Slow Cumulative Eye Position to Quantify Optokinetic Afternystagmus

In 30 normal subjects we computed the slow cumulative eye position (SCEP) of optokinetic afternystagmus (OKAN) that followed 60 seconds of full-field optokinetic stimulation at 60°/s. The mean SCEP was 112.8° ± 65.0°. The lower and upper fifth percentile limits for directional preponderance of the SCEP were −38.8% and 44.3%, respectively. The time constant, which we calculated by dividing the SCEP by the initial velocity, was 12.0 ± 7.4 seconds. This value is nearly identical to the time constant obtained from semilogarithmic regression of the decay of OKAN slow-phase velocity versus time. We conclude that the SCEP is a good measure of OKAN and that it reflects the substantial amount of variability and directional asymmetry observed in the optokinetic responses of normal subjects.