# Connecting the Brain with Augmented Reality: A Systematic Review of BCI-AR Systems
###### Abstract
This enhancement can involve multiple senses, such as sight, sound, and touch. AR can be described as a system that combines elements of the real and virtual worlds, allowing  
Figure 1: BCI focus of this paper. for real-time interaction and accurate 3D alignment between virtual and real objects. The additional sensory information can either enhance the natural environment (add virtual content in real-world elements) or mask it (hide or override real-world elements). The AR experience seamlessly blends with the physical world, creating an immersive perception within the real environment. Smart glasses offer two primary methods for displaying AR content: optical see-through and video see-through. Video see-through systems utilize cameras embedded within the head-mounted device to present video feeds. This is the conventional approach employed by smartphones for AR applications. Video see-through is particularly advantageous when remote experiences are desired, such as controlling a robot to fix something from a distant location or virtually exploring a potential vacation destination. It is also beneficial for utilizing image enhancement systems like night-vision devices. On the other hand, optical see-through systems combine computer-generated imagery with a real-world view seen through the glasses via a semi-transparent mirror. This method is useful in scenarios where concerns arise about potential power failures. An optical see-through solution allows users to maintain visual perception in every situation. Additionally, if high image quality is a priority, portable cameras and fully immersive head-mounted displays cannot match the experience of direct viewing provided by optical see-through technology. Various review attempts have been made in the literature to demonstrate the brain's connection with alternative realities. However, most of them focus on virtual reality and distinct applications like patient rehabilitation. A comparative analysis is presented in Table 1 to showcase the existing review attempts. [PERSON] et al. [8] conducted a review in 2012, highlighting the existing BCI-VR applications. The articles were categorized according to the neurophysiological signal used to drive the BCI (MI, P300, SSVEP). [PERSON] et al. [9] reviewed the use cases of virtual and augmented reality-based BCI applications for smart cities. The review was conducted in 2022, and the papers included were divided into two main categories depending on the type of reality (virtual or augmented). [PERSON] et al. [10] provided a comprehensive picture of the current state-of-the-art SSVEP BCIs in AR environments. The search was conducted on the Scopus database using the AR and SSVEP keywords and covering the last 6 years (2018-2023). Out of the 56 articles retrieved, 20 of them were thoroughly compared based on EEG acquisition, EEG processing, and BCI application. [PERSON] et al. [11] conducted a systematic review focusing on EEG-based BCI applications in immersive environments. The search was performed in four online databases (ACM, IEEE Xplore, PubMed, and Scopus), resulting in 2982 papers. The final number of articles to be assessed was 76, and they covered the last decade (2012 to 2022). The structure of the results contained the following sections: trend by year, application domains, trend by country, features of the VR/AR application, BCI paradigms, EEG acquisition, EEG signal processing, BCI interaction tasks, system evaluation, study findings, and challenges. This work presents a systematic review of EEG-based BCI applications in AR environments. To the best of our knowledge, this is the first systematic review focusing explicitly on immersive AR environments projected on HMDs. This review spans from 2012 to 2024 and exclusively includes applications involving only healthy participants. A search was performed in three online databases (IEEE Xplore, PubMed, and Scopus)