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How We See
Vision is the sense we know the most about, but how do we see? Most people would say, “That’s easy, our eyes see for us!” In truth, it is more complicated than that. It involves our brains as much as it does our eyes (if not more). Let’s take a look at how this works from the beginning.
Energy in the form of light enters the eye through the cornea. This light passes through the pupil, which can contract (close) and expand (open). This controls the amount of light that enters the eye. The light then passes through the lens, which helps to focus the image. Finally, the light hits the retina at the back of the eye, which is made of several different types of cells. There are two main types of cells - cones and rods. Cones are heavily involved in colour vision. They are found mainly in the fovea, which is at the center of the retina. The fovea is the area of the retina where an image falls when the viewer is looking directly at something. Rods are very important for seeing movement, but only transmit information to the brain in black and white. Rods tend to be found more at the edge of the retina. So, when you want to determine if a car is red or blue, your cones are at work. When you want to catch a baseball, your rods get really involved, even though in truth, both are active at any given time.
All of the information from the rods and cones leaves each eye through the optic nerves, which cross at the optic chiasm. This is so that both sides of the brain (left and right hemispheres) get information from each eye. From the optic chiasm, the information passes through the lateral geniculate nucleus, or LGN. Then it goes to the primary visual cortex (also known as V1), which is located in the occipital lobe.
So, why do we need a brain to see? Well, for one thing, when an image in the form of light energy hits the lens, it is flipped upside down and reversed from left to right. Information in this form wouldn’t be very helpful to us. One of the first things the brain does is to take the information sent from the eye and flip it upright and right to left. The occipital lobe in the brain can then process the now corrected information. Eventually, this information gets to a part of the brain where you become consciously aware of it. When it gets to this point, you can see! You have now experienced perception! This may not seem exciting because you achieve this complicated process continually and rapidly, but it really is quite amazing. Vision is incredibly complex and this section just scratches the surface of a widely studied, yet still very exciting research area.
Individuals with myopia are commonly known as being short-sighted or near-sighted. This is because they are able to focus on close objects but distant objects are out of focus. Myopia occurs when the light that enters the eye converges to a point in front of the retina instead of directly at the level of the retina as a normal eye would do.
Myopia can occur if the eye is too long front to back (axial myopia) or the power of the refractive tissues (cornea and lens) are too high. This causes the image to be focused in front of the retina, resulting in distant objects appearing blurry.
If a person is found to have myopia, the optometrist will issue a prescription for corrective lenses based on the person’s degree of myopia. Myopia is corrected using a negative (biconcave) spectacle lens. When the concave spectacle lens is placed in front of the eye, it acts to diverge (spread out) the light and push the focal point further back onto the retina. This re-focuses the image onto the retina, causing the individual to see clearly.
Individuals with Hyperopia are commonly known as being far-sighted. This condition occurs due to the eye being too short, or the power of the refractive tissue being too low. The condition results in the light converging to a focal point behind the retina. This causes nearby objects to be out of focus. In extreme cases, distant objects can also appear blurry.
Hyperopia can easily be corrected using refractive lenses. For an individual with hyperopia, the prescription issued by an optometrist will be for a positive (biconvex) spectacle lens. When placed in front of the eye, the convex lens acts to converge (bring together) the light and pull it further forward onto the retina, allowing the individual to see clearly at all distances.
During the aging process, the eye gets weaker and becomes less able to focus on nearby objects. Presbyopia usually begins in the 5th decade of life. This is when it is common for individuals to notice that close objects become increasingly difficult to see, especially in dim light or if there is fine detail. This natural aging process occurs as a result of the crystalline lens (the structure just behind the surface of the eye which helps refract (bend) light onto the retina) becoming less flexible. Presbyopia, which gradually deteriorates, can be easily corrected with positive (convex) spectacle lenses or contact lenses.
An alternate method for the correction of both myopia and hyperopia is contact lenses. These are thin lenses that are placed directly on the eye. When an individual has a special contact lens fitting, the lenses can be extremely comfortable to wear.
Some contact lenses are thrown away and replaced after one use. There are also contact lenses that can be cleaned and stored in a special solution overnight before being re-inserted the next day. Typically these re-usable contact lenses need to be thrown away after two weeks or a month, depending on their material.
There are a number of different reasons why individuals choose to wear contact lenses. These reasons include playing sports, not liking the appearance of glasses, and not wanting to limit peripheral vision. Contact lenses are a good alternative to glasses for many people, although often contact lenses cannot totally substitute for glasses. Most people choose to correct their vision with a combination of both types of lenses.
Spotlight on Innovation
Optogenetics is a field that combines genetics (the study of genes) and optics (the study of light). Researchers in this field study how light can be used to control cells in a body. It offers a way for neuroscientists (people who study the brain and nervous system) to map out the network of specialized cells in the brain called neurons. It also gives neuroscientists a better understanding of what neurons do, how they communicate and how they behave.
To communicate with one another, neurons send chemical and electrical signals to other neurons close by. Once one neuron receives a chemical or electrical message, it can become activated (turned on) and pass on this message to other neurons.
By using the methods of optogenetics, scientists can turn neurons that they are interested in on and off. The process typically begins by finding genetic material called a gene from an organism that produces molecules that can convert light into electrical signals (see Figure 20A). One group of molecules that can do this are the proteins called opsins. The genetic material is then placed into neurons of the scientist’s choosing (see Figure 20B). Once this is done, the neuron will now be able to produce these molecules. As a result, when light is shined at the neuron (see Figure 20C), the light sensitive molecules will produce an electrical signal. This signal will activate it and allow it to send out chemical and electrical messages to communicate with other neurons around it. Research in optogenetics can provide scientists with an understanding of neurological disorders and illnesses.