1. A method for improving memory or cognitive function in a subject diagnosed as having a disorder in which a diminished declarative memory is a symptom, comprising intracranially 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 cassette encoding a short hairpin RNA and comprising SEQ ID NOS: 37-40 or the short hairpin RNA encoded by said cassette, wherein the short hairpin RNA molecule specifically suppresses BACE1 gene expression in a cell of the nervous system of the subject, and wherein at least one attribute of said memory or cognitive function is improved.