The role of the oculomotor vermis in the control of saccadic eye movements.

The oculomotor vermis is a part of the posterior cerebellum, characterized by a low threshold (<10 micro A) for evoked saccades. It comprises vermal lobuli VIc and VIIA. Many Purkinje cells in this area show eye position or saccade-related responses or combinations of the two and usually lack responses to the presentation of visual targets, guiding the oculomotor behavior. The saccade-related responses are directionally selective and show preferences for saccade amplitude or duration, which differ widely between cells. However, at the population level, these saccade-related Purkinje cells give a very precise account of the timing of the saccadic eye movement and, specifically, of the time it ends. This population signal might therefore contribute to determining the end of the saccadic eye movement. Furthermore, by changing the duration of the population response, the amplitude of the saccade could be changed. In other words, saccadic adaptation could be a consequence of changing a representation of time in the cerebellum.