Direct Evidence for a Position Input to the Smooth Pursuit System.

Gunnar Blohm^1,2, Marcus Missal², Philippe Lefevre^1,2,3.

¹Centre for Systems Engineering and Applied
Mechanics (CESAME), Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium,
²Laboratory of Neurophysiology, Université catholique de Louvain, 1200 Brussels, Belgium,
³Laboratory of Sensorimotor Research, National Eye Institute, National Institutes of Health (NIH),
Bethesda, MD 20892, USA.

The principal input to the saccadic system is position error, whereas it is velocity error for the smooth
pursuit system. However, it has been shown recently that catch-up saccades to moving targets are
triggered and programmed by using velocity error in addition to position error. Similarly, an influence
of position error on smooth pursuit has been hypothesized but there has been no direct evidence of a
position input to the smooth pursuit system. Here, we developed an original experimental paradigm to
examine a possible role of position error signals on smooth pursuit. Therefore, we briefly flashed
(10ms flash) a target during ongoing pursuit eye movements in 2D. Once the flash appeared,
subjects were instructed to orient their eyes to the memorized position of the flash. We analyzed the
smooth eye movement perpendicular to the initial smooth pursuit trajectory in response to the
occurrence of the flash. We found a short-latency (~80ms) modulation of the smooth eye movement
that was directed toward the flash. This response appeared whether or not a saccade was executed
toward the memorized flash position. Furthermore, the response magnitude was proportional to the
position error of the flash. Our data also showed that the smooth response to the flash was
independent of the initial smooth eye velocity; thus the flash did not simply modify the trajectory of the
smooth eye movement. As a conclusion, our data provide direct evidence of a position input to the
smooth pursuit system. This is compatible with the observation that the saccadic and smooth pursuit
systems share common neural structures. These data will be discussed in terms of neural signals
responsible for this phenomenon.

Citation: Blohm G, Missal M and Lefèvre P, "Direct evidence for a position input to the smooth
pursuit system", 14th Annual Meeting of the Neural Control of Movement Society, Sidges (Spain),
2004