1. A method for improving memory or cognitive function in a subject in need thereof, comprising administering to the subject a therapeutically effective dose of a composition that decreases the expression of a beta amyloid cleaving enzyme type 1, or BACE1, in a cell of the nervous system of the subject, wherein the composition comprises a small interfering RNA molecule specific for a BACE1 gene and wherein the small interfering RNA molecule specifically suppresses BACE1 gene expression in a cell of the nervous system of the subject.