Methods for modeling the relationship between extracellular recording variability and impedance properties of chronic neural implants

A finite element model has been developed to investigate the theoretical relationship between changes in extracellular resistivity and electrical potential in a chronic extracellular recording scenario. The inputs to the model are experimental results obtained from chronic recording and complex impedance measurements in cerebral cortex of adult guinea pigs. Using the measured tissue-electrode impedance to set the resistivity in the model provides simulated extracellular potentials that are consistent with the measured spike amplitudes. In both the experimental and theoretical paradigms it was found that increased extracellular resistivity results in an increased potential at the recording electrode tip. Although the results of the two methods could not be directly correlated, they do suggest that a certain amount of variance can be accounted for by the increased resistivity values. The methods presented offer a powerful theoretical tool for understanding some of the factors, which may affect chronic extracellular recording stability