Platform Tilts Research Articles

Postural control deficits due to bilateral pyramidal tract lesions exemplified by hereditary spastic paraplegia (HSP) originate from increased feedback time delay and reduced long-term error corrections.

Pyramidal tract lesions determine the clinical syndrome of Hereditary Spastic Paraplegia (HSP). The clinical impairments of HSP are typically exemplified by their deficits in mobility, leading to falls and injuries. The first aim of this study was to identify the cause for postural abnormalities caused by pyramidal tract lesions in HSP. The second aim was to specify the effect of treadmill training for postural abnormalities. We examined nine HSP patients before and after treadmill training, as well as nine healthy control subjects during perturbed and unperturbed stance. We found that HSP was associated with larger sway amplitudes and velocities. Body excursions following platform tilts were larger, and upper body excursions showed a phase lead. Model-based analysis detected a greater time delay and a reduced long-term error correction of postural reactions in the center of mass. HSP patients performed significantly better in clinical assessments after treadmill training. In addition, treadmill training reduced sway amplitudes and body excursions, most likely by increasing positional and velocity error correction gain as a compensatory mechanism, while the time delay and long-term error correction gain remained largely unaffected. Moreover, the upper body's phase lead was reduced. We conclude that HSP leads to very specific postural impairments. While postural control generally benefits from treadmill training, the effect seems to mainly rely on compensatory mechanisms, whereas the original deficits are not affected significantly.

No evidence for stochastic resonance effects on standing balance when applying noisy galvanic vestibular stimulation in young healthy adults

Noisy galvanic vestibular stimulation (nGVS) at imperceptible levels has been shown to reduce body sway. This reduction was commonly attributed to the mechanism of stochastic resonance (SR). However, it has never been explicitly tested whether nGVS-induced effects on body sway consistently follow a SR-like bell-shaped performance curve with maximal reductions in a particular range of noise intensities. To test this, body sway in 21 young healthy participants was measured during varying nGVS amplitudes while standing with eyes closed in 3 conditions (quiet stance, sway referencing, sinusoidal platform tilts). Presence of SR-like response dynamics in each trial was assessed (1) by a goodness-of-fit analysis using an established SR-curve model and (2) by ratings from 3 human experts. In accordance to theory, we found reductions of body sway at one nGVS amplitude in most trials (75–95%). However, only few trials exhibited SR-like bell-shaped performance curves with increasing noise amplitudes (10–33%). Instead, body sway measures rather fluctuated randomly across nGVS amplitudes. This implies that, at least in young healthy adults, nGVS effects on body sway are incompatible with SR. Thus, previously reported reductions of body sway at particular nGVS intensities more likely result from inherent variations of the performance metric or by other yet unknown mechanisms.

Altered postural timing and abnormally low use of proprioception in lumbar spinal stenosis pre- and post- surgical decompression

BackgroundLumbar spinal stenosis (LSS) is frequently associated with postural instability. Although several studies evaluated patients’ functional impairments, underlying sensorimotor mechanisms are still poorly understood. We aimed to assess the specific set of postural control deficits associated with LSS during spontaneous and externally perturbed stance and evaluated post-surgical changes in postural behavior.MethodsWe analyzed postural control in eleven LSS patients (age 69 ± 8 years) pre- and post-laminectomy, correlated experimental data with functional tests and patient-reported outcomes, and compared findings to 15 matched, healthy control subjects (age 70 ± 6 years). Postural control was characterized by spontaneous sway measures and measures of perturbed stance. Perturbations were induced by anterior-posterior pseudorandom tilts of the body support surface. We used an established postural control model to extract specific postural control parameters.ResultsSpontaneous sway amplitude, velocity and frequency were abnormally large in LSS patients. Furthermore, patients’ postural reactions to platform tilts, represented by GAIN and PHASE were significantly altered. Based on simple feedback model simulations, we found that patients rely less on proprioceptive cues for stance regulation than healthy subjects. Moreover, their postural reactions’ timing is altered. After surgery, patients’ spontaneous sway amplitude was significantly reduced and their postural timing approximated the behavior of healthy subjects.ConclusionThe reduction in proprioceptive input for stance control due to stenosis-caused afferent dysfunction is a functional disadvantage for LSS patients – and may be the basis of increased spontaneous sway. This disadvantage may cause the timing of postural reactions to alter, with the intent of preventing rapid changes in stance regulation for safety reasons. After surgery, patients’ postural timing approximated those of healthy subjects, while the abnormally low use of proprioception remained unchanged. We suggest the post-surgery rehabilitation of proprioception, eg through balance exercises on unstable surfaces and reduced visual input.

Postural instability in adult ADHD – A pilot study

BackgroundApart from inattention, hyperactivity and impulsivity, ADHD in childhood presents with an impairment of motor coordination and balance functions. Until now, literature is scarce about sensorimotor deficits in adult ADHD. This is a pilot study that identifies and quantifies the role of sensory, motor, and central adaptation mechanisms for adult ADHD patients’ sensorimotor deficits in a systematic way, using postural control. MethodsWe analyzed spontaneous and externally perturbed stance in ten adult patients suffering from ADHD. Findings were compared to data from ten matched healthy subjects. ResultsSpontaneous sway amplitudes and velocities were larger in ADHD patients compared to healthy subjects. Furthermore, body excursions as a function of platform tilts were abnormally large in ADHD patients, specifically in the low frequency range. Based on simple feedback model simulations, we found that ADHD patients showed a larger time delay between platform tilts and body response, and a lower value of the integral part of the neural controller, which affects the long-term control of their posture. These postural abnormalities correlated well with the hyperactivity and impulsivity dimensions of the individual ADHD symptoms. ConclusionWe conclude that adult ADHD patients’ major postural deficit consists of an impairment of a stable, long-term sensorimotor behavior, which fits very well to the concept of impulsivity and hyperactivity.

Qualitative postural control differences in Idiopathic Parkinson’s Disease vs. Progressive Supranuclear Palsy with dynamic-on-static platform tilt

ObjectivesWe aimed to assess whether postural abnormalities in Progressive Supranuclear Palsy (PSP) and Idiopathic Parkinson’s Disease (IPD) are qualitatively different by analysing spontaneous and reactive postural control. MethodsWe assessed postural control upon platform tilts in 17 PSP, 11 IPD patients and 18 healthy control subjects using a systems analysis approach. ResultsSpontaneous sway abnormalities in PSP resembled those of IPD patients. Spontaneous sway was smaller, slower and contained lower frequencies in both PSP and IPD as compared to healthy subjects. The amount of angular body excursions as a function of platform angular excursions (GAIN) in PSP was qualitatively different from both IPD and healthy subjects (GAIN cut-off value: 2.9, sensitivity of 94%, specificity of 72%). This effect was pronounced at the upper body level and at low as well as high frequencies. In contrast, IPD patients’ stimulus-related body excursions were smaller compared to healthy subjects. Using a systems analysis approach, we were able to allocate these different postural strategies to differences in the use of sensory information as well as to different error correction efforts. ConclusionsWhile both PSP and IPD patients show abnormal postural control, the underlying pathology seems to be different. SignificanceThe identification of disease-specific postural abnormalities shown here may be helpful for diagnostic as well as therapeutic discriminations of PSP vs. IPD.

The final moments of landing in bumblebees, Bombus terrestris.

In comparison to other insects, like honeybees, bumblebees are very effective pollinators. Even though landing is a crucial part of pollination, little is known about how bumblebees orchestrate the final, critical moments of landing. Here, we use high-speed recordings to capture the fine details of the landing behaviour of free-flying bumblebees (Bombus terrestris), while landing on a flat platform with different orientations. We find that the bees have a fairly constant body and head orientation at the moment of leg extension, irrespective of platform tilt. At the same moment in time, the distance to the platform is held constant at around 8 mm (with the exception of low platform tilts). The orientation of the antennae and the first appendage that touches the platform vary between platform orientations, while the duration of the hover phase does not. Overall, the final moments of landing in bumblebees and their close relatives, the honeybees, are similar. However, the distance to the platform at the moment of leg extension and the duration of the hover phase are different in bumblebees and honeybees, suggesting that they are primarily adapted to land on surfaces with different orientations.

Effects of galvanic vestibular stimulation on postural limb reflexes and neurons of spinal postural network

Quadrupeds maintain the dorsal side up body orientation due to the activity of the postural control system driven by limb mechanoreceptors. Binaural galvanic vestibular stimulation (GVS) causes a lateral body sway toward the anode. Previously, we have shown that this new position is actively stabilized, suggesting that GVS changes a set point in the reflex mechanisms controlling body posture. The aim of the present study was to reveal the underlying neuronal mechanisms. Experiments were performed on decerebrate rabbits. The vertebral column was rigidly fixed, whereas hindlimbs were positioned on a platform. Periodic lateral tilts of the platform caused postural limb reflexes (PLRs): activation of extensors in the loaded and flexing limb and a decrease in extensor activity in the opposite (unloaded and extending) limb. Putative spinal interneurons were recorded in segments L4-L5 during PLRs, with and without GVS. We have found that GVS enhanced PLRs on the cathode side and reduced them on the anode side. This asymmetry in PLRs can account for changes in the stabilized body orientation observed in normal rabbits subjected to continuous GVS. Responses to platform tilts (frequency modulation) were observed in 106 spinal neurons, suggesting that they can contribute to PLR generation. Two neuron groups were active in opposite phases of the tilt cycle of the ipsi-limb: F-neurons in the flexion phase, and E-neurons in the extension phase. Neurons were driven mainly by afferent input from the ipsi-limb. If one supposes that F- and E-neurons contribute, respectively, to excitation and inhibition of extensor motoneurons, one can expect that the pattern of response to GVS in F-neurons will be similar to that in extensor muscles, whereas E-neurons will have an opposite pattern. We have found that ~40% of all modulated neurons meet this condition, suggesting that they contribute to the generation of PLRs and to the GVS-caused changes in PLRs.

Multisensory control of human upright stance

The interaction of different orientation senses contributing to posture control is not well understood. We therefore performed experiments in which we measured the postural responses of normal subjects and vestibular loss patients during perturbation of their stance. Subjects stood on a motion platform with their eyes closed and auditory cues masked. The perturbing stimuli consisted of either platform tilts or external torque produced by force-controlled pull of the subjects' body on a stationary platform. Furthermore, we presented trials in which these two stimuli were applied when the platform was body-sway referenced (i.e., coupled 1:1 to body position, by which ankle joint proprioceptive feedback is essentially removed). We analyzed subjects' postural responses, i.e., the excursions of their center of mass (COM) and center of pressure (COP), using a systems analysis approach. We found gain and phase of the responses to vary as a function of stimulus frequency and in relation to the absence versus presence of vestibular and proprioceptive cues. In addition, gain depended on stimulus amplitude, reflecting a non-linearity in the control. The experimental results were compared to simulation results obtained from an 'inverted pendulum' model of posture control. In the model, sensor fusion mechanisms yield internal estimates of the external stimuli, i.e., of the external torque (pull), the platform tilt and gravity. These estimates are derived from three sensor systems: ankle proprioceptors, vestibular sensors and plantar pressure sensors (somatosensory graviceptors). They are fed as global set point signals into a local control loop of the ankle joints, which is based on proprioceptive negative feedback. This local loop stabilizes the body-on-foot support, while the set point signals upgrade the loop into a body-in-space control. Amplitude non-linearity was implemented in the model in the form of central threshold mechanisms. In model simulations that combined sensor fusion and thresholds, an automatic context-specific sensory re-weighting across stimulus conditions occurred. Model parameters were identified using an optimization procedure. Results suggested that in the sway-referenced condition normal subjects altered their postural strategy by strongly weighting feedback from plantar somatosensory force sensors. Taking this strategy change into account, the model's simulation results well paralleled all experimental results across all conditions tested.

Pronounced overestimation of support surface tilt during stance

A veridical internal notion of the kinematic state of the foot support is essential for postural control. The means by which this is obtained is still a matter of debate. We therefore measured the conscious perception of support tilt during transient anterior-posterior rotations of a motion platform in six healthy subjects, using a psychophysical matching procedure. Furthermore, we evaluated subjects' postural responses (in terms of displacement of subjects' center of mass, COM, and their ankle torque, as represented by the center of foot pressure, COP). The platform tilts were applied in absence of visual and auditory orientation cues. The platform rotations consisted of smoothed position ramps with different dominant frequencies (0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 Hz) and different amplitudes (0.125 degrees, 0.25 degrees, 0.5 degrees, 1 degree, 2 degrees, 4 degrees, and 8 degrees) for the forward and backward directions, which yielded a 6x14 stimulus matrix. The stimuli were repeated five times in a random order. For the matching procedure, subjects tried to maintain an upright body orientation, while trying to orient a light-weight rod, which was attached to a belt around their waists, parallel to the perceived platform surface. We measured the stimulus-evoked angular excursions of the rod and of the subjects' COM as well as the COP shift. We found that the subjects' rod indications overestimated the platform tilts, particularly with small stimulus amplitudes. To characterize the overestimation, we compared the rod indications obtained while subjects stood on the tilting platform, to rod indications in a situation in which they stood next to the platform and tried to match the rod angle to the now visually perceived platform angle. From this comparison, we inferred that the subjects' kinesthetically derived notion of platform tilt overestimates the actual tilt by a factor of approximately 4. The estimates were linearly related to the angle between body (COM) and platform, i.e., to approximately the angle of the ankle joint, a finding which suggests a proprioceptive source of the overestimation. Further analyses supported this view; they showed that the onset latencies of the rod indications could be approximated by a theoretical indication mechanism with a reaction time of about 0.31 s, a velocity threshold of 0.099 degrees/s, and a displacement threshold of 0.12 degrees. These threshold values are well in line with previous work on the leg proprioceptive detection threshold of conscious perception of body sway. We therefore assume that the phenomenon of support tilt overestimation reflects a still unknown mechanism of leg proprioception in postural control.

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.

Human balance control during cutaneous stimulation of the plantar soles

Previous work on human postural control of upright stance, performed in the absence of visual and vestibular orientation cues, suggests that somatosensory cues in the feet enable subjects to maintain equilibrium during low-frequency platform tilts. Here we confirm earlier studies which indicated that stimulation of plantar cutaneous mechanoreceptors can lead to postural responses. Yet, this stimulation did not modify considerably the postural reactions of normal subjects and vestibular loss patients during platform tilts. We therefore suggest that it is necessary to differentiate between (i) cues from plantar cutaneous receptors involved in exteroceptive functions, like the evaluation of the support structure or of relative foot-to-surface motion, and (ii) cues from deep receptors which subserve proprioceptive functions like the control of center of pressure shifts within the limits of the foot support base.

The uropod righting reaction of the crayfishProcambarus clarkii (Girard): an equilibrium response driven by two largely independent reflex pathways

1. The uropod righting reaction of the crayfish,Procambarus clarkii, was investigated in response to stimulation of proprioceptors at the bases of the walking legs and to stimulation of the balance organs, the statocysts. 2. Tilting a platform beneath the legs of crayfish elicited movements of the exopodites of the uropods in the horizontal plane. These were produced by activity only in the slow opener and closer muscles of the exopodites, the fast muscles not being involved in the formation of the uropod pattern. Platform tilts (±20°) with the axis of rotation parallel to the transverse axis of the body elicited a symmetrical closing of the exopodites when the anterior leg groups were depressed, and a symmetrical opening when the same leg groups were levated. 3. Platform tilts parallel to the longitudinal axis of the body elicited asymmetrical movements of the uropods with the exopodite ipsilateral to the levated legs opening and the contralateral exopodite closing. In all experiments the responses of the uropods were only reliably elicited when accompanied by extension of the abdomen. 4. The influence of input from various leg groups was examined by raising them from the oscillating platform in various combinations. The strongest reflex drive came from the 4th and 5th pairs of walking legs. The major source of input came from the coxo-basipodite (C-B) joint of the walking legs. Blocking other leg joints (mero-carpopodite (M-C) and carpo-propodite (C-P)) had no obvious effects on the normal response. 5. Tilting animals in the roll plane (±20°), with no leg contact, produced an opening of the exopodite on the upward side and a closing of the exopodite on the downward side. Responses were again only reliably elicited when accompanied by abdominal extension, and were produced by activity only in the slow opener and closer muscles. 6. Previously identified statocyst interneurones (C1 and C2) were recorded in the circumoesophageal commissures. No modulation of their spontaneous activity by leg input could be detected. 7. Results suggest that separate pathways from the leg proprioceptors and the statocyst organs to the uropods exist and converge at a late stage in the pathway, within the terminal (6th) abdominal ganglion.

Reflex antennal movements in the spiny lobster,Palinurus elephas

1. Reflex antennal movements in the frontal plane, brought about by movement at the J1 joint, occur in the spiny lobster,Palinurus elephas, in response to tilting movements of a platform held under the animal's legs. Imposed movements of the J1 joint give rise to resistance reflexes. Experiments in which the two stimuli are delivered with different timing relations demonstrate that the two reflex pathways remain simultaneously operative. These results suggest a premotor organisation of motor patterns in the antennal muscles, and identify an integration step in the mechanics of the final movement. 2. On the basis of myographic recordings, the innervation of the two external and three internal rotator muscles that operate the J1 joint involve both the sharing of axons between synergists, and synergists receiving distinct but strongly coupled axonal supplies. 3. Two distinct modes of coordination are observed in the antennal rotator muscles. Alteration of antagonists, apparent not only in the general alternation of bursts but also in burst microstructure, occurs during reflex activity (Fig. 2B) and in the absence of imposed stimulation (Fig. 2A), while co-activation of antagonists occurs following general tactile stimulation of the body (Fig. 2A). Biases in the level of both tonic activity and reflex responsiveness towards either rotator muscle commonly occur under a variety of stimulus conditions (Fig. 2B). 4. The amplitude of the reflex responses to platform tilts is determined both by the amplitude of platform tilt and by the number of legs in contact with the platform. The reflex remains strongly phasic even at low frequencies of stimulation although at such frequencies an antennal muscle sometimes fires during movement of the platform in both directions (Fig. 3). 5. Resistance reflexes occurring in response to imposed antennal movement utilize the same pool of motoneurons as are involved in the responses to platform tilts. 6. Ramp platform tilts delivered during continuous sinusoidal movement of the antenna cause predictable changes in the pattern of resistance reflex discharges. An enhanced response in a muscle occurs when a ramp inducing discharge in a particular muscle is timed to coincide with the burst in that muscle, while a reduced burst together with an extra burst of firing in the antagonist muscle follows the application of a ramp movement in the opposite direction at the same relative time (Fig. 4A). An equivalent series of effects follows application of ramp antennal movements during continuous sinusoidal platform tilts (Fig. 4B). 7. Simultaneous sinusoidal movement of both platform and antenna such that the resistance reflexes are in phase with the responses to platform oscillation produces a pattern of simple reciprocation between the antagonist muscles of the moved antenna with clearly-defined non-overlapping bursts (Fig. 5C). When the two stimuli are arranged to conflict, the muscles are co-activated, each firing two bursts in each stimulus cycle (Fig. 5D). During such antiphase stimulation, reductions in the amplitude of platform tilt lead to enhancement of resistance reflex effects as well as reduction in the size of the bursts due to platform stimulation (Fig. 6).