Hypothesis (first porposed by Buzsaki, 1989): Memory consolidation has two steps. In the initial encoding, the hippocampus integrates spatial info afferent from the neocortex, favored by “exploratory” theta rhythms (~ 8 Hz). Next, during consolidation, recurrent excitation transfers the info back to the neocortex, resulting in hippocampus-independent long-term memory. The consolidation phase relies on sharp wave-ripple events (i.e., population bursts) in CA1 during slow wave sleep.
Evidence: First, enhancing the quality of slow wave sleep improves performance on hippocampus-dependent tasks after at least one night of sleep since learning. Also, after extensive learning, sharp-wave ripple events during sleep increase. And now, Ego-Stengel and Wilson have shown that disrupting sharp-wave ripple events disrupts learning. Rats were alternatively trained on two four-armed radial mazes, one of which was always followed by microstimulation of CA1 during sharp-wave ripple bursts during sleep, and one of which was a control learning maze. The number of errors per trial was significantly higher (p < 0.03, n = 12) for the CA1 microstimulation maze than for the control maze. This indicates that spatial learning is impaired when rats have fewer sharp-wave ripple bursts in the hippocampus during sleep. The learning curve for the microstimulation maze wasn’t completely flat, but it was right shifted as compared to the control curve. One interesting facet of their experiment is that most of the stimulations were during non-REM sleep (84%) or while the rats were awake (15%), and less than 1% were during REM sleep. Although REM sleep is often associated with “mental” rest, spatial memories at least do not appear to be dependent upon neural activity during REM.
Buzsaki G. 1989 Two-stage model of memory trace formation: A role for ‘‘noisy’’ brain states. Neuroscience 31:551–570.
Ego-Stengel V, Wilson MA. 2009 Disruption of ripple-associated hippocampal activity during rest. Hippocampus, Early View. DOI 10.1002/hipo.20707