Vision is one of our five special senses. Vertebrate eyes
have two major components to the vision pathway: the optical part and the
neural part. The optical part is responsible for gathering and focusing light
and the neural part is responsible for converting an optical image into a
neural code in the occipital lobe of the brain.
The pathway of light through the eye consists of passing
through the cornea and aqueous humor of the anterior chamber, passing through
the pupil, striking the lens, and then passing though the vitreous humor in the
posterior chamber. Finally, the focused light will hit the retina at the back
of the eye. The light must pass between ganglion bipolar cell layers in order
to reach the rods and cones, which are our photoreceptors. Photoreceptors are
responsible for capturing photons and transducing the light energy into
chemical energy in order to create action potentials and relay information to
other optic neurons in the retina. The photoreceptor I will be focusing on are
the cones.
Our eyes can only detect visible light, which has
wavelengths between 390 – 700 nm. It is estimated that humans can distinguish
around 2-7 million different colors through the use of cone cells. Humans
typically have three variations of cones: red, green, and blue and are
influenced by the expression of opsin genes (Jameson, 2009). However, it is
hypothesized that some humans are tetrachromats instead of trichromats. This
means that they have four variations of cones and can potentially identify and
perceive additional colors due to additional wavelength absorption and
signaling to the optic nerve, thalamus, and occipital lobe.
Many different species demonstrate tetrachromacy such as
birds, fish, insects and it isn’t unreasonable to suspect the evolutionary
development of tetrachromacy in humans based on development within other
species (Jameson & Wasserman 2006). There is limited research on
tetrachromacy in humans, but one famous artist has been identified as a
tetrachromat. Her name is Concetta Antico and she can supposedly differentiate
100 million more colors than trichromats.
Additional research is required to better understand the
purpose behind the development of tetrachromacy and since human subject
research would be required, it is important to keep biomedical ethics in mind.
There is no known physiological benefit to being a tetrachromat, so any
research would need to focus on non-maleficence and doing patients no harm when
there is little to no benefit to be gained.
References:
Jameson K. A. (2009). Human Potential for Tetrachromacy. Color (2.3).
Jameson, K. A., Winkler, A. D., Herrera, C., Goldfarb, K. (2014). The Veridicality of Color: A case study of potential human tetrachromacy. IMBS Technical Report Series. Retrieved from: https://pdfs.semanticscholar.org/313c/ebb88197fba22a2ec96fdcea9171395268da.pdf
Jameson, K. A., Bimler, D., & Wasserman, L. M. Re-assessing Perceptual Diagnostics For Observers With Diverse Retinal Photopigment Genotypes. Retrieved from: http://www.imbs.uci.edu/~kjameson/PICS.pdf
Concetta Antico - The Color Queen. Tetrachromatic Artist. (n.d.). Retrieved from https://concettaantico.com/#supervision.
I learned about this artist a few years ago and thought it was super cool. I’ve always wondered if the way any of us perceive color is exactly the same as anyone else or if we’ve just collectively decided that ‘orange’ is whatever color we see when we’re first introduced to the word. There’s a hypothesis out there that a certain percentage of women are tetrachromats, or at least have the genes for it, and are unaware. This may account for some disagreements about what color schemes are complementary (I don’t know if anyone else has that kind of disagreement in their families...). Apparently daughters of colorblind men who have a mutation in one kind of cone can inherit that along with the other two normal cones and a normal copy of the mutant one from their mother, though the mutation may render those cones completely ineffective. Anyway, I agree that it’s super interesting.
ReplyDeleteGabriele Jordan, Samir S. Deeb, Jenny M. Bosten, J. D. Mollon; The dimensionality of color vision in carriers of anomalous trichromacy. Journal of Vision 2010;10(8):12. doi: https://doi.org/10.1167/10.8.12.