Atypical language organization in temporal lobe epilepsy revealed by a passive semantic paradigm

Background Mesial temporal lobe epilepsy (MTLE) is the most common type of focal epilepsy in adults and can be successfully cured by surgery. One of the main complications of this surgery however is a decline in language abilities. The magnitude of this decline is related to the degree of language lateralization to the left hemisphere. Most fMRI paradigms used to determine language dominance in epileptic populations have used active language tasks. Sometimes, these paradigms are too complex and may result in patient underperformance. Only a few studies have used purely passive tasks, such as listening to standard speech. Methods In the present study we characterized language lateralization in patients with MTLE using a rapid and passive semantic language task. We used functional magnetic resonance imaging (fMRI) to study 23 patients [12 with Left (LMTLE), 11 with Right mesial temporal lobe epilepsy (RMTLE)] and 19 healthy right-handed controls using a 6 minute long semantic task in which subjects passively listened to groups of sentences (SEN) and pseudo sentences (PSEN). A lateralization index (LI) was computed using a priori regions of interest of the temporal lobe. Results The LI for the significant contrasts produced activations for all participants in both temporal lobes. 81.8% of RMTLE patients and 79% of healthy individuals had a bilateral language representation for this particular task. However, 50% of LMTLE patients presented an atypical right hemispheric dominance in the LI. More importantly, the degree of right lateralization in LMTLE patients was correlated with the age of epilepsy onset. Conclusions The simple, rapid, non-collaboration dependent, passive task described in this study, produces a robust activation in the temporal lobe in both patients and controls and is capable of illustrating a pattern of atypical language organization for LMTLE patients. Furthermore, we observed that the atypical right-lateralization patterns in LMTLE patients was associated to earlier age at epilepsy onset. These results are in line with the idea that early onset of epileptic activity is associated to larger neuroplastic changes

Effective Brain Connectivity from Intracranial EEG Recordings: Identification of Epileptogenic Zone in Human Focal Epilepsies

In the context of focal and drug-resistant epilepsy, surgical resection of the epileptogenic zone (EZ) may be the only therapeutic option for reducing or suppressing seizures. The aim of epilepsy surgery is the exeresis of the EZ, which is assumed to be the cortical region responsible for the onset, early organization, and propagation of seizures. EZ represents the minimum amount of cortex that must be resected in order to achieve seizure freedom; therefore, the correct identification of its extent and organization is a crucial objective. Nevertheless, the rather high rate of failure in epilepsy surgery in extra-temporal epilepsies highlights that the precise identification of the EZ is still an unsolved problem and that more sophisticated methods of investigation are required. In many patients, intracranial stereo-EEG recordings still represent the gold standard for the epilepsy surgery work-up, and, over the last 10 years, considerable efforts have been made to develop advanced signal analysis techniques able to improve the identification of the EZ. Since it is widely assumed that epileptic phenomena are associated with abnormal changes in brain synchronization mechanisms, particular attention has been paid to those methods aimed at quantifying and characterizing the interactions and causal relationships of neuronal populations, and initial evidence has shown that this can be a suitable approach to localizing the EZ. The aim of this review is to provide an overview of the different intracranial EEG signal processing methods used to identify the EZ, with particular attention being given to the methods aimed at characterizing effective brain connectivity using intracranial EEG recordings. Then, we briefly present our studies of the connectivity pattern associated with a particular form of focal epilepsy (type II focal cortical dysplasia), based on multivariate autoregressive parametric models and measures derived from graph theory