Anatomy of the Saccade System

Three important cortical areas are involved with saccade generation and include the frontal eye fields (FEF), the supplementary eye fields (SEF), and the parietal eye fields (PEF). Other zones of the association cortex that play a secondary, but important role in saccade generation are the prefrontal cortex (PFC) and posterior parietal cortex. The interrelationships of these structures are complex and are summarized with a clinical emphasis (Fig. 9-6 (Figure Not Available) ). y The FEF is located in the lateral part of the precentral sulcus at the posterior end of the middle frontal gyrus. It is primarily involved with intentional conjugate visual exploration to the contralateral visual space and is active in the disengagement from fixation that must precede each refixation saccade. The FEF receives input from the PFC, which functions in prediction, spatial memory, and inhibition of saccades; the SEF, wherein reside motor programs; and the PEF, which is specialized to integrate spatial vectors and facilitate reflexive visual exploration. y , y FEF output is directed to the superior colliculus (SC) and saccade generators (burst neurons) in the brain stem reticular formation.^] , y

The SEF is in the anterior part of the supplementary motor area in the posteromedial part of the superior frontal gyrus. y , y It receives input from the PFC and from the posterior hemisphere,^1 and it projects to the fEf^c1 the SC, and the brain stem reticular formation.y The SEF functions to coordinate visual with other sensory and motor inputs to maintain eye position in space, despite head and body movement during and between successive saccades. y y The PEF is situated in the superior part of the angular gyrus and supramarginal gyrus within areas 39 and 40 of Brodmann. y , y Lesions of this area result in increased latency of reflexive, visually guided saccadesy to the contralateral visual space, and this area has also been shown to be involved with disengagement from fixation as well as the triggering of reflexive visually guided saccades. The PEF seems more important in the reflexive exploration of the visual environment, while the FEF is more active with intentional (internally generated or willed) exploration.

The descending saccadic pathways most likely mimic those described in lower order primates. In these animals, the frontal lobes project to the SC, which is then projected to the saccade generators in the pons (paramedian pontine reticular formation [PPRF]) for horizontal saccades and the midbrain for vertical saccades (see later). There are two parallel systems for the FEF and SEF. A subcortical circuit has also been demonstrated from the frontal cortex to the caudate nucleus and projects to the substantia nigra (pars reticularis) that sends inhibitory output to the SC, suppressing unnecessary saccades.

In a clinical example, a patient with an acute unilateral right-sided cerebral infarction resulting in left-sided limb weakness may maintain his or her eyes and head turned to the right. This patient's gaze may be brought to near the midposition, but attempts at saccades with the left gaze field are not possible. The doll's head maneuver does, however, bring the eyes past the midline to the left. In contrast, a patient with an acute unilateral right-sided pontine infarct will have left-sided limb weakness also, but gaze preference will be to the left with impaired rightward horizontal saccades. The doll's head maneuver will not bring the eyes past the midline.

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