Functional representation of eye movements in the flocculus of monkeys (Macaca fuscata)

The flocculus of the monkey was mapped with microstimulation methods in chronic preparations. Three types of low threshold, disconjugate eye movements were elicited: (1) a lateral deviation of the ipsilateral eye, (2) a downward deviation of the ipsilateral eye and (3) counterclockwise rotation of the ipsilateral eye. Horizontal eye movements were obtained from folia 2, 3, 4 and 6-10 of the flocculus, and the region eliciting horizontal eye movements was traced in the sagittal plane over a distance of 4 mm in one animal. However, sites flanking these horizontal movement sites produced either vertical movements, rotatory movements or failed to produce a movement of either eye. These findings suggest a zonal organization of flocculus functions in monkeys that is consistent with the differential projections of flocculo-vestibular connections revealed by retrograde tracing studies [2].