# Cortical Connectivity Response to Hyperventilation in Focal Epilepsy: A Stereo-EEG Study
## 2 Materials  
This study considered a cohort of 10 _consecutive patients_ that underwent SEEG monitoring at IRCCS Institute of Neurological Sciences of Bologna from January 2022 to June 2024. The study protocol was approved by the local ethics committee (protocol number 89-2021, committee code 20230), and written informed consent was obtained from each patient. The SEEG implants included multiple electrodes, each presenting 5-18 recording sites, named _contacts_; the number and location of the electrodes were patient tailored, depending on the EZ localization hypothesis [17]. Each contact was 2.2 mm in length and was separated by 1.5 mm from neighboring contacts (Microdeep Intracerebral Electrodes-D08, Dixi Medical, Besancon, France).  
The SEEG implantation followed the workflow developed at Niguarda Hospital and involved the construction of a multimodal scene of the patient's brain. The multimodal scenes made it possible for clinicians to comprehensively evaluate all the anatomical information regarding the cortical area explored by each contact [18]. The SEEG signals were recorded using the Nihon Kohden EEG 2100 (Tokyo, Japan), using a maximum of 256 channels, and a sampling frequency of \(f_{\text{sampling}}=1000\) Hz. To correlate electrical and clinical features, a high-definition synchronized video of each patient was recorded for the whole duration of the SEEG monitoring (up to 20 days per patient).  
All patients underwent a standardized 1-hour-recording protocol during the second day of SEEG monitoring, which included two activation maneuvers, namely, HV and intermittent photic stimulation. Particularly, HV sessions consisted of a sequence of deep breaths, at a rate of approximately 20 breaths/minute, for a total period of 4 min. To ensure the correct progress of the maneuver, expert clinical personnel, normally a neurologist and a neurophysiological technician, were in charge of explaining the procedure to the patient and assisting him/her for the exam duration.  
In this work, we excluded all the SEEG contacts exploring the White Matter (WM) since they have only a propagator function [19], focusing only on contacts exploring the Grey Matter (GM). The discrimination between WM and GM, as well as the selection of the SEEG epochs, was performed by a board-certified neurophysiologist (L.F.). No other SEEG channel was excluded from the analysis, making our methodology agnostic to the specific SEEG implant and the clinical and demographic features associated with each patient. Hence, a subset of 26 SEEG epochs per patient was selected for the analysis, according to the following specifics:  
* One epoch associated with the _ictal transition_, namely the period that included the 20 s preceding and following the first ictal change;
* Five epochs associated with the HV maneuver;
* Ten epochs associated with recording periods during which the patient was awake;
* Ten epochs associated with recording periods during which the patient was asleep (N-REM phase).  
Each epoch lasted \(T_{\text{epoch}}=40\) seconds, resulting in a multidimensional signal, whose components, named _channels_, described the electrical activity generated by the cortical sites explored by the SEEG implant. An example of the epochs is reported in Figure 1.