image disparity. See also Chapter 6.

Nonfusional divergence An optomotor reflex to diverge the eyes when vision in 1 eye is blurred and fusion is disrupted on a long-term basis, as seen in cases of sensory exotropia or in spontaneous consecutive exotropia with amblyopia.

Vertical vergence An optomotor reflex to reduce vertical disparity and realign the eyes when fusion is disrupted, so that similar retinal images project on corresponding retinal areas.

Cyclovergence An optomotor reflex to reduce the cyclotorsional disparity of the eyes so that similar retinal images project on corresponding retinal areas. Cyclovergence includes 2 components: incyclovergence and excyclovergence. In addition to a motor reflex, each of these subsystems has a sensory component to compensate for cyclotorsional disparity, and for most targets the sensory component is the more important of the two.

Supranuclear Control Systems for Eye Movement

There are several supranuclear eye movement systems. The saccadic system generates all fast (up to 400°–500°/sec) eye movements, such as eye movements of refixation. This system functions to place the image of an object of interest on the fovea or to move the eyes from one object to another. Saccadic movements require a sudden strong pulse of force from the EOMs to move the eye rapidly against the viscosity produced by the fatty tissue and the fascia in which the globe lies. The study of saccadic velocity is used in some centers to quantitate muscle paresis and abnormal innervation. However, a careful examiner can confirm the presence of a paretic muscle without sophisticated instrumentation by clinically detecting a “floating saccade.”

The smooth pursuit system generates all following, or pursuit, eye movements. Pursuit latency is shorter than for saccades, but the maximum peak velocity of these slow pursuit movements is limited to 30°–60°/sec. The involuntary optokinetic system utilizes smooth pursuit to track a moving object and then introduces a compensatory saccade to refixate. Tests of this system, performed with an optokinetic stimulus, are often used to detect visual responses in an infant or child with apparent vision loss, such as with ocular motor apraxia (see Chapter 12). The vergence system controls disconjugate eye movement, as in convergence or divergence. Supranuclear control of vergence eye movements is not yet fully understood. There are also systems that integrate eye movements with body movements. The most clinically important of these systems is the labyrinthine (vestibuloocular) reflex system, which involves the semicircular canals of the inner ears. Other systems involve the utricle and saccule of the inner ears. The cervical, or neck, receptors provide input for this reflex control. See BCSC Section 5, Neuro-Ophthalmology, for in-depth discussion of these systems.