Effects of a fixation target oil torsional optokinetic nystagmus

PURPOSE. To investigate the effects of an imaginary and a visual target on torsional optokinetic nystagmus (tOKN) and directional symmetry of tOKN. METHODS. Torsional OKN was induced by a rotating random dot pattern (52 deg rees in diameter, constant angular velocity: +/-30 deg/sec to +/-52 deg/sec ) with an imaginary or a visual target in 11 eyes of 10 healthy humans by d ual-search coil methods. RESULTS. Intorsional OKN and extorsional OKN were symmetrical in their slow -phase gain. The mean slow-phase gain (0.037/0.041, intorsion/extorsion) of tOKN during fixation on a visual target at the center of the rotating rand om dot pattern was significantly (P < 0.002) smaller than that (0.051/0.052 , intorsion/extorsion) during fixation on an imaginary target at the center of the rotating random dot pattern. The mean tOKN slow-phase best duration (840 msec/724 msec, intorsion/extorsion) during fixation on the visual tar get was significantly (P < 0.002) longer than that (585 msec/543 msec, into rsion/extorsion) during fixation on the imaginary target. In seven eyes of six subjects, the mean slow-phase gain and beat duration (0.034 and 812 mse c) of tOKN during fixation on a visual target 6.5 degrees left or right fro m the center of the rotating random dot pattern were not significantly diff erent from those (0.037 and 825 msec) with a visual target at the center of the rotating random dot pattern (P > 0.3). CONCLUSIONS. A visual target spot suppresses tOKN by a nonpursuit visual sy stem. Intorsional and extorsional OKNs were symmetrical.