Odor- and context-dependent modulation of mitral cell activity in behaving rats

The projections and odor responses of mammalian olfactory receptor neurons, as well as the physiology of the bulb's principal neurons—the mitral cells (MCs)—are known from studies in slices and anesthetized animals. In behaving rats trained to discriminate between two odors associated with different reinforcers, we examined MC responses following alternated odor–reinforcer pairings. Whereas only 11% of the recorded MCs showed changes in odor-selective firing rate during the odor-sampling phase, 94% of MCs modulated activity during specific behaviors surrounding odor sampling. These cell- and odor-selective responses were not primary sensory responses; rather, they depended (reversibly) on the predictive value of each odor. MC activity thus depends critically on efferent influences linked to the animal's experience and behavior.