odorconcentration coding in the guineapig piriform cortex by optical imaging of intrinsic signals we demonstrated a possible code for odor concentration in the anterior piriform cortex of the guineapig odorinduced cortical activation which primarily originated in layer ii appeared in a narrow band beneath the rhinal sulcus over the lateral olfactory tract corresponding to the dorsal part of the anterior piriform cortex lower concentrations activated the rostral region of the band whereas higher ones generated caudally spreading activation and the site at which neural activation reached its maximum extent depended upon odor concentration different odors with low concentrations generated distinct but somewhat overlapping patterns in the rostral region of the band the limited extent of cortical activity may be one focal domain for each odor it was hard to judge however that odorspecific domains appeared in the anterior piriform cortex because the strong stimuli induced largely overlapping patterns furthermore the total area activated increased in proportion to concentrations raised to a power of 0509 importantly these imaging results were confirmed with unit recordings which indicated a rostrocaudal gradient in odorsensitivity among cortical neurons our results suggest that the dorsal part of the anterior piriform cortex may be associated with odor concentration therefore in addition to recruitment of more olfactory sensory cells and glomeruli in response to stronger stimuli a rostrocaudal gradient in axonal projections from mitraltufted cells andor in association fibers may play an important role in odorconcentration coding in the anterior piriform cortex