Dynamics of the hippocampal ensemble Code for space

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Dynamics of the hippocampal ensemble Code for
space

• Kissy Doyle
• Kenneth Mackenzie
• Delaine Ramesh
• Padmini singh

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UNDERSTANDING PLACE CELLS AND HOW THEY WORK
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Definition

• PLACE CELLS
• Hippocampal pyramidal cells
• Spatial receptive fields restricted to relatively
  small regions of specific environments Different
  cells active in different locations
• PLACE FIELDS
• Region of space in which cells are active

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How Place cells Work

• ENVIRONMENTAL BEHAVIOUR
• Consistent firing patterns over long periods of
  time
• Different place fields in different
  environments

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How Place Cells Work

• MOVEMENT
• Typically require active locomotion for spatial
  firing
• EMOTIONAL TIES
• Under conditions of severe restraint there is an
  absence of spatially related firing
• Significant influence of behavioral state on
  hippocampal activity

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How Place Cells Work

• SLEEPING
• Rhythmic activity and spatial selectivity lost
• PERSISTANCE
• Persist after removal of individual visual cues
• Persist as well as in total darkness
• Persist if individual cues are rotated

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How Place Cells Work

• SENSITIVITY or FAMILIARITY
• Show spatial specificity on the first exposure to
  spatial location
• Robustness of response improves significantly
  with repeated exposures
• Coordinates activity of ensembles of neurons
• Familiarity may be reflected in the degree of
  coordinated activation

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How Place Cells Work

• REPEATED TRAVERSAL
• During repeated traversal of fixed path firing of
  cells becomes highly directional
• High rates of firing when animal is moving in a
  particular direction
• Silent when animal moves in the opposing
  direction
• Emerges whenever an animal repeatedly follows a
  trajectory

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METHODS The Microdrive
Recording Device

• A Microdrive Assay consists of a center housing
  which holds both the wire electrodes and screws
  and teflon legs (glued directly to the skull)
• Electrode depth controlled by depth of screws in
  Teflon legs
• Diameter of each electrode 12 30 um
• Diameter of human hair 50 um
• Each electrode can pick up data from 5 20 units
• Cluster cutting sorts out data from individual
  neural units
• Entire apparatus lt 1.5 cm wide allows free
  movement of animal subject

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METHODS Preparation for the
Experiment

• 3 rats each had1 Microdrive implant
• Each Microdrive implant had 12 tetrodes
• Rats trained for 10 days to forage for chocolate
  pellets (in Box A only)
• While rats sleeping outside of Box apparatus,
  electrodes lowered to obtain stable recordings

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METHODS The Experiment

• Rats allowed to move freely around Box A while
  recordings are made
• 34, 24 and 40 of total CS cells from rats 1, 2
  and 3 respectively show statistically significant
  spatial relation
• Movement trajectories for rats predicted
• -Expected Rate Vector vs. Actual
  Rate Vector
• Factors which affected Mean Prediction Error
• -Intrinsic tracking error
• -Cellular activity variation
• -CS cell sample size
• Partition between Box A and Box B is removed
• Ensemble activity was again recorded
• Ensemble activity recorded a final time with rats
  isolated to Box A (partition replaced).
• 
  

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METHODS
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RESULTS
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RESULTS
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RESULTS

• Other things to note!
• Correlation between phase 2 and 3 of spatial
  distribution firing in box B was low. (unstable
  place fields)
• The behaviour did not differ significantly
  between rat 12. (qualitatively and
  quantitatively)

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CONCLUSIONS

• Information is distributed across many neurons
  during a behavioural task.
• Ensemble code for the firing of hippocampal place
  cells
• Hippocampus is active in spatial memory creating
  cognitive maps

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CONCLUSIONS

• Experience directs hippocampal representation
• Suggested synaptic enhancement
• Suppression of inhibitory interneurons - possible
  key to synaptic modification and spatial learning