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 [Image]     [Image]  Theta Rhythm and the Hippocampus              [Image]

                      Furthermore, because the hippocampus is
                      involved in memory processing, the presence
                      of theta rhythm during REM sleep in that
                      region of the brain might be related to that
                      activity. I suggested that the theta rhythm
                      reflected a neural process whereby
                      information essential to the survival of a
                      speciesãgathered during the dayãwas
                      reprocessed into memory during REM sleep.
 [Image]   Anatomy of [brain drawing]                               [Image]
            the brain
            and cross
           section of In 1974, by recording signals from the
                  the hippocampus of freely moving rats and
          hippocampus rabbits, I found the source from which theta
         show some of rhythm was generated in the hippocampus.
          the regions Together with the neocortex, the hippocampus
          involved in is believed to provide the neural basis for
         dreaming. In memory storage. The hippocampus (from the
                  the Greek word for "seahorse," which it resembles
         hippocampus, in shape) is a sequential structure composed
             incoming of three types of neurons. Information from
          information all sensory and associatonal areas of the
         is processed neocortex converges in a region called the
         sequentially entorhinal cortex; from there it is
               in the transmitted to the three successive neuronal
              dentate populations of the hippocampus. The signal
            gyrus and arrives first at the granule cells of the
          the CA3 and dentate gyrus, then at the CA3 pyramidal
              the CA1 cells (so called because of their triangular
            pyramidal shape) and finally at the pyramidal cells of
            cells. In CA1. After information is processed by this
           subprimate trio of cells, it is retransmitted to the
             species, entorhinal cortex and then back to the
         theta rhythm neocortex.
         is generated
               in the My studies showed that theta rhythm was
              dentate produced in two regions within the
            gyrus and hippocampus: the dentate gyrus and the CA1
              the CA1 neurons. The rhythms in these two areas were
              cells.  synchronous. Subsequently, Susan Mitchell and
                      James B. Ranck, Jr., of the State University
                      of New York Downstate Medical Center
                      identified a third synchronous generator in
                      the entorhinal cortex, and Robert Verdes of
                      Wayne State University discovered the brain
                      stem neurons that control theta rhythm. These
                      neurons transmit signals to the septum (a
                      forebrain structure) that activate theta
                      rhythm in the hippocampus and the entorhinal
                      cortex. Thus, the brain stem activates the
                      hippocampus and the neocortexãthe core memory
                      system of the brain.

                      To determine the relation between theta
                      rhythm and memory, I made a lesion in the rat
                      septum. Rats that had previously learned,
                      using spatial cues, to locate a particular
                      position in a maze were no longer able to do
                      so after their septums were disabled. Without
                      theta rhythm, spatial memory was destroyed.

                      Studies of the cellular changes that bring
                      about memory illustrated the role of theta
                      rhythm. In particular, the discovery in 1973
                      of long-term potentiation (LTP)ãa change in
                      neural behavior that reflects previous
                      activityãshowed the means by which memory
                      might be encoded. Timothy V. P. Bliss and A.
                      R. Gardner-Medwin of the National Institute
                      of Medical Research in London and Terje Lomo
                      of the University of Oslo found changes in
                      nerve cells that had been intensely
                      stimulated with electrical pulses.

                      [Image] Long-Term Memory Storage

[Copyright 1999, Peregrine Publishers, Inc. All Rights Reserved.]