Effect of source depth on the specificity of bipolar EEG measurements

The purpose of the present study was to evaluate how the brain sources located at different depths can be most effectively measured with bipolar EEG leads. The specificity of an EEG lead to detect sources was studied with a new parameter called region of interest sensitivity ratio (ROISR) by employing a spherical head model. We studied the specificity as a function of electrode distance and further as a function of scalp:skull:brain resistivity ratio. The simulations indicate that the closer to the surface of the brain the source is located, the shorter is the interelectrode distance in the optimal lead. Also in the case of superficial sources, the small misplacement of the electrodes results in a substantial decrease in specificity. The resistivity ratio has the largest effect on the specificity, when the source is located close to the surface of the brain. However in the case of deep sources, the resistivity ratio has only minimal effect on the specificity.