# Low Power EEG Data Encoding for Brain Neurostimulation Implants +
## 1 Introduction  
Neurostimulation is a therapeutic method recently endorsed for the treatment of central nervous system disorders, such as epilepsy, and acts by applying electrical pulses in the area of interest [1, 2]. It targets neuronal populations that reside in the vicinity of the implanted electrodes and aims in disrupting the onset and development of epileptic seizures [3]. In the field of epilepsy, two basic neurostimulation systems that involve electroencephalographic (EEG) data recording and transmission have been approved and are available for clinical use: the deep brain stimulation (DBS, Percept Medtronic) system and the responsive neurostimulation (RNS) system.  
The DBS is a neurostimulation device introduced for the treatment of epileptic seizures in cases where the epileptogenic area is diffuse or is not amenable to focal resective surgery [4]. The DBS is an open-loop programmable stimulation device, most often placed subcutaneously in the subclavicular region of the chest, which delivers electrical pulses through a cable to the electrodes placed in a specific part of the brain in a programmed periodic fashion with the aim to disrupt abnormal brain activity [5]. Although the DBS system has been approved to solely target the anterior nucleus of the thalamus, with one electrode in each nuclei in both hemispheres, it is currently used to treat a wide range of focal seizures, especially those that involve the frontal and the temporal lobes [6, 7]. The most recent DBS model (Percept Medtronic) incorporates baseline recording of cumulative spectral EEG data and transmission to an external device for evaluation [8].  
The RNS system is a closed-loop medical device that records electroencephalographic (EEG) signals, detects seizure patterns, and provides electrical stimulation within the implanted epileptogenic region [9]. The RNS system implements a strategy to terminate the ongoing epileptic seizure activity [10; 11], and has been approved for adults that are refractory to both anti-epileptic medication and mainstream surgical treatments. The RNS device is implanted on the skull and is specifically configured to respond to the seizure EEG patterns of the implanted patient [12]. The current implantation approach includes two electrodes placed over the presumed epileptogenic area. The electrodes are connected to the device amplifier and stimulator in a bidirectional manner, and the programmed stimulation is applied when a seizure pattern is detected [13]. Each patient is provided with an external computer to which they can daily transmit snapshots of the the recorded EEG data through a wireless telemetry probe. The transmitted EEG data are in turn uploaded to an online patient data management system (PDMS) that the neurologist can access to evaluate the patient's epileptic activity [14]. Depending on the physician's evaluation, the RNS system detection and stimulation parameters can be reprogrammed until seizure control noticeably improves [2].  
A common problem both neurostimulation systems face is the short life span of their batteries, which is a combined result of the patient's seizure frequency, the effective therapeutic stimulation intensity applied, and the daily rate of data transmission to the external computers. Battery replacements require recurrent surgical procedures, introducing significant costs for both the patient and the healthcare system [15]. Therefore, low energy consumption for wireless transmission and autonomy of these implanted devices becomes a goal worth pursuing. For that purpose, we set out to investigate improved methods to achieve low power consumption for such devices. We simulated a system for encoding and storing brain signals similar to the available neurostimulation devices and implemented data transmission using a low-power communication protocol. Our goal is to achieve EEG data wireless transmission at a significantly low power level [16].