MECHANISMS CONTROLLING HUMAN HEAD STABILIZATION .1. HEAD-NECK DYNAMICS DURING RANDOM ROTATIONS IN THE HORIZONTAL PLANE

1. Potential mechanisms for controlling stabilization of the head and neck include voluntary movements, vestibular (VCR) and proprioceptive (CCR) neck reflexes, and system mechanics. In this study we have teste d the hypothesis that the relative importance of those mechanisms in p roducing compensatory actions of the head-neck motor system depends on the frequency of an externally applied perturbation. Angular velocity of the head with respect to the trunk (neck) and myoelectric activity of three neck muscles were recorded in seven seated subjects during p seudorandom rotations of the trunk in the horizontal plane. Subjects w ere externally perturbed with a random sum-of-sines stimulus at freque ncies ranging from 0.185 to 4.11 Hz. Four instructional sets were pres ented. Voluntary mechanisms were examined by having the subjects activ ely stabilize the head in the presence of visual feedback as the body was rotated (VS). Visual feedback was then removed, and the subjects a ttempted to stabilize the head in the dark as the body was rotated(NV) . Reflex mechanisms were examined when subjects performed a mental ari thmetic task during body rotations in the dark (MA). Finally, subjects performed a voluntary head tracking task while the body was kept stat ionary (VT). 2. Gains and phases of head velocity indicated good compe nsation to the stimulus in VS and NV at frequencies <1 Hz. Gains dropp ed and phases advanced between 1 and 2 Hz, suggesting interference bet ween neural and mechanical components. Above 3 Hz, the gains of head v elocity increased steeply and exceeded unity, suggesting the emergence of mechanical resonance. 3. At low frequencies (<1 Hz) during MA, gai ns were very low, and phases indicated that the head was moving with t he trunk. A steady rise in gains and shift in phases toward a compensa tory response were observed as frequency increased. Between 1 and 2 Hz , the response of the neck moved toward compensation as gains observed during voluntary stabilization decreased, suggesting that reflex mech anisms were becoming the predominant controller of compensatory proces ses at this frequency range. Around 3 Hz, mechanical resonance was obs erved. 4. In VS, NV, and MA, electromyographic (EMG) activity steadily decreased in gain up to 1 Hz, then continuously increased at frequenc ies >1 Hz. This implied sustained participation of neural mechanisms i n the higher frequency range. Depending on the relative motion of the head with respect to space and to the trunk, either the vestibulocolli c or cervicocollic (proprioceptive) reflex were assumed to be present in the EMG output. 5. The patterns observed in the neck responses seco ndary to trunk perturbations were not apparent in the response dynamic s of voluntary head tracking. In VT, the most compensatory gains and p hases of both head velocity and muscle EMG responses appeared at the l owest frequencies of head movement. Gains steadily declined, and phase lags increased as frequency increased. 6. We acknowledge that the con tributions of the three mechanisms examined here cannot be completely separated by the paradigms used, but the data suggest that reflexes do participate in the stabilization process. Comparisons of the frequenc y responses of the cat and human showed that a model based on the pass ive mechanics of the cat's neck is applicable to these data even thoug h experimental conditions were different. Evidence of a similar patter n of gains and phases in our data to that of the animal model allow us to conclude that the observed activity of the head with respect to th e trunk in this series of experiments is indicative of a process of co mpensatory head stabilization as a consequence of trunk movements caus ed by external forces.