Scientists are peeking inside living brains to watch for the first time as a toxic duo of plaques and tangles interact to drive Alzheimer's disease -- and those tangles may predict early symptoms, a finding with implications for better treatments. It's not clear exactly what causes Alzheimer's. Its best-known hallmark is the sticky amyloid that builds into plaques coating patients' brains, but people can harbor a lot of that gunk before losing memories. Now new PET scans show those plaques' co-conspirator -- the tangle-causing protein tau -- is a better marker of patients' cognitive decline and the beginning of symptoms than amyloid alone. That's especially true when tau spreads to a particular brain region important for memory, researchers reported Wednesday in the journal Science Translational Medicine.