How do we see colours?

The classification of different types of light is done based on their wavelength, which can be anywhere from many meters in between each wave peak to lengths as short as the diameter of an atom. From high-energy short-wavelength light waves like cosmic and gamma rays to low-energy long-wavelength light waves like radio waves, the electromagnetic spectrum is vast.

The classification of different types of light is done based on their wavelength, which can be anywhere from many meters in between each wave peak to lengths as short as the diameter of an atom. From high-energy short-wavelength light waves like cosmic and gamma rays to low-energy long-wavelength light waves like radio waves, the electromagnetic spectrum is vast. We can see all the visible light spectrum ranges from violet light with about 400 nanometers to red light, of a longer wavelength of about 650 to 700 nanometers.

Cartoon illustrations of Rod and Cone cells

In our eyes, mainly two unique cell types known as the Rods and cones are light-sensitive cells located on the outer retina. Cones are for colour vision.  Our perception of light and dark, as well as peripheral vision, is controlled by rods. They are found in the centre of the retina, called the fovea, where light is focused. The fovea has millions of cone cells.

In human, cone comes in three varieties, each sensitive to a different range of light. They are known as blue, green, and red cones or, more appropriately (S)-short, (M)-medium and (L) for long-wavelength cones, respectively. Thus, human has a trichromatic vision. S cones make up only about 5% to 10% of the cones in the retina, and they are virtually absent from the centre of the fovea. Thus, we have more cones sensitive to the red light that means our vision is best for warmer colours like Reds, oranges and yellows. Like the rods, the cone cells are entirely colour blind as they simply respond to the number of photons that fall upon them.

 

Mantis Shrimp on the ocean floor

Other animals have different kinds and numbers of cones, which allows them to determine the globe differently. Dogs, for example, only have two forms of cones leading them to colour blind to differences between red and green. Butterflies have four kinds of cones in their eyes, letting them see UV. The animal with the best chromatic vision could be the mantis shrimp with 12 different varieties of cones.

The cone and rod cells have photopigments on their stacked membranous part. When light hits this photoreceptor known as rhodopsin in the rod cells, it contains a GPCR called opsin and a pigment retinal. This 11-cis retinal molecule absorbs the energy and changes to trans conformation, which leads to activation of opsin, which activates the G-protein, known as transducin. Transducin activates the phosphodiesterase, which cleaves the cGMP molecule, leading to the closure of cGMP dependent Na+ channels and causes hyperpolarisation in the cells. Thus, it stops generating a signal when light is available.

 

For cone cells, the situation is different; in rhodopsin, the photoreceptors are called photopsins. There are different opsins; other opsins bind to cis-retinal tunes the receptor to sense light of various wavelengths.  Rods and cones work maximally under different conditions of illumination. Rods have a lower threshold to light, whereas Cones have a higher point to light, meaning rods are more sensitive to light. A single photon in rod cells can lead to degradation of ~1000 cGMP molecules, but in cone cells, approximately 100 photons required to close the sodium channels. Thus, cones active throughout the day.

Rods are slower in response, but cones are very fast to respond to light. As rods even react to a single photon, they are saturated quickly, which means few photons lead to all channels’ closure. But the Na+ channels are also coupled with Ca2+ import. As Na+ channels are closing down, the Ca+2 concentration decreases and Ca2+ works in a feedback loop as lower calcium leads to lower phosphodiesterase activity, and it’s essential for the release of neurotransmitter. Eventually, the cGMP levels would increase, of the Na+ channels will open again.

Red cones respond equally to the 520nm and 600nm wavelength, but Green and Blue cones also react in this range. Green cones show a strong response to 520nm but weakly at 600nm. Blue cones do not respond to 600nm and show little response in 520 nm. Thus, the brain perceives it as a green signal. This varying combination of response from photoreceptors enables the brain to interpret a broad spectrum of colours.

Rainbow coloured hot air balloon

Rods and cones are also wired to your retinas in different ways. As many as 100 other rods may connect to a single ganglion cell, they all send their information to the ganglion at once. The brain can’t tell which individual activated. That is why they are not very good at providing detailed images. They only provide information about objects’ general shape and whether it’s light or dark.

The cones, by contrast, gets their ganglion cell which allows for very detailed colour vision if the lights are bright enough. Bright, brilliant colours are so intense that our photoreceptors will continue firing action potentials even after we close their eyes or look away. Our cones can get tired if we stare long enough at a brightly coloured image. Stimulation of green and blue cone for a long time, they stop responding after some time, if we look at a white canvas, we will see reddish afterimages of the previous object.

 

Cones synapse onto bipolar cells that in turn excite ganglion cells. Rods connect to specialised rod bipolar cells whose signals are conveyed through amacrine cells to the cone bipolar cells. These vertical excitatory pathways are modulated by horizontal cells that are primarily inhibitory. Through these lateral networks of inhibition and excitation, the ganglion cells send different responses in terms of frequency and duration of the action potential; these signals go to the cerebellum’s primary visual cortex through the hypothalamus. The brain analyses these responses to generate perceptions of different colours.

References:

  1.  Eric Kandel, James H. Schwartz, and Thomas Jessell, Principles of Neural Science Fifth Edition (2013)

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Arghya Das

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