FUNCTIONAL-PROPERTIES OF REGENERATED OPTIC AXONS TERMINATING IN THE PRIMARY OLFACTORY CORTEX

When the optic nerve of Rana pipiens is cut and deflected into the tel encephalon, the regenerating fibers terminate selectively in the super ficial neuropil of the primary olfactory cortex. These redirected fibe rs and their terminals on the dendrites of the cortical cells appear n ormal by LM and EM criteria. Electrical recording, done 2-16 months af ter surgery, shows visually evoked activity in the superficial neuropi l (Layer I) of the olfactory cortex, and visually excited responses in the deep cortical cell layer (Layer II). In the normal frog, the elec trical activity seen in the neuropil of the olfactory cortex consists of small transients about 2-3 X the noise level of the electrode conta ct. These occur spontaneously and are also excited by puffs of air to the nose. There is no such excitation by visual stimuli. Larger initia lly negative spikes eel above noise level are recorded in the cell lay er next to the ependymal surface, and these are also spontaneous, or e xicited by puffs of air to the nose, but not by visual stimuli. In the operated frog, the small transients in the neuropil appear and are ex cited by the puffs of air and by visual stimuli. Similarly the respons es in the cell layer are excited by both sorts of stimuli. But new typ es of electrical signals appear in the neuropil; they are driven only by visual stimuli presented to the affected eye. These are very large transients of the kind found in the tectal neuropil and have the two c haracteristic shapes which were classified as B and C types in the tec tum. Such large transients are never seen in the neuropil of the olfac tory cortex in normal frogs. The receptive fields of the small visuall y driven transients in the neuropil are not easy to make out because t he signal levels are so close to the noise level that different units cannot be reliably distinguished from each other. But the receptive fi elds of the much larger B and C type unit responses are as easy to cla ssify and plot as they are in tectum, even though on the average they are only about 2/3 as large as in tectum. The single-unit receptive fi elds belong to one or another of the several types of retinal ganglion cell classes distinguished in optic-nerve recordings. Four of the maj or classes normally project to the tectum and a fifth projects to the lateral geniculate complex. But all five are present in the ectopic pr ojection to the olfactory cortex. The various classes of ganglion cell operations are only crudely stratified in the olfactory cortex. The c lasses mediated by unmyelinated fibers (Classes I and II) are represen ted superficially in the neuropil; the myelinated contingent (Classes III and IV) are often found deeper. But a strict ordering by depth as seen in the tectum is not found in the cortex. In some of the operated frogs, several large, distinct B and C units, discriminable by size a nd shape, can be recorded at a single electrode position in the neurop il. Then both the single-unit receptive field (SURF) of each unit can be classified and plotted and also the multi-unit receptive field (MUR F) of their distribution in the visual field. However, the MURFs are n ot densely filled with single units, as they are in the tectum, and th ey subtend much wider areas in the visual field. Generally, the SURFs are also not related systematically to the position of the recording e lectrode on the cortical surface. Thus, there is no finely ordered ret inotopic map in the olfactory cortex. However, almost all the B and C units responded only to stimuli presented (monocularly) in the frontal part of binocular region of the visual field.