Abstracts

Rationale: Patients with mesial-temporal lobe epilepsy (mTLE) often exhibit central auditory processing (CAP) dysfunction. The contribution of unilateral hippocampal sclerosis to CAP dysfunction was systematically investigated using magnetoencephalography (MEG). Monaural 40-Hz auditory steady-state responses (ASSRs) of magnetic fields were recorded in controls and mTLE patients. Methods: Eighteen left mTLE patients, 11 right mTLE patients and 16 healthy controls (HCs) were examined in this study. Monaural repetitive clicks were presented to subjects at a rate of 40 Hz. Source localization from MEG data was focused on bilateral Heschl’s gyri. Phase-locking factor (PLF) and power values of 40-Hz ASSRs were analyzed in a time-frequency window using a non-parametric cluster permutation. Contra-laterality index (cLI) was calculated as the difference between contralateral and ipsilateral hemispheric responses normalized by the sum of responses. Linear discriminant analysis was performed to classify mTLE patients.  Results: Monaural 40-Hz ASSR demonstrated temporal frequency dynamics in both PLF and power data. More prominent responses in contralateral hemisphere than the ipsilateral hemisphere were evident in HCs in the time courses of PLFs (35-45 Hz) (Figure 1). Symmetrical hemispheric contralaterality was revealed by a non-parametric cluster permutation test in HCs (Figure 2A). However, predominant contralaterality was absent in mTLE patients (Figure 2B). Specifically, right mTLE patients exhibited a lack of contralaterality (i.e., reduced cLI) in response to left ear but not right ear stimulation, and vice versa in left mTLE patients. Linear discriminant analysis revealed that the maximum classification rate was 90% in right mTLE patients and 78% in left mTLE patients.  Conclusions: This is the first study to use 40-Hz ASSR for unilateral mTLE patients to clarify the relationship between CAP and epileptic focus. CAP dysfunction was delineated by a lack of contralaterality corresponding to epileptic focus. The current results suggest that monaural 40-Hz ASSR can provide useful information for localizing epileptic focus in mTLE patients.  Funding: Grant-in-Aid for Scientific Research on Innovative Areas MEXT KAKENHI 15H05875Research grant (2016) from The Epilepsy Research Foundation