Hippocampal ensembles represent sequential relationships among an extended sequence of nonspatial events

• Published in: Nature communications 2022-02, Vol.13 (1), p.787-787, Article 787
• Main Authors: Shahbaba, Babak, Li, Lingge, Agostinelli, Forest, Saraf, Mansi, Cooper, Keiland W., Haghverdian, Derenik, Elias, Gabriel A., Baldi, Pierre, Fortin, Norbert J.
• Format: Article
• Language: English
• Subjects: 631/378/116/2394; 631/378/1595/1554; Acoustic Stimulation - methods; Animals; Auditory Perception; Behavior; Coding; Electrodes, Implanted; Hippocampus; Hippocampus - physiopathology; Humanities and Social Sciences; Information processing; Learning algorithms; Machine Learning; Male; Memory; Models, Animal; multidisciplinary; Nerve Net - physiology; Neural coding; Neurobiology; Neurons; Neurosciences; Odorants; Olfactory Perception; Rats; Science; Science (multidisciplinary); Statistical methods; Stereotaxic Techniques; Temporal variations; Theta rhythms; Time Factors
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-022-28057-6

The hippocampus is critical to the temporal organization of our experiences. Although this fundamental capacity is conserved across modalities and species, its underlying neuronal mechanisms remain unclear. Here we recorded hippocampal activity as rats remembered an extended sequence of nonspatial events unfolding over several seconds, as in daily life episodes in humans. We then developed statistical machine learning methods to analyze the ensemble activity and discovered forms of sequential organization and coding important for order memory judgments. Specifically, we found that hippocampal ensembles provide significant temporal coding throughout nonspatial event sequences, differentiate distinct types of task-critical information sequentially within events, and exhibit theta-associated reactivation of the sequential relationships among events. We also demonstrate that nonspatial event representations are sequentially organized within individual theta cycles and precess across successive cycles. These findings suggest a fundamental function of the hippocampal network is to encode, preserve, and predict the sequential order of experiences. It remains unclear how hippocampal activity supports the temporal organization of our experiences. In this paper, the authors recorded from rats performing an odor sequence task and show that hippocampal ensembles represent the sequential relations among nonspatial events at different timescales.