Visual Sense

Human anatomy > Sensory organs > Visual Sense

Many people say that vision is the most importance sense. If you lose it, it will be very difficult to adapt to the colorful world. The eye is like a camera. The iris permits the entry of light; whereas the lens, cornea and humors focus it to the retina. However, humans do not see all light. Only the visible light found in the electromagnetic spectrum is perceptible to the human eye. To help you understand how the eye perceives the images around, here are some of the concepts you must know. 

Visual Sense

Image in the Retina

To focus an image in the retina, the eye should project a clear image. The light rays converge as it passes in the eye. The visual sense accomplishes the focusing by keeping the shape of the lens constant and adjusting the point at which the image will focus. The second way is by keeping the distance constant and changing the shape of the lens. Both ways create a perceivable image in the retina. 

Inverted Image

When the light rays are on the right focus, the image created is inverted. This makes the whole world inside the retina upside down. However, the action potential delivers the message to the visual cortex of the cerebrum and interprets the image as right side up. In wearing glasses that inverts the images, one person might see the world in an inverted orientation. But after a while, the brain can adapt to it and see the world again in the correct orientation. 


Light and dark adaptation occurs to assist the eye in adjusting to the light, especially when a person comes out from a darkened room. The mechanism involves the amount of available rhodopsin and pupillary reflex. The pupil enlarges in dim light to allow more light to enter the eye and contracts in bright light to receive less light. When you suddenly come out of a dark room and enter a brightly-illuminated room, your eyes have to accommodate the changes for it can totally adapt to it. 


Once your visual senses receive the input, the action potential passes through the left and right optic nerve II. The two converge to form the optic chiasm. In this area, the sensory perception from the nasal retina crosses the optic chiasm and the message goes to the opposite side of the brain. Beyond it, the optic tract receives the message and terminates it to the visual cortex in the occipital lobe, where analysis of input occurs. 

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