Olfactory-like Responses in Pyriform Area to Non-Olfactory Stimulation *

Considerable doubt continues to exist about the r6le of finely myelinated and unmyelinatedt visceral afferents in the function of the cerebral cortex. Recent evoked potential studies involving stimulation of the chorda tympani, "" the vagus, 7l and the splanchnic nerves' have failed to establish that impulses transmitted along the slow-conductingt components of these nerves reach the cortex. It is inferred, however, from clinical evidence that the slowly conducted signals are discriminated at the cortical level. The inability to resolve the foregoing problem by the usual physiological techniques indicated the need for other methods. For this reason it was decided to re-explore the bioelectrical properties of the olfactory system in the hope of gaining a clue to the physiology of visceral systems in general. Olfaction is unique in being both an exteroand visceroceptive sense. Its anatomical substratum has attributes that make it a prototype of a primitive visceral system. The primary olfactory neurone manifests a most primitive characteristic insofar as it assumes a position in the dermal layer. The olfactory nerves going to the bulb are entirely unmyelinated. From the primary cell station in the bulb a large proportion of fibers travel in the olfactory tract directly to a phylogenetically ancient part of the cortex. In short, one has in the olfactory apparatus a system that begins with entirely unmyelinated fibers and is irrefutably known to reach the cerebral cortex. The cortical responses in the pyriform area following natural olfactory stimulation are strikingly dissimilar to those obtained from the neocortex by natural activation of visual, auditory, or somesthetic receptors. In contrast to the latter, one finds an olfactory stimulus is followed by a sustained train of rhythmically recurring sinusoidal potentials of high voltage. Adrian, employing the hedgehog and the cat, appears to have been the first to describe such potentials as occurring in both the olfactory bulb and the pyriform cortex.' Figure 1 shows responses to olfactory stimulation recorded from the pyriform area in the opossum, rabbit, cat, and the squirrel