QUALITATIVE AND QUANTITATIVE FEATURES OF AXONS PROJECTING FROM CAUDALTO ROSTRAL INFERIOR TEMPORAL CORTEX OF SQUIRREL-MONKEYS

On the basis of cortical and subcortical connections and architectonic s, inferior temporal (IT) cortex of squirrel monkeys consists of a cau dal region, ITC, with dorsal (ITCd) and ventral (ITCv) subdivisions a rostral region, ITR; and possibly a third region intermediate to ITC a nd ITR, ITI (Weller and Steele, 1992; Steele and Weller, 1993). The pr esent study qualitatively and quantitatively examined the terminal arb orizations of 26 axons in ITR and ITI labeled by injections of biocyti n or, in one case, horseradish peroxidase, in ITCv. The majority of ax ons gave rise to a single terminal arbor, with a small number branchin g into two overlapping or nearby arbors. Presumptive terminal speciali zations consisted of rounded, bead-like swellings, most often located en passant. All axons terminated in layer 4 of cortex, and most had ad ditional terminations in layers 3 and 5. The total extent of each axon 's terminal arbor was 125-750 mu m dorsoventrally (mean = 360.6 mu m) and 150-725 mu m anteroposteriorly (mean = 328.1 mu m; all values unco rrected for shrinkage). In most axons, especially those with larger te rminal fields, boutons were not uniformly distributed, but formed 2-4 clumps (mean = 2.2), with a mean width of 149 mu m, separated by narro wer regions of fewer boutons. Based on a cluster analysis of character istics of the 26 axons, axons projecting from caudal (ITCv) to rostral (ITR or ITI) IT cortex of squirrel monkeys comprised three groups tha t we called Type I, Type II, and Type III. Type I axons, the smallest in areal extent of terminal arbor, terminated predominantly in dorsal ITR. Type III axons, largest in areal extent, and Type II axons, inter mediate in areal extent, terminated in ventral ITR and throughout ITI. The three classes of axons may correspond to different types of visua l information entering rostral IT cortex. The clumping of boutons sugg ests that individual axons terminate in limited patches within their t erminal fields.