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Have you ever wondered how dreams are stored, organized or played out during rest?  We know that the visual cortex is activated with visual imagery.  The auditory cortex is activated with sound.  It makes sense that reliving the experience activates the same cortex that brought vision and sound to the seat of memory formation – the hippocampus.

Is it possible that the hippocampus turns from a receiver of imagery to a sender of imagery?  Is this how memory is relived? How can we ever know the answer to this question?  The answer begins with well-formed questions such as, “What differences can be seen in brain activity during sleep and wakefulness?”

We have known for a long time that there are differences in the frequency and shapes of brainwaves produced in sleep and wakefulness.  By example, deep sleep is linked to slow brain wave activity.  Delta brain activity occurs between one half cycle per second and 4 cycles per second.  Delta waves can occur during sleep and when we are awake.  Just above delta waves are theta waves.   Theta waves are linked to intuition, as well as mental models of various items or relationships.  Theta waves range from four to eight cycles per second.  Infraslow signals are less than one tenth of a cycle per second.  As we transition from wakefulness to sleep there is a time of theta, followed slower delta and slower brain activity.

Delta (4–8 Hz) -band activity

Theta (0.5–4 Hz) range

Infraslow activity (<0.1 Hz)

One theory of brain activity suggests that low frequency activity is linked to a receiver requesting information.  Higher frequency activity is linked to sending information.  This theory sets the stage for research and a paper, which was presented in the Publications of the National Academy of Sciences in 2016.  Basic research showed that the human hippocampus produces relatively slow activity during wakefulness, and faster activity during sleep.  In contrast, several cortical regions, which are associated with information gathering and perception were observed doing just the opposite.  The cortical regions produced relatively faster action during wakefulness, and slower activity during sleep.   This pattern of activity was taken to mean that the hippocampus was a receiver during wakefulness and a sender during sleep.  The cortical regions, associated with sensation, perception, and consideration, were seen as organizing and sending information to the hippocampus during periods of wakefulness and receiving information during periods of sleep.

This is a very interesting.  Here we see functional differences in brain activity along with a plausible explanation of what the differences signify.  If this theory holds true then we are looking into one aspect of how the human brain is organized to process sensation, perception, experience, emotion, and thought.



Human cortical–hippocampal dialogue in wake and slow-wave sleep (2016).  Mitraa, A, Snydera,A.Z., Hackerc, C.D.,  Pahwac, M, Tagliazucchi, E., Laufse,F, Leuthardtg, E.C. and Raichlea,M.E.  Proc Natl Acad Sci U S A. 2016 Nov 1;113(44):E6868-E6876.