Vertical eye movement related unit activity in the rostral mesencephalic reticular formation of the alert monkey

Eye movement related unit activity was recorded in the rostral mesencephalic reticular formation (MRF) of the alert monkey. Most units (78 out of 117) were activated with a short activity burst starting before the eye movement and were otherwise silent. The activity was the same whether movements occurred spontaneously in the light or dark, or were fast phases of vestibular or optokinetic nystagmus, and could be related to parameters of a vector representing the eye movement such as amplitude, position changes along certain planes or direction of movements. Units coding position changes or direction of movement had their preferred direction always close to the vertical. Other units (18 out of 117) showed some tonic activity, which was also only related to vertical eye position. It is suggested that this region of the rostral MRF acts an an immediate supranuclear structure, mediating eye movements in the vertical plane.