how are people colour blind? – MRC Festival Zone 2019

Question: how are people colour blind?
- Keywords:
  colour blind,
  vision
Asked by anon-220323 on 19 Jun 2019.
- Ross Hill answered on 19 Jun 2019:
  
  In our eyes we have two types of light sensors
  
  Rods – The most powerful light sensors but they cannot tell the difference between colours
  
  Cones – there are far fewer of these light sensors and they can tell the difference between colours!
  
  So people who are colour blind have problems with the CONES. This can be because of a genetic illness, or due to damage to the back of the eye where the CONES are (the macula) or due to brain damage.
- Marianne King answered on 19 Jun 2019:
  
  Red/green colour blindness is most common in men because the gene that causes it is on the X chromosome. Men have XY chromosomes and women have XX. Therefore if men have a faulty gene on their X chromosome they don’t have their other X chromosome to compensate for it, whereas women do.
- Kate Timms answered on 19 Jun 2019: last edited 19 Jun 2019 1:22 pm
  
  Hi, the others have explained how colour blindness works, so I thought I would chip in with some tools to make sure the things you make are more colour blind friendly. In science we often use red for an increase in something and green for a decrease when we make complex graphs and figures. We also use red and green in lots of images of cells and tissues where we have done fluorescence imagine. This is terrible for people who are red-green colour blind as they can’t see anything in our figures.
  
  So recently there has been an effort to try to make our figure more accessible. Afterlife, it’s pretty useless presenting data that some people can’t see!
  
  This image shows what a difference making something magenta instead of red does to a colour blind person’s experience of the image. It’s definitely something we should all think about more!
- Kaitlin Wade answered on 19 Jun 2019:
  
  Our eyes are incredibly complex organs. They’re amazing. To see, we have two different types of cells at the back of our eyes – rods and cones. Rods detect light (and give the brain essentially a black and white picture of what is in front of us but also the brightness of that picture) and cones detect colour. People that are colour blind tend to have either problems with their cones (so they don’t work properly) or they have missing types of cones. The most common form of colour blindness is red-green colour blindness – so these people have difficult distinguishing between reds and greens (and everything that red and green contribute to).
- Nina Rzechorzek answered on 19 Jun 2019:
  
  Kate had raised a really key point about how we present data – thanks Kate! The others have also covered this nicely so I thought I’d bring in some species comparisons:
  To recap, in the retina (the light-sensitive layer at the back of the eye) there are two types of photoreceptor cells (rods and cones). Photoreceptor cells have two portions: the outer and inner segments. The outer segment is the photosensitive region, and in cone cells is composed mainly of membranous invaginations. In rod cells, the outer segment contains numerous flattened membranous sacs arranged like a stack of coins. The membrane of these invaginations and sacs contains photopigments, which convert a light stimulus to a receptor potential.
  
  Around 130 million rod and cone cells are present in the retina, but about 95% of the photoreceptor cells are rods. The photochemical in rod cells is rhodopsin, responsible for perception of shades of gray. Rhodopsin has a low threshold of excitability and is easily stimulated by low-intensity light (rod cells are about 300 times more sensitive to light than cone cells). Thus, rod cells are essential for night vision. Cone cells have a higher threshold of excitability than rod cells because their photochemical iodopsin requires relatively high intensity light to be stimulated. Cone cells are thus less sensitive to light than rod cells, but they enable colour perception.
  
  In primates, each cone cell has one of three opsins: a pigment primarily sensitive to blue, green, or red colours with maximum absorptions at 445 nm, 535 nm, or 570 nm respectively. The visual system is able to mix and contrast the effect of each cone cell, so ‘colour’ is the brain’s interpretation of differences in the wavelengths of light. When the light intensity decreases to a point that it is too weak to stimulate cone cells, colour vision disappears (why night and twilight scenes appear gray). In the dog, each cone has one of two opsins: a pigment sensitive to light with a wavelength of either violet (429–435 nm) or yellow–green (555 nm). Therefore the colour vision of dogs appears to be ‘dichromatic’ and dogs are unable to differentiate (1) between yellow, orange, green, yellow–green, or red and (2) gray from greenish blue. A guide dog at traffic lights may use the position of light signals and their relative brightness rather than the actual colour of the signals!
  The above was adapted from: Uemura, E. Fundamentals of Canine Neuroanatomy and Neurophysiology, 1st Edition (2015)
- Matthew Bareford answered on 20 Jun 2019:
  
  What everyone else said! 😂😂👍😊

Question: how are people colour blind?

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