Human Visual System

Human visual system consists of the Eyes, several parts of the brain, and the pathways connecting them.

The Eye

Eye consists of two systems:

1. Image forming system

2. Image transduction system

In this article, we will just discuss image forming system.

Image Forming System

This system consists of following structures:

Cornea

Cornea is the front surface of eye. Light enters the eye through cornea. It is a transparent protective tissue covering front of the eyeball. As the cornea is deeply curved, it bends the rays of incoming light and helps to focus them.

Pupil

Pupil is a circular opening which contracts or expands automatically, in response to incoming amount of light. Therefore, the size of pupil opening depends on the amount of light in the surroundings.

Iris

This is the colored part of the eye that ranges from light blue to dark brown.

Sclera

The sclera is commonly known as "the white of the eye." It is the tough, opaque tissue that serves as the eye's protective outer coat.

Lens

Located behind the pupil, focuses the light by changing its own thickness, through a process called Accommodation (the ability of the lens to vary its shape in order to focus incoming images on the retina)

Kind of accommodation depends on the location of the object

Distant objects → relatively flat lens

Close objects → Thick, rounder lens

Having traveled through pupil and lens, light waves reach its ultimate destination in the eye.

Retina

The retina is the series of the nerve cells at the back of the surface of the eye. Retina converts the electromagnetic energy of into useful information for the brain. There are two types of the light sensitive receptor cells in retina:

Rods

Rods are long and cylindrical in shape. Rods are more sensitive to light and work in dim light. They respond to difference in brightness and are most important for seeing in dim light. They are sensitive to light waves in 500nm range, or to blue and green light. The images produced by rods are in black, white and gray.

Cones

Cones are short, thick and cone shaped. Cones respond to color or hue. They require more light. They are responsible for sharp focus and color perception, particularly in bright light. Cones are more sensitive to wavelength of 550nm range but they respond to all colors. They are tightly packed together in the middle of retina and this is the area around fovea.

Fovea

Fovea is the part of retina where the concentration of cones is greatest. Images are seen more sharply when they fall on the fovea. It helps in focusing on something of particular interest.

Optic Nerve

Optic nerve is the fiber composed of many neurons and is located at the back of the eyeball that carries neural impulses (visual information) from the eye to the brain.