INFORMATION CODING IN THE RODENT PREFRONTAL CORTEX: I. SINGLE NEURON ACTIVITY IN ORBITOFRONTAL CORTEX. COMPARED WITH THAT IN PIRIFORM CORTEX Schoenbaum, Geoffrey and Howard Eichenbaum. Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3270, Center for Behavioral Neuroscience, State University of New York at Stony Brook, Stony Brook, NY 11794-2575.
APStracts 2:0094N, 1995.
SUMMARY AND CONCLUSIONS
1) Extracellular spike activity was recorded from 1942 single neurons in orbitofrontal (OF) and 591 single neurons in piriform (PIR) cortex over multiple sessions in rats performing an 8 odor discrimination task in which the stimulus sequence contained predictable associations between particular odor pairs. Neural firing patterns were examined in relation to task events on the current trial and variables associated with current sensory processing, events of recent past trials, and long-term associations involving the odor cues. 2) Overall, 34% of single neurons in OF and 30% of single neurons in PIR fired selectively during one or more salient trial events including trial initiation, odor-sampling, performance of the discriminative response, and water-consumption. The activity of other cells recorded in OF (13%) and PIR (10%) was suppressed for the duration of each trial. Although the proportion of some cell types differed between the two areas, the firing patterns of OF and PIR neurons were qualitatively indistinguishable. 3) Firing during odor- sampling and the discriminative response was influenced by the identity of the current odor. Some cells fired selectively to a single odor, but most cells were coarsely tuned such that they fired to several of the 8 odors to differing degrees consistent with previous reports. Considerable odor coding was observed in both OF and PIR. 4) Firing during trial initiation and odor- sampling was also influenced by the identity and reward association of the odor presented on the immediately preceding trial. The influence of past odor identity and valence was observed in both OF and PIR. 5) Firing during trial events was also influenced by the acquired associations between odors and their assigned reward contingencies and between pairs of odors involved in predictive relationships. The reward valence of the current odor significantly influenced firing during odor-sampling and the discriminative response; some cells responded preferentially to rewarded odors and others to non-rewarded odors. Firing during trial initiation and odor-sampling reflected whether or not the odor on the current trial had been predicted by the odor on the preceding trial. In addition, firing during odor-sampling reflected the expectation of reward on the following trial that could be inferred from the predictable associations between odors. Each of these properties was observed in both OF and PIR. 6) The findings on OF were consistent with the view that prefrontal subdivisions mediate the temporal organization of complex behaviors within specific informational domains. OF appears to be concerned with the specific domain of olfaction. Within this domain, neural activity in OF reflects ongoing sensory experience, recent past events, and acquired information bearing on future events. The patterns of neural activity demonstrated in OF closely parallel those found in prefrontal areas of primates also thought to play a critical role in the temporal organization of behavior. 7) Surprisingly, the activity patterns of neurons in PIR were virtually identical to those observed in OF. At a minimum, these findings indicate that piriform cortex performs functions beyond the role of odor recognition traditionally ascribed to it. The present findings may reflect the strong reciprocal connectivity between OF and PIR and other structures involved in olfaction and behavior. These observations, coupled with similar findings from studies of visuo-spatial processing in primate dorsolateral prefrontal and posterior parietal cortex, suggest the existence of distinct prefrontal-cortical systems that process the temporal stream of events in modality specific informational domains. 

Received 24 October 1994; accepted in final form 24 March 1995.
APS Manuscript Number J559-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.