Sleep – More Local and Complex than Previously Thought?

pocampal sharp wave ripples (hSWR) a replay of the memory trace occurs, which is then “transferred” into the cortex via sleep spindles and perhaps slow oscillations. At this point it is important to note that for one most evidence regarding this theory is of correlative nature and therefore cannot conclude causality. Up until now only two studies in rodents could show more than correlational evidence by actively disrupting SWR and therefore causing decreased memory consolidation (Girardeau et al., 2009; Ego-Stengel and Wilson, 2010). A more recent study in humans showed enhanced memory performance by stimulation of the entorhinal region during memory encoding (Suthana et al., 2012). Perhaps future studies could combine these two approaches. Another important caveat is that replay coinciding with hSWR is seen in wake-rest as well as sleep (Axmacher et al., 2008). In Axmachers study the highest incidence of ripples occurred during rest and the ripples during rest actually correlated with memory performance. However, studies in rodents reported that cortical replay of memory related neural-patterns only occurred during sleep-SWR (Peyrache et al., 2010). Perhaps hippocampal replay and thus consolidation occurs during wake and sleep-SWR, but this replay can only be coordinated with cortical replay and thus cortical consolidation during sleep, which provides the required network properties. Connectivity analyses in human fMRI studies could show general decreased cortical– cortical connectivity during slow wave sleep, but increased cortical–cortical connectivity during stage 2 light sleep (Spoormaker et al., 2010). And even more compelling, the same group could show increased hippocampalcortical connectivity during sleep spindles (Andrade et al., 2011). Surprisingly a very recent study in rodents presented that during sleep spindles the cortex seems to be functionally deafferented from its Sleep – more local and complex than previously thought?