Stimulus characteristics influence the gain of smooth pursuit eye movements in normal subjects

Impaired smooth pursuit eye movements are commonly believed to indicate a lesion of the central nervous system. Smooth pursuit performance, however, is strongly dependent on non-specific variables like cooperation, arousal and attentiveness. Therefore, disturbed smooth pursuit can be attributed either to lesions of the smooth pursuit system per se, or to the influence of non-controlled variables (non-structural disturbances). This renders the evaluation of smooth pursuit uncertain. In the present study we attempted to design a stimulus that yields smooth pursuit eye movements, which are not influenced by uncontrolled variations of state and input, for a better separation of structural lesions of the pursuit system and the effect of nonspecific variables. Our results suggest that a stimulus that leads to a centrally generated representation (percept) of motion is most suitable to elicit high gains of smooth pursuit (sigma pursuit), but only if attentiveness is optimal. Beta-motion (motion elicited by discrete steps of the target) or real target motion are capable to render the smooth pursuit performance optimal, even with low attentiveness, when the fixation point and its wider surroundings or enough discrete points in the neighbourhood move in the same direction in space.