12Encoding Head Movement: How It Stimulates

FBaseline rate and the push-pull of the canals

Each horizontal canal is normally paired with its partner in the opposite ear; a head turn excites one and inhibits the other, a push-pull arrangement that signals direction unambiguously. A vestibular implant recreates part of this from one side by setting a tonic baseline pulse rate, then increasing it for rotation one way and decreasing it for rotation the other. Because the implant can only modulate the firing it produces, the usable downward range is set by the baseline: a higher baseline gives more room to signal inhibitory (slowing) head movements. This rate-modulation strategy is the core of all current human vestibular implants and is what converts measured angular velocity into a neural command.[2015][2017]

TDriving the vestibulo-ocular reflex

The target of stimulation is the vestibulo-ocular reflex: the modulated afferent signal should make the eyes counter-rotate at the speed of the head so gaze stays fixed and the world stops bouncing. Stimulating a single canal electrode evokes an eye movement; the goal is for that movement to be in the plane of the canal and proportional to head velocity (an electrically evoked VOR, or eVOR). Human studies have shown that motion-modulated stimulation can restore a measurable, canal-appropriate eVOR and improve dynamic visual acuity during head movement, the functional antidote to oscillopsia. Whole-system trials further report improved posture, gait and quality of life when the implant runs continuously with motion modulation in daily life.[2014][2016][2021]

CAiming the response: current steering and precompensation

Current spread misaligns the eVOR, so a horizontal-canal electrode may produce an eye movement with unwanted vertical or torsional components. Current steering (shaping how charge is shared across electrode contacts) is used to focus stimulation on the intended ampullary nerve and reduce off-target recruitment. Precompensation deliberately offsets the command to counter known misalignment, so the net evoked eye movement points where the head movement says it should. These techniques trade fidelity against simplicity; perfect three-dimensional alignment from a single implanted side is not yet achievable, but partial alignment is functionally useful.[2017][2010]

TAdaptation, the brain's role, and honest fidelity

The artificial signal is imperfect and unfamiliar, so part of the benefit comes from the brain adapting and learning to interpret the new code over days to weeks. Responses to constant stimulation tend to fade (adaptation), so changing, modulated input is more effective than steady current at sustaining useful eye movements. Achieved fidelity is partial but functionally meaningful: VOR gains remain below the natural ~1.0 and alignment is imperfect, yet patients can gain steadier gaze, better balance and improved quality of life. Honest framing: the vestibular implant is still investigational, results vary between patients, and it restores a usable fraction of vestibular function rather than a normal sense of balance.[2015][2021][2020]