Abstract for Japanese Neuroscience Meeting Final version (August 12, 1991)

     Clustered arborizations of horizontal axons have been reported in the primary and secondary visual, primary auditory and motor cortices in cats and monkeys.  This study examined whether similar structure is present in a higher visual cortical area, the inferotemporal cortex(IT) of the monkey.  Biocytin (4%) was injected iontophoretically (7A x 25 min, 7 sec on-7sec off) into cytoarchitectonic areas TEO and TE of IT in 2 monkeys (Macaca fuscata).  The IT was cut tangentially and sections were treated with the avidin-biotin complex method.  Detailed structure of labeled fibers, terminals and cells was visualized as in best Golgi-impregnated materials.  Both TEO and TE injections resulted in patchy distributions of anterogradely labeled terminals within respective areas.  A single injection produced up to 8 patches, 0.3-0.8 mm in diameter and 0.8-3.6 mm from the center of the injection site.  Some injections retrogradely labeled many pyramidal cells.  These cells tended to be clustered in areas where labeled fibers and terminals were densely distributed.  We conclude that intrinsic horizontal axons arborise in a clustered manner in IT.  This organization may explain physiological observations that cells with similar stimulus selectivities are found in 2 or 3 separate groups along tangential electrode penetrations in area TE (Fujita, Cheng and Tanaka, 1991).