228 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”“time cells” as well as “place cells” strengthens the role of hippocampus
in episodic memory.^28
It is important to ask whether the hippocampus is involved in
recall other than encoding memory. Indeed, in a mouse study, it was
demonstrated that stimulating a population of neurons in the dentate
gyrus of the hippocampus is sufficient to trigger the recall of a fear
experience, suggesting the presence of memory engram (coding) in the
hippocampus.^29
As a rule, adult neurons do not divide. One exception occurs in the
dentate gyrus of the hippocampus (see Figs. 10.2 and 10.3), which con-
tinues to produce new neurons in the adult life. The ability of the hippo-
campus to undergo neurogenesis has been related to its ability to process
memory. The dentate gyrus is the first relay station in the hippocampus
for incoming information from the cerebral cortex. It is here where the
information is split up and distributed among many cells in other parts
of the hippocampus. The packaged information is then passed back to
the cerebral cortex for long-term storage.30 The newborn neurons in
the dentate gyrus are more responsive than the older neurons to incom-
ing excitatory input from the entorhinal cortex, providing evidence that
neurogenesis is important for processing recent memory.^31 On the other
hand, it has been suggested that, since the new neurons will have to inte-
grate into the existing hippocampal circuits, too much neurogenesis may
interfere with old memories already in place. Hence, a proper memory
function depends on an optimum degree of adult neurogenesis.^32
Most of the studies on the hippocampus were carried out in
rodents. The question is whether some of the evidence is also found
in human subjects. The answer is “yes.” It was demonstrated, for
example, that electrical stimulation of the human entorhinal cortex
enhances the encoding of space memory.^33 Functional imaging of
the human brain provided further evidence for the presence of grid
cells.^34 During active retrieval of space-relevant memory, neurons in
the human hippocampus that were originally encoded for the event
are reactivated.^35 Thus, the storage of life events in a spatiotemporal