Spatial information coding in the mammalian brain
• The hippocampus (HPC) is interesting functionally, clinically,
anatomically, physiologically and computationally.
• Admittedly, there are some pragmatic advantages too.
• The spatial coding properties of HPC neurons and networks
constitute a tangible example of rate coding in the CNS; their
rhythmic firing properties constitute a tangible example of temporal
coding in the CNS.
• The HPC is only one hub of an interconnected network of
structures with activity proportional to spatial parameters (e.g.
enthorhinal cortex and its grid cells, thalamus and its head direction
cells)
“You’ll never guess ‘oo I ‘ad in the back of my ‘ead”
Maguire EA et al. (1997) J. Neurosci. 17, 7103; see also Maguire et al. (2006)
But lesions have their limitations…
and fMRI has its limitations: spatial and temporal resolution
Place cells: O’Keefe & Dostrovsky, 1971
Place cells: a summary of the field
130 cm
10 Hz
0
0
0
170 cm
Probing rodent spatial memory in the water-maze
Place cells: stable over time
Place cells: follow (overt) visual cues
Place cells after dark
The Firing of Hippocampal Place Cells in the Dark Depends on the Rats Recent Experience
Quirk GJ, Muller RU, Kubie JL (1990)
J. Neurosci. 10: 2008-2017
Place cell changes with:
Experience
Attention
Reward
Rules
NMDA-R…
Network encoding in the hippocampus
x
p(x)
λ(x)
λ
location
probability density for
rat being at that location
mean firing rate at x
overall mean firing rate
Maze-based behaviours can be:
(1) used to infer learning and memory in rodents and thereby
(2) model the (episodic and declarative) memory deficits seen in patients with
hippocampal damage.
(3) Examine learning-dependent changes in neural activity.
Probing spatial encoding
MORPHING and
REMAPPING
Room
Box
Impaired place cell remapping in DG-NR1 KO mice
Human place cells?
Ekstrom AD et al. (2003)
Cellular networks underlying human
spatial navigation. Nature 425: 184-188
Err…Jennifer Aniston cells?!?
Quiroga et al. (2005) Invariant visual representation by single neurons in the human brain. Nature 435: 1102
‘Translocation’ of place fields towards reward locations
Lee I, Griffin AL, Zilli EA, Eichenbaum H & Hasselmo ME (2006) Neuron 51: 639-650
Accumulation of hippocampal place fields at the goal
location in an annular watermaze
Hollup, S. A. et al. J. Neurosci. 2001;21:1635-
Subfields of the hippocampus
Place cells in plenty of places
VI
pp
pp
sc
IV
III
CA3
*
pp
II
mf
* Trisynaptic pathway
DG
pp
*
Neocortex via parahippocampal
gyus and/or perirhinal cortex
Subiculum
*
rc
V
Entorhinal cortex
CA1
But who tells place cells about place?
Entorhinal cortical ‘grid cells’
Hafting T et al. (2005) Microstructure of a spatial map in the entorhinal cortex.
Nature 436: 801-806
Head direction cells
Is firing rate whilst running around all that matters?
The theta rhythm (4-12 Hz)
1s
Temporal coding: phase locking and phase precession
O'Keefe J & Recce ML (1993) Phase relationship between hippocampal place units and the EEG
theta rhythm Hippocampus 3, 317-330
Pyramidal cells: slaves to interneurons
Nature Reviews Neuroscience 5, 793-807 (October 2004)
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