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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.
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?
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?
Why would evolution have lead to extraretinal photoreception in mammals?
Do the terms "extraretinal photoreception" and "non-eye photosensitive cells" mean the same thing? Other terms for the same thing?
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.
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.