An illustrative system includes a brain interface system configured to be worn by a user and to output brain measurement data representative of brain activity of the user while the user is engaged in an electronic messaging session provided by an electronic messaging platform and a computing device configured to obtain the brain measurement data, determine, based on the brain measurement data, a graphical emotion symbol representative of a mental state of the user while the user is engaged in the electronic messaging session, and provide the graphical emotion symbol for use during the electronic messaging session.