ENTROPY, SURPRISE AND THE HIPPOCAMPAL LONG AXIS

The hippocampus exhibits functional specialization along its long axis; however, it is not clear how this determines changes in activity and connectivity in relation to predictability and expectation violation. Methods: twenty-six patients with intracranial electrodes in the hippocampus (ten with two electrodes along its long axis) performed a visual task where stimulus statistics translated into trialwise measures of entropy and surprise. Results: The head-body boundary (MNIy -20 to -25) exhibited pronounced gamma suppression with pre-stimulus entropy, while post-stimulus surprise elicited gamma response in anterior and posterior regions, though greater and longer-lasting in posterior. Pre-stimulus entropy was associated to increased coherence between anterior and posterior electrodes in the ripple band (80 Hz), and to lagged co-occurrence of sharp-wave ripples (SWRs) in both electrodes, being significant for the 'posterior precedes anterior' direction. Permutation tests indicated that posterior-to-anterior co-occurrent SWRs were associated to a significant phase shift and time delay for frequencies 2, 3 and 6 Hz in the same direction, suggestive of a travelling wave. Within 500 ms from stimulus onset, surprise was associated to reduced anterior/posterior coherence in frequencies 2 - 20 Hz; partial directed coherence was reduced in the anterior to posterior direction, also with decreased PAC (anterior beta / posterior gamma coupling). Dynamic causal modelling indicated that surprise de-coupled anterior and posterior regions. Conclusions: Entropy and surprise drove modulatory changes along the long axis. Pre-stimulus entropy associated to travelling SWRs from posterior to anterior hippocampus, whereas surprise induced de-coupling of the posterior hippocampus from anterior low-frequency activity.