The use of auto-correlation function to quantify periodicity in smooth pursuit.

Smooth pursuit eye movement was recorded with a DC amplifier during horizontal sinusoidal target movement at 0.3, 0.6, 0.8, 1.0 and 1.2 Hz. Eye movement was digitalized at 100 Hz and 12 bits accuracy. The digitalized eye movements were analyzed by the auto-correlation function, and the auto-correlation coefficient was used to quantify periodicity. In 13 healthy volunteers, the auto-correlation coefficient was nearly equal to 1.0 at 0.3 Hz but gradually decreased as the target frequency increased. In 8 patients with various neurological disorders, the auto-correlation coefficient was significantly smaller at all target frequencies than in the healthy volunteers. Less periodicity, as indicated by the decreased auto-correlation coefficient, suggests a disorder in the control visual feedback mechanism in smooth pursuit.