¹Dept of Psychology, Boston University, Boston MA  02215
²Dept of Psychology, Johns Hopkins University, Baltimore, MD  21218

Learning-mediated neuronal plasticity in adult neocortex has now been shown in a variety of paradigms in the visual, auditory, somatosensory., and recently in the olfactory system.  Evidence from anatomical, electrophysiological, and lesion studies point to the orbitofrontal cortex as a higher order olfactory area.  Neurons in this area respond differentially to odors in an olfactory discrimination task.  Moreover, the odor responses of these cells change within a session as rats learn new odor discrimination problems.  In this study, we examined whether the changes in odor responses were limited to situations in which the rats were still learning the task, or whether plasticity continued after learning had taken place.  Rats were tested on two different versions of a go/no-go odor discrimination task.  In the first version, eight odors were presented, four of which were rewarded with water while the other four were not rewarded.  In a second version, two or four odors were presented, one or two of which was not rewarded.  The two tasks also differed in where the water reward was delivered and how the rat needed to respond.  We examined the firing of cells in orbitofrontal cortex of the rat after the tasks were well-learned (performance > 85%), and compared the firing rates across odors and across successive blocks of trials within single testing sessions.  In both versions of the odor discriminatioon task, 10-15% of the neurons showed a significant interaction between odor and blocks of trials.  That is, these cells' firing profile across odors changed over the duration of the session, event though the animal consistently performed at the same high level.  While fewer cells show plastic changes after learning than during learning, these results suggest that plasticity is not limited to the period during which the animals initially learn the task.  Thus, reorganization of the neuronal ensembles encoding odors and reward values continues even after learning has taken place.  Supported by NIMH, NIA.