# Electrically Equivalent Head Tissue Materials for Electroencephalogram Study on Head Surrogates
## 1 Introduction  
Electroencephalograms (EEGs) and Magnetoencephalography (MEG) are used for sensing and localizing the neural activities of the brain where EEGs record the electrical field on the scalp and MEG records the magnetic field [1] of the brain tissue. These techniques are non-invasive, which provide excellent resolution for measuring brain functionality and connectivity. As such, with the advent of wearable sensors for real-time brain health monitoring, EEGs and MEG can be potential tools. EEGs or MEG are typically carried out in medical diagnostics where the system is used in a highly controlled and static environment. For mobile wearable applications, EEG or MEG studies need to be carried out in a dynamic and open environment. Currently, the clinical data of EEG/MEG as wearables or even as a mobile system are very limited. To advance the technologies further, researchers have leaned on applying EEGs or MEG on realistic phantom heads. Recently, phantom (alternatively termed as simulant) heads with matching material properties of head/brain tissues have been investigated for the confirmation of source localization techniques in inverse problems [2; 3; 4], analysis of EEG signal processing techniques [5], the validation of MEG data measurement [6], etc. In principle, human head tissue simulant materials should have the features and details of a real human head tissue. As shown in Figure 1, the major layers in a real head tissue include the skull, meninges, and brain tissue. The skull is like a sandwich structure where the spongy bone forms the core, and the cortical bone forms the outer layers. The meninges primarily include the dura mater, arachnoid mater, CSF, and pia mater. The most inner layer of tissue is the brain tissue, and it can be subdivided into white matter and grey matter. Over the years, many researchers have built various types of head tissue phantoms based on the intended applications. In other words, phantoms are only designed to match certain types of physical properties but not all. For our research, we focus on phantoms that mimic the electrical properties of head tissue. It can be understood that the design of the phantom head's shape and the corresponding composition of the head simulant can heavily influence the analysis of brain activities. Therefore, if the phantom material's properties do not match the properties of head tissue materials, the mismatch may yield irrelevant data. As such, the use of property-matching brain-simulant material is important to make a reliable phantom head. For the purpose of EEG analysis, electrical conductivities are the most important parameters in accurate source localization and construction [7]. It is also important for the accurate understanding of the electrical activity in the live and postmortem brain tissues using EEG and MEG [8], which is critical to understanding the accurate neurological functions [9], neurological and psychiatric disorders [10], probable treatment [11], and recovery observations [12; 13]. As such, the focus here is developing simulant materials with matching conductivities of head tissue materials. This is because any variation in Electrical conductivity in simulated human head layers has a huge impact on EEG source localization methods. In dipole reconstruction analysis, the conductivity of the skull and skin can impact the source localization up to 3 cm [14; 15]. The human brain conductivity of both the white matter and grey matter layers has less impact on source localization but it can impact the strength and orientation of the sources [11]. Obtaining a skull from a cadaver is often difficult, and the experimental setup is complex and limiting. For this, many studies have been conducted to mimic the electrical properties of human head tissue layers. In [5; 16], an effective single-layer head tissue model has been used. As different conductivity layers of head tissue can have different loss factors,  
Figure 1: The red dotted part shows the detailed human head tissue layers.