Frontal eye field efferents in the macaque monkey: II. Topography of terminal fields in midbrain and pons

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Abstract
Frontal eye field (FEF) projections to the midbrain and pons were studied in nine macaque monkeys that were used to study FEF projections to the striatum and thalamus (Stanton et al.: J. Comp. Neurol. 271:473‐492, '88). Injections of tritiated amino acids or WGA‐HRP were made into FEF cortical locations where low‐level microstimulation (≤50 μA) elicited saccadic eye movements, and anterograde axonal labeling was mapped. The injections were made into the anterior bank of the arcuate sulcus from dorsomedial sites where large saccades were evoked (lFEF) to ventrolateral sites where small saccades were evoked (sFEF). The largest terminal fields of FEF fibers were located in the ipsilateral superior colliculus (SC). Projections to SC were topographically organized: lFEF sites projected to intermediate and deep layers of caudal SC, sFEF sites projected to intermediate and superficial layers of rostral SC, and FEF sites between these extremes projected to intermediate locations in SC. Patches of terminal labeling were located ipsilaterally in the lateral mesencephalic reticular formation near the parabigeminal nucleus and the ventrolateral pontine reticular formation. These patches were larger from lFEF injections. Small, dense terminal patches were seen in the ipsilateral pontine gray, mostly along the medial and dorsal borders of these nuclei but occasionally in central and dorsolateral regions. Patches of label like those in the pontine nuclei were located ipsilaterally in the reticularis tegmenti pontis nucleus in lFEF cases and bilaterally in sFEF cases. Small terminal patches were found in the nucleus of Darkschewitsch and dorsal and medial parts of the parvicellular red nucleus in most FEF cases. In the pretectal region, labeled terminal patches were consistently found in the nucleus limitans of the posterior thalamus, but we could not determine if label in the nucleus of the pretectal area and dorsal parts of the nucleus of the posterior commissure marked axon terminals or fibers of passage. We found small, lightly labeled terminal patches in the pontine raphe between the rootlets of the abducens nerve (three cases) or in the adjacent paramedian pontine reticular formation (one case). Omnipauser cells in this region are important in initiating saccades. In one sFEF case, very small patches of label were located in the supragenual nuclei anterior to the abducens nuclei and in the ipsilateral nucleus prepositus hypoglossi posterior to the abducens nucleus. Presaccadic burster neurons in the periabducens region are known to fire immediately before horizontal saccades. Neither IFEF or SFEF projections appeared to terminate in the rostral interstitial nucleus of the medial longitudinal fasciculus, the interstitial nucleus of Cajal, or the motor nuclei of the extraocular muscles.

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