02 Jun Hippocampo-cortical coupling mediates memory consolidation during sleep
Each day we are bombarded with hundreds of pieces of new information. A good night’s sleep improves our ability to remember some of the more important details that run through our brain each day. Researchers at the RIKEN Brain Science Institute in Japan have discovered a brain circuit that governs how certain memories are consolidated during non-REM (deep) sleep. Published in Science, the study shows how experimentally manipulating this circuit during deep sleep can prevent or enhance memory retention in mice.
The ‘two-stage’ theory of memory posits that memory consolidation involves a dialog during sleep between the hippocampus, where traces are initially formed, and the neocortex, where they are stored for long-term retention. However, the causal role of a hippocampo-cortical dialog in memory consolidation has remained speculative.
The hippocampal network is most active during hippocampal sharp-wave ripples (SPW-Rs). The goal of this study was to examine the temporal correlation between SPW-Rs in the hippocampus and cortical delta waves and spindles in the mPFC during unperturbed SWS in rats. Delta waves reflect the down states of the slow oscillation, when cortical neurons stop firing. Consistent with previous reports, delta waves were prevalent in close-temporal proximity to hippocampal SPW-Rs. This suggests that the fine temporal relation between SPW-Rs and delta-spindle sequences is instrumental for communication between the hippocampus and neocortex. In other words, the cortex stops firing at the end phase of a delta wave and the hippocampus quickly fires sharp-waves, indicating a coupling of events.
This fascinating finding led the researchers to hypothesise that this coupling should increase when learning leads to memory consolidation. By measuring the incidence of coupled SPW-Rs and delta-spindle sequences following training on a hippocampus-dependent memory task, the researchers have shown that enhanced hippocampo-cortical coupling co-occurred with memory consolidation, as the occurrence of hippocampal and cortical rhythms selectively increased after complete, but not time-limited training.
The next step was to establish a causal link between increased hippocampo-cortical coupling and memory consolidation. This attempted with a closed-loop stimulation protocol to enhance the temporal coupling between SPW-Rs and delta spindles during SWS. The goal was to potentiate the consolidation of the weak memory traces by reinforcing the hippocampo-cortical oscillatory interactions during SWS following encoding. Coupled stimulations strongly enhanced the temporal correlation between hippocampal and cortical oscillations (above chance; see figure 1 below).
What does this mean? Effectively, dynamically enhancing the coupling between hippocampal SPW-Rs and cortical delta waves and spindles during SWS resulted in the consolidation of a labile memory trace. The results also show that show that long-term stabilisation of memory traces is enhanced by timed-functional interactions between the hippocampus and cortex during sleep. These results provide the first direct evidence for a causal role of a hippocampo-cortical dialog during sleep in memory consolidation, and indicate that the underlying mechanism involves a fine-tuned coordination between sharp wave-ripples, delta waves and spindles.
doi:10.1038/nn.4304
Figure 1: Discrimination index for the recall phase, computed during the first 2 min of exploration. Memory recall was observed only following coupled stimulation.
Source: http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.4304.html