New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that "primary" olfactory cortex functions like "association" cortex in other sensory systems

Associational connections of pyramidal cells in rat posterior piriform cort ex were studied by direct visualization of axons stained by intracellular i njection in vivo. The results revealed that individual cells have widesprea d axonal arbors that extend over nearly the full length of the cerebral hem isphere. Within piriform cortex these arbors are highly distributed with no regularly arranged patchy concentrations like those associated with the co lumnar organization in other primary sensory areas (i.e., where periodicall y arranged sets of cells have common response properties, inputs, and outpu ts). A lack of columnar organization was also indicated by a marked dispari ty in the intrinsic projection patterns of neighboring injected cells. Anal ysis of axonal branching patterns, bouton distributions, and dendritic arbo rs suggested that each pyramidal cell makes a small number of synaptic cont acts on a large number (>1000) of other cells in piriform cortex at dispara te locations. Axons from individual pyramidal cells also arborize extensive ly within many neighboring cortical areas, most of which send strong projec tions back to piriform cortex. These include areas involved in high-order f unctions in prefrontal, amygdaloid, entorhinal, and perirhinal cortex, to w hich there are few projections from other primary sensory areas. Our result s suggest that piriform cortex performs correlative functions analogous to those in association areas of neocortex rather than those typical of primar y sensory areas with which it has been traditionally classed. Findings from other studies suggest that the olfactory bulb subserves functions performe d by primary areas in other sensory systems.