Neuronal dynamics supporting formation and recombination of cross-modal olfactory-tactile association in the rat hippocampal formation.

Abstract

The present study is aimed at describing some aspects of the neural dynamics supporting discrimination of olfactory-tactile paired-associated stimuli during acquisition of new pairs and during recombination of previously learned pairs in the rat. To solve the task, animals have to identify one odor-texture (OT) combination associated with a food reward among three cups with overlapping elements. Previous experiments demonstrated that the lateral entorhinal cortex (LEC) is involved in the processes underlying OT acquisition, whereas the dorsal hippocampus (DH) is selectively involved in the recombination processes. In the present study, local field potentials were recorded form the anterior piriform cortex (aPC), LEC, and DH in freely moving rats performing these tasks. Signal analysis focused on theta (5-12 Hz)- and beta-band (15-40 Hz) oscillatory activities in terms of both amplitude and synchrony. The results show that cue sampling was associated with a significant increase in the beta-band activity during the choice period in both the aPC and the LEC, and is modulated by level of expertise and the animal's decision. In addition, this increase was significantly higher during the recombination compared with the acquisition of the OT task, specifically when animals had to neglect the odor previously associated with the reward. Finally, a significant decrease in coherence in the theta band between LEC and DH was observed in the recombination but not in the acquisition task. These data point to specific neural signatures of simple and complex cross-modal sensory processing in the LEC-DH complex. NEW & NOTEWORTHY This study is the first to describe electrophysiological correlates of cross-modal olfactory-tactile integration in rats. Recordings were sought from the lateral entorhinal cortex and the dorsal hippocampus because previous studies have shown their role in the formation and in the recombination of previously learned associations. We identified specific oscillatory-evoked neural responses in these structures in the theta and beta bands, which characterize acquisition and recombination of cross-modal olfactory-tactile pairs.