Treatment with testosterone can improve performance on tests of spatial ability in men with low testosterone levels and Alzheimer's disease. Improved performance on these tests may mean an improved ability to get around in one's environment without getting lost or injured. This could have a positive impact on both patients and those who care for them. We will investigate what areas of the brain are involved in these improvements in spatial ability. This will be done using a PET scan, which creates a 3-dimensional image of the brain that can allow us to see how the brain functions.