Dawn of vision – And why blue light might be so special
Virtually all mammalian species exhibit some kind of circadian rhythm that affects their activities. These rhythms parallel the cycle of day and night and are, thus, in most cases linked to the perception of light. However, major surprises were in store for scientists, when they investigated whether mutant mice, lacking the well known image-forming rod and cone cells in their retinas, still possessed a circadian rhythm.
Animals lacking these photoreceptive cell types not only continued to possess their circadian rhythms, but would respond to entrainments by light to a new rhythm. Since blue light was found to be most effective in such mice and only the total removal of the eye abolished all light-induced rhythmicity, some hitherto unrecognized photoreceptive cells had to reside in the retina. Such cells would not take part in image-formation, but had to have the sole task of detecting day and night changes.
It is perhaps surprising that after almost 200 years of research on the vertebrate eye, new discoveries on photoreception were still possible. Yet, it is a fact that the non-image forming ganglion cells of the vertebrate eye were only recently discovered and are still the subjects of intensive research. Take, for example, the question “Why does blue light (and not green or red light) play such a dominant role in regulating circadian activity?” Could it be that blue light sensitivity is a remnant of the times when photoreception first appeared hundreds of millions of years ago in the archaic sea? Yes, it is possible, for the oceans still represent the largest realm on Earth and the earliest life forms are generally thought to have evolved in them. Down-welling light in the water becomes more and more monochromatic as it penetrates the water, until at greater depths it is essentially blue (i.e., of a wavelength of around 480 nm). A receptor to the blue part of the light’s spectrum would not only have been a prime candidate to maximize available brightness for image formation, but more importantly would also have been the wavelength of choice for non-image forming processes, including the timing of clockworks governing circadian and feeding rhythms.
Blue light in totally clear water is detectable down to a depth of 1,000 m and, thus, a preferred candidate for the earliest evolving photoreceptive animals, but it is also the dominant wavelength at dawn and dusk and, therefore, of excellent potential for use by later-evolving terrestrial life forms and their circadian biorhythms. Had early non-image forming, rhythm-controlling photoreceptors been responding to longer wavelengths like, for instance, red light, then there would have been no excitatory inputs below a depth of 50 m. As a consequence, the living space of early life forms (evolving in the shallow waters of the archaic ocean and employing light as the key “zeitgeber” to entrain their circadian clockwork) would have been considerably smaller. Even now, embryologically the non-image forming receptors of the retina develop before the image-forming green-sensitive rods and colour-sensitive cone photoreceptors.
The importance of blue-light perception by the non-image forming ganglion cells to tune the circadian clockwork is nowadays undisputed, but blue light sensitivity may even have more surprises in store. Lockley and co-workers in an article for the journal “Sleep” reported in 2006 that tests carried out on human subjects with blue (460 nm) and green (555 nm) lights showed that the blue light’s alerting effect “sustains attention” considerably more than the green light. It, therefore, seems that some of the blue light sensitivity that we are unaware of, influences our behaviour to a much greater extent we ever imagined and that this is an echo of the very distant past from the dawn of vision. Could that also be the reason why blue lights are now often installed at train stations as it is believed blue lights can prevent suicides?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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