# AI-Based Electroencephalogram Analysis in Rodent Models of Epilepsy: A Systematic Review
## 2 Fundamental Concepts and Background
### EEG Signal  
There are two types of EEG recording: noninvasive and invasive. Noninvasive EEG has electrodes placed on the surface of the scalp. While noninvasive EEG is widely used for monitoring electrical activities in the brain, it is important to acknowledge its limitations. Surface EEG, in particular, is subject to inter- and intra-individual variability, which can result from factors such as electrode placement and individual physiological differences. Additionally, EEG's spatial resolution is limited, making it difficult to precisely localize neural activity, especially in deeper brain regions. These limitations should be considered when interpreting EEG findings and underscore the need for complementary imaging modalities to provide more comprehensive spatial information. The invasive variation is most commonly used to diagnose patients with refractory epilepsy [57; 58] and on animals in neurological research [59; 60]. Invasive EEG recording can be achieved in two ways: (1) by placing the electrode on the exposed cortex of the brain under the dura, known as electrocorticogram (ECoG), and (2) by intracranial EEG (iEEG), in which the electrodes are inserted into the brain parenchyma [61; 62]. EEG scalp electrodes cannot effectively detect seizure onset from the mesial temporal, orbitofrontal, or inter-hemispheric regions because of the enfolding of two-thirds of the cortex in sulci [63]. Following the proximity to the brain, the signals measured through invasion have better resolution with higher amplitude [64] and less susceptibility to biological artefacts (eye movements, muscle activities, and heartbeats) [62; 65] compared to the scalp electrodes. The advantages of invasive versus noninvasive include the following: spatial resolution (tenths of millimetres versus centimetres), broader bandwidth (0-500 Hz versus 0-50 Hz), and higher characteristic amplitude (50-100 \(\upmu\)V versus 10-20 \(\upmu\)V) [66]. P Preclinical procedures involving the use of animals and their care must obtain ethical approval and conform to institutional and international animal protection and care guidelines. Following the ethical approval, [PERSON] et al. [67] defined two procedural steps (electrode preparation and surgical implantation of invasive electrodes into the skull) before the inducement of seizures in rodent models of epilepsy or the EEG signal waveform recording of the genetic models of epilepsy. The implanted electrodes pick up the nearby neural activities to generate EEG signals; the EEG signal undergoes amplification, filtering, and digitisation before the transmission through wired or wireless telemetry [68]. The EEG signal from the different EEG recording electrodes (also known as channels) is characterised according to amplitude, frequency, morphology, synchrony, symmetry, continuity (rhythmic, intermittent, or continuous), location, and reactivity [69]. However, frequency, amplitude, and morphology are the most used pattern-recognition methods for classifying EEG waveforms [70]. Inherent in the EEG signals is different information describing the different states of the brain through the frequency domain [71]. Human brain waves measured through noninvasive EEG are mostly composed of low-frequency oscillation (delta, theta, alpha, beta, and gamma) in the range of 0-100 Hz [72]. Additional waveforms ensue when brain activity is calibrated using invasive EEG [72]. These waveforms are high-frequency oscillations (HFOs), are categorised as high gamma, ripple, and fast ripple, and have frequencies between 100 and 500 Hz [73; 74]. The HFOs are unconventional EEG frequencies discovered to be clinically significant through digital signal processing [69]. However, HFOs are often obscured by high-frequency artefacts; analysing these frequencies requires great care [73]. Furthermore, amplitude measures waveform voltage (magnitude).