# Connecting the Brain with Augmented Reality: A Systematic Review of BCI-AR Systems
###### Abstract  
The increasing integration of brain-computer interfaces (BCIs) with augmented reality (AR) presents new possibilities for immersive and interactive environments, particularly through the use of head-mounted displays (HMDs). Despite the growing interest, a comprehensive understanding of BCI-AR systems is still emerging. This systematic review aims to synthesize existing research on the use of BCIs for controlling AR environments via HMDs, highlighting the technological advancements and challenges in this domain. An extensive search across electronic databases, including IEEExplore, PubMed, and Scopus, was conducted following the PRISMA guidelines, resulting in 41 studies eligible for analysis. This review identifies key areas for future research, potential limitations, and offers insights into the evolving trends in BCI-AR systems, contributing to the development of more robust and user-friendly applications. augmented reality; AR; brain-computer interface; BCI; EEG; HMD; PRISMA; systematic review +
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Semi-invasive BCIs, on the other hand, are implanted inside the skull but positioned outside the brain's grey matter. These devices offer better signal resolution compared to non-invasive BCIs. In addition, the risk of scar tissue formation within the brain is lower in semi-invasive BCIs than in fully invasive ones. The least invasive method is the use of a set of electrodes, typically known as an electroencephalograph (EEG), which are attached to the top of the head [4]. These electrodes can detect and record brain signals. Regardless of the placement of the electrodes, the underlying mechanism remains the same: the electrodes measure small voltage differences between neurons. The signal is then amplified and filtered. Although the electrical signal is partially blocked and distorted by the skull, this non-invasive method is more widely accepted due to its relative advantages over the other techniques mentioned. The most important advantage is the safety of the procedure, as the electrodes do not require surgery to be placed [5]. Additionally, non-invasive BCIs are widely accessible and easy to use, making them suitable for a larger population without requiring extensive training. They do not restrict mobility or physical movement, allowing users to engage in various activities while using the interface. BCI systems examine the brain's electrical activity, which can be recorded using invasive, semi-invasive, or non-invasive techniques, such as electrodes positioned on top of the head. The signals are amplified and converted into digital form using preprocessing methods, and the applicable features of the signals are extracted, processed, and translated into commands capable of controlling external devices or applications. BCI systems can be categorized into three types: active, reactive, and passive. In active BCI systems, users participate in mental tasks that generate specific patterns of EEG signals. These patterns are then detected by the BCI system. The most commonly used method involves motor imagery (MI), where participants imagine moving body parts without physically carrying out the movements [6]. On the other hand, reactive BCI involves regulating brain activity in response to external stimuli provided by the BCI system. The prevalent paradigm in this category is the P300 speller, where symbols or letters are displayed sequentially on a screen, and participants focus their attention on the desired symbol. Passive BCI [7] involves solely monitoring the EEG activity of users without requiring them to engage in any mental tasks. In passive systems, the EEG activity is not intentionally manipulated for a specific purpose but rather used to extract information such as the user's emotional state. The BCI focus of this paper is presented in Figure 1.  
Augmented reality (AR) is an interactive encounter with the actual surroundings in which computer-generated perceptual information enhances the objects present in the real world.