The cerebellum in eye movement control: nystagmus,coordinate frames and disconjugacy

In this review we discuss several aspects of eye movement control in which the cerebellum is thought to have a key role, but have been relatively ignored. We will focus on the mechanisms underlying certain forms of cerebellar nystagmus, as well as the contributions of the cerebellum to binocular alignment in healthy and diseased states. A contemporary review of our understanding provides a basis for directions of further inquiry to address some of the uncertainties regarding the contributions of the cerebellum to ocular motor control.The cerebellum is an important structure within a widely distributed neural network that controls movements including those of the eyes. Both the immediate online control of movement and the adjustments necessary to optimize motor performance in the long term are under its purview. Not surprisingly, ocular motor abnormalities are prominent on clinical examination of most cerebellar patients. Attempts to localize particular eye movement abnormalities to specific parts of the cerebellum—traditional clinical–anatomical localization—combined with results of experimental lesions in animals, have led to inferring functions to specific cerebellar structures. While these approaches facilitate topical diagnosis, they do not necessarily translate to a better understanding of the specific role of the cerebellum in eye movement control. In fact, we are still relatively ignorant about how the cerebellum performs its functions. Here, we will consider the role of the cerebellum in binocular control—both to create disconjugacy when it is necessary, and to prevent ocular misalignment when it is unnecessary and disruptive. We will also touch on the implications of evolving into frontal-eyed creatures, with the competing demands of binocular, foveal vs retinal, full-field stabilization of images. Furthermore, we suggest that the phylogenetically old vestibular anlage persists in a rudimentary form within our human brains and its vestiges can be uncovered in neurological disease.^{1, 2, 3, 4} These issues bear on interpretation of pathological nystagmus, which depends on the coordinate system in which the nystagmus is couched (foveal: eye frame vs full-field retina: head frame), and also on the types of ocular misalignment seen in cerebellar patients.