The triplex theory of vision: the fourth dimension

In 1993, based on my original 1981 research on serial sections of human embryos, I proposed that vision may have three components, rather than just 2—the rods and cones. I termed the third component nonvisual retinal ganglion cells (NVRGCs)(ipRGCs: intrinsically photosensitive retinal ganglion cells NVRGCs: nonvisual retinal ganglion cells, PACAP: pituitary adenylate cyclase-activating polypeptide RHT: retinohypothalamic tract); subsequent researchers identified the same phenomenon, but used a different term, intrinsically photosensitive retinal ganglion cells (ipRGCs). I also reported nonvisual photopigments, one of which, with more recent techniques, has been identified as melanopsin. The purpose of this paper is to synthesize my earlier research with more recent findings to establish the triplex theory of vision and to describe the fourth, nonvisual, dimension of vision and its possible future applications for affecting circadian rhythm, pupillary light reaction, hormonal activities, mood changes, thermal regulation, sleep, and other nonvisual functions.