Cerebellar Control of Saccades in Health and Disease

Vision is one of the senses that are used for gathering information about the surrounding environment. Patterns of light reflecting from the environment enter the eye and are projected onto the retina. The retina harbors photoreceptors, which transform the patterns of light into the electrical activity that neurons use to convey visual information. This information enables humans to observe and interact with the environment. To gather detailed visual information about the properties of an object of interest, it must be projected on the fovea. The fovea is the part of the retina that has the highest density of photoreceptors and therefore provides the highest visual acuity. The projection of (moving) objects can be kept on the fovea using smooth pursuit eye movements, and new objects can be targeted by using saccadic eye movements. Eye movements can be recorded and quantified with relative ease. Current technology enables us not only to record eye movements in healthy adults in a laboratory setting, but also in more challenging subjects such as children or patients, or to record concurrently with functional imaging in a MR-scanner. Eye movements can be studied at multiple levels, ranging from the quantification of reflexive behavior to the inference of cognitive strategies involved in voluntary eye movement control. With properly designed studies it is possible to assign a functional role to the different stages in the visuo-oculomotor pathways. These characteristics propelled a large number of electrophysiological and behavioral studies on eye movements in non-human primates and humans. Functional imaging studies during the last twenty years have opened new ways for further investigations of the oculomotor system. The accumulating knowledge from these electrophysiological, behavioral and the more recent imaging studies has led to a detailed, but not complete, understanding of the neuronal pathways that subserve the oculomotor system. This also led to the appreciation that eye movements exhibit specific abnormalities when a confined part of the nervous system that is involved in these eye movements is affected by disease or trauma. Therefore, eye movement abnormalities have a clear clinical value and can contribute to the diagnostic process in a clinical setting. This thesis focuses on the neuronal pathways and clinical use of saccadic and smooth pursuit eye movements.