Current source density analysis: Methods and application to simultaneously recorded field potentials of the rabbit's visual cortex

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Abstract
This paper deals with the application of current source density (CSD) analysis to simultaneously recorded intracortical field potentials of the rabbit's visual cortex. Recordings were made with multielectrodes with either 8 contacts at distances of 300 μm, or 16 contacts at distances of 150 μm on one carrier needle. For synchronized activities, a spatial resolution of 150 μm turned out to be sufficient to record all depth-varying details of the field potentials; for seizure potentials even a spacing of 300 μm was adequate in most cases. For practical application, an appropriate spacing of the measuring points has to be chosen for a satisfactory estimation of the first and second derivatives of the field potentials. For this reason an interpolation procedure is applied to reduce the spacing from 300 μm or 150 μm electrode contact distances, respectively, and to obtain intermediate values at 75 μm distances. With this spacing satisfactory estimations of the second derivative are obtained. Theoretically, CSD analysis has to be made three-dimensionally, but under certain conditions which are discussed, a one-dimensional analysis can be applied. An unknown quantity is σz, the vertical conductivity. It turned out that average values obtained from different experiments are not representative and that the vertical conductivity has to be measured in every experiment. This is caused by the great individual differences of the cortices even if the same stereotactic coordinates are chosen. Therefore, in every experiment relative conductivity measurements are performed. The influence of different conductivity values within the various layers and the influence of a conductivity gradient is discussed and demonstrated by examples.