Biology Reference
In-Depth Information
input is believed to be integrated within the hippocampus itself (Robertson
et al.
, 1998;
Mizumori
et al.
, 1999).
An interaction between the head direction system and location information system is
believed to be necessary for efficient spatial navigation. The head direction system would
facilitate information to place cells to set their firing in relation to distal external cues,
facilitating in this way efficient navigation under specific conditions. This interaction is
clearly reflected in all proposed models of spatial navigation (McNaughton
et al.
, 1996;
Redish & Touretzky, 1997; Sharp, 1999; McNaughton
et al.
, 2006). The external sensory
cues are necessary to initialize this idiothetic representation of space (Quirk
et al.
, 1990;
Markus
et al.
, 1994). Additionally it is demonstrated that motor input (Foster
et al.
, 1989;
Bassett
et al.
, 2005) serves as another source for the internal reference to the spatial
orientation.
Several models have proposed the subicular region as a key structure that
integrates movement, place information and direction (McNaughton
et al.
, 1996; Sharp, 1999;
O'Mara, 2005; Barry
et al.
, 2006) (Fig 8). Subicular units seem to code for these three
elements (Sharp and Green, 1994). Similarly the hypothetical 'boundary vector cells' in
subiculum are believed to integrate different sources of spatial orientation in allocentric
system that controls the development of hippocampal place fields (Hartley
et al.
, 2000; Barry
et al.
, 2006). Error-mediated repeated interaction between idiothetic representation and place
fields is proposed to stabilize the path integration system, preventing place cells firing drift
(Knierim
et al.
, 1995).
Figure 8. Path integration models. A. Diagram of a model proposed to explain hippocampal place cell
firing properties. The hippocampal place cells are assumed to be linked through excitatory synapses to
form a two-dimensional attractor surface. Each spatial configuration becomes attached to the stimuli in
the environment it represents through Hebbian mechanisms. Path integration is accomplished through
synaptic alterations of the input that receives information about place, directional heading, and
movement. The entorhinal cortex integrates sensory information from different brain areas interacting
with the internal map in the hippocampus which receives directional and motor information from the
subiculum. In this way, the subiculum is seen as the bridge between different subsystems (Adapted
from (McNaughton
et al.
, 1996) and (Sharp, 1999)). B. In the second path integration model, the
entorhinal cortex and the subiculum are proposed as the anatomical bases of the universal spatial map.
The subiculum integrates directional and motor input which is used by the system to implement path
integration extrapolating similar spatial representation across different environments. Here, the
entorhinal cortex and subiculum are assumed to work together to form a stable attractor and perform
path integration, in the same way that the hippocampus and subiculum were assumed to work in A.
Input from the universal map in the entorhinal cortex determines the hippocampal place field
configuration. Environmental stimuli and events also play a role in defining which place cells will be
active for particular location. Projections from the hippocampal place cells back to the place x direction
x movement (subicular) layer assures the universal map to maintain the same rotational orientation each
time the animal visits a environment with a familiar context (adapted from (Sharp, 1999)).
