Young RE, Roper CFE, Walters JF, (1979) Eyes and extraocular photoreceptors in midwater cephalopods and fishes: Their roles in detecting downwelling light for counterillumination. Mar Biol (Berlin) 51:371–380.
Early study providing evidence that, in addition to signals coming from the eyes, extraocular photoreceptors also provide signals that contribute to the ability of some species of squid to be able to adjust the amount of light they produce to match the downwelling light in the environment.