Dependency of human neck reflex responses on the bandwidth of pseudorandom anterior-posterior torso perturbations.

The vestibulocollic (VCR) and cervicocollic (CCR) reflexes are essential to stabilize the head-neck system and to deal with unexpected disturbances. This study investigates how neck reflexes contribute to stabilization and modulate with perturbation properties. We hypothesized that VCR and CCR modulate with the bandwidth of the perturbation and that this modulation is maintained across amplitudes and influenced by the eyes being open or closed. Seated subjects were perturbed in an anterior-posterior direction. The perturbations varied in bandwidth from 0.3 Hz to a maximum of 1.2, 2.0, 4.0, and 8.0 Hz, at three amplitudes, and with eyes open and closed. Frequency response functions of head kinematics and neck muscle EMG demonstrated substantial changes with bandwidth and vision and minor changes with amplitude, which through closed-loop identification were attributed to neural (reflexive) modulation. Results suggest that both reflexes were attenuated when perturbations exceeded the system's natural frequency, thereby shifting from a head-in-space to a head-on-trunk stabilization tendency. Additionally, results indicate that reflexive and mechanical stiffness marginally exceed the negative stiffness due to gravity; a stabilization strategy which minimizes effort. With eyes closed, reflexes were attenuated further, presumably due to a reduced ability to discriminate self-motion, driving the system to a head-on-trunk stabilization strategy at the highest bandwidth. We conclude that VCR and CCR modulate with perturbation bandwidth and visual feedback conditions to maintain head-upright posture, but are invariant across amplitude changes.