The primary position of the eyes, the resetting saccade, and the transverse visual head plane. Head movements around the cervical joints.

Photographic and video analyses show that the primary position of the eyes is a natural constant position in alert normal humans, and the eyes are automatically saccadically reset to this position from any displacement of the visual line. The primary position is not dependent on fixation, the fusion reflex, gravity, or the head position. The primary position is defined anatomically by head and eye planes and lines that are localized by photography, magnetic resonance imaging, and x-rays of the head and neck. The eyes are in the primary position when the principal (horizontal) retinal plane is coplanar with the transverse visual head (brain) plane (TVHP), and the equatorial plane of the eye is coplanar with a fixed orbital plane (Listing's plane). Evidence is presented to indicate an active neurologic basis for the primary position instead of passive mechanical forces. A different understanding of the primary position and the conception of the TVHP may be valuable in analyzing oculomotor defects.