English: Proposed Mechanism for Quantum Magnetoreception in Birds. Cryptochrome in cells in the retina of the eye absorb blue light and form radical pairs of electrons which can spin either in opposition or in parallel, providing an extremely sensitive quantum detector for the Earth's weak magnetic field. The direction of the field influences the proportion of the cytochrome molecules that enter different states. The direction is signalled via a cascade of neurotransmitter molecules, and then to the brain via nerve impulses in the optic nerve.