Thalamocortical impulse propagation and information transfer in EEG and MEG

Measures of functional connectivity and information transfer between the thalamus and the cortex can provide detailed insight into brain function. Employing magnetoencephalography and electrical median nerve stimulation, it has been recently proposed that impulse propagation along the thalamocortical fiber tract can be described by a single moving dipole source. Other studies, however, using electroencephalography observed dipole clustering in the thalamus and the cortex. To assess the source of these conflicting results, we simultaneously recorded somatosensory evoked potentials and fields in 12 healthy volunteers. Using a single dipole model for the time interval of 10 to 30 milliseconds after stimulus onset, we found continuous thalamocortical dipole movement in 10 volunteers and dipole clustering in the precortical near thalamic and cortical regions in 2 volunteers. Thus, independent of the recording method, both clustering and movement can be observed. The degree of temporal overlap between the precortical near thalamic and the cortical activity distinguished the volunteers exhibiting clustering and those exhibiting movement. In a subsequent simulation study, we could show that both dipole clustering and dipole movement can occur, depending on the temporal overlap of the precortical and cortical activities. In conclusion, we propose a two-dipole model to better account for precortical and cortical activity and information transfer.