A set of 16 recording points 50 µm apart in a vertical track spanning from 250 µm above to 500 µm below the st. pyramidal was placed at the center of the population in parallel to the somatodendritic axis. The value of field potential measured at each point was calculated as follows:

where sigma is the extracellular conductivity, I_mijis the total transmembrane current at the jth compartment of neuron i, and r_ij is the distance from the recording point to that compartment. Thus, compartments are treated as point sources into a medium of homogeneous conductivity sigma. If the conductivity tensor were known for the CA1 tissue, a more detailed expression could be derived to calculate the field potential with higher accuracy (see Nunez, 1981). These calculations would have a high computational cost due to the presence of complex Bessel functions in the integration of the field potentials.
 

The current density can be obtained (as in the experimental current source density analysis) through a Poisson equation which relates the field potential with the volumetric average of the transmembrane currents:

In a laminated structure such as the CA1 region the overall extracellular currents flow only in the dendrosomatic direction (z axis) and the analysis can be simplified in one dimension:

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