Abstracts

Rationale: The limbic system to which mesial temporal lobe epilepsy (MTLE) belongs consists of the hippocampus-mesencephalon-parahippocampal gyrus-retrosplenial network (HMPRN; Papez circuit), temporal lobe-amygdala-orbitofrontal cortex network (TAON; Yakovlev circuit), and the dorsomedial default mode network (DMN). Although certain cognitive functions in humans arise from activity in specific frequency bands, we hypothesized that abnormal neural activity on the epileptic network may likewise differ from one frequency band to another. Therefore, we hypothesized that changes in the brain network of patients with MTLE occur in specific frequency bands and that such changes may be reflected as changes in FC. We hypothesized that (1) elevated FC on the affected side indicates the formation of an abnormal network due to epilepsy, (2) decreased FC bilaterally indicates normal network dysfunction due to the abnormal network, and (3) elevated FC on the healthy side indicates the formation of a compensatory network for the resulting cognitive and mental dysfunction.

The purpose of this study was to clarify the network changes occurring between the hippocampal and limbic networks in patients with MTLE using magnetoencephalographic FC analysis, which can evaluate neural activity in different frequency bands.


Methods: Participants were 9 patients aged 18 years or older with MTLE who had preoperative magnetoencephalography and subsequent temporal lobectomy and achieved postoperative seizure free (Engel class I) and 9 patients from an age- and sex-matched magnetoencephalogram healthy cohort. The recorded magnetoencephalography was divided into seven frequency bands: delta (2-4 Hz), theta (4-8 Hz), alpha (8-14 Hz), beta (14-30 Hz), gamma (30 -80 Hz), ripple (80-250 Hz), and fast ripple (250-500 Hz), FC changes between the hippocampus and HMPRN, DMN, and TAON were analyzed using magnitude-square coherence and statistically compared between patients with MTLE and healthy controls.


Results: FC changes between the affected hippocampus and HMPRN and DMN were broadly elevated in the gamma to ripple band. FC changes between the healthy hippocampus and HMPRN and DMN were broadly decreased across the entire band.

FC change between the affected hippocampus and TAON was broadly elevated in the gamma to ripple band, and FC change between the healthy hippocampus and TAON was broadly decreased in the fast ripple band.


Conclusions: The affected hippocampus was thought to form an abnormal epileptic network in the gamma to ripple band with the limbic network. On the other hand, the healthy hippocampus was thought to show dysfunction due to epileptic network impairment in different frequency bands with different limbic networks. It is well known that patients with MTLE suffer from cognitive and psychosomatic disorders such as memory impairment and depression, and these brain network changes may suggest such dysfunction.


Funding: JSPS KAKENHI Grant Number 23K15663 (principal investigator: Tomotaka Ishizaki).