# Minimally-Invasive Neural Interface for Distributed Wireless Electrocorticogram Recording Systems
## 1 Introduction  
The neural interfaces enable us to build a direct communication pathway between the human brain and external world, through which we can monitor neural activities, supplement the nervous system using a neuroprosthetic device, and even assist and augment human cognitive or sensory functions by brain-computer interface (BCI). In practice, the brain's spontaneous or stimulated activities should be closely monitored to provide the important neurofeedback for rehabilitation or for parts of evaluation and titration of therapy in neurological disorders such as epilepsy, Parkinson's disease, strokes, etc. The same neural interface systems can be also useful tools for neuroscience studies in exploring nervous systems and understanding brain circuits and their complex activities and connectivity [1; 2]. The development of neural interface systems gives a trade-off between signal fidelity and subject safety, depending on where and how the electrodes are being deployed and located, e.g., electroencephalogram (EEG), electrocorticogram (ECoG), or single unit action potential (SUAP). The SUAP gives us the most accurate neural activity information; however, the system must be severely invasive due to the necessity of using micro-machined electrodes implanted to reach single neurons [3], and may lead to tissue reaction. The EEG potential can be obtained on the surface of thescalp without penetration of a cranium, allowing a non-invasive system. However, the bio-information achieved by the EEG system is limited in temporal and spatial resolution because it can only collect the ensemble activities of neuron groups [4]. On the other hand, the ECoG recently gains its attention as a compromised solution for balancing between signal fidelity and safety [5; 6; 7; 8; 9; 10; 11; 12; 13]. The effectiveness of the ECoG recording to decode neural signals by using spectrograms has been demonstrated in recent articles [14; 15; 12]. The ECoG system can record brain activities on the surface of the cortex, not penetrating brain. Therefore, the ECoG system is less invasive than the SUAP recording system, and the recorded signals can provide more abundant bio-information than surface EEG signals. In addition, the recent observation of spikes and field potentials of the cortical neurons from the brain surface has been reported, which makes the surface recording more effective method [16]. The ECoG brain signals typically have a low amplitude (\(\sim\)100 uV) and a relatively low bandwidth (<500 Hz). Thus, it is important to provide low-noise signal amplification and digitization for the given signal range and bandwidth. In addition, in order to increase the temporal and spatial resolution for useful clinical and research applications, multiple locations need to be simultaneously monitored up to >200 electrodes over the large area of the brain [17]. Although much better than the SUAP monitoring in terms of safety, ECoG is still regarded as an invasive method since it needs a craniotomy to place electrode arrays and implantable interface modules on top of cortex. It is desirable for the implanted system to be realized in a small form factor with an elongated lifetime so that it can operate for weeks or even for years once implanted. In order to prevent possible infection, it is essential to completely remove the transcutaneous connection between the implanted device and an external system by implementing a low-power wireless data transmission scheme in a minimized volume. Several groups have reported the implantable interface circuits and system level integration for ECoG/EEG recording with the on-chip spectral analysis function to provide the processed information with wireless telemetry for applications such as motion, epileptic seizure, and depression detection [5; 6; 7; 8; 9; 18; 19; 20]. However, these approaches consume huge power and they can only allow a limited number of parallel recording channels at the given power budget and exhibit a bulky system volume.