Three-dimensional kinematics of saccadic eye movements in humans with cerebellar degeneration.

During saccades, the orientation of the eye velocity axis varies depending on the instantaneous eye position. The means by which this eye velocity axis tilting arises remains contentious. Some have argued that muscle pulleys in the orbit implement the tilts, since they cause the pulling directions of the extraocular muscles to change in a manner that depends on instantaneous eye position. Others have suggested that the tilting is centrally programmed. In the current study, three-dimensional eye and head rotation data were acquired, using the magnetic search coil technique, to confirm the presence of eye position-dependent eye velocity axis tilting during saccades. We studied normal humans and humans with inherited or sporadic cerebellar degeneration. While the humans with cerebellar degeneration were noted to have abnormalities in the two-dimensional metrics and consistency of their saccades, the eye velocity axis tilts were similar to those observed in the normal subjects. Our findings suggest that the cerebellum does not encode eye velocity axis tilting during saccades, further supporting the notion that these phenomena arise due to the effects of muscle pulleys.