Neck muscle activity in eye--head coordinated movements.
The electromyographic (EMG) activity of different neck muscles in relation to gaze orientation has been studied in alert trained cats. When the head is kept fixed, the activity of these muscles is proportional to eye eccentricity in the horizontal as well as in the vertical planes. On basis of this tonic activity, a preferential orientation can be attributed to each muscle: upward and lateral for biventer, rectus and complexus, and downward and lateral for longissimus, splenius and obliquus capitis cranialis. Fluctuations in this modulation of the EMG activity by eye position can be observed. When the head is free to move, the muscles show phasic discharges having similar preferential orientations. For a given muscle, this orientation covers a quite large angle: many muscles contribute to a given movement. The timing of the discharge of the different muscles as a function of the direction of the head movement was examined. It was found that the latency, i.e. the delay between the discharge and movement onset, progressively increases as the movement direction diverges from the preferential orientation of the muscle. It has been noted that the muscles having an upward preferential orientation may show, in relation to downward movements, inhibition occurring prior to the onset of the head movement. The same muscles may also increase their activity around the midcourse of downward movements. Thus, the head motor system controls the direction and amplitude parameters not only by selectively activating the appropriate muscles but also by sequencing their activity in a subtle way to start, control the trajectory and stop the movement, reminiscent of what has been described for limb movements.