Further observations on the effects of head position on vertical OKN and OKAN in normal subjects.

Vertical optokinetic nystagmus (VOKN) and vertical optokinetic after-nystagmus (VOKAN) were studied in three head positions (upright, lateral, and head-hanging) in normal subjects by use of full-field OKN stimulation. Mean vertical OKN gain asymmetry (downbeating greater than upbeating) was noted for the upright position at stripe velocities of 40, 50, and 60 deg/sec. In three subjects, mean VOKN gain asymmetry was eliminated in the head-hanging position, but only slightly reduced in the lateral position. In one subject, mean gain asymmetry was reversed in the head-hanging position. Down-beating VOKAN (usually about 2 to 20 beats) was found in 11 of 21 subjects in the upright position, but was not generally affected by position change. Upbeating VOKAN never occurred following stripes-down stimulation (appropriate stimulation). Inappropriately directed upbeating VOKAN did occur, however, after stripes-up stimulation at 60 deg/sec for 2 of 21 subjects. Downbeating after-nystagmus that was opposite to the direction of previously elicited optokinetic nystagmus (inappropriately directed after-nystagmus), but which occurred from about 2 to 10 seconds after lights-off, was seen in seven subjects.