Instrumentation for the non-invasive direct measurement of skull impedance

A model is presented for the local flow of electric current, based on the known electrical properties of the tissues which comprise the human head. It is applied to the design of an instrument for the direct non-invasive measurement of the skull's impedance, addressing the need for exceedingly high impedances at frequencies in the 10 to 100 kHz range. It is shown how the design achieves impedances in excess of those of the usually achieved with operational amplifiers, even when their open loop gains are strictly limited. The performance characteristics of a hardware realisation of the design are presented.