In 7 healthy subjects, horizontal smooth pursuit eye movements (SPEM) and horizontal optokinetic ocular reflex (OKR) were measured in the dark under the passive attitude which subjects tried to fixate at the center of a screen (passive SPEM and passive OKR). A sinusoidal spot target oscillation and a sinusoidal narrow-band (7 deg bandwidth in vertically) pattern oscillation were employed to elicit passive SPEM and passive OKR, respectively. The pattern consisted of randomly arranged fine dots. For the control, SPEM was elicited by following the spot target actively (active SPEM). Nine sinusoidal oscillations with frequencies ranging from 0.2 to 1.6 Hz and peak velocities ranging from 19 to 150 deg/s were employed. Passive OKR was clearly demonstrated, and the velocity gain of passive OKR was slightly lower than that of active SPEM under frequencies less than 0.8 Hz and was similar at 1.6 Hz. Common frequency-phase profiles were observed between passive OKR and active SPEM. However, the velocity gain of passive SPEM was much lower than that of passive OKR and depended on the amplitude of target oscillation i.e. the lower amplitude provided a higher gain. We concluded that poor response of the passive SPEM and amplitude dependency of the passive SPEM response was caused by the absence of assistance from the voluntary saccade in the passive attitude. The SPEM response for a small moving target may depend on the voluntary saccade function which maintains a target at the fovea while supplementing SPEM.
