Oculomotor SystemThe visual system informs us about the wiring of sensory circuits along the T < 0 portion of the mapping timeline of brain electrical activity, and the oculomotor system is a great place to begin understanding the other side, because it has been extensively studied and well characterized. The predictive side of the mapping timeline is associated both with staged motor behaviors that have a high probability of executing, and sensory input related to predictions that can modify or terminate those behaviors in flight. Most closely associated with motor predictions is the cerebellum, which has more neurons than all the other brain areas combined.
Human vision, while precise, is also associated with retinal imperfections and small movements during fixation, causing displacements of the visual image. When the eyes move, they move against forces that tend to restore them to their equilibrium (resting) positions. These forces need to be countered by active optimization, to keep the eyes on target for both pursuit and image stabilisation. Lately, much has been made of the involvement of the superior colliculus in eye movements, it's a small structure at the roof of the midbrain that's involved in target selection. But selecting targets is only a small part of the picture, and overall eye movements are a cooperative effort between the brainstem and the cerebellum, under control of various areas of the cerebral cortex. Eye movements are generally categorized according to fixation, pursuit, saccades, and a series of reflexes like the vestibulo-ocular reflex that keeps the eyes on target when the head moves. Many of the reflexes are organized in and around (and by) the superior colliculus, and these may include things like the looming reflex related to threat avoidance.
In this section we'll review the architecture of the human oculomotor system, with a view towards integrating its functions into the electrical activity timeline. After a quick overview, we'll work bottom up, describing first the eye muscles and then the system of motor neurons that drives them, before returning to the superior colliculus and exploring it in detail. We'll also journey through the oculomotor areas of the cerebellum, noting some fascinating neural architecture (and behavior) along the way.
|