Jarzembski, William B., Larson, Sanford J. and Sances Jr, Anthony. Evaluation of specific cerebral impedance and cerebral current density. Annals of the New York Academy of Sciences. 170:476-490, 1970.
Sensor electrodes were chronically implanted in the sagittal planes in the region of the medial thalamus and in sagittal and coronal planes in the region of the motor cortex of the brains of 30 monkeys. Electrodes were placed externally at the nasion and inion. Currents varying from 0.1 to 100 mA were generated with frequencies ranging from 10 to 5,000 Hz.
They found that the current densities in the brain at all frequencies tested were linearly proportional to the applied currents, but with the thalamic current density slightly higher than the cortical current density. Calculations indicated that from 42% to 46% of the total applied current entered the brain.