Figure 4. Intrinsic and Extrinsic Hippocampal Reactivation in Rodents.

A. Patterns of hippocampal place cell firing during sleep before learning (Pre-Learning Sleep), during wakeful learning (Learning), and after learning during sleep (Post-Learning Sleep). Top row: Effective connectivity matrix of 42 random hippocampal place cells from a single rat for each of the above three experimental phases, illustrating significantly increased effective connectivity during sleep after, relative to before, learning. Lines demonstrate significant (adjusted p<0.05) cross-correlation between cells (dots), with color of line reflecting strength of correlation (red – high; blue – low). Bottom row: Mean cross-correlation between hippocampal place cell pairs during each of the three experimental phases described above. Black bars indicate cell pairs that did not have overlapping (correlating) place fields during learning phase, while white bars indicate cell pairs that did have overlapping place fields during the learning phase that selectively express increased connectivity in sleep after learning. Adapted from Wilson and McNaughton, 1994. [44] B. Multiunit firing sequences during NREM sleep (right panels) demonstrating recapitulation of firing patters observed during prior learning while awake (left panels) in both the cortex (upper row) and hippocampus (lower row) cells. Note time scales, which reflect a compression of firing patterns in sleep, relative to learning during wake. Adapted from Ji, et al., 2007. [147] C. Extrinsic cued reactivation of previously learning memories during NREM, biasing NREM replay. During wake, rodents were trained to go to the left or right side of a track to seek a reward, dependent upon a specific paired auditory cue. During NREM sleep (left panel), a proportion of replay events could be manipulated to show a left or right bias as a function of which auditory cue was presented. Mean bias (right panel) of replay activity (left or right of center of the track) as a function of the specific auditory cue presented during sleep. Adapted from Bendor, et al. 2012. [82]