Information coding in the rodent prefrontal cortex. I. Single-neuron activity in orbitofrontal cortex compared with that in pyriform cortex

1. Extracellular spike activity was recorded from 1,942 single neurons in orbitofrontal cortex (OF) and 591 single neurons in pyriform cortex (PIR) over multiple sessions in rats performing an eight-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 in 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 eight 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 in 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 nonrewarded odors. Firing during trial initiation and odor sampling reflected whether or not the odor in the current trial had been predicted by the odor in the preceding trial. In addition, firing during odor sampling reflected the expectation of reward in 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 in 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.(ABSTRACT TRUNCATED AT 400 WORDS)