Neural control of head rotation: electromyographic evidence

Electromyograms, representing samples of neurological control signals, can be used to predict head rotation that can be either time optimal or non-time optimal, depending upon a given subject's intent. Kinematic factors such as operating levels in terms of horizontal and vertical position of the head from which horizontal rotations are made, importantly influence the tonic and phasic aspects of the EMG signal. Pairs of head-rotating muscles, splenius and sternocleidomastoideus muscles, demonstrate reciprocal innervation, particularly for time optimal head rotations. Head rotations of the same amplitude but different velocities correlate with pulse height and pulse width of envelopes of the rectified EMG. Rise of the agonistic EMG correlates in detail with rise of the acceleration transient for a given fast or slow head rotation. Our data demonstrate both the optimal control of details of head movements and the influence of the stretch reflex that strongly relates to initial head position and the velocity of the movements around the primary position. The results support the idea of higher level programmed control of head rotation in human subjects.