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