Eyes!

Visible light makes up a small fraction of the electromagnetic spectrum. In human vision, light enters the eye through the cornea, a transparent structure that helps the eye to focus. The iris controls the amount of light that enters the eye. The iris functions much like the shutter on a camera. The iris changes size in order to change the size of it's opening, called the pupil, which lets light into the back of the eye. The lens's function is to focus the incomming light onto the retina, which is the photoreceptive portion of the eye. The retina contains specialized cells called rods and cones.

In the rods and cones, light is transformed into neural energy. Attached to the rods and cones are the bipolar cells. These cells provide synaptic connections to the rods and cones. The bipolar cells connect to the ganglion cells. The ganglion cells form the optic nerve, which leads to the occipital lobe of the brain. The occipital lobe is located in the back of your head, and interprets the impulses from the optic nerves. The occipital lobe contains cells that are specialized in the detection of light patterns. Simple cells respond to a singular line of light. Complex cells respond to a bar of light that is moving is a specific direction. Hypercomplex cells respond to a moving bar of light that is a particular length. And feature detectors respond to specific stimulus patterns.

There is an area on the retina called the blind spot where there aren't any receptors. This is where the optic nerve exits the eye. By looking at the picture below, (not to close though) you can demonstrate the blind spot. Close your left eye, and stare at the cross with your right eye. Now slowly move toward the screen. The airplane should disappear. By moving closer, though, it should reappear.

Color Vision

The color that an object appears to the eye is due to the object's wavelength. Violet has a wavelength of about 400 nanometers. Red is about 650 nanometers. The wavelength of light corresponds to it's hue. Brightness and saturation also change the appearence of color objects. Brightness is the intensity of the light. Saturation refers to the complexity of the light, namely how many different wavelengths appear.

One theory on why we see colors proposes that color is simply the relative excitement of red-, blue-, or green-sensitive cones. If there is a high level of excitement in the blue-sensitive cones, then an object will appear blue. This theory is called the Young-Helmholtz or Trichromatic Theory. Additive color is a color whose wavelengths are the addition of other wavelengths. Subtractive color is color made from subtracting wavelengths. An example of subtractive color would be mixing paints.

Additive Color and Subtractive Color

Another color theory, the Oppenent-Process Color Theory, states that colors are arranged into three pairs: red and green, blue and yellow, and white and black. Color blindness is an example of this. Color blindness is caused by missing or deficient pigments in the cones of the eye. There are different types of color blindness, depending on which cone system(s) are dysfunctional. Monochromacy is where people only have black and white vision, while dichromacy is when a person has some color vision, but cannot see red and green, or blue and yellow. Color blindness is hereditary and sex linked.

Dark/Light Adaption

One way that our eyes react to a rapid change in the amount of light in our environment is by the regulation of the amount of light that reaches our eyes. To do this, the size of the pupil is altered. In a dark environment, our pupil will grow larger. In a bright environment, it will become contract and become smaller. This is not enough to adequately adjust to the many different levels of brightness that we may encounter. Thus, we need another way of regulating light sensitivity. Photopigments on the rods and cones allow this. In a bright invironment, these light-sensitive pigments are broken down. In the dark, however, they regenerate, and can make the eye up to 100,000 times more sensitive to light. Light adaptation is faster than dark adaptation.

Visual Pathways to the Brain

The part of the occipital lobe which recieves visual information is called the visual cortex. The optic chiasm is the point where the optic nerves cross over, and project to the opposite hemisphere of the brain.