ABSTRACT
This chapter discusses the means for experimental determination of current distributions in living tissue. It examines the theories of direct, loaded probe method and the indirect, three and four electrode methods. The chapter demonstrates that the in vivo measurement of current density, with presently available techniques, is not a simple task. To develop reasonably accurate maps of current pathways suitable for analysis of the mode of action of electrical stimulation, it is necessary to make a large number of measurements. The current density within various brain structures cannot be determined by means of classical field theory. Although finite element methods may yield insight, experimental measurements are required for definitive evaluation of current distributions. Relaxation methods developed from the theory of finite differences may be applied to the solution of homogeneous electric field problems. The theoretical bases for all the methods for determining resistivity and current density assume that the media is linear, homogeneous, and isotropic.
