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Balance in the Blood not just the Ear

Galvanic Vestibular Stimulation. For those of us familiar with it, it's a possible panacea to cure simulation sickness, and rope the sense of balance of the user directly into the simulation. Two weak electrical fields, generated by units placed just behind the earlobes, and modulated with intensity, control the vestibular system, making the sense of balance do anything it's told to.

When a weak DC current is delivered to the mastoid behind a person's ear, their body responds by shifting their balance toward the anode. The stronger the current, the more powerful its pull. If it is strong enough, it throws them off balance, or recreates the feeling of nausea or a barrel roll with equal skill.

It is possible using such a system to produce very realistic sensations of movement for cockpits and ground-based driving simulators, accurately replicating the feel of travel with little or no actual movement.

Since the sense of balance via such stimulation is keyed to the same update frequency as the rest of the simulation's sensory outputs, the age old problem where vertigo is produced when the visual sense and the balance sense are out of alignment no longer occurs.

Unfortunately, it seems there is a fly in the mixture. When the vestibular system is manipulated, it trips a secondary system in the body, to alter how blood flows round the brain. The organs of the inner ear have a direct effect on brain blood flow, independent of blood pressure and CO2 levels in the blood.

Dr. Jorge Serrador, from Harvard Medical School, worked with a team of researchers, including NASA scientists, to investigate the effects of stimulation of the otoliths and semi-circular canals on cerebrovascular response. What they found was that the blood flow patterns of the brain changed in direct response.

Changes in cerebral flow velocity were dependent on the frequency of vestibular stimulation and were in opposition to changes in blood pressure and not directly related to changes in end tidal CO2.

Speaking about the implications of these results, Serrador said, "Standing up places the head above the heart and thus makes it harder to provide blood flow to the brain. Having a connection between the otoliths, which tell us that we are standing, and the cerebrovasculature may be part of the adaption that allows us to maintain our brain blood flow when upright.

"This connection might explain the reduced cerebral blood flow in some people. For example, ageing is associated with vestibular loss that might contribute to reductions in global cerebral blood flow. Similarly, patients with orthostatic intolerance could have underlying vestibular impairment that exacerbates cerebral hypoperfusion when upright."

Whilst we don't have enough hands on data yet of the effects of sustained use of galvanic vestibular stimulation, these results thus far presented, are worrying. It may actually be possible to cause damage to the brain via the increased pressure from blood flow, when it is not actually needed. Such effects would of course be likely only under long-term exposure to undesirable conditions, such as being upside down for an extended period in the simulation.

Whether the body can detect the problem and correct it, or if the adaptation can be disabled via a brain machine interface is also unknown at this time. Still, it does give pause for thought when using GVS.


Balance organs affect brain blood flow

Vestibular effects on cerebral blood flow

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