The investigators want to characterize high-frequency oscillations (HFOs) in intracranial recordings, which may occur as markers of epileptogenic tissue and also under physiological stimulation.

The investigators want to show that recording of high-frequency oscillations (HFOs) is feasible in our project population both intraoperatively and during presurgical physiological conditions.

Inclusion Criteria:

• Patients intend to be treated at the Klinik für Neurochirurgie
• Patients fall into one of the patient groups: (1) Patients with symptomatic epilepsy with focal and/or secondary generalized seizures resistant to medication where a neurosurgical resection is considered as an intervention of epilepsy surgery. (2)Patients where a tumor is planned to be resected that has caused preoperative seizures. (3) Patients where a tumor is planned to be resected that has not caused preoperative seizures.
• Male and Female subjects 18 years to 99 years of age
• Written informed consent after participants' information

Exclusion Criteria:

• contraindications on ethical grounds
• known or suspected non-compliance, drug or alcohol abuse,
• enrolment into a clinical trial within last 4 weeks

1. Patients with epilepsy where a surgical resection is considered.
2. Patients where a tumor will be resected that has caused preoperative seizures.
3. Patients where a tumor will be resected that has not caused preoperative seizures.