The main purpose of this study is to evaluate the safety of LY3954068 in participants with early symptomatic Alzheimer's Disease (AD). The study will also investigate how much LY3954068 gets into the bloodstream and will test the effects of LY3954068 on markers of AD. The study will be comprised of two parts, A and B. Each enrolled participant in Part A will receive a single dose of LY3954068 or placebo (no active drug) given into the spinal fluid. Each participant in Part B will receive 2 doses of either LY3954068 or placebo administered into the spinal fluid. Participants will have the opportunity to join an optional bridging period to a separate potential study where participants would receive LY3954068. The study will last up to approximately 45 weeks for Part A, and 100 weeks for Part B, including the screening period.