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: 4. Population coding: Each feature  many neurons. Each neuron  several features. 1. Sparse spiking, no precise patterns. 2. Spatially organized probalistic spiking patterns. 3. Position  Not related  direction sensitivity (unlike afferents) May facilitate integration of multiple whiskers.

Mutual Information How much information two things share. ~ a more sophisticated correlation. A measure of how knowledge about one thing reduces your uncertainty about another thing. :.... ?

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 Rodent somatosensory cortex. Rodent somatosensory cortex. Single whisker  discrete structures (whisker barrels) separated by regions called septa. Single whisker  discrete structures (whisker barrels) separated by regions called septa. Same geometric order as whiskers. Same geometric order as whiskers. Model system for cortical columns. Model system for cortical columns. Organized into layers. Layer IV (L4): individual neurons -- consistent trial-to-trial, strong directional tuning. Organized into layers. Layer IV (L4): individual neurons -- consistent trial-to-trial, strong directional tuning. However, L2/L3 layer does not. 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? Conclusion: (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. Stimulated whisker Septa near whisker. Nearby whiskers

Little individual direction tuning. Tuning amount varied by individual. Tuning corrected for Spiking rate. No spatial organization for directional tuning.

Question: Sparse/Dense responses? (1) Varies trial-to-trial (2) Varies greatly between cells. (3) Onset-Offset active cells may vary. Conclusions:

Fraction active by location: Stimulus – different. Spontaneous – similar. Assuming independence does not match the data.  Correlated Activity Subsets activated not consistent trial-to-trial.

Distance between neurons  means little. Distance from BCC  significant meaning. For both spontaneous and stimulus. Correlations present in spontaneous, but increased with 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: No. < 40µm pair distance

Classification accuracy improves with population size considered. Stimulus detection always > false positive. % small errors increased with size. % large errors decreased with size.

Summary Spatially organized – but probalistically Correlated spiking, but variant No discrete subpopulations observed.