Our brains have evolved to construct a version of reality that actively fills the gaps in our perception. They asked whether sparking those cells alone could make the brain “see” something that wasn’t really there. “Instead, illusion—and perception in general—is the brain interpreting sensory evidence based on your beliefs about the sensory world,” she says. Reality is socially anchored—it feels real because the physical bodies of other individuals are a continual confirmation for our brains. It causes a strain on the brain systems that normally keep our shared world feeling steady.