Additional Study Information: This is a randomized, double-blind, placebo-controlled, parallel-group study that will evaluate the efficacy and safety of gantenerumab versus placebo in participants with prodromal (very early form) to mild AD. Participants will have a chance of receiving either injections of gantenerumab or placebo as per the visit schedule. The primary efficacy assessment will be performed after about 2 years. It is anticipated that participants will then have the option of enrolling in an open-label extension study, in which they will get gantenerumab.