The contribution of the cortical layers to the generation of the EEG: Field potential and current source density analyses in the rabbit's visual cortex

Abstract

Intracortical spontaneous field potentials (EEG) were simultaneously recorded from the different layers of the rabbit's visual cortex using multielectrodes produced by thin-film technology. The signals were subjected to spectral analyses. Additionally, current source density analysis was applied to spontaneous delta waves. The results are based on 10 experiments. The spontaneous EEG of the rabbit's visual cortex consists mainly of slow delta waves. The spectral analytical studies of the delta band showed maximum power in layers I and II and in layers V and VI. Significant power minima were found in the middle layers IV and/or III. Essentially, the zones of power minima agree with the zones of minimum coherence and those zones where a phase reversal is observed. These properties of the intracortical spontaneous delta activity are best described as dipole-like. The spontaneous delta waves can be surface negative or surface positive; accordingly they show different source-sink distributions from which different generation mechanisms can be inferred. The possible mechanisms for the generation of these current source-sink distributions (dipoles) are discussed, taking into consideration the cortical anatomy. In many cases these discussions must be speculative because, for an unambiguous explanation of the observed phenomena, a much better knowledge of the cortical anatomy, the fibre connections with other cortical areas and with the deep cerebral structures is necessary.