The growing interest in the Metaverse has generated momentum for members of academia and industry to innovate toward realizing the Metaverse world. The Metaverse is a unique, continuous, and shared virtual world where humans embody a digital form within an online platform. Through a digital avatar, Metaverse users should have a perceptual presence within the environment and can interact and control the virtual world around them. Thus, a human-centric design is a crucial element of the Metaverse. The human users are not only the central entity but also the source of multi-sensory data that can be used to enrich the Metaverse ecosystem. In this survey, we study the potential applications of Brain-Computer Interface (BCI) technologies that can enhance the experience of Metaverse users. By directly communicating with the human brain, the most complex organ in the human body, BCI technologies hold the potential for the most intuitive human-machine system operating at the speed of thought. BCI technologies can enable various innovative applications for the Metaverse through this neural pathway, such as user cognitive state monitoring, digital avatar control, virtual interactions, and imagined speech communications. This survey first outlines the fundamental background of the Metaverse and BCI technologies. We then discuss the current challenges of the Metaverse that can potentially be addressed by BCI, such as motion sickness when users experience virtual environments or the negative emotional states of users in immersive virtual applications. After that, we propose and discuss a new research direction called Human Digital Twin, in which digital twins can create an intelligent and interactable avatar from the user's brain signals. We also present the challenges and potential solutions in synchronizing and communicating between virtual and physical entities in the Metaverse.

翻译：对元宇宙日益增长的兴趣为学术界和工业界成员创新实现元宇宙世界提供了动力。元宇宙是一个独特、连续且共享的虚拟世界，人类在其中以数字形态存在于在线平台。通过数字化身，元宇宙用户应能在环境中拥有感知存在感，并能与周围虚拟世界互动并进行控制。因此，以人为中心的设计是元宇宙的关键要素。人类用户不仅是核心实体，也是可用于丰富元宇宙生态系统的多感官数据来源。在本综述中，我们研究了脑机接口（BCI）技术的潜在应用，这些技术可增强元宇宙用户的体验。通过直接与人体中最复杂的器官——人脑进行通信，BCI技术有潜力实现以思维速度运行的最直观的人机系统。BCI技术可通过这一神经通路为元宇宙实现各种创新应用，例如用户认知状态监测、数字化身控制、虚拟交互以及想象语音通信。本综述首先概述了元宇宙和BCI技术的基础背景。接着，我们讨论了元宇宙当前可通过BCI潜在解决的挑战，例如用户在虚拟环境中体验时的晕动症，或用户在沉浸式虚拟应用中的负面情绪状态。之后，我们提出并讨论了一个名为“人类数字孪生”的新研究方向，其中数字孪生可通过用户脑信号创建智能且可交互的化身。我们还呈现了元宇宙中虚拟与物理实体同步及通信的挑战与潜在解决方案。