  Over the past decade, extended reality (XR) has emerged as an assistive
technology not only to augment residual vision of people losing their sight but
also to study the rudimentary vision restored to blind people by a visual
neuroprosthesis. To make the best use of these emerging technologies, it is
valuable and timely to understand the state of this research and identify any
shortcomings that are present. Here we present a systematic literature review
of 227 publications from 106 different venues assessing the potential of XR
technology to further visual accessibility. In contrast to other reviews, we
sample studies from multiple scientific disciplines, focus on augmentation of a
person's residual vision, and require studies to feature a quantitative
evaluation with appropriate end users. We summarize prominent findings from
different XR research areas, show how the landscape has changed over the last
decade, and identify scientific gaps in the literature. Specifically, we
highlight the need for real-world validation, the broadening of end-user
participation, and a more nuanced understanding of the suitability and
usability of different XR-based accessibility aids. By broadening end-user
participation to early stages of the design process and shifting the focus from
behavioral performance to qualitative assessments of usability, future research
has the potential to develop XR technologies that may not only allow for
studying vision loss, but also enable novel visual accessibility aids with the
potential to impact the lives of millions of people living with vision loss.
