1. A method of detecting a neurodegenerative disease in a mammal comprising: activating brain tissue of the mammal by application of radiation from a laser through an opening or a thinned portion of the mammal's skull under conditions effective to promote a simultaneous multiphoton excitation of the brain tissue and to emit a fluorescence characteristic, wherein the radiation is at an intensity level capable of being achieved by a titanium sapphire mode locked solid state laser and has a wavelength in the visible red to the infrared region of the light spectrum and is pulsed at a pulse width between about 10 −9 to 10 −15 second, said fluorescence characteristic being achieved by combining photons; comparing the fluorescence characteristic to a standard fluorescence emitted by exciting healthy brain tissue of the mammal under the same conditions used to carryout said activating; and identifying the brain tissue where the fluorescence characteristic differs from the standard fluorescence as potentially having a neurodegenerative disease.