LESIONS IN THE CONNECTED BRAIN A network perspective on brain tumors and lesional epilepsy

Pharmaco-resistant temporal lobe epilepsy (TLE) is often treated with surgical intervention at some point. As epilepsy surgery is considered a last resort by most physicians, a long history of epileptic seizures prior to surgery is not uncommon. Little is known about the effects of ongoing TLE on neural functioning. A better understanding of these effects might influence the moment of surgical intervention. Functional connectivity (interaction between spatially distributed brain areas) and network structure (integration and segregation of information processing) are thought to be essential for optimal brain functioning. We report on the impact of TLE duration on temporal lobe functional connectivity and network characteristics. Functional connectivity of the temporal lobe at the time of surgery was assessed by means of interictal electrocorticography (ECoG) recordings of 27 TLE patients by using the phase lag index (PLI). Graphs (abstract network representations) were reconstructed from the PLI matrix and characterized by the clustering coefficient C (local clustering), the path length L (overall network interconnectedness), and the ‘small-world index’ S (network configuration). Functional connectivity (average PLI), clustering coefficients, and the small-world index were negatively correlated with TLE duration in the broad frequency band (0.5–48 Hz). Temporal lobe functional connectivity is lower in patients with longer TLE history, and longer TLE duration is correlated with more random network configuration. Our findings suggest that the neural networks of TLE patients become more pathological over time, possibly due to temporal lobe changes associated with long-standing lesional epilepsy.network analysis have shown to be a promising tool in studying epilepsy and brain tumor patients. Changes in network characteristics and functional connectivity have been associated with both epilepsy and brain lesions [9, 10, 132, 136]. We therefore consider that changes in functional connectivity and network configuration may be a marker of possible progression of TLE. We hypothesize that functional interactions in the brain are correlated to temporal lobe epilepsy duration. We expect to demonstrate a correlation between TLE duration and changes in network characteristics of the temporal lobe. We expect less functional connectivity and a more random configuration of the temporal neural networks as TLE duration increases.