Hippocampal Signal Complexity in Mesial Temporal Lobe Epilepsy: A Noisy Brain is a Healthy Brain

Abstract

Patients with mesial temporal lobe epilepsy (mTLE) show structural and functional abnormalities in hippocampus
and surrounding mesial temporal structures. Brain signal complexity appears to be a marker of functional integrity
or capacity. We examined complexity in 8 patients with intracranial hippocampal electrodes during performance
of memory tasks (scene encoding and recognition) known to be sensitive to mesial temporal integrity. Our patients
were shown to have right mesial temporal seizure onsets, permitting us to evaluate both epileptogenic (right) and
healthy (left) hippocampi. Using multiscale entropy (MSE) as a measure of complexity, we found that iEEG from
the epileptogenic hippocampus showed less complexity than iEEG from the healthy hippocampus. This difference
was reliable for encoding but not for recognition. Our results indicate that both functional integrity and cognitive
demands influence hippocampal signal complexity.