work with rodless, coneless human subjects hence has also opened the door into image-forming (visual) roles for the ganglion cell photoreceptor. The discovery that there are parallel pathways for vision was made: one classic rod- and cone-based arising from the outer retina, the other a rudimentary visual brightness detector arising from the inner retina. The latter seems to be activated by light before the former. Classic photoreceptors also feed into the novel photoreceptor system, and colour constancy may be an important role as suggested by Foster. It has been suggested by the authors of the rodless, coneless human model that the receptor could be instrumental in understanding many diseases, including major causes of blindness worldwide such as glaucoma, a disease which affects ganglion cells. In other mammals, photosensitive ganglia have proven to have a genuine role in conscious vision. Tests conducted by Jennifer Ecker et al. found that rats lacking rods and cones were able to learn to swim toward sequences of vertical bars rather than an equally luminescent gray screen. Violet-to-blue light Most work suggests that the peak spectral sensitivity of the receptor is between 460 and 484 nm. Lockley et al. in 2003 showed that 460 nm (blue) wavelengths of light suppress melatonin twice as much as 555 nm (green) light, the peak sensitivity of the photopic visual system. In work by Zaidi, Lockley and co-authors using a rodless, coneless human, it was found that a very intense 481 nm stimulus led to some conscious light perception, meaning that some rudimentary vision was realized. Discovery In 1923, Clyde E. Keeler observed that the pupils in the eyes of blind mice he had accidentally bred still responded to light. The ability of the rodless, coneless mice to retain a pupillary light reflex was suggestive of an additional photoreceptor cell. In the 1980s, research in rod- and cone-deficient rats showed regulation of dopamine in the retina, a known neuromodulator for light adaptation and photoentrainment. Research continued in 1991, when Russell G. Foster and colleagues, including Ignacio Provencio, showed that rods and cones were not necessary for photoentrainment, the visual drive of the circadian rhythm, nor for the regulation of melatonin secretion from the pineal gland, via rod- and cone-knockout mice. Later work by Provencio and colleagues showed that this photoresponse was mediated by the photopigment melanopsin, present in the ganglion cell layer of the retina. The photoreceptors were identified in