**3.1 Radiofrequency ablation (RFA)**

Stereotactic lesioning has long been a surgical treatment to focal epilepsy. Examples include laser interstitial thermal therapy (LITT) and SEEG-coupled RFA. Lesioning technique is more suitable for lesional epilepsy as the lesion could be identified and approached by the SEEG electrode, whereas in non-lesional epilepsy, the electrodes are often placed according to the epileptogenic network. It could be helpful in cases when open surgery is relatively contraindicated. For example, in temporal lobe epilepsy in dominant hemisphere, stereotactic lesioning might be able to treat the mesial temporal structure where a dominant hippocampectomy is contraindicated. On the other hand, deeply located pathology such as hypothalamic hamartoma and periventricular nodular heterotopia could be treated by stereotaxic lesioning instead of making large corticotomy or passing through important structures such as basal ganglia. More importantly, stereotactic lesioning is *not* a contraindication to subsequent surgery i.e., resection or neuromodulation. SEEG-guided RFA is done by applying radiofrequency thermocoagulation between two contiguous electrode contacts to

make a precise lesion. SEEG signals would be recorded and the likely ictal onset zone together with the MRI-found epileptogenic lesion would be concluded to be the EZ. Direct electric stimulation as previously discussed could also give us the guidance of where the EZ is and if any eloquent areas are nearby. When the target of lesioning is decided, power would be applied with patient being awake until impedance increases i.e., when the coagulum is formed. The duration would usually be less than 1 minute. Sometimes, patient could even hear the crackling sound when the coagulum is made. In an in vitro study by Staudt MD, *et al.* in Operative Neurosurgery found that smaller power, longer duration, closer distance, bipolar thermocoagulation form a larger lesion [20].

#### **3.2 Outcomes**

In the systematic review of Pierre B, *et al.* in Epilepsia in 2018, six retrospective studies and 296 patients were included. Permanent neurological deficit was charted in 2.5% patients. Seizure-free outcome was achieved in average 23% while seizure response rate was up to 58% [21]. Greatest efficacy was observed in periventricular nodular heterotopia while lowest in non-lesional cases. Studies showed high heterogeneity concerning case selection and, therefore factors for good outcome are still unknown.

In the cohort study done by Alexis Moles, *et al.* in 2018, patients with temporal epilepsy with SEEG done was selected to the group of anterior temporal lobectomy and RFA as what procedure the patient had undergone. Three-quarters of patients in lobectomy group achieve seizure freedom in contrast to 0% in the RFA group [22]. Yet around half of the patients in the RFA were responder with no memory impairment recorded. In the cohort, SEEG-RFA is the first choice of treatment after SEEG implantation in the patients who were enrolled later after this treatment policy is employed. They would perform anterior temporal lobectomy if there is failure of the RFA and this would regard as treatment failure. So, the difference in the treatment outcome is not due to the different characteristics, but the treatment per se. Therefore, SEEG-RFA for temporal epilepsy could never achieve the same outcome as the well-established anterior temporal lobectomy and amygdalohippocampectomy but could offer some improvement in seizure control in the patients where lobectomy is contraindicated e.g., temporal lobe epilepsy in dominant hemisphere, apart from directly subjecting patients to neuromodulation. The target of this treatment is the ictal onset zone identified in the SEEG recordings instead of dealing with the entire EZ.

## **4. Brain-computer interface**

Brain-computer interface (BCI) is a rapidly developing field in neuroscience. One area would the neuroprosthesis which receives electrical signals from the brain and perform tasks that patients with neurological deficits are not able do e.g., speech and movement. The hardware to receive the electrical signals include scalp EEG, ECoG and, of course, SEEG. Motor imagery, P300 and steady-state visual evoked potential (SSVEP) are some of the types of BCI [23]. Motor neuroprosthesis is the one that is better developed but recently the study by Moses DA, *et al.* showed the possibility of decoding the brain speech areas and produce speech for patient who was anarthric for a long time [24].
