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  Jorge Luis Borges wrote: “To give a blind man an image / is to give something so tenuous it can be infinite / something so vague it can be the universe.”

The October edition of The Scientist Magazine is all about the eye. Sight is definitely an all consuming sense. The world around us is defined by the sights that we become accustomed to. Science is also consumed by sight, we are continuously trying to better our sight via glasses and contacts or surgery. We also continuously attempt to give sight to those that may never have had it or may have lost it. We are astounded by creatures that share the world with us that may not have sight. Blind bats and moles are difficult for us to comprehend.  Just how do they use sounds to identify where they are and hunt for prey and everything else they need to do to survive?

labeled eye

It is awe inspiring to understand how the eye actually works. Simply put, light travels through the cornea and bends through the pupil and the lens. The lens focuses the light on to the retina, or the back of the eye where cells detect light and carry an electrical impulse along the optic nerve to the brain on the opposite side. From here the information to carried to the back of the brain where the visual cortex is. Here the brain takes over and decides what it is seeing. So someone who cannot see may have an issue in any portion of this pathway. It might be the eye, it might be the brain, or it might be something in between. It’s not so simple.

colored light

The fact that we can distinguish one color from another is a whole other story. We can only see all of the colors of the rainbow because we have a three cone system. This system only exists in some primates other than humans. All other mammals only have 2 genes that control the photoreceptor cells (cells that sense the colors of light). Each of the 3 cones allows us to see a different set of colors, one for blue (short wavelength), green (medium wavelength), and red (long wavelength) of light.

dog eye

Dogs actually see color. [Visual Neuroscience, 3:119-25, 1989] They have a similar system of cones to the one that we have. The difference being that they can see far fewer colors. They have only two types of cones. Chickens have 4.[PNAS, 89:5932-36] Many birds have this extended ability so that they can see ultraviolet light. Oil that is found on their cones works like a filter. There are other animals that don’t even have any of these particular cone cells to see color. They have other types of cells that control the ability to see color.

Bats that are blind can see using sound waves. There has been research that shows that “sounds activate visual cortex and improve visual discrimination.” This research was published in the Journal of Neuroscience 34:9817-24, 2014

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