Complementary laminar terminations of afferents to area 17 originating in area 18 and in the lateral geniculate nucleus in squirrel monkey

The projections from area 18 and the lateral geniculate nucleus onto area 17 of the squirrel monkey (Saimiri) were investigated with retrograde (horseradish peroxidase) and anterograde (tritiated proline) labelling techniques, and the (Fink-Heimer) silver impregnation method for degenerating axons and their terminals. The association fibers from area 18 terminated in all layers of area 17 except in layer IV and in the lower aspect of layer IIIc. The greatest number of terminals were in layers I, V and VI. The bulk of geniculocortical fibers terminated in layer IV and the lower aspect of layer IIIc; a minority of the geniculocortical fibers terminated in layer VI and the lower aspect of layer IIIb. Thus, the majority of fibers from the two sources investigated terminate in a complementary laminar fashion in area 17. The portion of area 17 on the lateral surface of the hemisphere, where the central visual field is represented, received a less dense projection from the geniculate nucleus than the striate cortex in the calcarine fissure, where the peripheral visual field is represented. Ocular dominance columns were not apparent in the striate cortex. No evidence was found that the lateral geniculate nucleus projects to area 18. The results of combined injections of horseradish peroxidase and tritiated proline in area 17 indicated a point-to-point reciprocity between area 17 and the ipsilateral lateral geniculate nucleus.