Rapidly advancing AR technologies are in a unique position to directly mediate between the human brain and the physical world. Though this tight coupling presents tremendous opportunities for human augmentation, it also presents new risks due to potential adversaries, including AR applications or devices themselves, as well as bugs or accidents. In this paper, we begin exploring potential risks to the human brain from augmented reality. Our initial focus is on sensory and perceptual risks (e.g., accidentally or maliciously induced visual adaptations, motion-induced blindness, and photosensitive epilepsy), but similar risks may span both lower- and higher-level human brain functions, including cognition, memory, and decision-making. Though they have not yet manifested in practice in early-generation AR technologies, we believe that such risks are uniquely dangerous in AR due to the richness and depth with which it interacts with a user's experience of the physical world. We propose a framework, based in computer security threat modeling, to conceptually and experimentally evaluate such risks. The ultimate goal of our work is to aid AR technology developers, researchers, and neuroscientists to consider these issues before AR technologies are widely deployed and become targets for real adversaries. By considering and addressing these issues now, we can help ensure that future AR technologies can meet their full, positive potential.