Neural substrates, dynamic and thresholds of galvanic vestibular stimulation in the behaving primate

(Image: Torsional eye movements in response to sinusoidal GVS - while applying sinusoidal GVS between surface electrodes placed on the mastoid processes behind the ears, the animal's eye movements were recorded while it was fixating)

Whether standing, walking or driving, the vestibular system (i.e. our balance organ) provides information to our brain about our orientation and movement in space. This information is essential to our ability to orient, move and interact with our environment. To study the vestibular system in isolation, researchers and clinicians use galvanic vestibular stimulation (GVS), which delivers electrical currents behind the ears to selectively activate the vestibular system. Despite the growing popularity of GVS for the assessment and treatment of a wide range of clinical disorders, including Parkinson’s disease, stroke, concussion and even obesity, exactly how this non-invasive tool modulates vestibular signals remains an open question.

In the new paper published in Nature Communications, Cluster members from Johns Hopkins (Kathleen Cullen), Erasmus MC (Patrick Forbes) and UBC (Jean-Sébastien Blouin) recorded the activity of vestibular neurons in monkeys during both GVS and motion to reveal the neural substrate underpinning GVS-evoked perceptual, ocular and postural responses. The authors show for the first time that vestibular signals of both rotation and translation respond to the externally delivered stimulus with constant GVS-evoked neuronal detection thresholds. This resolves several conflicts regarding the neural mechanisms underlying GVS, and rejects a prevailing assumption in the literature that neurons encoding translation are preferentially activated during GVS. Instead, these results demonstrate that all vestibular neurons transmit equivalent levels of information to central vestibular pathways to detect GVS-evoked sensations of self-motion. The authors further demonstrate that neural response tuning to GVS differs markedly from those produced during actual head motion. Using a simple model of vestibular neural activity, the authors were able to explain the main trends observed in GVS-evoked responses and identify future research directions to address how detailed cellular channels contribute to neural responses. Overall, these findings provide key information for developing physiologically accurate models of GVS activation of the vestibular system. Such models are essential for the advancement and accurate application of this technique as a clinical tool in humans.

The full paper can be accessed here.