A note on the problem of olfactory associations of the orbitofrontal cortex in the monkey

Abstract

Evidence has recently been reported suggesting (1) that a circumscript posterolateral region of the monkey's orbitofrontal cortex has the functional significance of a higher-order olfactory association cortex (2) that olfactory information is conveyed to this cortical region over a conduction route that by-passes the thalamus. The present paper reports an attempt to identify sources of afferents to the above region that conceivably could function as links in such a specific olfacto-frontal conduction route. The attempt was made by comparing the patterns of retrograde cell-labelling resulting from horseradish peroxidase injections confined to, respectively, the lateroposterior quadrant of the orbitofrontal cortex and a more anterior orbitofrontal region with each other and with published reports of similar studies involving the frontal convexity. Both orbitofrontal injections labelled numerous cells in the medial subdivision of the nucleus mediodorsalis, and lesser numbers of cells in the nucleus ventralis anterior thalami, in the basal amygdaloid nuclei, substantia innominata, and temporal isocortex. Only in the case of injection in the lateroposterior quadrant of the orbitofrontal cortex did a considerable number of additional labelled cells appear in the medial bank of the rhinal sulcus (prorhinal cortex of Van Hoesen and Pandya). Since the prorhinal cortex lies in the path of known olfactory circuitry, this finding suggests that it gives rise to an olfactory conduction route selectively directed at the lateroposterior orbitofrontal cortex.