Eye-head coupling in humans. II. Phasic components.

A tonic coupling between the horizontal component of eye position and dorsal neck muscle activity has been demonstrated in animals and humans. In addition, a transient saccade related coupling has been found in animals. In order to investigate such a phasic component of the eye-head synergy in humans, we have recorded the activity of isolated motor units in the splenius muscle during large horizontal eye movements in head fixed subjects. Eye movement recording was achieved by conventional binocular electro-oculography and the activity of the right splenius muscle was recorded with Bronks coaxial electrodes inserted manually at the C4-C5 intervertebral level. We found two main types of motor unit discharge patterns in the splenius (SPMU), the first type (type A, 14 SPMUs) shows a phasic modulation of firing rate during saccades with a triphasic profile composed of a pre-saccadic suppression, a per-saccadic burst and a post saccadic tonic discharge proportional to eye position. The second type (type B, 6 SPMUs) exhibits little, if any, modulation of firing rate with either fixation or saccades. These results suggest that eye-head coupling is present not only during the fixation period but also during saccades and that a phasic activity or suppression related to saccadic eye velocity is present in dorsal neck muscle EMG.