11 July 2012

"I spy with my AMAZING eye..."

Human vision is incredible, with our eye being one of the most sophisticated structures for capturing light that has ever evolved. Our eyes are capable of detecting a single photon of light at night and can create complex and unbelievably definite images during the day, being able to make out structures 1/10th of a hair-span wide. Our eyes allow us to see the world in a detail that almost no other mammal (or animal) can imagine and, since humans rely mostly on our vision to interact with and perceive the world, are extremely important to our survival and quality of life.

Although light is detected by the eye, and is essential for it to function, too much light can be very damaging to our vision. Thus the size of the pupil, the black hole in its centre where light enters, can be controlled by the iris that surrounds it. The iris contains light absorbing pigments and determines how much light can enter the eye. The eye ball itself is protected by being shrunk back into its socket, with our brow protruding over it to reduce the likelihood of it being physically struck. In addition, our eyelids have eyelashes that help to stop dust and debris from falling onto its surface.

The human eye is classified as a 'lens eye', since it forms images using a biological lens that is suspended behind the pupil. Lens eyes are the most complex form of visually perceiving light and have only evolved in primates, birds and some Cephalopods (squid and octopuses), being unique in their ability to alter their focus to produce crisp images of close up objects and those that are much further away. Most organisms have a 'fixed focus' system where any object that is not at a certain and specific distance from their eye will appear blurred. The lens makes this focus possible by bending the light that passes through it so that it is refracted neatly onto the fovea (which is at the centre of the macula), at the back of the eye. The fovea is a small area of the retina where the light receptor cells that make make up the retina are particularly dense, and produces the most detailed image of our surroundings. Thus, by becoming thicker for closer up objects and thinner for those further away, the lens can refract light so that most of it lands on the fovea and a sharp, clear image is formed.

To focus on close up objects the lens is made thicker so that its refractive power is increased. This is accomplished by a contraction of the ciliary bodies so that the tension on the suspensory ligaments is reduced. This means that they pull the lens less taught and it contracts. Likewise, to focus on far away objects the converse is true: the ciliary bodies relax, causing the suspensory ligaments to tighten and the lens is pulled upon, stretching it out. This then decreases the refractory power of the lens and light is bent less.

The mammalian visual system is made up from 2 different types of photoreceptor cell: cones, which are responsible for seeing colour during the day (or in other conditions of high light intensity); and rods, which work in 'black and white' and are responsible for our night vision. These photoreceptors contain 4 different photopigments that split when photons of light hit them, producing an electrical charge. This charge is then magnified into a nerve impulse and is sent to the optic chiasm in the brain (via the optic nerve), where it is collated with other impulses from the eyes and processed to form an image. The type of photopigments present in the photoreceptor depends upon its type. Rod cells only contain rhodopsin, which is made from vitamin A (the reason why carrots, which are high in vitamin A, can improve your night vision). Rhodopsin is very sensitive and can detect a single photon of light, responding best to light at 498nm. Although this isn't enough to form an image, it shows just how sensitive human eyes are and explains why our eyes sting when we go from the dark into the light: the sudden increase in light intensity splits all of the photopigment and prevents it from being reconstructed. This is known as 'bleaching' and takes about 15 minutes to be reversed, which is why it takes your eyes a while to adjust to seeing in the dark. Rod cells show the opposite distribution to cone cells and are less concentrated in the fovea, becoming more abundant towards the edge of the retina. Thus, human night vision is at its best in its periphery and objects often become less clear to us in the dark when we look at them directly! This fact is how many scientists explain those incidences where you seem to see something out of corner of your eye that vanishes when you look to see what is was...

The remaining 3 photopigments then, are involved in colour vision and work together to form the 'pallet' of colours that humans can see. This complimentary system is called a trichromatic system and each photopigment responds best at a different wavelength of light, so that most of the electromagnetic spectrum is covered. Long Wave Sensitive (LWS) opsin responds best to wavelengths of 564nm and sees red light; Medium Wave Sensitive (MWS) opsin responds best to wavelengths of 533nm and sees green light; and Short Wave Sensitive (SWS) opsin responds best to wavelengths of 433nm and sees blue light. The brain mixes the signals coming from the 4.5 million cone cells in the retina of each eye and produces colour. Interestingly, this is the same system that early (tube) colour television sets used to form colour picture! They used thousands of units of 3 triangles placed side-by-side that were coloured red, green and blue respectively.

Many nocturnal predators, such as canids and felids (like the cat in this picture), possess a reflective layer of cells beneath their retina called the tapetum lucidum. These cells reflect light back through the retina so that each photon is detected twice. This greatly improves their night vision and explains why their eyes seem to glow in the dark.

However, despite human vision being one of the best visual systems in the world that allows us to see with a clarity experienced by very few other organisms, it is fundamentally limited. This is believed to be due to a phenomenon called 'nocturnal bottlenecking', which occurred over millions of years during the rein of the dinosaurs. Due to the size and ferocity of the dinosaurs the mammals present in this era remained very small and were only active at night to avoid predation. This meant that many genes for colour photoreception were lost, since they were not needed and were not selected for. Thus, the mammalian colour visual system had to be 'rebuilt' from only the 3 photopigments that we had left when the dinosaurs had died out and we began to display diurnal activity. This unfortunately means that our perception of the electromagnetic spectrum is very limited and, as a result, mammals cannot see infra-red or ultraviolet light (UV) light like many of the organisms in the other classes of animals.

However our evolution on the flat African savannah plains has helped to compensate for this and we have  developed a fantastic visual system, which evolved as our primary sense. Our visual system is far superior in terms of quality to that of most other organisms, even if we cannot perceive as much of the electromagnetic spectrum as them; and personally speaking, I would much rather be able to see in higher quality than in more colours so our bottlenecking may actually have worked out for the best!

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