The human vertical optokinetic afternystagmus and the effect of gravity during pitch tilt

Background: Findings concerning human vertical optokinetic afternystagmus (VOKAN) are often not consistent and in some cases even contradictory. Method: VOKAN was studied in nine normal subjects using corneo-retinal potential technique and an infrared video camera detection apparatus (ISCAN). VOKAN responses were measured at three angles of pitch head (and body) tilt: 0° (upright), 90° (supine) and 135° (45° below horizontal). The optokinetic stimulus was stripe movement with a velocity of 60°/s. Results: Upward slow phase velocity (SP-Up) VOKAN was evoked in 6/9 subjects (67%) at 0° tilt and at 90° tilt, and in 4/9 (44%) at 135° tilt. Downward slow phase velocity (SP-Down) VOKAN was elicited in 3/9 subjects (33%) at 0° tilt; in 2/9 subjects (22%) at 90° tilt and in 0/9 subjects (0%) at 135° tilt. Conclusion: The pitch tilt, as a gravity effect, influences significantly on the appearance and the duration of VOKAN.