An effective correlation dimension and burst suppression ratio of the EEG in rat. Correlation with sevoflurane induced anaesthetic depth

Abstract

Although brain signals measured under the skull (electrocorticogram, ECoG) and signals measured on top of the scalp (electroencephalogram, EEG) stem from the same brain activity, they are different. We investigated how we can produce EEG when we know ECoG ("forward problem") and how we can produce ECoG when we know EEG ("inverse problem"). We modeled the head as three concentric spheres, representing the brain, skull and scalp. Brain activity is simulated by a dipole. The forward method links the ECoG potentials on the inner sphere to the EEG potentials on the outer sphere via a transfer matrix, based on the geometries and the conductivities of tissues involved. Results showed that the error between analytically computed EEG and EEG produced from analytically produced ECoG with the forward method, is smaller at electrodes close to the source, compared to electrodes far away from the source. The higher the resolution of an ECoG electrode grid, the better the forward model works. Another finding was that the forward model is more accurate or surface sources, compared to deep sources. This result is of practical importance, since most cognitive interesting sources stem from the cortex (the outermost layer of the brain). In the inverse model, the transfer matrix is inverted and additional regularization constraints are applied to compute ECoG from simulated EEG. We showed that the inverse model gives good results. The forward method is tested with data measured from an epileptic patient at the University of Freiburg. Results show that the forward model gives better results at the EEG electrode overlying the ECoG grid compared to the electrode posterior to the grid. Further research is needed to make errors smaller.