The effect of stimulus predictability and age on human tracking eye movements

Human tracking eye movements were recorded in healthy volunteers of varying age. The target motion was either predictable (pure sinusoids or sinusoidal frequency sweep) or unpredictable (pseudorandomized). Gain and phase values for the tracking and the smooth pursuit eye movement signal were calculated as a function of target frequency from 0.2 to 2 Hz. Max. target velocity was 20 or 40 deg/sec. In the low frequency area the smooth pursuit gain for predictable stimulation was higher than the gain elicited by the pseudorandomized stimulation. In the high-frequency area, gain values did not differ significantly. For the predictable stimulation, the phase of the smooth component was always a lag, increasing with increasing frequency. At low frequencies of the pseudorandomized signal a phase lead was observed. At higher frequencies the lead turned into a lag that was greater than for the predictable stimulation. The young and old groups showed reduced smooth pursuit gain values, compared with the main group specially when the target waveform was pseudorandomized. The reduced smooth pursuit ability was mostly compensated for by an increased amount of saccades.