An Experimental and Theoretical Analysis of Effects of Volume Conduction in a Non-Homogeneous Medium on Scalp and Cortical Potentials Generated in the Brain
Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
James D. Horgan
Robert W. Lade
A variety of electroencephalographic techniques are used routinely in clinical and research situations to observe and/or record the man1festat1ons of the electrical activity arising in the brain. It is assumed that these electrophysiologic signals reflect an integrated response of the clinical state of the brain and hence are indicative of the subject's state-of-being with respect to his internal and external environments. Because such signals represent the superposition of the action currents generated by numerous distributed bioelectric sources contained in a non-homogeneous volume conductor, it is postulated that the efficacy of information retrieval could be enhanced if it were possible to make a quantitative determination of the extent to which such spatially disparate sources contribute to the potentials measured at arbitrary sampling points. It was therefore the purpose of this work to provide a more complete understanding of the degree to which passive field effects in the brain and its overlying structures might tend to influence typical neural potentials as measured on the cortex or scalp.
An experimental investigation of the passive spread of potentials in the head was accomplished using monkeys. Sites of bioelectric signal generation were simulated using surgically implanted sources driven with subthreshold sinusiodal currents. Scalp and cortical potentials due to sources at several locations were sampled with standard and specialized recording arrays. Based upon experimental results involving cortical and scalp potential distributions and signal ratios it was determined that signal spread to all regions of the head is possible via the passive conduction phenomenon. It was also demonstrated that the ultimate shape and magnitude of potential distributions is a strong function of source depth and angular displacement with respect to the mid-sagittal plane.
A theoretical analysis of the passive spread of potentials was accomplished following the development of a mathematical model of the head. Based upon a spherical geometry, the model consists of four distinct regions of differing conductivity representing the brain, cerebro-spinal fluid layer, skull and scalp. In the formulation of the model, special considerat1on was accorded source representation and impedance discontinuities. Utilizing the model, a theoretical simulation of the monkey experimentation was undertaken, and an excellent agreement of results as obtained. The effects on scalp potentials of variations in the thickness and resistivity of the brain's integumental layers given different source locations were analyzed theoretically. It was determined that the skull exerts the greatest attenuating effect on scalp potentials while the cerebro-spinal fluid layer may give rise to signal smearing, especially for cortical-level sources. It was concluded that typical clinical recording schemes are far more sensitive to cortical-level than in-depth sources, although the latter may be sampled using special techniques. The passive spread of neurally generated potentials appears to extend throughout the head; the degree to which a particular source contributes to an observed signal depends upon the depth of the source in the brain and the geometric distance between the source and recording site.