Topography of pyramidal neuron intrinsic connections in macaque monkey prefrontal cortex (areas 9 and 46)

Abstract

An understanding of the normal organization of prefrontal cortex is essential to the recognition of pathology underlying human behavioral disorders believed to depend on this region. We have therefore studied the pattern of intrinsic intra- and interlaminar pyramidal neuron connectivity in prefrontal areas 9 and 46 (of Walker) in macaque monkey cerebral cortex (anterior to the arcuate sulcus between the principal sulcus and midline). We made focal (200–400 μm) injections of biocytin and mapped the pattern of orthogradely transported label. Injections made into the superficial layers label wide-ranging lateral projections within the same areas of prefrontal cortex. Projections local to such small injections form a narrow band of terminals in layers 1–3 (200–400 μm wide, 2–4 mm long) centered on the injection site. Collateral fibers spread orthogonal to this terminal band, making frequent bifurcations, to establish a series of parallel bands of terminals with uninnervated bands between, spaced regularly across the cortex (center to center 500–600 μm). The entire pattern of terminal label is stripelike, with occasional narrower interbands and crosslinks between the bands, and can extend over 7–8 mm across the cortex. These projections arise from pyramidal neurons in layers 2, 3, and 5 and terminate in layers 1–3. The stripelike pattern contrasts with patchlike patterns in other cortical regions (V1, V2, V4, motor, somatosensory) and is smaller in scale than stripelike zones of corticocortical afferent terminals to this region, reported to be 300–750 μm wide and spaced 1.0–1.5 mm center to center.