Physiology of Vision

Introduction

The human eye is an amazing organ that can see the light, detect colours and focus on objects. It can change its shape to accommodate near or far things of vision. The retina at the back of the eye converts light into electrical signals. These signals are sent through optic nerve fibres to various areas in your brain for processing. It is interesting to explore how this process occurs by looking at some critical physiological aspects of vision.

Physiology of Human Vision

The first step in seeing is the collection of light by the eye. The cornea and lens function by bending or refracting light onto the retina. This image is then focused on the back of the eye by adjusting the shape of the cornea and lens. When you look at something, your eyes move to bring the object of interest into the centre of your vision. This is called a saccade and it occurs very quickly, in fractions of seconds. When you look at an object continuously without blinking, there are several parts on the retina that receive light information about this image: – Fovea: the small, central area of the retina that has the highest concentration of cones and handles sharp vision – Macula: an oval-shaped region below the fovea that contains cones and handles colour vision – Optic disc: a small, dark spot at the back of the eye where the optic nerve enters the retina Your brain processes the light information collected by these different areas on the retina in several ways. The visual cortex is located at the back of your brain. It receives and interprets the signals from the optic nerve to create an image. Ganglionic layer-This layer is made up of nerve ganglia. These nerve ganglia form synapses with axons of bipolar neurons. Axons of all nerve cells combine to form optic nerve. This nerve penetrates the retina and goes to brain. At the point, at which retina is pierced by optic nerve, cones and rods are absent. So no image will be formed at that place this point is known to be “Blind spot “/Optic disc. Just above the blind spot at the optical axis of the eyeball, there is a place, where only cones are present. So this place is known as yellow spot or macula lutea or Area centralis. This image is not actually what you see when you look around. It is an interpretation of the electrical signals that your brain receives. This is why when you look at something, the image may be a little blurry or faded. Your brain also considers several factors, such as the distance of the object from you, its size and how bright it is, to create an accurate image. For example, if you were to look at a tiny object close up, you would not see as much detail as if it was larger and further away. The same is true of the brightness of an image versus how bright it appears in your eyes. For example, if you are in a dark room looking at someone’s face lit by the light of an open door behind them. Their face will appear much brighter than when there is no other light source. This process occurs through your pupil dilating and contracting. It allows more or less light into your eyes, depending on where you are looking. When light enters the eye, it passes through the cornea. It is a curved, transparent tissue covering the front of the eye. The pupil is located in the centre of the cornea. It regulates how much light enters the look by changing size (dilation or contraction). Behind the pupil is the lens, which helps focus the light on the retina. The lens is flexible and changes shape. It becomes more convex to bend or refract incoming light. It does so to focus on a single point at the back of the eye called the fovea centralis.

Types of Photoreceptors

The retina contains two types of photoreceptor cells, namely rods and cones. They are specialised cells for sensing light. There are nearly 120 million rods in the human retina, accountable for peripheral and night vision. The cones are concentrated in the central part of the retina and there are about six million of them. Cones are essential for colour vision and daytime visual acuity. When light hits the photoreceptors, they convert it into electrical impulses, which are then sent to the posterior of the eye via the optic nerve. The retina is a layered tissue that contains both rods and cones in a specific ratio depending on where it is located along this pathway. For example, only rods are present at the peripheral part of the retina, whereas cone cells exist near its central portion. The electrical impulses generated by the photoreceptors are transmitted along the optic nerve to several different areas in the brain for processing. These include the primary visual cortex (Brodmann area 17), located at the brain’s back. It is responsible for essential vision functions such as detecting light, colour and movement. The secondary visual cortex (Brodmann area 18) is responsible for object recognition. There are other areas in the brain associated with vision, including Brodmann Area 19, where visual memories are processed and stored, and Brodmann Area 37, which appears to be involved in emotional responses to images of objects or scenes.

Binocular Vision Physiology

The physiology of binocular vision is an exciting topic to explore. When our eyes work together as a team, we can see in three dimensions. This occurs because each eye sees a slightly different image due to the placement of the pupil and lens. Our brains then process these two images into one cohesive picture. Without this ability, we would have no depth perception. Vision is one of the most important senses that humans possess. This allows us to explore our surroundings and take in all types of information about them. We can see where objects are located relative to ourselves and determine what they are made up of by using binocular vision physiology. This ability is crucial for driving, walking and climbing.

Conclusion

Vision is a complex process that involves many parts of the body. There are several different types of photoreceptors located in the eye. They work together to provide us with sight. The retina contains both rods and cones, which allow for colour discrimination and depth perception via binocular vision physiology.