Odor Discrimination and Odor Quality Perception in Rats with Disruption of Connections between the Olfactory Epithelium and Olfactory Bulbs

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Rats were trained using olfactometry and operant conditioning to discriminate among homologous fatty acids, homologous aldehydes, and a series of unrelated odors. Their memory for the positive and negative assignment of each odor (tested under extinction) was assessed before and after they had received selective lesions of the olfactory bulbs or injection of the olfactory epithelial toxin 3-methyl indole (3-MI). Response accuracy on the memory test provided a measure of the extent to which treatments altered the remembered perceptual quality of the odors. The degree of deafferentation of the bulb by treatment with 3-MI was assessed using anterograde transport of horseradish peroxidase applied to the olfactory epithelium. Rats treated with 3-MI had a detectable reaction product only in varying numbers of glomeruli on the lateral and, in some cases, posterior medial walls of the olfactory bulb. Bulbar lesions destroyed the dorsal and dorsomedial bulbar areas that have been identified in optical and electrophysiological studies as showing responses to fatty acids. Rats with bulbar lesions had good to near perfect retention on their post-treatment memory test on all odor pairs, as did 3-MI-treated rats that still had substantial input to glomeruli on the lateral or posterior medial wall of the bulb. 3-MI-treated rats with substantially fewer afferent connections had severe retention deficits, particularly for the aldehyde and fatty acid odors, but this loss was secondary to deficits in the ability to discriminate among these odors. The results indicate that input to bulbar areas that are activated by a series of homologous odors may not be essential for odor discrimination and that deafferentation of the majority of bulbar glomeruli may be primarily without effect on odor quality perception as assessed by the memory test. These outcomes point to a much higher degree of redundancy within the olfactory bulb than that envisioned by current combinatorial or odotopic hypotheses of odor quality coding or, alternatively, to mechanisms of odor coding used in the awake behaving animal that have not yet been elucidated.