Fastigiofugal fibers encoding horizontal and vertical components of saccades as determined by microstimulation in...

To identify the routes by which oculomotor vermis signals control eye movements (saccadic signals), saccades evoked by microstimulation were studied in the region of the uncinate fasciculus (UF) and juxtarestiform body (JB) in the macaque monkey. Anatomical pathways of axons from the fastigial oculomotor region (FOR) were studied by anterograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP). The routes were identified by comparing maps of low threshold for evoking saccades with the anatomical map of anterogradely labeled axons arising from the FOR. Microstimulation of a region of the UF and JB demonstrated that saccadic signals are carried exclusively by decussated FOR axons which leave the cerebellum via the contralateral UF. The fibers in the JB do not carry saccadic signals. The horizontal component of saccadic signals is conveyed by fibers in the descending limb of the UF, while the vertical component is conveyed by a smaller group of fibers which separate from the UF and enter the midbrain with the contralateral superior cerebellar peduncle.