A non-thalamic olfactory pathway to the orbital gyrus in the cat.

Extracellular unit responses were recorded from the cortical layer of the orbital gyrus following stimulation of the piriform cortex fronting on that gyrus. The responses were obtained only from the dorsal bank of the rhinal sulcus. The responses were presynaptic axon and postsynaptic soma spikes with latencies of about 4 and 4.8 msec, respectively. When the dorsal bank of the rhinal sulcus was stimulated, antidromic responses with a latency of about 4.4 msec were recorded from the superficial and deep soma layers of the piriform cortex. Following injections of horseradish peroxidase into the dorsal bank of the rhinal sulcus, labeled cells were found in the piriform cortex, the lateral, basolateral and central amygdaloid nuclei, and the prelimbic area. These results indicate that the piriform cortex projects directly to part of the orbital gyrus by way of association fiber pathways.