September 26, 2011 | 2 First published on February 05, 2006. In
mammals, apart from visual photoreception (that is, image
formation), there is also non-visual photoreception. The
receptors of the former are the rods and cones that you all
learned about in middle school. The receptors for the latter are
a couple of thousand Retinal Ganglion Cells (RGCs) located in
the retina in each eye. Each of these cells expresses a
photopigment melanopsin (the cryptochrome challenger apparently
lost the contest about a year ago after several years of frantic
research by proponents of both hypotheses). The axons – nerve
processes – from these cells go to and make connections in three
parts of the brain. One is the brain center that controls
pupillary reflex – when the light is bright the pupils
constrict, while in the dark the pupils dilate. The second is
the brain center involved in the control of mood. There is still
a lot to work out about this center, but that is probably the
place where exposure to light helps alleviat