Efferent cortico‐cortical fiber connections of area 18 in the squirrel monkey (Saimiri)

Abstract Association fibers, degenerating as a result of laminar heat lesions in area 18 of Saimiri , were traced to their respective cortical terminal sites. A strong intrinsic fiber system connected the supragranular cell layers with the underlying layer V of area 18. In addition, the supragranular layers of area 18 projected ipsilaterally to area 17, the territory of area 19 and area MT in the posterior bank of the superior temporal sulcus. Sparse fibers were traced to the cortex in the intraparietal sulcus and to area 8. The projection upon area 17 was strictly topographical; the termination of these fibers was restricted to layer I of area 17. The projection upon area 18 also showed signs of a topographical organization, though less distinct. The evidence for the existence of Brodmann's classic area 19 or its possible replacement as suggested by Allman and coworkers is discussed. Callosal fibers terminated in a topographically organized manner in a strip of cortex posterior in area 18, i.e., the OBg of von Bonin, and in a strip of cortex probably in anterior area 18. A sparse projection connected area 18 with the contralateral area MT.