primary position (of gaze)

The primary position of gaze is the position the eyes take when looking straight ahead with the head and body erect.¹ The fixation point lies straight ahead on the horizon.² Adducted, abducted, elevated and depressed positions of the globe are secondary positions, and oblique positions are tertiary positions.¹

Primary position is the reference for describing the actions of the extraocular muscles. The primary action of a muscle is its major effect with the eye in primary position, and in that position the visual axis forms an angle of 23° with the orbital axis.² Primary position also has a stricter kinematic definition.¹ Under Listing's law, the eye assumes only orientations that it could reach from primary position by a single rotation about a fixed axis in Listing's plane, which lies perpendicular to the primary gaze direction.³

Extraocular muscles do not rest in primary position. Electromyography shows that the muscles are never electrically silent with the eyes fixed in primary position and keep up a tonic activity, which stops completely only in deep sleep or deep anesthesia.⁴