Abstract
The hippocampus and amygdala are key brain structures of the medial temporal lobe, involved in cognitive and emotional processes as well as pathological states such as epilepsy. Despite their importance, it is still unclear whether their neural activity can be recorded non-invasively. Here, using simultaneous intracerebral and magnetoencephalography (MEG) recordings in patients with focal drug-resistant epilepsy, we demonstrate a direct contribution of amygdala and hippocampal activity to surface MEG recordings. In particular, a method of blind source separation, independent component analysis, enabled activity arising from large neocortical networks to be disentangled from that of deeper structures, whose amplitude at the surface was small but significant. This finding is highly relevant for our understanding of hippocampal and amygdala brain activity as it implies that their activity could potentially be measured non-invasively.
Highlights
Computational modeling studies suggested that mesial structures, such as amygdala and hippocampus, produce a signal that could be non-invasively recorded at the surface applying appropriate protocols[9,10] and techniques[11]
A method of choice for validating the results of surface Independent component analysis (ICA) is the simultaneous recording of intracranial and surface signals, where SEEG represents a “ground truth” obtained directly within deep brain structures, with anatomical precision of electrode contact localization at the millimeter scale confirmed by magnetic resonance imaging (MRI)
To evaluate detectability of hippocampus and amygdala on MEG, we performed ICA on MEG signals triggered by SEEG hippocampus or amygdala spikes as previously marked on the simultaneous intracranial recording (“SEEG-triggered analysis”)
Summary
Computational modeling studies suggested that mesial structures, such as amygdala and hippocampus, produce a signal that could be non-invasively recorded at the surface applying appropriate protocols[9,10] and techniques[11]. Evidences of mesial brain structures surface detectability have been mainly obtained by evoking activity in hippocampus[12,13,14] or amygdala[15,16,17] through-specific experimental protocols supposed to activate these structures. An issue with TLE is that amygdala and hippocampus are often activated within a larger network involving other neocortical structures, as evidenced by SEEG18,19. It is very difficult to ensure that the observed surface signals effectively come from the hippocampus or amygdala and not from the nearby neocortical structures. A method of choice for validating the results of surface ICA is the simultaneous recording of intracranial and surface signals, where SEEG represents a “ground truth” obtained directly within deep brain structures, with anatomical precision of electrode contact localization at the millimeter scale confirmed by magnetic resonance imaging (MRI)
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