Subjects were trained to identify by assigned number common substances presented as vapor phase stimuli via an orthonasal or a retronasal route. Following training, odorant identification learning was evaluated by measuring ability to correctly identify to a criterion. Those who met the criterion were then tested first with the stimuli presented to the nares that differed in location from the nares used in training, and second to the nares that corresponded in location to the nares used in

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... ing. It was found that, under conditions of natural retronasal breathing, orthonasally trained subjects made correct identifications on -80% of the trials upon retronasal testing, but for the following orthonasal testing identifications were significantly more frequent, approaching 100% correct. After subsequent retronasal training, the same subjects' orthonasal identifications remained significantly higher, although identifications improved to -92% correct on retronasal trials. Other subjects were instructed in a breathing technique designed to enhance retronasal stimulation. After orthonasal training, retronasal testing of these subjects still gave significantly fewer correct identifications than orthonasal testing, notwithstanding the modified retronasal breathing, but after subsequent retronasal training correct identifications by these subjects no longer differed significantly between orthonasal and retronasal testing. Efficacy of modified retronasal breathing was confirmed in two subsequent experiments. The observed substantial positive transfers between retronasal and orthonasal odorant identification training and testing loci demonstrate that these odorant pathways do not subserve completely independent olfactory systems, while the less accurate identifications via the retronasal route, unless instruction in retronasal breathing was given, suggest a difference in the efficiency with which odorants are normally delivered to the olfactory mucosa. According to Cain (1987) , there exist 'two modalities that problem; numerous unresolved questions remain. An "stand guard" over what we eat'. These are taste and intriguing aspect of olfaction is the existence of two distinct olfaction. Cain asserts that smell presents the more complex pathways through which stimuli can arrive at the olfactory