A Finnish firm Valkee sells light-ear-plugs against thing such as jetlag. I asked a researcher in Aalto university how do they really work and he responded "why would evolution have lead to photoreceptive cells in ears?" -No direct answer. I asked a well-respected professor who said she did not know this area well enough. Now Valkee guided me to the publications such as Penetration of light into the brain of mammals (1963) and Spectral characteristics of visible radiation penetrating into the brain and stimulating extraretinal photoreceptors (1979). Related to the first one, I found this (1980) from the references:

"It is now fully accepted that the perception of light by extraocular photoreceptors plays a significant role in synchronizing endogenous rhythms with the environmental light-dark cycle in non-mammalian vertebrates"

Things such as certain birds and lizards apparently have extraretinal photoreception aka photosensitive cells not in eyes (this is how I understand it). Now the publication continues

"The limited number of mammalian species tested to date and the near exclusive reliance on nocturnal animals leaves open the possibility of extraocular photoreception in some adult mammals (Rusak & Zucker, 1975; 1979)."

Now according to a skeptical researcher in my university, there is only one paper supporting photosensitivity in mammals' brains: Wade et al (PNAS 85 (1988) 9322-9326 with rats. My professor in system sciences was extremely scornful when I even asked this question on a seminar course about brain -- he did not specify his reasons and pretty much labelled my thinking as inexperience. Now I am not sure whether researchers are even speaking about the same issues: too large disparities between opposing and proposing teams for the assumed effect apparently through the mechanism called "extraretinal photoreception in mammals". I am very curious.

Helper questions

  1. What are the mechanisms by which a led in ear would affect a mammal such as a homo sapiens? You don't get D vitamin because of no UV light. You get very-very light heat because of tosslink connection. So it cannot sense the heat as antidote against things such as SAD and jetlag. Other mechanism?

  2. Is the "extraretinal photoreception in mammals" just placebo or does there exist scientific proofs for it particularly with large mammals about the size of homo sapiens?

  3. Why would evolution have lead to extraretinal photoreception in mammals?

  4. Do the terms "extraretinal photoreception" and "non-eye photosensitive cells" mean the same thing? Other terms for the same thing?

  5. Now eyes are developed very late in cell-division with mammals. Do born-blind mammals and later-blinded mammals experience extraretinal photoreception differently? If led-light (non-UV light) has an effect on large mammals, then I expect this may be possible to see by analyzing results of mammals with different-developed visual-cortexes.

  6. Does this statement "Light penetrates deep brain areas, eye's receptors have developed from receptors of old CNS." by Humancharger justify the extraretinal photorecetpion?

P.s. I presupposed in this question that extraretinal photoreception is the effective mechanism by which light-in-ear would affect a mammal. It is also possible that there are other mechanisms -- I am not an expert with the terminology here, anatomically and physiologically challenging.


2 Answers 2


Its pretty well established that there are photoreceptors in cells besides the cones and rods in the retina of the eye. Humans and most animals have four light receptor genes known (so far). In addition to Rhodopsin - there are the short, med and long wavelength opsin genes.

While they are mostly expressed in the retina of the eye, they can be found in many other tissues as well. The first image from GeneAtlas below shows the relative amount of RNA found for short wavelength opsin in a variety of tissues - its expressed in immune and nerve cells ( cyan and forest green respectively) relatively well too. This might imply that neurons are light reactive. Compare that to that of medium wavelength opsin, which is primarily much more common in the retina.

blue light opsin med wavelength opsin

This is not a psychosomatic effect. Light receptors in the skin are known to help with seasonal affective disorder - shine a bright or blue light behind your knees. These receptors are not connected to optical nerves, and so you don't get any image from them, but the information can affect your biochemistry anyway.

The idea of unconscious receptor inputs from other parts of the body probably applies to many sorts of receptors. This last year there has been a great deal of interest in taste receptors which are expressed in the gut. They can taste sweetness and other flavors a second time and register the gustatory response in the brain. Its not a conscious input, but it registers in the brain in MRI.

Why would evolution do this? It seems to me that this is a new way of looking at the individual life of a cell and makes a good deal of sense. If every cell has all the genes' DNA why wouldn't a little bit of receptor expression be found in any cell which could use the information? The conscious processes of the brain probably only take in a small fraction of the information that is sent in and there maybe hundreds of other such senses from various parts of the body to integrate, only a fraction of which we are aware of.

In addition, there are probably lots of cases where receptor signals are only used locally by cells that are sensing their local environment. It really doesn't make sense that the individual cells must blind themselves. Individual bacteria and fungi have scores of receptors. It makes sense that cells that are part of an organism would have as many or more senses as well.

  • 1
    $\begingroup$ -1. It is well-established that opsins are expressed widely, but their functions are not known. It hasn't been convincingly shown anywhere that human opsins confer light sensitivity to any cells other than those in the retina. The knee study you mentioned has been refuted (see my comment here: biology.stackexchange.com/a/704/72). Also, a point of nomenclature, photoreceptors are cells that are sensitive to light, and opsins are called photopigments when they are bound to a chromophore and are sensitive to light. $\endgroup$
    – yamad
    Feb 5, 2013 at 17:48

Do born-blind mammals and later-blinded mammals experience extraretinal photoreception differently?

This article in Scientific American here where they refer to "Circadian and vision neu­ro­scientist Russell G. Foster of the University of Oxford" outlines:

"In 1999 we found that mice lacking rods and cones were nonetheless able to synchronize their circadian rhythm to the light/dark cycle. These observations led to the discovery of an additional photo­receptor system in the retina of humans and other mammals consisting of a small number of photosensi­­tive retinal ganglion cells (called pRGCs). These cells are most sensitive to blue light, and, significantly, blue light is most effective in alleviating the symptoms of SAD." (I added italics to interesting points)

WSJ repeats the blue-light thing and recites:

"In 2002, German researchers isolated one from green algae—a class of proteins called channelrhodopsins—that responded only to blue light. -- Depending on just how researchers tailor the virus that carries the light-sensitive protein, they now can target almost any type of neuron they want to study."

So differently in a way that they lack the cones/rods in retina and some links to visual-cortex but no in a way of SAD: evolution have apparently developed methods for blind-born mammals to survive and to sync their inner clock. This resonates with the earlier facts by shigeta but I don't know whether there is any other thing to light -- perhaps UV radiation -- that could tune the internal clock.

It looks that they are trying to find ways to engineer bionic-visions things similar to dobelle without physical modification of skull. The viruses tries to propagate certain photo-receptive protein into some spot where they try to activate them though external radiation. This is called optogenetics where they are trying to control certain behavioral things with optics.

New puzzles

  1. why do they use protein? Why not other things?

  2. do we have many kind of photoreceptivity? Certain muscles (requires protein) activated by EM radiation? What about other cells such as fat-cells?

P.s. this is pretty fruitless to advance unless someone provides precise power spectrum of Valkee (requires apparently spectrum-analyzer) and polarization experiments. WSJ referred to trials where they use monochromatic light and coherent light in testing while Valkee-light can be pretty much anything.

  • $\begingroup$ There may be some light receptor cells at skin (well known, high sunlight cause more melanin in skin and also ultraviolet helps Vitamin-D synthesis). But I can't "see" any trace of light through skin, even if I tightly close my eyes. $\endgroup$ Sep 19, 2016 at 18:50

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