Co-Occurrence of Sleep Oscillations in Hippocampus and Prefrontal Cortex in a Memory Consolidation Task

Abstract

The co-occurrence of hippocampal sharp-wave ripple oscillations (100-250 Hz) and cortical spindle oscillations (12-15 Hz) is thought to be a key mechanism for memory consolidation. The present study uses simultaneous recordings from dorsal hippocampal area CA1 and from the prelimbic area of the medial prefrontal cortex (mPFC) in adult male Brown Norway rats to test the hypothesis that learning-related increases in oscillation density enhance the cross-correlation between ripples and spindles. We first confirm that ripple and spindle density increase during sleep after learning in a spatial navigation task on an open field maze. Same-day recordings of learning and non-learning tasks show no significant enhancement of ripple-spindle cross-correlation during post-task sleep. The percentages of spindles with co-occurring ripples and of ripples occurring during or near spindles do not vary significantly with task. However, ripples appear to occur before, during, and after spindles at rates greater than chance. This suggests that the statistics of ripple and spindle co-occurrence may be fixed, with increased oscillation densities potentially enhancing single unit interactions between regions.