Abstracts

Rationale: Our previous study (Chatani et al, NeuroImage 2016) demonstrated that the hippocampal sclerosis (HS) significantly modulated the auditory processing in patients with mesial temporal lobe epilepsy (mTLE). M100 in the auditory cortex (AC), a major component of auditory-evoked magnetic fields (AEF) to tone-burst stimulation, was attenuated in the patients with HS compared with healthy controls (HC) and disease controls. These data mainly analyzed the M100 responses in the AC contralateral to the auditory stimulation. Here, we report that monaural auditory stimulation provides more diagnostic information on epileptic lateralization in mTLE when neural synchronization in the bilateral ACs are analyzed.  Methods: We reanalyzed the previous data (Chatani et al, 2016) and recruited additional subjects. Thus, 17 left mTLE patients (age range; 28 - 58 years, 13 females), 12 right mTLE patients (age range; 18 - 71 years, 5 females) and 24 HC (age range;21 - 68 years, 14 females) participated in this study. Tone-burst stimuli of 500-Hz frequency with 100-ms duration were monaurally presented with 1000-ms inter-stimulus intervals through plastic tubes (Figure 1). AEFs were obtained by averaging approximately 120 responses (2 minutes). We analyzed signal sources of M100 in bilateral AC. Source waveforms were extracted via dynamic statistic parametrical mapping, followed by continuous wavelet transformation for each single-trial data using complex Morlet. We calculated the phase-locking factor (PLF), which represents phase synchronization with respect to the stimuli, and phase-locking value (PLV), which represents phase synchronization between the region of interests (ROI; bilateral ACs). We defined M100 as a component with a peak between 80 - 160 ms (Kikuchi et al, Hear Res 2017) and also the time window of interest for PLF and PLV were set as the same. PLFs and PLVs were calculated in theta (4 - 7 Hz), alpha (8 - 13 Hz) and beta (14 - 30 Hz) band frequencies. Statistical analyses were performed using the repeated-measures analysis of variance (ANOVA). For the M100 amplitude and PLFs, a three-way ANOVA was performed with group (HC vs. left mTLE vs. right mTLE), hemisphere (left ROI vs. right ROI) and stimulation side (left ear vs. right ear) as three factors. For the PLVs, a two-way ANOVA were carried out with group (HC vs. left mTLE vs. right mTLE) and stimulation side (left ear vs. right ear) as two factors. P < 0.05 was considered significant. Results: The main effect of group was not significant in the M100 amplitude. There was a significant main effect of group in theta, alpha and beta ranges in PLF. There was also a significant main effect of group in theta and alpha range in PLV. No significant interactions between group and stimulation side were found in the M100 amplitude, PLF and PLV. A post-hoc analysis revealed that mTLE patients showed the attenuated responses in each frequency range in PLF and PLV compared with HC, respectively. In addition, PLV in the theta range was significantly lower in right mTLE than left mTLE (Figure 2).  Conclusions: We found that mTLE patients showed the altered neural synchronization for auditory stimulation compared with HC. More specifically, right mTLE exhibited more desynchronization than left mTLE in theta frequency. Auditory response to pure tone is reported to be right hemispheric predominance, therefore, this finding may suggest that dysfunction of AC is modulated especially in right mTLE. The simple task using monaural auditory stimulation can be useful as a diagnostic tool for epileptic lateralization in mTLE patients. Funding: The Japan Epilepsy Research FoundationNon-linear Neuro-oscillology