In one aspect the present invention provides methods for assessing a physiological state of a mammalian retina in vivo, the methods of this aspect of the invention each include the steps of: (a) irradiating a portion of a mammalian retina, in vivo, with light having a wavelength in the range of from 600 nm to 1000 nm at an intensity sufficient to stimulate two-photon-induced fluorescence in the retina; and (b) assessing a physiological state of the retina by analyzing the fluorescence.