Self and Memory 227“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityand his colleagues. This finding provides a mechanism for episodic
memory, which, in essence, stores experiences in a space–time context.
When live rodents are allowed to freely behave in a new environment,
certain pyramidal cells in the hippocampus become active, as detected
by intra-cerebral electrode recording. A cell will fire only when the ani-
mal is in a particular location and position, and many cells are recruited
to form a cognitive map of the entire space. For each environmental set-
ting, a specific cognitive map is formed that can later be used for recall.^22
What is interesting is that the storage of these space memories requires
LTP in the neurons.^23
Further studies have identified at least four types of pyramidal cells
in the rodent hippocampal formation responsible for space memory: the
place cells which fire when the animal is situated in a unique location;
the grid cells which fire periodically in a grid-like pattern as the ani-
mal traverses at a given speed; the head direction cells which are active
when the head is in a specific orientation regardless of the coordinate;
and the border cells which react when a border is in the proximity.^24
Whereas place cells are located in the hippocampus proper, the other
three types (grid, head direction, and border cells) fire most intensely
in the entorhinal cortex, the gateway to the hippocampus.^25 Using the
highly specific optogenetic study, there is evidence that signals from
the three entorhinal cell types converge on the hippocampal pyrami-
dal place cells, which integrate the input to generate a reliable place
field for spatial memory. The speed sensitivity of grid cells further adds
a time element to the place field. Thus, place cells have access to both
self-motion and landmark-based information during navigation.^26 The
importance of place cells in the formation of abstract memory has also
been suggested.^27
More direct evidence for the recording of time in the hippocam-
pus has recently been obtained. When rats were trained in a two-part
task with a 10-second gap in between, electrical recordings showed that
many hippocampal neurons were sequentially active during this “silent”
period, suggesting the encoding of time interval. Thus, the presence of