Objective: 

Analysis of local activity and cortical activity recorded from electrodes in subthalamic nucleus.

Methods: 

The patients were operated to implant electrodes in subthalamic nucleus to treat parkinsonism. Three patients were anesthetized with propofol, three with sevoflurane and three with desflurane without other simultaneous drugs. EEG was recorded with Neuroscan TM or Nervus ™ digital EEG equipment simultaneously from scalp and depth electrodes.

Results: 

Burst suppression was recorded between two depth electrodes and between two scalp electrodes. The patterns of bursts in these two traces resembled each other closely, and suppression occurred synchronously. During suppression we could see patterns in depth electrodes which were not visible on scalp.

Conclusion: 

The similarity of burst suppression EEG recorded with scalp and depth electrodes can only be explained with mainly cortical origin of the signal. During suppression we can see transients and rhythms in depth electrodes which are not seen on scalp and we conclude that these are so called local field potentials. Mathematical modeling of the intracerebral electrical fields shows why cortical activity is recorded with high amplitude between two depth electrodes. Intracerebral electrical currents are, however, very complex due to, among other things, anisotropy of subcortical brain. Improved models of these fields are necessary to understand the signals recorded with subcortical electrodes, the effect of fields created by stimulation currents in these electrodes, as well as in understanding the subcortical components of evoked potentials.