  Epileptogenic lesions have higher concentrations of sodium than does normal
brain tissue. Such lesions are palpably recognized by a surgeon and then
excised in order to eliminate epileptic seizures with their associated abnormal
electrical behavior. Here we study the frequency-dependent electrical
conductivities of lesion-laden tissues excised from the brains of epilepsy
patients. The low-frequency (< 1000 Hz) conductivity of biological tissue
primarily probes extracellular solvated sodium-cations traveling parallel to
membranes within regions bounded by blockages. This conductivity rises
monotonically toward saturation as the frequency surpasses the rate with which
diffusing solvated sodium cations encounter blockages. We find that saturation
occurs at dramatically higher frequencies in excised brain tissue containing
epileptogenic lesions than it does in normal brain tissue. By contrast, such an
effect is not reported for tumors embedded in other excised biological tissue.
All told, epileptogenic lesions generate frequency-dependent conductivities
that differ qualitatively from those of both normal brain tissues and tumors.
