Decoding neuronal population activity in rat somatosensory cortex: role of columnar organization

The present study asks in what way the activity of a neuronal population responding to a sensory stimulus could be most efficiently decoded, or 'read off', by the target neurons. A simple solution to this problem has been proposed - pooling the activity of responding neurons. However, pooling can be inefficient if sensory information is encoded by the 'label' of each neuron firing a spike. We have tested the efficiency of pooling by quantifying the extent to which information about a sensory stimulus is diminished when the identity of the individual neurons is lost by pooling. Analyzing the response of small groups of neurons in rat barrel cortex to single-whisker deflection, we found that pooling neurons within the same column is efficient for representing stimulus position; it causes a loss of only 1% of the information about whether the principal whisker was stimulated, and a loss of 5-12% of the finer information about which of nine possible whiskers (the principal and its neighbors) was stimulated. Cross-column pooling led to larger information losses, in the range of 25-55%. Thus, to decode stimulus position from the discharge of barrel cortex populations, 'downstream' neurons could pool the activity arising from neurons of the same column, while maintaining the activity arising from neurons of separate columns at least partially segregated. Since such parcellation is present in some of the projections from barrel cortex, these findings suggest that columnar organization of barrel cortex serves to facilitate decoding of the location of the stimulated whisker.