Phobic postural vertigo

Patients with phobic postural vertigo (PPV) often report a particularly increased unsteadiness when looking at moving visual scenes. Therefore, the differential effects of large-field visual motion stimulation in roll plane on body sway during upright stance were analyzed in 23 patients with PPV, who had been selected for the integrity of their vestibular and balance systems, and in 17 healthy subjects. Visual motion stimulation induced a sensation of apparent body motion (roll vection) in all patients and normal subjects. Normal subjects showed an increased lateral sway path with a lateral shift of the center of pressure (COP) in stimulus direction (mean 1.67 cm, SD 1.63). The patients also exhibited an increase in sway path during visual motion stimulation; however, their body sway differed from that of normals in that there was no lateral displacement of COP (mean 0.19 cm, SD 0.73). The lateral displacement of COP and the increase in RMS of body sway during visual motion stimulation were significantly greater in normals than in the patients ( p<0.05). The patients' increased body sway without COP deviation does not imply an increased risk of falling. Two explanations are conceivable for this increased body sway without body deviation in patients with PPV: (a) the patients rely more on proprioceptive and vestibular rather than on visual cues to regulate upright stance; or (b) they depend on visual, vestibular, and proprioceptive information, but the threshold at which they initiate a compensatory body sway opposite in direction to a perceived body deviation is lower than in normal subjects. The data support the second explanation.     

Differential integration of visual and kinaesthetic signals to upright stance

The present experiment was designed to assess the effect of active (deliberate) maintenance of a small forward (FL) or backward body lean (BL) (about 2° ankle flexion) with respect to the spontaneous direction of balance (or neutral posture, N) on postural balance. We questioned whether BL and FL stances, which impose a volitional proprioceptive control of the body-on-support angle, could efficiently reduce mediolateral displacements of the centre of pressure (CoP) induced by the visual motion of a room and darkness. Subjects (n = 15) were asked to stand upright quietly feet together while confronted to a large visual scene rolling to 10° on either side in peripheral vision (and surrounding vertical visual references in central vision) at 0.05 Hz. CoP displacements were recorded using a force platform. Analysis of medio-lateral CoP root-mean square showed that the effect of the moving room depends on the subject’s postural stability performance in the eyes open N stance condition. Two significant postural behaviours emerged. (1) The most stable subjects (G1) were not affected by the conditions of altered vision, but swayed more in BL stance than in the N stance. (2) The unstable subjects (G2) exhibited (i) larger CoP displacements in altered visual conditions and a greater coupling of the CoP with the motion of the visual scene, (ii) enhanced visual dependency with postural leaning, and (iii) decreased CoP displacements when leaning forward in the eyes open motionless scene. Interestingly, the visual quotient positively correlated with the proprioceptive quotient, indicating that the more the subjects relied heavily on the visual frame of reference (FOR) the more they were influenced by body leaning. This result suggested hence a lesser ability to use efficiently body-ground proprioceptive cues. On the whole, the present findings indicate that body leaning could provide a useful mean to assess the subject’s ability to use body-ground proprioceptive cues not only to improve postural stability during eyes opening (especially during forward leaning), but also as a mean to disclose subjects’ visual dependency and their associated difficulties to shift from visual to proprioceptive-based FOR.     

Activity of pursuit neurons in the caudal part of the frontal eye fields during static roll-tilt

The smooth-pursuit system and vestibular system interact to keep the retinal target image on the fovea during head and/or whole body movements. The caudal part of the frontal eye fields (FEF) in the fundus of arcuate sulcus contains pursuit neurons and the majority of them respond to vestibular stimulation induced by whole-body rotation, that activates primarily semi-circular canals, and by whole-body translation, that activates otoliths. To examine whether coordinate frames representing FEF pursuit signals are orbital or earth-vertical, we compared preferred directions during upright and static, whole-body roll-tilt in head- and trunk-restrained monkeys. Preferred directions (re monkeys’ head/trunk axis) of virtually all pursuit neurons tested (n = 21) were similar during upright and static whole-body roll-tilt. The slight shift of preferred directions of the majority of neurons could be accounted for by ocular counter-rolling. The mean (±SD) differences in preferred directions between upright and 40° right ear down and between upright and 40° left ear down were 6° (±6°) and 5° (±5°), respectively. Visual motion preferred directions were also similar in five pursuit neurons tested. To examine whether FEF pursuit neurons could signal static whole-body roll-tilt, we compared mean discharge rates of 29 neurons during fixation of a stationary spot while upright and during static, whole-body roll-tilt. Virtually all neurons tested (28/29) did not exhibit a significant difference in mean discharge rates between the two conditions. These results suggest that FEF pursuit neurons do not signal static roll-tilt and that they code pursuit signals in head/trunk-centered coordinates.     

The contribution of vestibular input to the stabilization of human posture: a new experimental approach

An experiment was designed to evaluate the vestibular contribution to the stabilization of upright stance in normals and in two patients with loss of vestibular function. A forward or backward displacement of a load (2 kg) by a torque motor attached to the subject induced opposing movements in the head and trunk. The small linear acceleration of the head in space of about 0.1 g was followed, with a latency of 50-65 ms, by EMG responses in the tibialis anterior and rectus femoris (backward acceleration) or gastrocnemius muscles (forward acceleration). These responses were absent in patients with a vestibular deficit. It is suggested that the observed EMG responses are due to fast acting vestibulospinal reflexes involved in the regulation of upright stance. For comparable head accelerations the integrated EMG responses induced by the vestibulospinal mechanism are about ten times smaller than those induced by spinal stretch reflexes during displacement of the feet. Vestibulospinal reflexes would appear, therefore, to play only a minor role in the compensation of stumbling.     

Visual and vestibular contributions to pitch sway stabilization in the ankle muscles of normals and patients with bilateral peripheral vestibular deficits

Summary Vestibular, visual, and proprioceptive influences on muscle activity correcting for backwards body tilt were investigated in normals and patients with bilateral peripheral vestibular deficits. Body tilt was induced by a dorsi-flexion rotation of the feet about the ankle joints while the subject stood on a force measuring platform. Ankle muscle activity and torque were monitored as upright stance was reestablished, and correlated with head angular accelerations and neck muscle activity. In normals with eyes closed, soleus stretch reflex activity at 50–80 ms was followed by two bursts of tibialis anterior (TA) EMG activity at ca 80 and 125 ms from the onset of 36 deg/s, 3 deg amplitude platform rotations. Neck muscle activity rotated the head backwards at the same time as TA activity rotated the body forwards about the ankle joints. Under the influence of vision, i.e. eyes open, slight increases in the second burst of TA activity, and ankle torque were observed. When the subjects sat, and were instructed to activate TA rapidly on onset of the platform movement, TA EMG activity increased gradually at ca. 150 ms and not as a burst. In patients with long-lasting bilateral vestibular deficits, both bursts of TA activity were significantly less than normal with eyes closed. Consequently sway correcting torques were abnormally low and all but one of the patients fell over backwards. With eyes open, TA activity was slightly less than, and ankle torques were approximately equal to normal values. In contrast to normals, TA responses obtained in standing and sitting positions were not significantly different. Neck EMG activity varied from normal, consisting of a long burst 100 ms in duration. The present data indicate that a coordinated pattern of ankle, and neck muscle activity occurs during the first 150 ms following induced backward tilt. Ankle muscle activity corrects for the body sway, and neck muscle activity attempts to stabilise the head with respect to earth fixed coordinates. It is proposed that the vestibulo-spinal reflex system predominantly underlies the genesis and coordination of this muscle activity.     

Improving balance function using vestibular stochastic resonance: optimizing stimulus characteristics

Stochastic resonance (SR) is a phenomenon whereby the response of a non-linear system to a weak periodic input signal is optimized by the presence of a particular non-zero level of noise. Stochastic resonance using imperceptible stochastic vestibular electrical stimulation, when applied to normal young and elderly subjects, has been shown to significantly improve ocular stabilization reflexes in response to whole-body tilt; improved balance performance during postural disturbances and optimize covariance between the weak input periodic signals introduced via venous blood pressure receptors and the heart rate responses. In our study, 15 subjects stood on a compliant surface with their eyes closed. They were given low-amplitude binaural bipolar stochastic electrical stimulation of the vestibular organs in two frequency ranges of 1–2 and 0–30 Hz over the amplitude range of 0 to ±700 μA. Subjects were instructed to maintain an upright stance during 43-s trials, which consisted of baseline (zero amplitude) and stimulation (non-zero amplitude) periods. Measures of stability of the head and trunk using inertial motion unit sensors attached to these segments and the whole body using a force plate were measured and quantified in the mediolateral plane. Using a multivariate optimization criterion, our results show that the low levels of vestibular stimulation given to the vestibular organs improved balance performance in normal healthy subjects in the range of 5–26% consistent with the stochastic resonance phenomenon. In our study, 8 of 15 and 10 of 15 subjects were responsive for the 1–2- and 0–30-Hz stimulus signals, respectively. The improvement in balance performance did not differ significantly between the stimulations in the two frequency ranges. The amplitude of optimal stimulus for improving balance performance was predominantly in the range of ±100 to ±400 μA. A device based on SR stimulation of the vestibular system might be useful as either a training modality to enhance adaptability or skill acquisition, or as a miniature patch-type stimulator that may be worn by people with disabilities due to aging or disease to improve posture and locomotion function.     

Influence of dynamic tilts on the perception of earth-vertical

The aim of this study was to test the hypothesis that optimal activation of both the semicircular canals and the otoliths provides reliable vestibular cues about self-orientation in space. For this, we measured the ability of subjects to estimate the subjective vertical immediately, 20 s and 90 s after a rapid tilt (180°/s²) from upright into different roll positions between 90° left and right side down. Subjects had to estimate the earth-vertical and earth-horizontal direction in the dark by (a) setting a luminous line, (b) performing saccades, and (c) verbally declaring body position relative to gravity. The mean error curves from the three paradigms showed consistent E (Müller)- and A (Aubert)-effects, which did not significantly change over time. Horizontal and vertical saccade tasks exhibited different response characteristics, as previously reported by others, which likely reflect different computation mechanisms. The verbal estimation paradigm yielded complementary results to those of the luminous line paradigm and vertical saccade task. The E-effect of the luminous line and the vertical saccade paradigm might be explained by a bias towards earth-vertical due to interactions of vestibular and neck afferent signals. The invariably small A-effect of the luminous line and the vertical saccade paradigm probably reflects somatosensory signals that had relatively weak influence in our experiments. We conclude that phasic activation of the vestibular system reduces the influence of non-vestibular cues observed in low tilt velocity or static experiments. Although this activation generates an E-effect, the total error in the range of ±90° is reduced. Electronic Publication     

Precision contact of the fingertip reduces postural sway of individuals with bilateral vestibular loss

Contact of the hand with a stationary surface attenuates postural sway in normal individuals even when the level of force applied is mechanically inadequate to dampen body motion. We studied whether subjects without vestibular function would be able to substitute contact cues from the hand for their lost labyrinthine function and be able to balance as well as normal subjects in the dark without finger contact. We also studied the relative contribution of sight of the test chamber to the two groups. Subjects attempted to maintain a tandem Romberg stance for 25 s under three levels of fingertip contact: no contact; light-touch contact, up to 1 N (approximately 100 g) force; and unrestricted contact force. Both eyes open and eyes closed conditions were evaluated. Without contact, none of the vestibular loss subjects could stand for more than a few seconds in the dark without falling; all the normals could. The vestibular loss subjects were significantly more stable in the dark with light touch of the index finger than the normal subjects in the dark without touch. They also swayed less in the dark with light touch than when permitted sight of the test chamber without touch, and less with sight and touch than just sight. The normal subjects swayed less in the dark with touch than without, and less with sight and touch than sight alone. These findings show that during quiet stance light touch of the index finger with a stationary surface can be as effective or even more so than vestibular function for minimizing postural sway.     

Vestibular, visual, and somatosensory contributions to human control of upright stance

We investigated the changes of human posture control of upright stance which occur when vestibular cues (VEST) are absent and visual and somatosensory orientation cues (VIS, SOM) are removed. Postural responses to sinusoidal tilts of a motion platform in the sagittal plane (+/-2 degrees, f=0.05, 0.1, 0.2 and 0.4 Hz) were studied in normal subjects (Ns) and patients with bilateral vestibular loss (Ps). We found that absence of VEST (Ps, visual reference) and removal of VIS (Ns, no visual reference) had little effect on stabilization of upright body posture in space. In the absence of both VEST and VIS (Ps, no visual reference) somatosensory graviception still provided some information on body orientation in space at 0.05 and 0.1 Hz. However, at the higher frequencies Ps qualitatively changed their behavior; they then tended to actively align their bodies with respect to the motion platform. The findings confirm predictions of a novel postural control model.     

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.     

Vestibular influences on human postural control in combinations of pitch and roll planes reveal differences in spatiotemporal processing

The present study examined the influence of bilateral peripheral vestibular loss (BVL) in humans on postural responses to multidirectional surface rotations in the pitch and roll planes. Specifically, we examined the effects of vestibular loss on the directional sensitivity, timing, and amplitude of early stretch, balance correcting, and stabilizing reactions in postural leg and trunk muscles as well as changes in ankle torque and trunk angular velocity following multidirectional rotational perturbations of the support surface. Fourteen normal healthy adults and five BVL patients stood on a dual axis rotating platform which rotated 7.5° at 50°/s through eight different directions of pitch and roll combinations separated by 45°. Directions were randomized within a series of 44 perturbation trials which were presented first with eyes open, followed by a second series of trials with eyes closed. Vestibular loss did not influence the range of activation or direction of maximum sensitivity for balance correcting responses (120–220 ms). Response onsets at approximately 120 ms were normal in tibialis anterior (TA), soleus (SOL), paraspinals (PARAS), or quadriceps muscles. Only SOL muscle activity demonstrated a 38- to 45-ms delay for combinations of forward (toe-down) and roll perturbations in BVL patients. The amplitude of balance correcting responses in leg muscles between 120 and 220 ms was, with one exception, severely reduced in BVL patients for eyes open and eyes closed conditions. SOL responses were decreased bilaterally for toe-up and toe-down perturbations, but more significantly reduced in the downhill (load-bearing) leg for combined roll and pitch perturbations. TA was significantly reduced bilaterally for toe-up perturbations, and in the downhill leg for backward roll perturbations. Forward perturbations, however, elicited significantly larger TA activity in BVL between 120 and 220 ms compared to normals, which would act to further destabilize the body. As a result of these changes in response amplitudes, BVL patients had reduced balance correcting ankle torque between 160 and 260 ms and increased torque between 280 and 380 ms compared to normals. There were no differences in the orientation of the resultant ankle torque vectors between BVL and normals, both of which were oriented primarily along the pitch plane. For combinations of backward (toe-up) and roll perturbations BVL patients had larger balance correcting and stabilizing reactions (between 350 and 700 ms) in PARAS than normals and these corresponded to excessive trunk pitch and roll velocities. During roll perturbations, trunk velocities in BVL subjects after 200 ms were directed along directions different from those of normals. Furthermore, roll instabilities appeared later than those of pitch particularly for backward roll perturbations. The results of the study show that combinations of roll and pitch surface rotations yield important spatiotemporal information, especially with respect to trunk response strategies changed by BVL which are not revealed by pitch plane perturbations alone. Our results indicate that vestibular influences are earlier for the pitch plane and are directed to leg muscles, whereas roll control is later and focused on trunk muscles. Electronic Publication     

Multisensory determinants of orientation perception in Parkinson's disease

Perception of the relative orientation of the self and objects in the environment requires integration of visual and vestibular sensory information, and an internal representation of the body's orientation. Parkinson's disease (PD) patients are more visually dependent than controls, implicating the basal ganglia in using visual orientation cues. We examined the relative roles of visual and non-visual cues to orientation in PD using two different measures: the subjective visual vertical (SVV) and the perceptual upright (PU). We tested twelve PD patients (nine both on- and off-medication), and thirteen age-matched controls. Visual, vestibular and body cues were manipulated using a polarized visual room presented in various orientations while observers were upright or lying right-side-down. Relative to age-matched controls, patients with PD showed more influence of visual cues for the SVV but were more influenced by the direction of gravity for the PU. Increased SVV visual dependence corresponded with equal decreases of the contributions of body sense and gravity. Increased PU gravitational dependence corresponded mainly with a decreased contribution of body sense. Curiously however, both of these effects were significant only when patients were medicated. Increased SVV visual dependence was highest for PD patients with left-side initial motor symptoms. PD patients when on and off medication were more variable than controls when making judgments. Our results suggest that (i) PD patients are not more visually dependent in general, rather increased visual dependence is task specific and varies with initial onset side, (ii) PD patients may rely more on vestibular information for some perceptual tasks which is reflected in relying less on the internal representation of the body, and (iii) these effects are only present when PD patients are taking dopaminergic medication.     

Conventionally assessed voluntary activation does not represent relative voluntary torque production

The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT), which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects (n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship between superimposed and voluntary torque. This includes very high contraction intensities (90–100%VA), which are difficult to consistently obtain in regular healthy subjects (VA of ∼90%). Subjects were tested at 30, 60, and 90° knee angles on two experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation (triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles. Maximal VA was similar and very high across knee angles: 97 ± 2.3% (mean ± SD). At high contraction intensities, the increase in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque increased from 79.6 ± 6.1 to 100%MVC, superimposed torque decreased from 8.5 ± 2.6 to 2.8 ± 2.3% of resting triplet. Therefore, an increase in VA of 5.7% (from 91.5 ± 2.6 to 97 ± 2.3%) coincided with a much larger increase in voluntary torque (20.4 ± 6.1%MVC) and EMG (33.9 ± 6.6%max). Moreover, a conventionally assessed VA of 91.5 ± 2.6% represented a voluntary torque of only 79.6 ± 6.1%MVC. In conclusion, when maximal VA is calculated to be ∼90% (as in regular healthy subjects), this probably represents a considerable overestimation of the subjects’ ability to maximally drive their quadriceps muscles.     

Hind leg heat balance in obese Zucker rats during exercise

 To analyse the effect of obesity on exercise-derived heat dissipation, lean and obese Zucker rats were exercised on an inclined treadmill until they would no longer run with gentle prodding. We measured their oxygen consumption, water vapour loss, the concentrations of adenosine tri- and diphosphate, creatine phosphate, and lactate in quick-frozen leg muscles, and the temperature of muscle, skin and blood in the aorta. We determined blood flow to leg muscle, fat and skin by measuring the entrapment of fluorescent microspheres. From the measurements we calculated heat flow rates between hind leg muscle, blood, fat and skin and the environment. The obese rats weighed twice as much as the lean (340–400 g and 175–200 g respectively) and ran half as fast (113 ± 7 m versus 257 ± 17 m). The differences between the two groups for basal oxygen consumption (lean: 6.7 ± 0.9 μmol/min, obese: 5.0 ± 1.9 μmol/min) and exercising oxygen consumption (lean: 37.8 ± 5.6 μmol/min, obese: 22.2 ± 3.8 μmol/min) were not significant. Both groups stopped running after the same time at their maximal speed (lean: 4.5 ± 0.3 min, obese: 4.2 ± 0.2 min). During exercise, lean rats had higher increases in core temperature (lean: 0.7°C, obese: 0.4°C) and muscle temperatures (lean: 1.3°C, obese: 0.7°C) than the obese rats. The calculated heat flows indicated a predominant conductive transfer of heat from muscle through the skin in lean rats but a higher proportion of heat transfer to the blood in obese rats. It is concluded that muscle heat accumulation did not cause fatigue in either case. Received: 30 July 1997 / Received after revision: 24 October 1997 / Accepted: 27 October 1997     

Sensorimotor integration in human postural control

It is generally accepted that human bipedal upright stance is achieved by feedback mechanisms that generate an appropriate corrective torque based on body-sway motion detected primarily by visual, vestibular, and proprioceptive sensory systems. Because orientation information from the various senses is not always available (eyes closed) or accurate (compliant support surface), the postural control system must somehow adjust to maintain stance in a wide variety of environmental conditions. This is the sensorimotor integration problem that we investigated by evoking anterior-posterior (AP) body sway using pseudorandom rotation of the visual surround and/or support surface (amplitudes 0.5-8 degrees ) in both normal subjects and subjects with severe bilateral vestibular loss (VL). AP rotation of body center-of-mass (COM) was measured in response to six conditions offering different combinations of available sensory information. Stimulus-response data were analyzed using spectral analysis to compute transfer functions and coherence functions over a frequency range from 0.017 to 2.23 Hz. Stimulus-response data were quite linear for any given condition and amplitude. However, overall behavior in normal subjects was nonlinear because gain decreased and phase functions sometimes changed with increasing stimulus amplitude. "Sensory channel reweighting" could account for this nonlinear behavior with subjects showing increasing reliance on vestibular cues as stimulus amplitudes increased. VL subjects could not perform this reweighting, and their stimulus-response behavior remained quite linear. Transfer function curve fits based on a simple feedback control model provided estimates of postural stiffness, damping, and feedback time delay. There were only small changes in these parameters with increasing visual stimulus amplitude. However, stiffness increased as much as 60% with increasing support surface amplitude. To maintain postural stability and avoid resonant behavior, an increase in stiffness should be accompanied by a corresponding increase in damping. Increased damping was achieved primarily by decreasing the apparent time delay of feedback control rather than by changing the damping coefficient (i.e., corrective torque related to body-sway velocity). In normal subjects, stiffness and damping were highly correlated with body mass and moment of inertia, with stiffness always about 1/3 larger than necessary to resist the destabilizing torque due to gravity. The stiffness parameter in some VL subjects was larger compared with normal subjects, suggesting that they may use increased stiffness to help compensate for their loss. Overall results show that the simple act of standing quietly depends on a remarkably complex sensorimotor control system.     

Interdependence of torque,joint angle,angular velocity and muscle action during human multi-joint leg extension

Purpose

Force and torque production of human muscles depends upon their lengths and contraction velocity. However, these factors are widely assumed to be independent of each other and the few studies that dealt with interactions of torque, angle and angular velocity are based on isolated single-joint movements. Thus, the purpose of this study was to determine force/torque–angle and force/torque–angular velocity properties for multi-joint leg extensions.

Methods

Human leg extension was investigated (n = 18) on a motor-driven leg press dynamometer while measuring external reaction forces at the feet. Extensor torque in the knee joint was calculated using inverse dynamics. Isometric contractions were performed at eight joint angle configurations of the lower limb corresponding to increments of 10° at the knee from 30 to 100° of knee flexion. Concentric and eccentric contractions were performed over the same range of motion at mean angular velocities of the knee from 30 to 240° s^?1.

Results

For contractions of increasing velocity, optimum knee angle shifted from 52 ± 7 to 64 ± 4° knee flexion. Furthermore, the curvature of the concentric force/torque–angular velocity relations varied with joint angles and maximum angular velocities increased from 866 ± 79 to 1,238 ± 132° s^?1 for 90–50° knee flexion. Normalised eccentric forces/torques ranged from 0.85 ± 0.12 to 1.32 ± 0.16 of their isometric reference, only showing significant increases above isometric and an effect of angular velocity for joint angles greater than optimum knee angle.

Conclusions

The findings reveal that force/torque production during multi-joint leg extension depends on the combined effects of angle and angular velocity. This finding should be accounted for in modelling and optimisation of human movement.     

Antihysteresis of perceived longitudinal body axis during continuous quasi-static whole-body rotation in the earth-vertical roll plane

Estimation of subjective whole-body tilt in stationary roll positions after rapid rotations shows hysteresis. We asked whether this phenomenon is also present during continuous quasi-static whole-body rotation and whether gravitational cues are a major contributing factor. Using a motorized turntable, 8 healthy subjects were rotated continuously about the earth-horizontal naso-occipital axis (earth-vertical roll plane) and the earth-vertical naso-occipital axis (earth-horizontal roll plane). In both planes, three full constant velocity rotations (2°/s) were completed in clockwise and counterclockwise directions (acceleration = 0.05°/s², velocity plateau reached after 40 s). Subjects adjusted a visual line along the perceived longitudinal body axis (pLBA) every 2 s. pLBA deviation from the longitudinal body axis was plotted as a function of whole-body roll position, and a sine function was fitted. At identical whole-body earth-vertical roll plane positions, pLBA differed depending on whether the position was reached by a rotation from upright or by passing through upside down. After the first 360° rotation, pLBA at upright whole-body position deviated significantly in the direction of rotation relative to pLBA prior to rotation initiation. This deviation remained unchanged after subsequent full rotations. In contrast, earth-horizontal roll plane rotations resulted in similar pLBA before and after each rotation cycle. We conclude that the deviation of pLBA in the direction of rotation during quasi-static earth-vertical roll plane rotations reflects static antihysteresis and might be a consequence of the known static hysteresis of ocular counterroll: a visual line that is perceived that earth-vertical is expected to be antihysteretic, if ocular torsion is hysteretic.     

Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats

Previous studies conducted in our laboratory have demonstrated that the vestibular system plays a significant role in controlling arterial pressure (AP) in conscious rats under conditions of transient microgravity. The vestibular system is known to be highly plastic, and on exposure to different gravitational environments, the sensitivity of the vestibular system-mediated AP response might be altered. In order to test this hypothesis, rats were maintained in a 3-G or a normal 1-G environment for 2 weeks, and the AP responses to free drop-induced microgravity were determined. In 1-G rats, the microgravity increased the AP by 37 ± 3 mmHg; this pressor response was significantly attenuated by vestibular lesion (VL) (24 ± 3 mmHg) or body stabilization (29 ± 2 mmHg). Thus, the microgravity-induced pressor response was mediated by both the vestibular and nonvestibular systems; the input of the latter system was blocked by body stabilization. In the 3-G rats, the pressor responses were significantly suppressed compared to those in the corresponding 1-G rats; i.e., the AP increased by 24 ± 2 mmHg in freely moving 3-G rats, by 10 ± 4 mmHg in 3-G rats with VL, and by 13 ± 4 mmHg in stabilized 3-G rats. Furthermore, there was no difference between the 1- and 3-G rats in terms of the pressor response induced by stressors such as a loud noise or an air jet. These results indicate that pre-exposure to 3-G for 2 weeks induces plasticity in both the vestibular- and nonvestibular-mediated AP responses to microgravity.     

Human postural responses to motion of real and virtual visual environments under different support base conditions

The role of visual orientation cues for human control of upright stance is still not well understood. We, therefore, investigated stance control during motion of a visual scene as stimulus, varying the stimulus parameters and the contribution from other senses (vestibular and leg proprioceptive cues present or absent). Eight normal subjects and three patients with chronic bilateral loss of vestibular function participated. They stood on a motion platform inside a cabin with an optokinetic pattern on its interior walls. The cabin was sinusoidally rotated in anterior-posterior (a-p) direction with the horizontal rotation axis through the ankle joints (f=0.05-0.4 Hz; A (max)=0.25 degrees -4 degrees ; v (max)=0.08-10 degrees /s). The subjects' centre of mass (COM) angular position was calculated from opto-electronically measured body sway parameters. The platform was either kept stationary or moved by coupling its position 1:1 to a-p hip position ('body sway referenced', BSR, platform condition), by which proprioceptive feedback of ankle joint angle became inactivated. The visual stimulus evoked in-phase COM excursions (visual responses) in all subjects. (1) In normal subjects on a stationary platform, the visual responses showed saturation with both increasing velocity and displacement of the visual stimulus. The saturation showed up abruptly when visually evoked COM velocity and displacement reached approximately 0.1 degrees /s and 0.1 degrees , respectively. (2) In normal subjects on a BSR platform (proprioceptive feedback disabled), the visual responses showed similar saturation characteristics, but at clearly higher COM velocity and displacement values ( approximately 1 degrees /s and 1 degrees , respectively). (3) In patients on a stationary platform (no vestibular cues), the visual responses were basically similar to those of the normal subjects, apart from somewhat higher gain values and less-pronounced saturation effects. (4) In patients on a BSR platform (no vestibular and proprioceptive cues, presumably only somatosensory graviceptive and visual cues), the visual responses showed an abnormal increase in gain with increasing stimulus frequency in addition to a displacement saturation. On the normal subjects we performed additional experiments in which we varied the gain of the visual response by using a 'virtual reality' visual stimulus or by applying small lateral platform tilts. This did not affect the saturation characteristics of the visual response to a considerable degree. We compared the present results to previous psychophysical findings on motion perception, noting similarities of the saturation characteristics in (1) with leg proprioceptive detection thresholds of approximately 0.1 degrees /s and 0.1 degrees and those in (2) with vestibular detection thresholds of 1 degrees /s and 1 degrees , respectively. From the psychophysical data one might hypothesise that a proprioceptive postural mechanism limits the visually evoked body excursions if these excursions exceed 0.1 degrees /s and 0.1 degrees in condition (1) and that a vestibular mechanism is doing so at 1 degrees /s and 1 degrees in (2). To better understand this, we performed computer simulations using a posture control model with multiple sensory feedbacks. We had recently designed the model to describe postural responses to body pull and platform tilt stimuli. Here, we added a visual input and adjusted its gain to fit the simulated data to the experimental data. The saturation characteristics of the visual responses of the normals were well mimicked by the simulations. They were caused by central thresholds of proprioceptive, vestibular and somatosensory signals in the model, which, however, differed from the psychophysical thresholds. Yet, we demonstrate in a theoretical approach that for condition (1) the model can be made monomodal proprioceptive with the psychophysical 0.1 degrees /s and 0.1 degrees thresholds, and for (2) monomodal vestibular with the psychophysical 1 degrees /s and 1 degrees thresholds, and still shows the corresponding saturation characteristics (whereas our original model covers both conditions without adjustments). The model simulations also predicted the almost normal visual responses of patients on a stationary platform and their clearly abnormal responses on a BSR platform.     

Regulation of bipedal stance: dependency on “load” receptors

Summary According to recent observations, influence of body load has to be taken into account for the neuronal control of upright stance in addition to the systems known to be involved in this regulation (e.g. afferent input from vestibular canals, visual and muscle stretch receptors). The modulation of compensatory leg muscle electromyographic (EMG) responses observed during horizontal body posture indicates the existence of a receptor system which responds to loading of the body against the supporting platform. This receptor should be located within the extensor muscles because a compensatory EMG response and a loading effect on this response was only present following translational, but not rotational impulses. As the EMG responses were identical to those obtained during upright stance, it is argued that these load receptors activate postural reflexes. According to recent observations in the spinal cat, this afferent input probably arises from Golgi tendon organs and represents a newly discovered function of these receptors in the regulation of stance and gait.