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Spatial Organization of Neuronal Population Responses in
Layer 2/3 of Rat Barrel Cortex
Jason N. D. Kerr, Christiaan P. J. de Kock, David S. Greenberg, Randy M. Bruno, Bert
Sakmann, and Fritjof Helmchen
Take Home Points:
1. Sparse spiking, no precise patterns.
2. Spatially organized probalistic spiking patterns.
3. Position Not related direction sensitivity (unlike afferents)
4. Population coding:
Each feature  many neurons.
Each neuron  several features.
May facilitate integration of multiple whiskers.
Mutual Information :
How much information two things share.
....
?
A measure of how knowledge about one thing reduces
your uncertainty about another thing.
~ a more sophisticated correlation.
Rationale:
Large single cell variability.
Sparse and short-lived patterns
BUT could have an unambiguous pattern,
but requires many neurons.
(Think sample size)
Methodologically:
Development of spatial maps of neural activity.
– normally impossible with extracellular recording.
Rat Barrel Cortex
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Rodent somatosensory cortex.
Single whisker  discrete structures (whisker barrels) separated
by regions called septa.
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Same geometric order as whiskers.
Model system for cortical columns.
Organized into layers. Layer IV (L4): individual neurons -consistent trial-to-trial, strong directional tuning.
However, L2/L3 layer does not.
1: Woolsey and Van der Loos, 1970
Methods
Calcium
Indictors
Two-Photon
Microscopy
Location of all cells
Single-cell and single-spike resolution.
Skull exposed, optical imaging while stimulating whisker.
Patch-clamp recordings – visually targeted.
Random whisker deflected for 500ms. Interstim ~3-6 sec
Cortex sectioned and area of WB determined.
….. Then a significant variety of analysis.
Layout & Identification
Deflection  transients similar
to spontaneous ones.
Electrical and microscopy
produced similar results.
Question: Spatial organization?
Stimulated
whisker
Septa near
whisker.Conclusion:
Nearby
whiskers
(1) Depends on distance from BCC
(2) Highly variable
(3) For both onset & offset
(4) Highly significant topology.
(5) Offset ~ onset, but smaller.
(6) Spontaneous: all similar.
Little individual
direction tuning.
Tuning amount
varied by individual.
Tuning corrected for
Spiking rate.
No spatial organization
for directional tuning.
Conclusions:
Question: Sparse/Dense
responses?
(1) Varies trial-to-trial
(2) Varies greatly between cells.
(3) Onset-Offset active cells may vary.
Fraction active by location:
Stimulus – different.
Spontaneous – similar.
Assuming independence does not
match the data.
 Correlated Activity
Subsets activated not
consistent trial-to-trial.
Correlations present in spontaneous,
but increased with stimulus.
Distance between neurons
Distance from BCC
 means little.
 significant meaning.
For both spontaneous and stimulus.
“During sensory stimulation, neighboring neurons may be bound together
by common inputs.”
Question: Is effect of distance to BCC on
correlation a result of pairs in the BCC being
packed closely?
Conclusion:
< 40µm
pair distance
No.
Stimulus detection always > false
positive.
Classification accuracy improves with
population size considered.
% small errors increased with size.
% large errors decreased with size.
Summary
Spatially organized – but probalistically
Correlated spiking, but variant
No discrete subpopulations observed.