Current source-density and neuromagnetic analysis of the direct cortical response in rat cortex.

The electrophysiological basis of macropotentials produced by the direct cortical response (DCR) to electrical stimulation was studied using a combination of current source-density (CSD) and neuromagnetic analysis. Current source-density analysis indicated the locations of extracellular sources and sinks in the cortical depth giving rise to each temporal component of the DCR complex. Information about intradendritic currents was obtained from extracranial magnetic field measures. These data indicate that the DCR is composed of a sequential activation of pyramidal cells at different cortical depths. The complex begins with depolarization of cells in the upper and middle layers, followed by depolarization of deeper pyramidal cells with apical dendrites extending near the cortical surface. The complex ends with a positive-negative slow wave sequence indicating possible afterhyperpolarization of surface dendrites and hyperpolarization of cell bodies and basilar dendrites in the depth. These data demonstrate a unique way in which electrical and magnetic measures may be combined to provide complementary information about the spatially and temporally organized cellular currents within local neuronal networks.