COMING AT LIGHT
In the visual system there are non image-forming pathways, which involve rod and cone cells and the intrinsic photoreceptive retinal ganglion cells. Sekaran and colleagues show in a recent work published in Current Biology that two weeks before rod and cone cells become sensitive to light, the intrinsic photoreceptive retinal ganglion cells provide the earliest light detection in the mammalian retina (Melanopsin-dependent photoreception provides earliest light detection in the mammalian retina. Current Biology 15, 1099-1107, 2005).
The authors found that the photo-pigment melanopsin is expressed in the photo-ganglion cells at birth, enabling them to detect light before development of sensitivity in rod and cone cells.
The photo-ganglion cells project mainly to the suprachiasmatic nucleus, the central circadian pacemaker, and provide a measurement of environmental brightness at dawn and dusk allowing circadian time to be aligned with environmental time.
It is really fascinating to think of a functional non-image forming pathway active at birth, and many hypotheses can arise about its significance.