Glossary

frontal eye field Motor cortex for eye movements. In man and monkey, the frontal eye field lies in premotor cortex, just rostral to the hand representation in motor cortex.

neural integrator Brain stem circuit that maintains the current position of the eye by integrating all eye velocity commands. Its tonically spiking output neurons (tonic neurons) are located in the nucleus prepositus hypoglossi, the interstitial nucleus of Cajal, and the medial vestibular nucleus.

nystagmus Any to-and-fro movement of the eyes. Nystagmus can be normal (e.g., continued optokinetic stimulation causes optokinetic nystagmus, such as watching a train go by) or pathological, both congenital and acquired. The waveform of pathological nystagmus often indicates the damaged structure (e.g., sawtooth nystagmus indicates vestibular system damage).

optokinetic reflex/nystagmus Smooth, relatively slow eye movements elicited by movement of the whole visual field. Prolonged optokinetic reflex becomes optokinetic nystagmus when the slow movement alternates with quick-phase movements in the opposite direction.

response field The spatial parameters for the sensory stimulation and/or motor behavior associated with the spiking (or maximal spiking) of a particular neuron.

saccade generator Circuitry in brain stem reticular formation that underlies all rapid eye movements. Its principal output cells are short-lead excitatory burst neurons (EBNs): Horizontal EBNs are in the caudal paramedian pontine reticular formation near the abducens nucleus, and vertical EBNs are in the midbrain near the oculomotor nucleus. Other key cell types are inhibitory burst neurons, long-lead excitatory burst neurons, and omnipause neurons.

saccades Short-duration, high-velocity (rapid) eye movements that quickly move the eyes to a new position; can include both voluntary saccades and the quick phase of vestibular or optokinetic nystagmus. Special saccade types include memory saccades, in which the eye movement is directed to the location of a remembered target not currently visible, and antisaccades, in which the movement is deliberately directed opposite to the target's location.

smooth pursuit Smooth, continuous eye movements used to track a moving target.

superior colliculus Midbrain visuomotor structure for triggering visually guided saccades.

vestibuloocular reflex Smooth, relatively slow eye movements elicited by the vestibular sense that serve to cancel movements of the visual stimulus on the retina (retinal slip) caused by head movements.

This article on the human brain's widely distributed circuitry for controlling the eyes stresses the functional significance of eye movements. It takes an evolutionary perspective by emphasizing the primacy of the image-stabilization system (vestibuloocular reflex, optokinetic reflex, and quick phases of nystagmus) and how the relatively new eye movements of primates (saccades, smooth pursuit, fixation, and vergence) serve the primate's high-resolution foveal vision by engaging the far older image-stabilization circuits. The anatomy and physiology of principal oculomotor structures are reviewed, with emphasis on the frontal eye field region of primate neocortex.

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Do Not Panic

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