Behavior and physiology of the macaque vestibulo-ocular reflex response to sudden off-axis rotation: Computing eye translation

The vestibulo-ocular reflex (VOR) has historically been considered a computationally simple reflex: to stabilize images on the retina against imposed head rotation, the eyes must be counterrotated by an equal amount in the opposite direction. During almost any head rotation, however, the eyes are also translated. We show that the VOR compensates for 90% of this translation, and suggest a computational scheme by which this is done, based on a temporal dissection of the VOR response to sudden head rotation. An initial response that corrects only for imposed rotation is refined by a series of three temporally delayed corrections of increasing complexity. The first correction takes only head rotation and viewing distance into account; the second, head rotation, viewing distance, and otolith translation; and the third, head rotation, viewing distance, otolith translation, and translation of the eyes relative to the otoliths. Responses of type I gaze velocity Purkinje (GVP) cells in the cerebellar flocculus and ventral paraflocculus of rhesus monkeys were recorded during sudden head rotation. We show that cell discharge was modulated both by axis location and by viewing distance, suggesting that GVP cells play a role in the VOR response to rotation- induced eye translation.