In literature, it has been suggested that the CNS anticipates spontaneous change in body position during quiet stance and continuously modulates ankle extensor muscle activity to compensate for the...

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Importance of body sway velocity information in controlling ankle extensor activities during quiet stance

The vestibular system is crucial for postural control; however there are considerable differences in the task dependence and frequency response of vestibular reflexes in appendicular and axial muscles. For example, vestibular reflexes are only evoked in appendicular muscles when vestibular information is relevant to postural control, while in neck muscles they are maintained regardless of the requirement to maintain head on trunk balance. Recent investigations have also shown that the bandwidth of vestibular input on neck muscles is much broader than appendicular muscles (up to a factor of 3). This result challenges the notion that vestibular reflexes only contribute to postural control across the behavioral and physiological frequency range of the vestibular organ (i.e., 0–20 Hz). In this review, we explore and integrate these task-, muscle- and frequency-related differences in the vestibular system’s contribution to posture, and propose that the human nervous system has adapted vestibular signals to match the mechanical properties of the system that each group of muscles controls.

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Task, muscle and frequency dependent vestibular control of posture

Galvanic vestibular stimulation serves to modulate the continuous firing level of the peripheral vestibular afferents. It has been shown that the application of sinusoidally varying, bipolar galvanic currents to the vestibular system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galvanic vestibular stimulation was applied to nine healthy young subjects. Three different stochastic vestibular stimulation signals, each with a different frequency content (0–1 Hz, 1–2 Hz, and 0–2 Hz), were used. The stimulation level (range 0.4–1.5 mA, peak to peak) was determined on an individual basis. Twenty 60-s trials were conducted on each subject – 15 stimulation trials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position with their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) under each subject’s feet. Cross-spectral measures were used to quantify the relationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulation signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimulation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consistent significant coherency between the stochastic vestibular stimulation signals and the anteroposterior COP time series. This work demonstrates that, in subjects who are facing forward, bipolar binaural stochastic galvanic stimulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2-Hz stochastic vestibular stimulation signal may be due to the intrinsic dynamics of the quasi-static postural control system. In particular, it may result from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulation signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as ”responders” to galvanic vestibular stimulation.

The effects of stochastic galvanic vestibular stimulation on human postural sway.

In the nineteenth century Pierre–Jean–Marie Flourens (1825) and Ernst Mach described the vestibular system and its peripheral organs while Robert Barany, rewarded by the Nobel prize in 1914, was the first to investigate vestibular disorders with caloric tests making surgical treatments of the vestibular organ possible. Recently, Graf and Klam (2006) have reminded us that this ancient sensory system appeared more than 500 million years ago. Logically its influence would most likely not be restricted to balance reflexes at the brainstem level; it must have also shaped our brain. The vestibular system is the one sensory organ dedicated to gravity perception, which along with light and oxygen served as a motor of evolution. In the 1950s the groups of Otto–Joachim Grusser in Germany, Wilder Penfield in Canada, and later the group of Alain Berthoz in France, demonstrated in elegant experiments on awake monkey (Guldin and Grusser, 1998), epileptic patient (Penfield, 1957), and neurologically-normal human (Lobel et al., 1999) the existence of vestibular projections to the cortex and how they combine with visual and proprioceptive information. An increasing number of researchers, often fervent disciples, have built on these findings to produce a spate of publications that have consolidated the evidence for a sense of verticality and three-dimensional body representations within the vestibular cortical areas. In the 1990s Paul Smith and colleagues examined vestibular processing in the hippocampus and its role in spatial memory. Exploring this topic in the rodent (Smith, 1997), they began to elucidate the secrets and the previously silent functions of the vestibular system. These findings led to increasing clarity about how vestibular degeneration may be related to some aspects of dementia (Previc, 2013), psychiatric diseases (Gurvich et al., 2013), and cognitive impairments in the elderly (Bigelow et al., 2015; Semenov et al., 2015). The research by Marianne Dieterich and Thomas Brandt has examined the bilateral organization of multiple multisensory cortical areas and revealed the vestibular dominance of the non-dominant hemisphere (Dieterich et al., 2003). They addressed the following questions: how is one global percept of motion and orientation in space formed, and does this dominance determine the lateralization of brain function such as handedness (Brandt and Dieterich, 2015)? A vestibular contribution to the most crucial aspects of the human sense of self and self-consciousness has recently been highlighted by neurological and neuroscientific investigations: vestibular signals contribute to the experience that the self is located within the boundaries of the body (Blanke et al., 2004; Lopez et al., 2008) and may even be involved in self-other discrimination and interactions (Lenggenhager and Lopez, 2015).

In this Frontiers in Integrative Neuroscience Research Topic initiated by Sidney Simon, twenty-four articles highlight recent discoveries in the field of vestibular cognition, including: (1) Anatomy of the vestibulo-cortical pathways; (2) Spatial navigation and memory; (3) Spatial cognition, bodily and self-motion perception; (4) Vestibular stimulation and rehabilitation; (5) Posture and motor control; (6) Vestibular disorders and compensation; and (7) Development of vestibular function.

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Editorial: The Vestibular System in Cognitive and Memory Processes in Mammalians.

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

/pdf/trigeminal-visceral-and-vestibular-inputs-may-improve-3sedsnb163.pdf

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis.

M Sázel

Papers

Galvanic vestibular stimulation