Human trichromacy revisited

Proc Natl Acad Sci U S A. 2013 Jan 15;110(3):E260-9. doi: 10.1073/pnas.1214240110. Epub 2012 Dec 19.


The presence of a photopigment (melanopsin) within certain retinal ganglion cells was a surprising and significant discovery. This pigment is routinely described as "nonvisual" to highlight its signaling role in pupil dilation and circadian rhythms. Here we asked whether light absorbed by melanopsin can be seen by healthy human subjects. To answer this requires delivering intense (above rod saturation), well-controlled lights using four independent primaries. We collected detection thresholds to many four-primary stimuli. Threshold measurements in the fovea are explained by trichromatic theory, with no need to invoke a fourth photopigment. In the periphery, where melanopsin is present, threshold measurements deviate from trichromatic theory; at high photopic levels, sensitivity is explained by absorptions in four, not three, photopigment classes. We consider a series of hypotheses to explain the tetrasensitivity at high photopic levels in the human peripheral field. The most likely hypothesis is that in healthy human subjects melanopsin absorptions influence visibility.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Color Vision / physiology*
  • Female
  • Fovea Centralis / physiology
  • Humans
  • Male
  • Models, Biological*
  • Optical Phenomena
  • Photic Stimulation
  • Retina / physiology
  • Retinal Cone Photoreceptor Cells / physiology
  • Retinal Pigments / physiology
  • Retinal Rod Photoreceptor Cells / physiology
  • Rod Opsins / physiology


  • Retinal Pigments
  • Rod Opsins
  • melanopsin