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& Buzsáki, 2006) and are updated by entorhinal cortex-mediated environmental signals
(Hafting
et al.
, 2005; Zugaro
et al.
, 2005). The sequence in which several temporally-linked
cell assemblies, each representing spatial fields, will be recalled is triggered by the
environmental input of the previous locations by way of the entorhinal cortex (Frank
et al.
,
2000; Hafting
et al.
, 2005). The temporal features of spike timing-dependent plasticity (Levy
& Steward, 1979; Magee & Johnston, 1997; Markram
et al.
, 1997; Bi & Poo, 1998) ensure
that the recalling of learned associations will be in the same sequence as it has been encoded
during the learning experience (Mehta
et al.
, 1997). A consequence of the oscillatory
temporal organization of cell assemblies is the theta phase precession of spikes of single place
cells. Dynamic plasticity processes during theta cycles link continuously experience-
dependent hippocampal assemblies in unidirectional sequence that represents spatial
representations in time (Hasselmo
et al.
, 2002; Zugaro
et al.
, 2005; Dragoi & Buzsáki, 2006;
Johnson & Redish, 2007). Synaptic strengths across assemblies, representing different spatial
representations and discharging in different gamma cycles, can determine both their time
order within the theta cycle and the distances between the respective place fields (Lisman &
Idiart, 1995; Harris
et al.
, 2003). The phase shifts of cells assemblies in hippocampus indicate
that hippocampal neurons do not just represent a highly processed image of the sensory
environment, but generate sequence information that integrates subsequent episodes. The
hippocampal formation can also encode relative spatial location, without reference to sensoy
external cues, by the integration of linear and angular self-motion (path integration). This
issue will be discussed next.
7.
P
ATH
I
NTEGRATION
Place cells use environmentally-stable sensory stimuli as a directional reference to
provide a rodent's orientation in space (Jeffery
et al.
, 1997; Goodridge
et al.
, 1998). Beside
external sensory cues, and especially in the cases when these cues are unstable, the
hippocampal network relies on an internal direction sense or idiothetic (body-, or head-
motion) stimuli (Quirk
et al.
, 1992; Suzuki
et al.
, 1997; Young
et al.
, 1997; Xiang & Brown,
1998; Jeffery, 1999; Jeffery & O'Keefe, 1999; Mizumori
et al.
, 1999).
The head direction system is composed of neurons whose firing rate increases only
whenever the animal head is pointed in a specific direction (Taube
et al.
, 1990). This type of
cell, is found in different structures of the parahipocampal complex as well as in other
subcortical structures (Taube, 1995a; Stackman & Taube, 1998; Taube, 1998). The firing of
these neurons conveys information about where the animal's head is pointing. They seem to
use environmental cues to calibrate their directional firing (Goodridge
et al.
, 1998) and they
depend on vestibular input without which their firing disappears. Head-direction tuned
neurons are present in the presubiculum and parasubiculum, regions that encode location and
direction (Cacucci
et al.
, 2004). This region could synthesise spatial information and direction
information, forming the bridge between both systems. Subiculum (Hartley
et al.
, 2000, our
own unpublished observations), hippocampal CA1 (Leutgeb
et al.
, 2000) and entorhinal
cortex (Hafting
et al.
, 2005) are also areas proposed to integrate place as well as head
direction. Finally the sequence of idiothetic episodes, even in the absence of external sensory
