Understanding hippocampal network dynamics by studying sharp-wave ripples across the proximal-distal axis of area CA1

Abstract

The area CA1 in the hippocampus is known to have qualitatively different inputs between the proximal and distal regions. Differences corresponding to these differing input patterns have been seen in the functional aspects of area CA1 across the proximal-distal axis. There might be differences in the networks that connect to the proximal and distal regions of area CA1 that encode and consolidate these memories. Sharp-wave ripples (SWR) in the area CA1 of the hippocampus reflect the activity of neurons at a network level. To study possible differences in the activity of the networks that connect to area CA1 across its proximal-distal axis, we recorded and analysed SWR across the proximal-distal axis of area CA1. We found that when SWR occur in one region of area CA1, there was a reduction in the likelihood of co-occurrence and in the relative amplitude of ripples as a function of distance from that region. Interestingly, we found that the difference in ripple peak time did not follow a similar trend; when SWR occurred in any region, the corresponding ripples in the distal region seemed to occur earlier while those in the proximal region seemed to occur later. Although our result regarding difference in peak time is ambiguous, our results on co-occurrence of SWR and relative ripple amplitudes across the proximal-distal axis imply that SWR initiated in one region of area CA1 progress to the other regions, and that the extent to which they progress depends on the distance from the region of initiation. These results are indicative of the possible differences in the connectivity and activity of the underlying networks that connect to area CA1.