of SPV in the seated position.     

Serum enzyme activity of healthy subjects during modeling of the effects of weightlessness

The enzymic activity of blood of healthy male volunteers was examined during 8-day bed rest in the horizontal and head-down (-6 degrees) position, water immersion up to the neck and 6-hour head-down tilt (-15 degrees). Alkaline phosphatase, cholinesterase (CE), leucine arylamidase (LA), glutamate dehydrogenase (GDH) and gamma-glutamyl transpeptidase (GGTP) were measured. During horizontal bed rest the activities of all the enzymes, except for GDH, decreased in a moderate degree which was very distinct at an early stage of exposure. The activity of GDH and CE decreased significantly after the exposure. The enzymic activity tended to decline during head-down tilt at -6 degrees. The LA and GGTP activity decreased to a greater extent, being statistically significant during head-down tilt at -6 degrees and in the recovery period. The enzymic activity insignificantly increased during water immersion and 6-hour head-down tilt at -15 degrees, remaining in some cases elevated during 5 days after exposure. The lower activity of enzymes (which was significant for some of them) during horizontal and antiorthostatic bed rest was primarily associated with diminished motor activity, whereas increased enzymic activity was related to the gravity-induced blood shift to the intrathoracic area.     

The effect of head-down tilt and water immersion on intracranial pressure in nonhuman primates.

A headward fluid shift occurs in humans exposed to space-flight. This shift is thought to be the stimulus for the observed reduction in plasma volume, and may produce a sustained rise in intracranial pressure (ICP). In order to determine the effect of head-down tilt (HDT) and water immersion on intracranial pressure (ICP), five anesthetized rhesus monkeys were fitted with intracranial pressure-monitoring transducers and subjected to -6 degrees HDT followed by head-out thermoneutral (34.7 degrees) water immersion. ICP was elevated from 3.8 +/- 1.1 mm Hg in the horizontal control period to 5.3 +/- 1.3 mm Hg (p less than 0.05) during the 15 min pre-immersion HDT. When seated in the empty immersion tank, ICP stabilized at -6.3 +/- 1.3 mm Hg for the control period and increased to -2.2 +/- 1.9 mm Hg (p less than 0.05) when the water level was maintained for 30 min at the sternal notch. The ICP returned toward pre-immersion levels (-5.5 +/- 1.4 mm Hg) as the tank was drained. Mean blood pressure (MBP) remained constant during the pre-immersion tilt test. In the pre-immersion control period, MBP was 91 +/- 3 mm Hg compared to 90 +/- 4 mm Hg (p greater than 0.05) during immersion and 82 +/- 1 mm Hg (p less than 0.05) in the post-immersion recovery period. In summary, exposure of rhesus monkeys to either head-down tilt or water immersion produced a prompt rise in ICP during the stimulus period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systolic time intervals during combined hand cooling and head-up tilt.

Cardiac effects initiated by hand cooling and head-up tilt, separately and in combination, were studied in human subjects. Systolic and diastolic arterial pressures, heart rate, pre-ejection period (PEP), and left ventricular ejection time (LVET) were measured while subjects were resting supine, supine with hand immersed in 10 degrees C water, tilted 70 degrees head up, and hand immersed during head-up tilt. Hand immersion and head-up tilt individually increased diastolic blood pressure and concomitantly increased the duration of PEP. During the combined maneuver, the increase in diastolic pressure was greater than observed during either separate maneuver; however, the combined maneuver had no comparable summative effect on PEP prolongation. An interaction appears to occur during the combined maneuver which counteracts the Frank/Starling effect on PEP due to decreased venous return during head-up tilt.     

Cooling hyperthermic firefighters by immersing forearms and hands in 10 degrees C and 20 degrees C water

INTRODUCTION: Firefighters experience significant heat stress while working with heavy gear in a hot, humid environment. This study compared the cooling effectiveness of immersing the forearms and hands in 10 and 20 degrees C water. METHODS: Six men (33 +/- 10 yr; 180 +/- 4 cm; 78 +/- 9 kg; 19 +/- 5% body fat) wore firefighter 'turn-out gear' (heavy clothing and breathing apparatus weighing 27 kg) in a protocol including three 20-min exercise bouts (step test, 78 W, 40 degrees C air, 40% RH) each followed by a 20-min rest/cooling (21 degrees C air); i.e., 60 min of exercise, 60 min of cooling. Turn-out gear was removed during rest/cooling periods and subjects either rested (Control), immersed their hands in 10 or 20 degrees C water (H-10, H-20), or immersed their hands and forearms in 10 or 20 degrees C water (HF-10, HF-20). RESULTS: In 20 degrees C water, hand immersion did not reduce core temperature compared with Control; however, including forearm immersion decreased core temperature below Control values after both the second and final exercise periods (p < 0.001). In 10 degrees C water, adding forearm with hand immersion produced a lower core temperature (0.8 degrees C above baseline) than all other conditions (1.1 to 1.4 degrees C above baseline) after the final exercise period (p < 0.001). Sweat loss during Control (1458 g) was greater than all active cooling protocols (1146 g) (p < 0.001), which were not different from each other. DISCUSSION: Hand and forearm immersion in cool water is simple, reduces heat strain, and may increase work performance in a hot, humid environment. With 20 degrees C water, forearms should be immersed with the hands to be effective. At lower water temperatures, forearm and/or hand immersion will be effective, although forearm immersion will decrease core temperature further.     

Effect of longitudinal physical training and water immersion on orthostatic tolerance in men

To test the hypothesis that moderately intense physical training has no effect on orthostasis, orthostatic and fluid-electrolyte-endocrine responses to 60 degrees head-up tilt were compared before and after 6 h of water immersion (34.5 +/- 0.1 degrees C) up to the neck following 6 months of exercise training. During the tilt test the five male subjects (27-42 years) each wore a lower-body positive-pressure suit (MAST-111A antishock trousers). The tilt procedure consisted of a 40-min supine control period (suit deflated), followed by a maximum 90-min tilt period (suit inflated to 50 +/- 5 mm Hg for 30 min, then deflated for 60 min or until presyncope). The mean +/- S.E. pretraining cycle ergometer peak VO2 was 3.20 +/- 0.14 L.min-1 (39 +/- 2 ml.min-1.kg-1), 3.36 +/- 0.27 L.min-1 (42 +/- 4 ml.min-1.kg-1) after 3 months (N.S.), and increased by 18% to 3.78 +/- 0.36 L.min-1 (48 +/- 5 ml.min-1.kg-1, +22%, p less than 0.05) posttraining. During pretraining, water immersion tilt tolerance decreased from 74 +/- 16 min before to 34 +/- 9 min (delta = 40 min, p less than 0.05) after immersion. During posttraining, water immersion tilt tolerance decreased similarly from 74 +/- 16 min preimmersion to 44 +/- 13 min (delta = 30 min, p less than 0.05) postimmersion (74 vs. 74 min, N.S.; 34 vs. 44 min, N.S.).(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in regional and central hemodynamics during a 7-day immersion in water

Regional and central hemodynamics were assessed at bi- and tetrapolar rheography and tacho-oscillography during 7-day "dry" immersion and 8-day -8 degrees head-down tilt. Blood redistribution evident from enhanced pulse filing of the brain, lungs and arms was most pronounced on day 3-5. The onset of the blood outflow to the liver was observed on immersion day 5 due to compensatory and adaptive reactions. Lack of exercise tolerance of cardiovascular system through its insufficient training was similar in the immersion and head-down tilt for all the 6 healthy males studied (aged 41-49) despite more obvious changes in regional hemodynamics during the immersion, which recovered on its fifth day.     

Visual sensations of roll rotation during complex vestibular stimulation

BACKGROUND: In aviation, vestibular-induced spatial disorientation is a significant cause of accidents. Recreating flight-like vestibular stimuli in simulators might be a means for training pilots to respond adequately in disorienting situations. Due to the physical constraints of land-based simulators, the question arises whether a given illusion may be created in different ways. For instance, is it possible to induce sensations of tilt by rotary stimuli? The present study concerns the relationship between sensations of rotation and tilt during complex vestibular stimulation. METHODS: The visual sensation of roll rotation was quantified by means of a velocity-matching procedure. In a large gondola centrifuge eight subjects underwent four runs (2 G, 2 min) with different heading positions (forward, backward, centripetally, and centrifugally). The inclination of the gondola persistently corresponded with the vector sum of the Earth gravity force and the centrifugal force (60 degrees at 2 G). Thus, the semicircular canal stimulus in roll was combined in different ways with stimuli in yaw and pitch, as well as with an increasing or decreasing G vector. RESULTS: The magnitude of the responses was only dependent on the roll component of the stimulus. The gain, defined as the ratio between the response and the roll stimulus, was 7-10%. The responses decayed with a time constant ranging from 4 to 5.5 s. CONCLUSION: The visual sensation of roll rotation reflects the roll plane canal velocity stimulus independently of other stimulus components. This is in contrast to earlier findings on the sensation of changes in position (roll tilt).     

Rescue lifting system (RLS) might help to prevent death after rescue from immersion in cold water

OBJECTIVE: In order to prevent sudden death after rescue from immersion in cold water, victims should be handled carefully avoiding additional cardiovascular stress. In this study we investigated if a new double-sling rescue system ("Rescue Lifting System-RLS) was superior to conventional single-sling techniques. METHODS: We studied 14 healthy male subjects in good physical condition aged 21 to 40 years. They were lifted up from the ground with the new RLS and two conventional techniques ("Lifesling" and a navy rescue system used in SAR helicopters). Heart rate was determined by QRS detection (Polar Precision Performance device; Polar Electro Oy, Kempele, Finland) and blood pressure by sphygmomanometry. RLS and "Lifesling" were tested under conditions of dry land and immersion in 18 degrees C water. RESULTS: Rescue with RLS induced only moderate heart rate changes which were significantly lower (about 30 bpm) than with conventional techniques. These findings could be reproduced under "wet" condition. DISCUSSION: RLS enables rescue in a supine position avoiding extensive orthostatic stress. It might therefore be favourable in preventing sudden death after rescue from immersion in cold water.     

The influence of the aquatic environment on the center of pressure,impulses and upper and lower trunk accelerations during gait initiation

Gait initiation is defined as the transition from stationary standing to steady-state walking. Despite the frequent use of therapy pools for training walking in early stages of rehabilitation, none have been reported on the effects of immersion on gait initiation. We aimed to analyze the center of pressure (COP) trajectories, the vertical and anteroposterior impulses and upper and lower trunk accelerations during anticipatory (APA) and execution phases of gait initiation. In the COP trajectory, the execution (EXE) phase was further subdivided in two phases: predominantly mediolateral (EXE1), and predominantly anteroposterior (EXE2). Able-bodied participants initiated gait while standing on a force plate and walked approximately 4 steps following a visual cue. The participants were wearing three inertial sensors placed on the lower and upper trunk, and on the stance shank. Individuals performed 10 trials each on land and in water, in two consecutive days. The lengths and velocities of COP trajectories increased in water compared to land during APA, while the COP length increased and the COP velocity reduced in water during EXE2. The anteroposterior impulses increased in water during EXE. Lower trunk acceleration was smaller in water while the upper trunk acceleration did not differ, resulting in the larger ratio of upper to lower trunk acceleration in water during EXE. Overall, immersion in water increases COP length during gait initiation, and reduces COP velocity during EXE2, indicating a new postural strategy in water. The aquatic medium may be favorable for individuals who need weight support, gradual resistance and a longer time to execute gait initiation.     

Diving bradycardia is not correlated to the oculocardiac reflex

Both facial immersion in cold water and pressure on the eyeball cause reflex bradycardia. These reflexes are called diving reflex and oculocardiac reflex, respectively. The latter is sometimes used in diving medicine to estimate the risk of severe diving bradycardia. The purpose of this study was to quantify the effects of both reflexes on heart rate in 15 subjects. All subjects performed four tests: (1) breath-holding (2) breath-holding and facial immersion in water of 10 degrees, 15 degrees, and 20 degrees C; (3) facial immersion in water and snorkeling; (4) application of pressure of 30, 50, and 70 mmHg on the eyeball. In seven subjects an additional test was done: (5) eyeball pressures during breath-holding. It was shown that the intensity of the oculocardiac reflex is not a good indication of the bradycardia that can be expected during diving. It is proposed that breath-holding with facial immersion in water of 20 degrees C or colder during at least 10 s is a more appropriate test to assess the possibility of severe diving bradycardia and cardiac arrhythmias.     

The effects of roll vs. pitch rotation in humans under orthostatic stress.

BACKGROUND: It has been known since 1953 that pre-exposure to less than +1 Gz will reduce subsequent +Gz-tolerance. With few exceptions, during operational flying, the transition from hypogravity to hypergravity involves roll as well as pitch rotation. We examined the effect of roll vs. pitch rotation while undergoing transition from hypogravity to +1 Gz on a tilt table. METHODS: Twelve subjects (28-47 yr old) were rotated at 45 degrees x s(-1) from head-up (HU) at 15 degrees relative to gravitational vertical to 135 degrees head-down (HD) and back to the HU position after different HD dwell times. HD dwell times were set at 7, 15, and 30 s. The subject was rotated about the interaural axis (pitch) and about the naso-occipital axis (roll). Both the HD dwell times and axes of rotation were randomized within and across subjects. BP and heart rate were recorded during the HU-HD-HU maneuver. RESULTS: Analysis of variance, repeated measure design revealed that the rate and magnitude of BP decrease induced by the HD to HU maneuver is significantly higher (p < 0.01) in roll than in pitch during all HD dwell times. The decrease of BP at 7s is significantly (p < 0.01) higher than at 15s and 30s. Heart rate increases significantly higher (p < 0.01) in pitch than in roll at 7s-dwell time. CONCLUSION: Our results suggest that the compensatory mechanism to orthostatic stress is more efficient in response to pitch than roll rotation. This is reflected from the findings that the mean magnitude of OH (orthostatic hypotension) and the rate of BP decrease induced by the HD-HU maneuver is significantly greater in roll rotation than pitch rotation. The mean HR increase post HD-HU rotation is significantly higher in the pitch than the roll rotation. The significant rate of BP decrease during HD-HU roll rotation could have important implications for maintaining G-tolerance and spatial orientation during subsequent exposure to hypergravity.     

Immersion in thermoneutral water: effects on arterial compliance

BACKGROUND: Since thermoneutral head-out water immersion induces an increase in central blood volume and cardiac output along with a decrease in peripheral vascular resistance, we examined here whether thermoneutral immersion leads rapidly to an increase in arterial compliance. METHODS: Changes in hemodynamic status and arterial compliance induced by thermoneutral head-out water immersion to the midchest were studied in 26 healthy volunteers (15 women, 11 men) using a non-invasive pulse wave measurement system. Several hemodynamic parameters including stroke volume (SV) and large and small artery compliance were studied in a quiet room with a stable environmental temperature (25 degrees C) and during thermoneutral (34.5 degrees C) head-out water immersion. RESULTS: In a preliminary study, an agreement between SV estimated by radial pulse contour analysis and that obtained by Doppler echocardiography was observed in ambient air. An increase in SV led to an increase in cardiac output (12%) 10 min after the start of immersion. Heart rate and arterial pressure remained unchanged. Systemic vascular resistance was significantly decreased and total arterial compliance estimated by the SV to aortic pulse pressure ratio was significantly increased. Radial pulse contour analysis confirmed these results with an increase in both large and small artery compliance. CONCLUSION: In conclusion, radial pulse contour analysis provides an interesting method for assessing hemodynamic changes under environmental constraints. Our results were consistent with a prompt increase in arterial compliance during acute water immersion.     

Status of human hemodynamics during water immersion in different postures of immersion

Central and peripheral hemodynamics was investigated in 16 essentially healthy volunteers who performed a routine tilt test or a tilt test in water immersion. Unlike tilt tests carried out before water immersion, the supine to up-right transfer in water did not change cardiac rhythm, cardiac output, leg blood flow or other circulation parameters. The fact that there are no posture-related circulation changes in water immersion suggests that the horizontal and upright positions in water can be viewed as hemodynamically similar.     

Ventilatory and gas exchange dynamics in response to head-down tilt with and without venous occlusion

The purpose of these experiments was to examine ventilatory and gas exchange responses to acute head-down tilt. Subjects (N = 5) participated in two series of head-down tilt experiments. Both experimental protocols involved movement from a near-vertical position (120 degrees) to a -30 degrees head-down position. One set of experiments employed venous occlusion (VO) in the legs during the first 30 s of tilt, while the second set was performed without occlusion (control experiment). Oxygen uptake (VO2), carbon dioxide output (VCO2) and expired ventilation (VE) increased significantly (p less than 0.05) above vertical values in both experimental conditions during the first 75 s following tilt. Further, VO2, VCO2, and VE were greater (p less than 0.05) during the VO experiment compared to control immediately following removal of venous occlusion. Although end-tidal CO2 tension (PETCO2) tended to increase following tilt, mean PETCO2 averaged over 15-s periods remained unaltered (p greater than 0.05) during tilt in both experimental protocols compared to the vertical position. These data demonstrate that acute head-down tilt results in an increase in VE and the general maintenance of isocapnia.     

Central and general hemodynamics in healthy persons during simulated weightlessness

The pumping and contractile functions of the left ventricle were investigated during combined exposure to water immersion and head-down tilt (at -6 degrees) that simulated microgravity. This 7-day exposure caused noticeable changes in central and systemic circulation which developed as a function of time. It is assumed that the deficiency of venous blood return seen on test days 1-3 was a compensatory response to initial blood displacement which was induced by a reduction of the circulating blood volume. The reduction was in turn a result of responses from volumoreceptors of the heart and intrathoracic vessels to "excessive" intrathoracic blood volume.     

Optokinetic drum tilt hastens the onset of vection-induced motion sickness

BACKGROUND: Under optokinetic drum conditions, a stationary participant views the patterned interior of a rotating drum. Quickly, most participants perceive illusory self-rotation in the direction opposite to the drum's true rotation (vection). It has been documented that up to 60% of participants experience motion sickness-like symptoms under optokinetic conditions perhaps because of conflicting sensory information from the visual and vestibular systems. METHODS: Keeping rotation speed constant (10 RPM), drum tilt relative to the axis of rotation was systematically manipulated (0 degrees, 5 degrees, 10 degrees), producing a wobble effect. Overall well-being and eight motion sickness symptoms were assessed every 2 min using subjective scales. RESULTS: Participants reported 1) a complex type of circular vection that included a "wobble" or "sway" component and 2) a quicker onset of motion sickness-like symptoms as tilt increased. CONCLUSION: In a tilted drum, the vestibular system correctly indicates that the participant is stationary while the visual system indicates a complex type of self-rotation. This type of sensory conflict is more severe than what takes place under typical optokinetic drum conditions (no tilt). Results suggest that as visual/vestibular sensory conflict increases, so does the speed at which motion sickness symptoms occur.