Cytoarchitectural characteristic of the frontal eye fields in macaque monkeys

The cytoarchitecture of the prearcuate gyrus, including the region of the physiologically defined frontal eye fields (FEF), was studied in four macaque monkeys (Macaca fascicularis, M. mulatta) to determine if the FEF could be anatomically identified. Brain sections were stained with standard Nissl and, in some cases, myelin stains. Two nonstandard planes of section were used: one tangential to the prearcuate gyrus and the second normal to the most posterior bend of the prearcuate gyrus. The first plane of section was advantageous for studying the location of the FEF with reference to the entire medial-lateral extent of the gyrus and the second allowed good comparisons of the FEF to adjacent anterior and posterior cortical areas. Frontal plane sections through the prearcuate gyrus were also examined in 15 macaque monkeys for comparison with sections cut normal to the posterior bend of the gyrus and tangential to the gyrus. Intracortical microstimulation was performed in three monkeys. The FEF was defined as the area from which low-threshold (less than or equal to 50 microA) saccades could be evoked. The area extended about 10 mm along the anterior bank of the arcuate sulcus. Within the area, saccade amplitudes were represented in a mediolateral, large-to-small topography. No topography of saccade direction was noted within FEF but reversals of saccade direction for any given electrode pass were found. These results confirm the results from our earlier mapping study of FEF (Bruce et al.: J. Neurophysiol. 54:714-734, '85). Cell bodies of large pyramidal cells in layers III and V of the prearcuate gyri from three hemispheres were measured with the aid of an image-combining computer microscope. The distribution of cells of greater than 22 microns diameter or cross-sectional areas of greater than 500 microns 2 were plotted. In one monkey, marker lesions made at microstimulation sites within the FEF or in adjacent non-FEF areas were also plotted. The location of the FEF appeared to coincide with the concentration of large layer V pyramidal cells in the prearcuate gyrus rather than with any previously mapped cytoarchitectonic area. The numbers of large pyramids in layer V were noticeably reduced along the lip of the prearcuate gyrus and at dorsomedial and ventrolateral locations which were outside the physiologically defined FEF.