Subclasses of eye movements

To understand how the cerebellum influences eye movements, it is useful to define the different subclasses of eye movements and to review ocular motor control signals and their underlying anatomical substrate within the brainstem. It is on these circuits that the cerebellum exerts its influence (Leigh and Zee, 1999). Saccades are the rapid eye movements that quickly change the line of sight, placing the image of an object of interest on the fovea, where it can be scrutinized and its saliency determined (“fight or flight”). Saccades are characterized by their size, speed, and latency. The quick phases of nystagmus resemble saccades in their dynamic characteristics.

Vestibular slow phases are smooth eye movements that are generated by the vestibulo-ocular reflex (VOR); they act to stabilize images on the fovea when the head is rotating (rotational or r-VOR) or translating (translational or t-VOR). Vestibular slow phases are characterized by their amplitude, direction, and timing (or phase) relative to head motion. Optokinetic (OKN) slow phases are full-field visual following movements that can be elicited by rotation of the visual scene (e.g., with an optokinetic drum) around a subject. In natural circumstances, optokinetic slow phases act to supplement the r-VOR during sustained head rotation in the light. OKN slow phases, too, are characterized by amplitude, direction, and timing.

Pursuit is a smooth tracking eye movement that keeps the image of a moving object on the fovea. Pursuit movements are often characterized by their response to an abrupt change in target speed (step-ramp stimulus). The initial movement of the eye during the first ∼100 msec of such tracking reflects the open-loop performance of the system (before visual feedback can be used to modify ongoing behavior). The acceleration of the eyes characterizes the response during this open-loop period. Sustained pursuit of a target moving at a constant-velocity is a measure of closed-loop behavior (which uses visual feedback to improve performance) and is characterized by a gain (eye velocity/target velocity). Predictive patterns of target motion, such as sinusoids, are also used to probe pursuit capabilities.

Vergence eye movements rotate the eyes by different amounts, usually in opposite directions, shifting gaze between objects at different distances and following objects moving in depth. This serves to maintain an image on both foveas at the same time and facilitates depth perception. Disparity (a difference between the location of an image on the two retinas), blur of images on the retina, and other features of the target (e.g., relative size) are cues that are used to signal its distance from the observer and to drive vergence.