MECHANISMS CONTROLLING HUMAN HEAD STABILIZATION .2. HEAD-NECK CHARACTERISTICS DURING RANDOM ROTATIONS IN THE VERTICAL PLANE

1. In this study we have tested the hypothesis that the mechanisms con trolling stabilization of the head-neck motor system can vary with bot h the frequency and spatial orientation of an externally applied pertu rbation. Angular velocity of the head with respect to the trunk (neck) and myoelectric activity of two neck muscles (semispinalis capitis an d sternocleidomastoid) were recorded in eight seated subjects during p seudorandom rotations of the trunk in the vertical (pitch) plane. Subj ects were externally perturbed with a random sum-of-sines stimulus at frequencies ranging from 0.35 to 3.05 Hz. Four instructional sets were presented. Voluntary mechanisms were examined by having the subjects actively stabilize the head in the presence of visual feedback as the body was rotated (VS). Visual feedback was then removed, and the subje cts attempted to stabilize the head in the dark as the body was rotate d (NV). Reflex mechanisms were examined when subjects performed a ment al arithmetic task during body rotations in the dark (MA). Finally, su bjects performed a voluntary head tracking task while the body was kep t stationary (VT). 2. In VS and NV, gains and phases of head velocity indicated good compensation for the perturbation at frequencies up to 2 Hz. Between 2 and 3 Hz, gains dropped slowly and then steeply descen ded above 3 Hz as phases became scattered. 3. In MA, gains were lower and exhibited more scatter than in VS and NV at frequencies <1 Hz. Pha ses around -180 degrees indicated that compensatory activity was occur ring even with these low gains. Between 1 and 2 Hz, response gains ste eply ascended, implying that reflex mechanisms were becoming the predo minant mechanism for compensation in this frequency range. Above 2 Hz, gains dropped off to 0.5 and lower, but phases remained close to -180 degrees, suggesting that the reflex mechanisms were not dominant in t his frequency range, but that they were still contributing toward comp ensation for the trunk perturbation. 4. Neck muscle electromyographic (EMG) responses were similar in VS, NV, and MA, demonstrating decreasi ng gains between 0.35 and 1.5 Hz, and then increasing beyond the previ ous high level of activation. This U-shaped response pattern implies a n enhanced participation of neural mechanisms, probably of reflex orig in, in the higher frequency range. 5. Patterns observed during externa l perturbations of the trunk were not apparent in the response dynamic s of voluntary head tracking. In VT, subjects successfully tracked the stimulus only at the lowest frequencies of head movement. A gradual a nd consistent decline was exhibited as frequency increased. EMG activa tion continued throughout the frequency range, however, suggesting a c ontinued effort to track the target. 6. A comparison of response dynam ics revealed that the greatest distinction between responses to pseudo random rotations in the horizontal and vertical planes existed at very low (<0.5 Hz) frequencies and at frequencies >2 Hz. Low-frequency dif ferences reflected improved gains in the vertical plane. High-frequenc y differences reflected the presence of resonant oscillations in the h orizontal but not in the vertical plane. Response dynamics at these fr equencies might have been the result of a stiffer head-neck system in the vertical plane due to the combination of smaller rotational amplit udes and greater muscle moment arms than in the horizontal plane. 7. T he results of this study suggest that head stabilizing mechanisms are related to both the frequency and orientation of an external perturbat ion. Neck reflexes exhibit a greater operational bandwidth in the vert ical than in the horizontal plane and may function to damp mechanical resonance and free the voluntary mechanisms for producing an efficient time-matched response to a continually changing environment.