Motor learning is the process by which movements become smooth and accurate through practice. Motor learning is important during early childhood development, and continues throughout adulthood, because the neural circuits controlling our movements need to be recalibrated in response to changes in the brain or body due to injury, disease, or the normal aging process. Motor learning depends on a brain region called the cerebellum, and patients with cerebellar dysfunction have clumsy, uncoordinated movements. One of the two main inputs to the cerebellum is the climbing fiber input from the inferior olive in the brainstem. An influential theory of cerebellar function suggested that the climbing fibers carry the error signals that control motor learning. However, recent evidence suggests that motor learning can occur in the absence of instructive signals in the climbing fibers. Thus, there seems to be more than one way to implement motor learning in the brain. The goal of this project is to determine which aspects of motor learning are controlled by the activity of the climbing fibers, and which aspects of learning rely on other neural mechanisms. This question will be addressed by studying the eye movement responses to vestibular stimuli (i.e., the sensory signals encoding movements of the head) and their regulation by motor learning. Most, if not all, movements are guided by vestibular signals. The eye movement response to a vestibular stimulus is called the vestibulo-ocular reflex (VOR). This vestibular reflex functions to stabilize visual images on the retina, and is essential for maintaining good vision during movements of the body. Both the amplitude and the timing of the eye movements driven by the VOR can be adaptively modified by cerebellum-dependent learning, and thus the VOR serves as a model system for studying the neural mechanisms controlling movement amplitude and timing more generally.