• Describe the anatomy of the visual system
• Understand how light waves are related to vision
• Describe the main theories about color vision
• Understand monocular and binocular cues and the perception of depth

The visual system constructs a mental representation of the world around us. This contributes to our ability to successfully navigate through physical space and interact with important individuals and objects in our environments.

Anatomy of the Visual System

The eye is the major sensory organ involved in vision (Figure 1). There are several parts of the eye from the front to the back side, including the cornea, pupil, iris, lens, retina, fovea, and optic nerve. The cornea, pupil, iris, and lens are situated toward the front of the eye. At the back are the retina, fovea, and optic nerve.

Click through the following slideshow to learn the anatomy of the eye, and practice what you’ve learned.

Now let us dive into each of the parts in detail.

Anatomy of the eye

• The cornea is the transparent covering over the eye. It serves as a barrier between the inner eye and the outside world, and it is involved in focusing light waves that enter the eye. Light waves are transmitted across the cornea and enter the eye through the pupil.
• The pupil is the small opening in the eye through which light passes, and the size of the pupil can change as a function of light levels as well as emotional and physiological arousal. When light levels are low, the pupil will become dilated, or expanded, to allow more light to enter the eye. When light levels are high, the pupil will constrict, or become smaller, to reduce the amount of light that enters the eye.
• The iris is the colored portion of the eye. It is connected to the muscles that control the pupil’s size.
• The lens is a curved, transparent structure that serves to provide additional focus for light entering the eye. Light crosses the lens after passing through the pupil. The lens is attached to muscles that can change its shape to aid in focusing light that is reflected from near or far objects.
• The retina is the light-sensitive lining of the eye and is located at the back of the eye.
• The fovea, which is part of the retina, is a small indentation in the back of the eye. In a normal-sighted individual, the lens will focus images perfectly on fovea. The fovea contains densely packed specialized photoreceptor cells, known as cones, which are light-detecting cells. Another type of photoreceptor is rods. See Figure 2.
• Cones are specialized types of photoreceptors that work best in bright light conditions. Cones are very sensitive to acute detail and provide tremendous spatial resolution. They also are directly involved in our ability to perceive color.
• Rods are specialized photoreceptors that work well in low light conditions, and while they lack the spatial resolution and color function of the cones, they are involved in our vision in dimly lit environments as well as in our perception of movement on the periphery of our visual field.

Note that while cones are concentrated in the fovea, where images tend to be focused, rods, another type of photoreceptor, are located throughout the remainder of the retina.

Sighted people have all experienced the different sensitivities of rods and cones when making the transition from a brightly lit environment to a dimly lit environment. Imagine going to see a blockbuster movie on a clear summer day. As you walk from the brightly lit lobby into the dark theater, you notice that you immediately have difficulty seeing much of anything. After a few minutes, you begin to adjust to the darkness and can see the interior of the theater. In the bright environment, your vision was dominated primarily by cone activity. As you move to the dark environment, rod activity dominates, but there is a delay in transitioning between the phases. If your rods do not transform light into nerve impulses as easily and efficiently as they should, you will have difficulty seeing in dim light, a condition known as night blindness.

Rods and cones are connected (via several interneurons) to retinal ganglion cells (see Figure 2 again). Axons from the retinal ganglion cells converge and exit through the back of the eye to form the optic nerve.

Optic Nerve

The optic nerve carries visual information from the retina to the brain. There is a point in the visual field called the blind spot (not shown in Figure 1): Even when light from a small object is focused on the blind spot, we do not see it. We are not consciously aware of our blind spots for two reasons: First, each eye gets a slightly different view of the visual field; therefore, the blind spots do not overlap. Second, our visual system fills in the blind spot so that although we cannot respond to visual information that occurs in that portion of the visual field, we are also not aware that information is missing.

The optic nerve from each eye merges just below the brain at a point called the optic chiasm. As Figure 3 shows, the optic chiasm is an X-shaped structure that sits just below the cerebral cortex at the front of the brain. At the point of the optic chiasm, information from the right visual field (which comes from both eyes) is sent to the left side of the brain, and information from the left visual field is sent to the right side of the brain.

Once inside the brain, visual information is sent via a number of structures to the occipital lobe at the back of the brain for processing. Visual information might be processed in parallel pathways which can generally be described as the “what pathway” (the ventral pathway) and the “where/how” pathway (the dorsal pathway). The “what pathway” is involved in object recognition and identification, while the “where/how pathway” is involved with location in space and how one might interact with a particular visual stimulus (Milner & Goodale, 2008; Ungerleider & Haxby, 1994). For example, when you see a ball rolling down the street, the “what pathway” identifies what the object is, and the “where/how pathway” identifies its location or movement in space.