  Subcortical structures play a critical role in brain function. However,
options for assessing electrophysiological activity in these structures are
limited. Electromagnetic fields generated by neuronal activity in subcortical
structures can be recorded non-invasively using magnetoencephalography (MEG)
and electroencephalography (EEG). However, these subcortical signals are much
weaker than those due to cortical activity. In addition, we show here that it
is difficult to resolve subcortical sources, because distributed cortical
activity can explain the MEG and EEG patterns due to deep sources. We then
demonstrate that if the cortical activity can be assumed to be spatially
sparse, both cortical and subcortical sources can be resolved with M/EEG.
Building on this insight, we develop a novel hierarchical sparse inverse
solution for M/EEG. We assess the performance of this algorithm on realistic
simulations and auditory evoked response data and show that thalamic and
brainstem sources can be correctly estimated in the presence of cortical
activity. Our analysis and method suggest new opportunities and offer practical
tools for characterizing electrophysiological activity in the subcortical
structures of the human brain.
