Thomas Mergner

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.

Vascular Risk Factors and Arteriosclerotic Disease in Idiopathic Normal-Pressure Hydrocephalus of the Elderly

BACKGROUND AND PURPOSE There is some evidence from previous studies that idiopathic normal-pressure hydrocephalus (NPH) of the elderly might be linked to vascular leukoencephalopathy. The purpose of this study was to examine the prevalence and impact of vascular risk factors and vascular diseases in idiopathic NPH compared with a control cohort. METHODS The prevalence of arterial hypertension; diabetes mellitus; hypercholesterolemia; hyperlipidemia; smoking; obesity; and cardiac, cerebrovascular, and other arteriosclerotic diseases was assessed in 65 patients with idiopathic NPH. The findings were compared with those of 70 patients with comparable age distribution. To describe the differences of the prevalences of vascular risk factors, odds ratios were obtained by univariate and multivariate analyses. RESULTS The univariate analysis revealed significant associations between idiopathic NPH and arterial hypertension (prevalence, 54 of 65 [83%]; control group, 25 of 70 [36%]; P < .001) and diabetes mellitus (prevalence, 31 of 63 [49%]; control group, 20 of 70 [29%]; P < .015) but not with other vascular risk factors. After multivariate regression analysis, only hypertension remained significantly associated with NPH (P < .0001). There was also a significant association between NPH and cardiac (P < .001), cerebral arteriosclerotic (P = .007), and other arteriosclerotic diseases (P = .001). A positive association was found between the severity of clinical symptoms of NPH and the presence of hypertension, especially for gait disturbance. The presence of hypertension was not related to the duration of NPH. CONCLUSIONS Our data show a highly significant association between idiopathic NPH and arterial hypertension. Arterial hypertension might be involved in the pathophysiological mechanisms promoting idiopathic NPH.

The role of canal-neck interaction for the perception of horizontal trunk and head rotation

SummaryThe present report considers the conscious perception of passive horizontal rotations of the trunk, the head, or both, by human observers. It examines in particular how this perception depends on the interaction of canal and neck afferents.Three sets of sinusoidal stimulations (0.2 Hz) were applied to subjects (Ss): Rotations of (1) whole body (pure labyrinthine stimuli, λ), of (2) only the trunk with the head stationary in space (pure neck stimuli, ν), and of (3) both head and trunk, each with an amplitude and a direction of its own, giving rise to various in-phase and counter phase combinations of λ and ν. — The Ss were to estimate the magnitude of their turning sensations (ψ). In doing so, they were to concentrate either on the rotation of their trunk in space (TS) or of their head in space (HS), or of the head relative to the trunk (HT).The TS and HS turning sensations induced by pure λ-stimuli were essentially the same as to magnitude and direction. Pure ν-stimulation also led to TS and HS turning sensations. However, the former had the direction of the trunk-to-head

What is pathological with gaze shift fragmentation in Parkinson's disease?

\left( {\hat - TS} \right)

Canal-neck interaction in vestibular neurons of the cat's cerebral cortex.

deflection, the latter that of the head-to-trunk deflection. The ν-induced HS turning sensation represented an illusion, since the head remained stationary in space. When the λ- and ν-stimuli were combined, the interaction could be described by a linear summation of their effects. The estimates of TS turning followed the equation ψTS ∼ λ-ν, thus well reflecting the actual TS rotation. The estimates of HS could be described by ψHS ∼ λ+kv; the term kv represents the “ν-illusion” contaminating the HS turning sensation. The estimates of HT turning were roughly proportional to ν alone and, therefore, close to the actual HT rotation.We conclude that humans may derive a rather faithful information about trunk rotation from the combined activation of canal and neck afferents, but that the sensation of passive head rotation is contaminated by an (illusionary) contribution from neck afferents. These additive and subtractive modes of interaction have parallels in postural reflexes as well as in neuronal responses that are known from cat.

Impaired standing balance in elderly: A new engineering method helps to unravel causes and effects

Abstract During biped stance or locomotion, humans show remarkable skills in reactive balancing upon external disturbances. Mainly four types of external disturbances are relevant for stance: the field force gravity, contact forces such as a push or pull against the body, as well as body support surface rotation and translational acceleration. It is known from clinics that sensory loss severely impairs the balancing. Three sensory inputs are instrumental: vestibular, joint angle, and joint torque. System identification studies currently try to understand how humans are able to flexibly cope with changes in, and superposition of the disturbances. A solution is presented in this article. The article first describes the control problem and then reviews recent evidence for a PD (proportional-derivative) controller, for multisensory feedback, and for sensory reweighting as a key to understand the flexibility. On this basis, a recent disturbance estimation and compensation (DEC) model is introduced. It builds on two concepts from previous psychophysical studies of human self-motion perception. First, inputs from several sensory transducers are fused to establish sensors that provide explicit measures of the physical variables (sensor concept). Second, a processing level, interleaved between sensors and feedback (meta level), performs online sensory estimations of the external disturbances. These estimations are then fed into a local proprioceptive feedback loop, yielding corresponding disturbance rejections. Previous work on using the DEC model to describe human reactive balancing data is briefly reviewed. Then, novel work is presented, in which voluntary control over the reactive balancing is added to the DEC model. A prediction method for anticipating self-produced and external disturbances is suggested and corresponding software and hardware (robot) simulations are presented. The results serve as guidelines for future human experiments. Since the DEC model is very simple, we concluded that its behavioral flexibility and fault tolerance goes together with computational parsimony, an equally important biological constraint.

A modeling approach to the human spatial orientation system.

Abstract. Oculomotor dysfunction in Parkinsons disease (PD) is mainly characterized by a fragmentation of memory-guided gaze shifts (target is reached by several hypometric saccades). Since this phenomenon can also be observed in normal subjects, we scrutinized its pathophysiological significance in PD patients. We recorded horizontal eye movements in eleven mildly- or moderately-affected PD patients and eleven control subjects. A quantitative assessment of gaze shift fragmentation was made possible by increasing its incidence over a sequence of two visually- and two subsequent memory-guided gaze shifts. Basic saccade measures (latency, velocity, etc.) were similar in the two subject groups as well as in fragmented versus non-fragmented gaze shifts. Fragmentation probability is increased in the second memory-guided gaze shift, and this clearly more so in patients than in controls. The fragmentation shows a typical gain pattern (uniform increase of gain of saccadic amplitudes across correction saccades towards 1.0 with the last saccade of the gaze shift) independent of subject group, stimulus mode, and fragmentation degree. Gaze shift fragmentation represents a physiological phenomenon, which has thus far been overlooked. It reflects a robust correction mechanism, which assures that target is reached even if the pre-oculomotor drive through the basal ganglia to the superior colliculus becomes abnormally weak or under inadequately strong inhibition – as is postulated for PD. Thus, only the abnormally high incidence of fragmentation, and of the associated amplitude reduction of the primary saccades, rather than the phenomenon per se, can be used as a diagnostic criterion in early stages of PD.

Vestibular memory-contingent saccades involve somatosensory input from the body support.

SummaryNeuronal responses to natural stimulation of neck proprioceptors were studied in the region of the small cell group x in the dorsolateral medullary brainstem of slightly anesthetized and paralyzed cats. Stimulation consisted of horizontal trunk rotations about C1 with the head fixed in space. Out of 74 neurons recorded, 92% showed an increase in discharge rate with ipsilateral neck stretch and a decrease with contralateral stretch (Type N I responses); 8% showed the reverse response pattern (Type N II responses). In the primary head-to-trunk position, almost all neurons had tonic activity that probably stemmed from prestretched neck proprioceptors. Responses to sinusoidal stimulation and position trapezoids showed a static (position-sensitive) as well as a dynamic (essentially velocitysensitive) component. The relative weight of the two components varied considerably among the neurons. It was not possible to distinguish discrete neuronal populations on the basis of the dynamic characteristics. There was no evidence of a convergent input from other receptor systems, such as the horizontal canal system. Several neurons responded to “muscle tapping” and showed an increase of the velocity component following systemic injection of succinylcholine. We take this as evidence that they may receive input from muscle spindle receptors.