Augmenting the visible spectrum

A scorpion is brownish and it is difficult to spot in its environment made of fallen leaves and branches. Not so if we were to look at it under UV light, as birds can do. Credit: National Geographic

Spectral filtering in bird cones. a) A flat-mounted chicken retina under brightfield illumination that shows the distinctive pigmentation of the cone oil droplets. (b) A diagram of the avian single cone photoreceptors showing the relative position of the oil droplet within the cells (top) and a representation of the spectral filtering cutoff effects of the droplet (bottom). Credit: Matthew B Toomey et al./eLife

Some animals can see wavelengths (light colors) we cannot see. The visible spectrum for us is comprised between the “red” and the “violet” (with the rainbow providing a good, first hand, approximation). Shorter wavelength, infrared, and longer wavelengths (ultraviolet) are outside our eyes possibility to grasp.

Scientists are looking into the mechanisms adopted by other species to capture other wavelengths, hence to get a different view of the world. This is not necessarily and “expanded” view, since with very few exceptions animals like a bee can see in the ultraviolet range but cannot see wavelengths we can see.

Several species of birds, on the other hand, seem to be able to have the capability to capture a broader spectrum than the one we can. Finch, sparrows and more can capture the same spectrum as we do plus part of the ultraviolet wavelengths.

Scientists have discovered that these birds can “see” ultraviolet light thanks to an extra type of cone in their retina. In addition to the red, green and blue cone they share with us, they have a fourth cone sensitive to violet and ultraviolet light. The protein involved is still an “opsin” with a specific structure that makes it sensitive to UV light. Also, they have an oil droplet on the UV cone that acts as a filter making the cone even more sensitive to just the UV light (like a digital camera sensor that is covered with a filter to be sensitive to just a small range of wavelengths).

Clearly we cannot hope for an extra type of cone to be planted in our retina but these studies are opening the door to a better understanding of the “opsins” molecules and how to leverage and modify them. In the end this may result in ways to patch some issues with vision deriving from genetic drifts. It also appears that our cones, the blue one, in principle would be able to capture a broader wavelength reaching into the UV, but this is impossible because our lens is blocking UV rays (as well as the sunglasses that we wear from time to time). People who had their lens (as it happen to be my case) replaced by an artificial one no longer filter UV so that in principle the “blue” cone may detect UV light. 

Indeed, after surgery I noticed that the light has become suddenly brighter and that created problems leading me to wear sunglasses till I got used to it.

However, the processing of the data captured by our eyes is done in the brain, and our brain has not been used to process UV related signal so even if my eyes are now transmitting data related to UV I remain “UV blind”.

A science fiction writer could imagine that the UV signals now captured by my retina could be intercepted by a computer for processing and the result sent to the brain for further processing thus making UV signals meaningful.  This science fiction is what “System Metabolic Engineering” is actually trying to do… The boundaries between science fiction and science are getting more and more blurred.

Author - Roberto Saracco

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