Editing Hippocampus (section)

=== Sharp waves ===
{{Main|Sharp waves and ripples}}

During sleep or during resting, when an animal is not engaged with its surroundings, the hippocampal EEG shows a pattern of irregular slow waves, somewhat larger in amplitude than theta waves. This pattern is occasionally interrupted by large surges called ''sharp waves''.<ref>{{cite journal | vauthors = Buzsáki G | title = Hippocampal sharp waves: their origin and significance | journal = Brain Research | volume = 398 | issue = 2 | pages = 242–252 | date = November 1986 | pmid = 3026567 | doi = 10.1016/0006-8993(86)91483-6 | ref = refBuzsaki1986 | s2cid = 37242634 }}</ref> These events are associated with bursts of spike activity lasting 50 to 100 milliseconds in pyramidal cells of CA3 and CA1. They are also associated with short-lived high-frequency EEG oscillations called "ripples", with frequencies in the range 150 to 200&nbsp;Hz in rats, and together they are known as [[sharp waves and ripples]]. Sharp waves are most frequent during sleep when they occur at an average rate of around 1 per second (in rats) but in a very irregular temporal pattern. Sharp waves are less frequent during inactive waking states and are usually smaller. Sharp waves have also been observed in humans and monkeys. In macaques, sharp waves are robust but do not occur as frequently as in rats.<ref name="Skaggs2007" />

Sharp waves appear to be associated with memory.<ref name="Wilson">{{cite journal | vauthors = Wilson MA, McNaughton BL | title = Reactivation of hippocampal ensemble memories during sleep | journal = Science | volume = 265 | issue = 5172 | pages = 676–679 | date = July 1994 | pmid = 8036517 | doi = 10.1126/science.8036517 | ref = refWilson1994 | s2cid = 890257 | bibcode = 1994Sci...265..676W }}</ref> Numerous later studies, have reported that when hippocampal place cells have overlapping spatial firing fields (and therefore often fire in near-simultaneity), they tend to show correlated activity during sleep following the behavioral session. This enhancement of correlation, commonly known as ''reactivation'', has been found to occur mainly during sharp waves.<ref>{{cite journal | vauthors = Jackson JC, Johnson A, Redish AD | title = Hippocampal sharp waves and reactivation during awake states depend on repeated sequential experience | journal = The Journal of Neuroscience | volume = 26 | issue = 48 | pages = 12415–12426 | date = November 2006 | pmid = 17135403 | pmc = 6674885 | doi = 10.1523/JNEUROSCI.4118-06.2006 | ref = refJackson2006 }}</ref> It has been proposed that sharp waves are, in fact, reactivations of neural activity patterns that were memorized during behavior, driven by strengthening of synaptic connections within the hippocampus.<ref>{{cite journal | vauthors = Sutherland GR, McNaughton B | title = Memory trace reactivation in hippocampal and neocortical neuronal ensembles | journal = Current Opinion in Neurobiology | volume = 10 | issue = 2 | pages = 180–186 | date = April 2000 | pmid = 10753801 | doi = 10.1016/S0959-4388(00)00079-9 | ref = refSutherland2000 | s2cid = 146539 | authorlink1 = Grant Robert Sutherland }}</ref> This idea forms a key component of the "two-stage memory" theory,<ref name="Two-stage">{{cite journal | vauthors = Buzsáki G | title = Two-stage model of memory trace formation: a role for "noisy" brain states | journal = Neuroscience | volume = 31 | issue = 3 | pages = 551–570 | date = January 1989 | pmid = 2687720 | doi = 10.1016/0306-4522(89)90423-5 | s2cid = 23957660 }}</ref> advocated by Buzsáki and others, which proposes that memories are stored within the hippocampus during behavior and then later transferred to the [[neocortex]] during sleep. Sharp waves in [[Hebbian theory]] are seen as persistently repeated stimulations by presynaptic cells, of postsynaptic cells that are suggested to drive synaptic changes in the cortical targets of hippocampal output pathways.<ref name="Two-stage"/> Suppression of sharp waves and ripples in sleep or during immobility can interfere with memories expressed at the level of the behavior,<ref name="suppression of hippocampal ripples">{{cite journal | vauthors = Girardeau G, Benchenane K, Wiener SI, Buzsáki G, Zugaro MB | title = Selective suppression of hippocampal ripples impairs spatial memory | journal = Nature Neuroscience | volume = 12 | issue = 10 | pages = 1222–1223 | date = October 2009 | pmid = 19749750 | doi = 10.1038/nn.2384 | s2cid = 23637142 }}</ref><ref name="impair spatial learning study">{{cite journal | vauthors = Ego-Stengel V, Wilson MA | title = Disruption of ripple-associated hippocampal activity during rest impairs spatial learning in the rat | journal = Hippocampus | volume = 20 | issue = 1 | pages = 1–10 | date = January 2010 | pmid = 19816984 | pmc = 2801761 | doi = 10.1002/hipo.20707 }}</ref> nonetheless, the newly formed CA1 place cell code can re-emerge even after a sleep with abolished sharp waves and ripples, in spatially non-demanding tasks.<ref name="pmid27760158">{{cite journal | vauthors = Kovács KA, O'Neill J, Schoenenberger P, Penttonen M, Ranguel Guerrero DK, Csicsvari J | title = Optogenetically Blocking Sharp Wave Ripple Events in Sleep Does Not Interfere with the Formation of Stable Spatial Representation in the CA1 Area of the Hippocampus | journal = PLOS ONE | volume = 11 | issue = 10 | pages = e0164675 | date = 19 Nov 2016 | pmid = 27760158 | pmc = 5070819 | doi = 10.1371/journal.pone.0164675 | doi-access = free | bibcode = 2016PLoSO..1164675K }}</ref>