The brain is impossibly complicated
- if it were simple enough to
understand, we'd be too simple to
understand it.
- Lyall Watson
Spatial representation

“We live in it, move through it, explore it, defend
it… yet we find it difficult to come to grips with
space” (O’Keefe and Nadel, 1978).

Space is not a sensory modality (we do not have
sensory organs for it – it is a construct of mental
processing).

Memory system – evolved in response to
environmental demands.
Bast, 2007
Intra-hippocampal anatomy/circuitry
Place cells

Single neurons that fire when the rat moves through a
specific location (O’Keefe and Dostrovsky, 1971).
– Pyramidal & Granule

Area where a cell fires is its “place field”.

Still fire when landmarks removed, light turned off
(Hafting et al., 2005).

Most studies in environments about 1m across (small area
for a rat) – this study uses an 18m track –larger spatial
scales.
Place cells

Replay - Cells coactivated during awake behavior have
correlated activity during sleep (consolidation?, Wilson and
McNaughton, 1994).

Preplay - firing in a T-maze went ahead in the direction of
reward location (Johnson and Redish, 2007).
– Retrieval of stored spatial representations.
– Hippocampus – active problem solving.
Remapping

Memory interference – similar memories.

Place fields can appear, disappear, or move
randomly (seemingly).
– Rate remapping
– Global remapping

Depends upon differences in environment –
memory separation.
Place cell plasticity
Neves et al., 2008
Head-direction cells

Neurons that fire when the animals head faces a
specific direction in the horizontal plane.

Independent of movement / behavior.

Vestibular input critical to direction signal.
Taube, 2008
Goal cells?
Grid cells

Medial entorhinal cortex – multiple firing fields,
forming a grid.

Firing fields dispersed over the entire
environment.

Firing fields are generally equally spaced apart,
distance from one firing field to all adjacent
firing fields is approximately the same.
Spatial representation system

Place Cells – hippocampus.

Head direction cells – presubiculum.

Grid – entorhinal cortex.
– Thalamus
– Parietal cortex

Cell types together provide info for a mapping system.
– Location, distance/direction between locations.
Aims

Ventral place cells?
– How is space represented along the dorsoventral
axis of the hippocampus?
Materials and methods

Twenty-one male Long-Evans rats ~350-400 g.

Testing occurred in the dark phase.

Rats trained to run back and forth on an 18 m long and 12
cm wide linear track.

Running maintained by chocolate crumbs at each of the
turning points. On recording trials, the rats ran
approximately 10 consecutive laps in each direction.

On the 18 m track, the rat’s position was tracked by an eyesafe invisible laser beam placed at one end of the track.
Materials and methods
Materials and methods

The head stage was connected to a 10m long cable that
moved freely.

An experimenter consistently followed 1 m behind the rat
to ensure that there was no strain from the cable on the
rat’s head.

The tetrodes were lowered in steps of 50 μm or less until
single neurons could be isolated at appropriate depths.

After each recording, the tetrodes were moved further
until new well-separated cells were encountered. The most
ventral recording locations were encountered 30-90 days
after the start of tetrode turning.
Materials and methods
Histology.

The rats were perfused (with electrodes in?).

Sectioned (30 mm) using a cryostat.

The positions of the tips of the recording electrodes were
determined.
Blue=CA3
Red=CA1
Purple=Subiculum
CA3 Place fields
Left runs = “Red”
Right runs = “green”
Top = firing rate as function of
position
Bottom = spike density on
individual laps
Most ventral = 5-10m place field
Theta phase precession

Theta 4-12Hz oscillation, EEG.

Neuron fires in relation to theta cycle (0-360 degrees).

As a rat moves through a place field the neuron fires
earlier and earlier in the cycle.

Each place cell will fire at a different phase of theta determining location with good precision and possibly
providing temporal code.
Theta phase precession
dorsal
Intermediate
Ventral
Composite rate maps
dorsal
Intermediate
Ventral
Spatial scale as function of position along
longitudinal axis of hippocampus
Conclusions

Ventral place cells? – yes

Dorsal hippocampal neurons – mean of 98cm.

Ventral hippocampal neurons (>10m).
Conclusions

Behavioral differences – dorsal / ventral
hippocampus.
– Smaller scales (watermaze) – Dorsal place fields.
– Larger scale (contextual conditioning) – ventral
place fields.
 Fire everywhere in one room and nowhere in another.