Science of the Senses: Sight

Eyesight is one of those things that is often taken for granted, myself included until recently. If you’ve read my previous post, you’ll know I had surgery recently and thankfully it was a success. I had a chronic detached retina and had no vision in the top half of my right eye. Luckily the detachment hadn’t reached a certain point (macula) so once the blurriness has worn off, it should leave no lasting damage to my vision. While at the eye infirmary, waiting to go down to theatre I got talking to the other ladies on my ward. One was in to have her eye ‘pulled straight’ as the muscles in one side had caused her eye to turn inwards on itself and the other was in for laser surgery to reduce the pressure on her eyes following a routine eye test at the opticians where she just assumed she needed a new pair of glasses. It has fascinated me how one small change can have a massive effect on the wonder of sight.

But how do we see in the first place?

I covered this as part of my undergrad so I’ll give it a go at explaining the wonder of sight.

The eye is like a camera. It focusses light onto a light sensitive surface (retina) using the cornea and lens. The aperture (pupil) alters in size depending on the amount of light entering. In broad terms, vision as a whole is then followed by the light energy converted into an electrical signal and neural processing of the signal to produce an image.

The cornea and the lens work together to bend the light so it focuses on the retina which contains photoreceptors – rods and cones. The retina is like a film over the back of the eye and in the middle is the macula. This acts like a pair of sunglasses to capture UV rays and stop them from damaging the retina. Rods are responsible for vision in low lighting (black and white vision) while cones are responsible for sharp, coloured vision in the daytime. These photoreceptors contain pigments that change the light energy entering the eye into an electrical signal. Cones have pigments for 3 colours: red, green and blue light. The pigments are excited by differing light and it is this excitement at a particular wavelength that relates to a particular colour – why we see an object as blue, yellow, pink, purple etc.

Image: Google Images

The electrical signal then travels along the optic nerves to be processed in the visual cortex of the brain. The signal has all the information our brain needs to understand what it is we’re seeing and produces an image. While travelling along the optic nerves, the signal enters the brain at the optic chiasm where some of the information from each eye is crossed over to the opposite side of the brain for processing in the binocular zone. The view of the objects we’re seeing are slightly different in each eye, so processing the information on the opposite side of the brain allows the image produced to have a 3D perspective.

So we actually see with our brains but it’d be impossible without the collection of visual information from our eyes. Sight happens at the speed of light so we’re able to see in real-time. An intricate and fascinating mechanism, which is much more complicated than I’ve explained but I hope I’ve done the topic justice.

Until next time,

 

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