Abstracts

Rationale: While early components of auditory evoked responses (AERs) to speech stimuli are related to low-level processing in the auditory areas, at least some of the late components are thought to reflect language processing. Language lateralization in epilepsy patients is often determined from these late components of AERs. However, the influence of epileptic networks on AERs is not known. We sought to determine whether hemispheric asymmetries in source-activations related to early (50-150ms) and late (300-600ms) components of AERs are significantly influenced by the laterality of epileptic networks. Methods: Participants comprised 35 patients with drug-resistant epilepsy (17M;17 with left hemispheric epileptic networks) who underwent a comprehensive presurgical evaluation which included a MEG study of language at the Medical College of Wisconsin between 2012 and 2015. All patients provided written informed consent for research use of the clinical data. During the MEG study, patients performed a modified continuous recognition memory (CRM) language task in which auditory word stimuli and spectrally matched noise stimuli were randomly interleaved. Source modeling of the AERs to word and noise stimuli was performed using Freesurfer-segmented patient-specific head models and dynamic Statistical Parametric Mapping (dSPM). A threshold corresponding to half the maximal dSPM value (during the selected time-window) was applied to the activation maps. Hemispheric laterality indices for each time-window and stimulus type were calculated based on above-threshold voxels within a perisylvian region of interest. We examined correlations between the laterality of epileptic networks and both the laterality of early (50-150ms) and late (300-600ms) components of responses to words as well as early responses to noise-stimuli. Results: We found a significant correlation between the laterality of epileptic networks and laterality of early (50-150ms) noise-related responses (p=0.02) and late (300-600ms) language-related responses (p=0.02). There was also a trend toward significance for the correlation between laterality of epileptic networks and early (50-150ms) language-related responses (p=0.09). The effect of epileptic-network laterality on AERs did not differ between patients with seizures of right versus left hemispheric origin. Conclusions: The laterality of epileptic networks significantly influences asymmetries in source-activations related to both early and late components of AERs. This finding has important methodological implications to determining language dominance from late components of AERs using MEG. It is possible that the biasing effect of seizure laterality on source activations may be a greater concern with auditory paradigms since most patients with drug-resistant epilepsy have epileptic networks in the temporal lobe. Funding: None.