Mechanisms controlling human head stabilization during random rotational perturbations in the horizontal plane revisited

Neck stabilization through sensory integration of vestibular and visual motion cues

BACKGROUND

To counteract gravity, trunk motion, and other perturbations, the human head-neck system requires continuous muscular stabilization. In this study, we combine a musculoskeletal neck model with models of sensory integration (SI) to unravel the role of vestibular, visual, and muscle sensory cues in head-neck stabilization and relate SI conflicts and postural instability to motion sickness.

METHOD

A 3D multisegment neck model with 258 Hill-type muscle elements was extended with postural stabilization using SI of vestibular (semicircular and otolith) and visual (rotation rate, verticality, and yaw) cues using the multisensory observer model (MSOM) and the subjective vertical conflict model (SVC). Dynamic head-neck stabilization was studied using empirical datasets, including 6D trunk perturbations and a 4 m/s2 slalom drive inducing motion sickness.

RESULTS

Recorded head translation and rotation are well matched when using all feedback loops with MSOM or SVC or assuming perfect perception. A basic version of the model, including muscle, but omitting vestibular and visual perception, shows that muscular feedback can stabilize the neck in all conditions. However, this model predicts excessive head rotations in conditions with trunk rotation and in the slalom. Adding feedback of head rotational velocity sensed by the semicircular canals effectively reduces head rotations at mid-frequencies. Realistic head rotations at low frequencies are obtained by adding vestibular and visual feedback of head rotation based on the MSOM or SVC model or assuming perfect perception. The MSOM with full vision well captures all conditions, whereas the MSOM excluding vision well captures all conditions without vision. The SVC provides two estimates of verticality, with a vestibular estimate SVCvest, which is highly effective in controlling head verticality, and an integrated vestibular/visual estimate SVCint which can complement SVCvest in conditions with vision. As expected, in the sickening drive, SI models imprecisely estimate verticality, resulting in sensory conflict and postural instability.

CONCLUSION

The results support the validity of SI models in postural stabilization, where both MSOM and SVC provide credible results. The results in the sickening drive show imprecise sensory integration to enlarge head motion. This uniquely links the sensory conflict theory and the postural instability theory in motion sickness causation.

Modeling Movement-Induced Errors in AC Electromagnetic Trackers

Error analysis of electromagnetic motion tracking systems is of growing interest to many researchers. Under sensor movement, it is logical to presume that the error in position and orientation measurements will increase due to the linearization used in the algorithms, among other reasons. In this article, we analyze theoretically the error, that results from linearization, in position measurement of the Polhemus tracking system for a moving sensor. We derive formulas to estimate this error in terms of the sensor position and speed. Then, we verify these formulas by numerical simulations.

Directional differences in head stabilisation in Acanthodactylus pardalis lizards

Running inevitably causes the animal trunk to undulate. The consequential head rotations have to be stabilised in space for a steady gaze and an accurate sense of self-motion for balance. The ecology and anatomy of the species determine the necessity to stabilise the head in yaw, pitch, and roll direction. Terrestrial lizards, running with a sprawled body posture, are especially prone to undulations in the horizontal (yaw) plane. Measurements on an experimental oscillation platform show that Acanthodactylus pardalis lizards stabilise their head less in pitch direction (54% stabilisation) than in yaw and roll direction (66% and 64% stabilisation, respectively). Because we performed these experiments in darkness, the lizards based their head stabilisation on vestibular information. Hence, we hypothesised that their vestibular system is less sensitive in pitch direction than in yaw and roll direction. Yet, this was not confirmed by a detailed Fluid-Structure Interaction model of the membranous labyrinth, which showed that not pitch sensitivity (88% of yaw sensitivity), but roll sensitivity (73% of yaw sensitivity) is the lowest. So why is the head stabilisation in darkness almost as good in roll direction as in yaw direction? While this may be due to neurological nonlinearities, it seems worth noticing that the moment of inertia is lowest in roll direction due to the elongated head shape. Hence, less torque is needed to stabilise a head rotation in roll direction than in the other two directions.

Rigid head-neck responses to unpredictable perturbations in patients with long standing neck pain does not change with treatment

In a previous study we have shown that patients with long standing non-specific neck-pain display more rigid neck movement behavior than controls in response to unpredictable perturbations. In the present study we investigated head/neck motor control in patients with neck-pain during a course of physiotherapy intervention and the associations with pain, neck disability and kinesiophobia. In this longitudinal observational study, 72 patients with non-specific neck-pain were exposed to unpredictable horizontal rotations by means of an actuated chair in three conditions; with a visual reference, and without vision with and without a cognitive task before first consultation with physiotherapist, after 2 weeks and 2 months of intervention. The neck movements were analyzed in the frequency domain to cover voluntarily and reflex controlled responses. Questionnaires encompassed Neck Disability Index, Tampa Scale of Kinesiophobia, and the Numerical Rating Scale for current pain. The results showed that the response pattern for the amplitudes of movement between head and trunk across frequencies did not change over time, whereas some changes in timing were found for some frequencies. Pain, neck disability, and kinesiophobia improved after intervention, but were not significantly associated with neck movement responses to perturbations across time or condition. Although physiotherapy intervention improved self-reported function, the rigid responses to unpredictable perturbations remained unchanged. This indicates altered function in reflex mediated control mechanisms, i.e., the vestibulocollic and the cervicocollic reflex systems that control the head in space and on the trunk. Future research should further investigate pain related changes in reflex systems and whether alterations in these systems are modifiable.

Head stabilization in small vertebrates that run at high frequencies with a sprawled posture

Small animals face a large challenge when running. A stable head is key to maintenance of a stable gaze and a good sense of self-motion and spatial awareness. However, trunk undulations caused by the cyclic limb movements result in involuntary head movements. Hence, the head needs to be stabilized. Humans are capable of stabilizing their head up to 2–3 Hz, but small animals run at cycle frequencies that are up to six times higher. We wondered how natural selection has adapted their head stabilization control. We observed that the relative contributions of vision, on the one hand, and vestibular perception and proprioception, on the other hand, remain the same when lizards undergo fast or slow body undulations in an experimental set-up. Lizards also maintain a short phase lag at both low and high undulation frequencies. Hence, we found no indication that they use a different control mechanism at high frequencies. Instead, head stabilization probably remains possible owing to faster reflex pathways and a lower head inertia. Hence, the intrinsic physical and neurological characteristics of lizards seem to be sufficient to enable head stabilization at high frequencies, obviating the need for evolutionary adaptation of the control pathways. These properties are not unique to lizards and might, therefore, also facilitate head stabilization at high frequencies in other small, fast animals.

Frequency-dependent deficits in head steadiness in patients with nonspecific neck pain

Motor control impairments are reported in patients with nonspecific neck pain but the particular deficits in underlying regulatory systems are not known. Head steadiness is controlled both by voluntary and reflex systems that are predominantly effective within different frequency intervals. The aim of the present study was to investigate within which frequency range(s) potential motor control deficits may reside. The ability to keep the head stationary in space in response to unpredictable perturbations was tested in 71 patients with nonspecific neck pain and 17 healthy controls. Participants were exposed to pseudorandom horizontal rotations across 10 superimposed frequencies (0.185-4.115 Hz) by means of an actuated chair in three conditions; with a visual reference, and without vision with, and without a cognitive task. Below 1 Hz, patients kept the head less stable in space compared to healthy controls. Between 1 and 2 Hz, the head was stabilized on the trunk in both groups. Patients kept the head more stable relative to the trunk than relative to space compared to healthy controls. This was interpreted as higher general neck muscle co-activation in patients, which may be explained by altered voluntary control, or/and upregulated gamma motor neuron activity which increases the contribution of reflex-mediated muscle activation. Alternatively, increased muscle activity is secondary to vestibular deficits.

Mechanisms controlling human head stabilization during random rotational perturbations in the horizontal plane revisited

This study repeats the experimental protocol for investigation of head stabilization in healthy humans, described by Keshner and Peterson (1995) but with a modification of the analysis. Head movements were considered with respect to the room instead of relative to the trunk. The aim was to investigate the approximate contribution of reflex and voluntary control across perturbing frequencies and conditions with modulation of visual information and mental attention and discuss the resulting outcome while comparing methods. Seventeen healthy individuals were asked to keep the head steady in space while subjected to pseudorandom rotational perturbations in the horizontal plane, firmly seated on an actuated chair. Both methods confirmed the results for gain in previous studies showing fair ability to keep the head steady in space below 1 Hz with vision. Compensation deteriorated when vision was removed and worsened further with addition of a mental task. Between 1 and 2 Hz, unity gain occurred between head and trunk movements, whereas above 2 Hz the head moved more than the trunk. For phase angles, the original method demonstrated a phase split occurring from ~1 Hz, a purely mathematical artifact that caused subjects with virtually identical movements to appear as significantly different. This artifact was eliminated by analyzing the head‐room relative to trunk‐room rather than head–trunk relative to trunk‐room angles, thus preventing potentially erroneous interpretations of the results.