Acute and Long Term Effects of VNS on Memory in Patients With Refractory Epilepsy

Previous studies underlined the potential of both invasive as transcutaneous auricular vagus nerve stimulation to ameliorate certain cognitive functions. In this randomized, controlled cross-over within-subjects study, a memory test is conducted in patients with refractory epilepsy who are implanted with a vagus nerve stimulation. The memory test consists out of a word recognition paradigm based on the study of Clarck et al. published in 1999 in Nature Neuroscience. Testing is performed at baseline (before start of the stimulation) and after 6 weeks of treatment with vagus nerve stimulation. During the first session, the patients complete the word recognition task during three interventions: * Invasive vagus nerve stimulation * Transcutaneous vagus nerve stimulation (cymba concha) * Sham vagus nerve stimulation (no stimulation) During the second session, the patients complete the word recognition task during two interventions: * Invasive vagus nerve stimulation * Sham vagus nerve stimulation (no stimulation) The goal is to investigate if invasive vagus nerve stimulation and transcutaneous nerve stimulation can influence (i.e. improve) the performance on the memory task and if this performance is improved after 6 weeks of VNS treatment.