The brain is impossibly The brain is impossibly complicated - if it were simple complicated - if it were simple enough to understand, we'd be enough to understand, we'd be

too simple to understand it. too simple to understand it.

- Lyall Watson

Spatial representationSpatial representation

““We live in it, move through it, explore it, We live in it, move through it, explore it, defend it… yet we find it difficult to come to defend it… yet we find it difficult to come to grips with space” grips with space” (O’Keefe and Nadel, 1978).(O’Keefe and Nadel, 1978).

Space is not a sensory modality (we do not Space is not a sensory modality (we do not have sensory organs for it – it is a construct have sensory organs for it – it is a construct of mental processing).of mental processing).

Memory system – evolved in response to Memory system – evolved in response to environmental demands.environmental demands.

Bast, 2007

Intra-hippocampal Intra-hippocampal anatomy/circuitryanatomy/circuitry

Place cellsPlace cells

Single neurons that fire when the rat moves through Single neurons that fire when the rat moves through a specific location (O’Keefe and Dostrovsky, 1971).a specific location (O’Keefe and Dostrovsky, 1971).– Pyramidal & GranulePyramidal & Granule

Area where a cell fires is its “place field”.Area where a cell fires is its “place field”.

Still fire when landmarks removed, light turned off Still fire when landmarks removed, light turned off (Hafting et al., 2005).(Hafting et al., 2005).

Most studies in environments about 1m across (small Most studies in environments about 1m across (small area for a rat) – this study uses an 18m track –larger area for a rat) – this study uses an 18m track –larger spatial scales.spatial scales.

Place cellsPlace cells

ReplayReplay - Cells coactivated during awake behavior - Cells coactivated during awake behavior have correlated activity during sleep have correlated activity during sleep (consolidation?, (consolidation?, Wilson and McNaughton, 1994)Wilson and McNaughton, 1994)..

Preplay Preplay - firing in a T-maze went ahead in the - firing in a T-maze went ahead in the direction of reward location direction of reward location (Johnson and Redish, (Johnson and Redish, 2007)2007). .

– Retrieval of stored spatial representations.Retrieval of stored spatial representations.

– Hippocampus – active problem solving.Hippocampus – active problem solving.

RemappingRemapping

Memory interference – similar memories.Memory interference – similar memories.

Place fields can appear, disappear, or move Place fields can appear, disappear, or move randomly (seemingly).randomly (seemingly).

– Rate remappingRate remapping– Global remappingGlobal remapping

Depends upon differences in environment – Depends upon differences in environment – memory separation.memory separation.

Place cell plasticityPlace cell plasticity

Neves et al., 2008

Head-direction cellsHead-direction cells

Neurons that fire when the animals head Neurons that fire when the animals head faces a specific direction in the horizontal faces a specific direction in the horizontal plane.plane.

Independent of movement / behavior.Independent of movement / behavior.

Vestibular input critical to direction signal.Vestibular input critical to direction signal.

Taube, 2008

Goal cells?Goal cells?

Grid cellsGrid cells

Medial entorhinal cortex – multiple firing Medial entorhinal cortex – multiple firing fields, forming a grid.fields, forming a grid.

Firing fields dispersed over the entire Firing fields dispersed over the entire environment.environment.

Firing fields are generally equally spaced Firing fields are generally equally spaced apart, distance from one firing field to all apart, distance from one firing field to all adjacent firing fields is approximately the adjacent firing fields is approximately the same.same.

Place Cells – hippocampus.Place Cells – hippocampus.

Head direction cells – presubiculum.Head direction cells – presubiculum.

Grid – entorhinal cortex.Grid – entorhinal cortex.

– ThalamusThalamus– Parietal cortexParietal cortex

Cell types together provide info for a mapping Cell types together provide info for a mapping system.system.– Location, distance/direction between locations.Location, distance/direction between locations.

Spatial representation systemSpatial representation system

Aims Aims

Ventral place cells?Ventral place cells?

– How is space represented along the How is space represented along the dorsoventral axis of the hippocampus?dorsoventral axis of the hippocampus?

Materials and methodsMaterials and methods

Twenty-one male Long-Evans rats ~350-400 g.Twenty-one male Long-Evans rats ~350-400 g.

Testing occurred in the dark phase.Testing occurred in the dark phase.

Rats trained to run back and forth on an 18 m long and Rats trained to run back and forth on an 18 m long and 12 cm wide linear track.12 cm wide linear track.

Running maintained by chocolate crumbs at each of the Running maintained by chocolate crumbs at each of the turning points. On recording trials, the rats ran turning points. On recording trials, the rats ran approximately 10 consecutive laps in each direction.approximately 10 consecutive laps in each direction.

On the 18 m track, the rat’s position was tracked by an On the 18 m track, the rat’s position was tracked by an eye-safe invisible laser beam placed at one end of the eye-safe invisible laser beam placed at one end of the track.track.

Materials and methodsMaterials and methods

Materials and methodsMaterials and methods

The head stage was connected to a 10m long cable that The head stage was connected to a 10m long cable that moved freely.moved freely.

An experimenter consistently followed 1 m behind the An experimenter consistently followed 1 m behind the rat to ensure that there was no strain from the cable on rat to ensure that there was no strain from the cable on the rat’s head.the rat’s head.

The tetrodes were lowered in steps of 50 μm or less until The tetrodes were lowered in steps of 50 μm or less until single neurons could be isolated at appropriate depths. single neurons could be isolated at appropriate depths.

After each recording, the tetrodes were moved further After each recording, the tetrodes were moved further until new well-separated cells were encountered. The until new well-separated cells were encountered. The most ventral recording locations were encountered 30-most ventral recording locations were encountered 30-90 days after the start of tetrode turning.90 days after the start of tetrode turning.

Materials and methodsMaterials and methods

Histology.Histology.

The rats were perfused (with electrodes in?).The rats were perfused (with electrodes in?).

Sectioned (30 Sectioned (30 m) using a cryostat.m) using a cryostat.

The positions of the tips of the recording electrodes The positions of the tips of the recording electrodes were determined.were determined.

BlueBlue=CA3=CA3

RedRed=CA1=CA1

PurplePurple=Subiculum=Subiculum

CA3 Place fieldsCA3 Place fields

Left runs = “Red”Left runs = “Red”

Right runs = “green”Right runs = “green”

Top = firing rate as function of Top = firing rate as function of positionposition

Bottom = spike density on Bottom = spike density on individual lapsindividual laps

Most ventral = 5-10m place field Most ventral = 5-10m place field

Theta phase precessionTheta phase precession

Theta 4-12Hz oscillation, EEG.Theta 4-12Hz oscillation, EEG.

Neuron fires in relation to theta cycle (0-360 Neuron fires in relation to theta cycle (0-360 degrees).degrees).

As a rat moves through a place field the neuron fires As a rat moves through a place field the neuron fires earlier and earlier in the cycle.earlier and earlier in the cycle.

Each place cell will fire at a different phase of theta - Each place cell will fire at a different phase of theta - determining location with good precision and determining location with good precision and possibly providing temporal code.possibly providing temporal code.

Theta phase precessionTheta phase precession

dorsal Intermediate Ventral

dorsal

Intermediate

Ventral

CompositeComposite rate maps rate maps

Spatial scale as function of position Spatial scale as function of position along longitudinal axis of along longitudinal axis of

hippocampushippocampus

ConclusionsConclusions

Ventral place cells? – yesVentral place cells? – yes

Dorsal hippocampal neurons – mean of Dorsal hippocampal neurons – mean of 98cm.98cm.

Ventral hippocampal neurons (>10m). Ventral hippocampal neurons (>10m).

ConclusionsConclusions

Behavioral differences – dorsal / ventral Behavioral differences – dorsal / ventral hippocampus.hippocampus.

– Smaller scales (watermaze) – Dorsal place Smaller scales (watermaze) – Dorsal place fields.fields.

– Larger scale (contextual conditioning) – Larger scale (contextual conditioning) – ventral place fields. ventral place fields.

Fire everywhere in one room and nowhere in Fire everywhere in one room and nowhere in another.another.