The Eye and the Visual World

The eye is our window to the universe. No other sense do we so richly associate with the human experience than sight. From our first memories as children to the first rendering of a Picasso, our eyes reveal part of the mystery of life. Vision and the world around us has baffled, exhilarated, and inspired humans throughout the millennia. How the eye functions though, has been more elusive. In the following sections, we will overview the functions of the eye, its connections to the brain, and see how we turn radiant electromagnetic energy into Monet's Waterlilies.

Where it all begins…Light.
We would have no vision if it wasn't for our friend, radiant electromagnetic energy, or light. Of course, only a small portion of the light spectrum is visible by the eye. Light has physical characteristics similar to sound. Like sound waves, light moves in continuous waves. These light waves are like the tide of an ocean. They consist of a rise and a subsequent fall. The rise and fall of light waves is called frequency. The respective frequency of light waves determines its wavelength.

Light has different qualities that have unique effects on the human eye. Wavelength, for instance, is what the eye perceives as color. Different wavelengths create different colors for the eye (we will discuss color perception later). The intensity of the light waves, on the other hand, refers to the brightness perceived by the eye. The more intense a light source, the brighter we humans experience it (Schiffman, 49).

Light Reception and the Anatomy of the Eye
The eye is a complex structure. In this section we will outline the different parts of the human eye and their functions.

The Cornea
The commonly referred "white" of the eye is actually called the sclera. The sclera meets up with a translucent membrane called the cornea. For our eyes to convert light into vision, light waves must be refracted into focus at the back of the eye. The cornea refracts these rays to the surface in the back of the eye, called the retina.

The Iris
The iris is the concentric ring that is the color of our eyes. The iris is also one of the best ways to identify us as individuals, even better than fingerprints (Schiffman, 53). The iris is disk-shaped and consists of two muscles. Like a diaphragm for light, the iris regulates how much light comes into our eyes. In bright conditions, the muscles of the iris limit how much light comes in. In lowlit conditions, the muscles of the iris increase the amount of light comming in. In this picture the iris is compared to a camera's diaphragm.

Source: http://www.photo.net/photo/edscott/vis00010.htm

The Pupil
The pupil is the black opening surrounded by the iris. It is where light is emitted into the eye. Pupil size is controlled by the iris and its muscles.

The Lens
The lens is located behind the cornea and contains two transparent fluids: aqueous humor and vitreous humor. Aqueous humor maintains the shape of the eye and provides the metabolic requirements of the cornea. The vitreous humor is located behind the lens and it helps to hold the lens in place (Schiffman, 55). The lens works to fit visual information into a meaningful manner to the eye.

The Retina
The retina is the place to which the lens reflects light. The retina is composed of photoreceptors that absorb light and transform that light into neural messages in the brain. The retina is where we finally get a link from the outside world of light to the human perception. The retina has two kinds of receptors: rods and cones.

Rods and Cones
Rods and cones both have light-absorbing pigments. Rods are found in the peripheral part of the retina (Schiffman, 55). Rods are used for scotopic vision, which can be described as night vision or black and white vision. Rods have a high sensitivity to light due to their use in low light conditions. Cones on the other hand are primarily found in a small indentation called the fovea (the focal point of images entering the eye). Cones are used for photopic vision or high intensity light, like that of daytime light. They have a lower sensitivity to light.

Also found in the retina is the macula. The macula is a yellowish pigmented section found on the fovea. The macula functions to clarify images on the fovea. Deterioration of the macula leads to blindness.

We have discussed the anatomy of the eye and the nature of light passing into it. We will now focus on the eye's connection to the brain and how and why we see what we see. From light on the retina to the conscious recognition of one of your friends, perception begins and ends in the mind.

The Optic Disk
The optic disk is the place where all the optic nerves and their messages meet and enter the brain. It is also called the blind spot because no light receptors exist here and therefore, nothing can be seen. The optic nerves pass through the optic disk and then reach the optic chiasm.

The Optic Chiasm
Once the optic nerves pass the optic disk, they reach an X-shaped region called the optic chiasm. As pictured, images from the left visual field enter both eyes. On the left eye, the image enters in the nasal half. On the right eye it enters the temporal side. The different sides (nasal and temporal) refer to where that image is past along to the occipital lobe in the brain.

Pay no attention to the LGN, or optic tracts and radiations labeled on the picture, they will be discussed in the next section. The important part to realize is that each half of the visual field is projected on the opposite side of each eye and onto the opposite side of the occipital lobe of the brain (Schiffman, 72).

The Lateral Geniculate Nucleus
The lateral geniculate nucleus is a cluster of neurons located in the thalamus of the brain. Cells stemming from the retina synapse at the lateral geniculate nucleus. The lateral geniculate nucleus is basically an interpretation of the retina's signals. The cells of the lateral geniculate nucleus lead to the visual cortex or the optic radiations.

The Visual Cortex or Optic Radiations
The fibers from the lateral geniculate nucleus lead to a group of fibers called the optic radiations. The optic radiations are a cluster of neurons that end up or synapse in the occipital lobe. The visual cortex is another name given to this area. It is in the occipital lobe that vision reaches consciousness. It seems like a far way to go for something that appears instantaneous.

You see with your eyes, not your hands…
Well, actually it's a little bit more complicated than that, as you can see. We actually see with our brain. Our eyes take in light (electromagnetic radiant energy), the light is projected to the back of the eye, or retina. The projection of the retina is then interpreted by cells that carry a signal along a pathway to the brain. Then, in the occipital lobe, light is turned into information that can be interpreted by the brain.