Vestibulocollic reflexes in the absence of head postural 5 control

50 Percutaneous electrical vestibular stimulation evokes reflexive responses in appendicular 51 muscles that are suppressed during tasks where the muscles are not contributing to balance 52 control. In neck muscles, which stabilize the head on the torso and in space, it is unclear if 53 similar postural task dependence shapes vestibular reflexes. We investigated whether 54 vestibulocollic reflexes are modulated during tasks where vestibular information is not directly 55 relevant to maintaining the head balanced on the torso. We hypothesized that vestibulocollic 56 reflexes would be 1) evoked when neck muscles are not involved in balancing the head on the 57 torso, and 2) invariant across synergistic neck muscle contraction tasks. Muscle activity was 58 recorded bilaterally in sternocleidomastoid and splenius-capitis muscles during head-free and 59 head-fixed conditions while subjects were exposed to stochastic electrical vestibular stimulation 60 (±5mA,0-75Hz). Significant vestibular reflex responses (P<0.05) were observed during head61 free and head-fixed trials. Response magnitude and timing were similar between head-free and 62 head-fixed trials for sternocleidomastoid, but splenius-capitis magnitudes decreased with the 63 head-fixed by ~25% (P<0.05). Nevertheless, this indicates that vestibulocollic responses are 64 evoked independent of the requirement to maintain postural control of the head on the torso. 65 Response magnitude and timing was similar across focal muscle contractions (i.e. axial66 rotation/flexion/extension) provided the muscle was active. In contrast, when subjects co67 contracted neck muscles, vestibular-evoked responses in sternocleidomastoid decreased by ~3068 45% (P<0.05) compared to focal muscle contractions but remained unchanged in splenius69 capitis. These results indicate robust vestibulocollic reflex coupling, which we suggest functions 70 through its closed-loop influence on head posture to ensure cervical spine stabilization. 71