The invention relates to the use of gene therapy vectors to improve vision by introducing into healthy rod photoreceptor cells of a patient suffering from cone photoreceptor dysfunction and/or degeneration a nucleic acid encoding a gene product that is light-sensitive and/or that modulates endogenous light-sensitive signaling in a photoreceptor cell, such that the range of light intensities to which the rod photoreceptor responds is extended and/or the speed at which the rod photoreceptor responds to light is increased.