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The efferent connections from area 6 and adjacent regions of area 9 were investigated in 9 adult monkeys (Macaca fascicularis) using the autoradiographic technique. Injection fields throughout these regions projected ipsilaterally to area 7 in the parietal lobe, n. caudatus, claustrum, n. ventralis anterior, n. parafascicularis, n. medialis dorsalis, n. reticularis thalami, colliculus superior, griseum centralis, griseum pontis and n. reticularis tegmenti pontis. The dorsolateral and medial aspects of areas 6 and 9 projected to the caudal cingulate gyrus, n. ventralis lateralis pars caudalis and n. interstitialis of Cajal. Axons originating in the ventrolateral cortex of areas 6 and 9 terminated in the orbital cortex. Projections arising from area 6 were traced to area 4, into the ventral bank of the rostral cingulate sulcus, to putamen, n. centralis superior lateralis, area X and its caudal continuation within n. ventralis posterior lateralis pars oralis, to n. ruber pars parvocellularis as well as the bulbar reticular formation. Adjacent prefrontal areas projected to the medial aspect of area 24, to areas 19, 20, 21 and 49/27 and to parts of the lateral thalamic nuclear group. This prefrontal projection pattern was in many respects also seen in cases injected into the dorsolateral portion of area 6, while its medial and ventrolateral subdivisions showed additional projections to n. ventralis medialis, the medial parts of centrum medianum as well as spinal and prorhinal projections, respectively.Furthermore, extensive bilateral connections were found within the frontal lobe, to basal ganglia, n. parafascicularis, n. centralis thalami, n. ruber, griseum pontis, n. reticularis tegmenti pontis, bulbar reticular formation and, more selectively, to colliculus superior and n. medialis dorsalis. Evidence was also presented for direct prefrontal connections to amygdala and corticonigral projections arising from areas 6 and 9. The results are discussed with respect to the intermediate position of area 6 between primary motor and prefrontal cortex as well as to differences of projection patterns within area 6 itself and, in particular, the supplementary motor area.
Heinz Künzle (Thu,) studied this question.