Perception Laboratory: Visual Processing

Purpose: The purpose of this laboratory is to familiarize you with the visual system, from the eye to the brain. You will also learn about various disorders associated with the visual system.

1. Anatomy of the Eye

Illustration Courtesy of National Eye Institute, National Institute of Health

Use this figure to identify the major structures of the eye. On your lab worksheet, identify each of the structures labeled with a letter, then describe the function of the structure. You should use the web resources to attempt to provide information beyond that found in your textbook.

To aid you in better understanding the function of each of the structures, here are some web resources:
Peter Kaiser's web-book The Joy of Visual Perception...especially Master Diagram of the Eye.
Webvision's Gross Anatomy of the Eye.
Cole Eye Institute's Eye Anatomy Tutorial
eMedicineHealth's Anatomy of the Eye.

2. Anatomy of the Retina

The primary function of the eye is transduction (changing light energy into neural energy). Transduction takes place in the photoreceptors of the retina. Thus, the retina is a vitally important part of the eye. On your lab worksheet, identify each of the structures in the highly schematic figure below.

To learn more about the retina (and review some material on the eye), go through a tutorial developed by Diana Molavi on the Eye and Retina. Stop when you get to section F (on lateral inhibition). Later on we'll return to the concept of lateral inhibition. Pay particular attention to the figure of the retina. Note the actual cross section of the retina and the way that researchers label the various layers of the retina.

Ready to explore the retina in greater detail? Let's use a couple of sites to learn more about the retina. Though the sites provide more detail than you'll need for a basic understanding of retinal function, it's good to gain a sense of the web resources available to you.

One site is Lance Hahn's Retina page. The other site is the Webvision (Kolb, Fernandez, & Nelson). Using these two sites, learn more about each of the retinal structures and write down what you've learned on your worksheet. (For instance, how many different types of horizontal cells might exist?) Try to write down some information about each structure that is not found in your textbook.

3.Disorders of the Eye

The optical structures of the eye work to provide us with visual acuity (the ability to see fine details in a scene). However, visual acuity is dependent on the proper functioning of the whole array of structures that we have investigated in this laboratory. For example, if the image is not focused clearly on the retina (particularly the fovea) we will not see clearly. If the image is focused clearly on the retina, but the optic nerve is damaged, then we will experience some visual deficit. If the visual information reaches the visual cortex, but an area of the visual cortex is damaged, then we will experience some visual deficit (which will likely porduce a deficit in acuity).

Visual acuity is typically expressed as a ratio, with 20/20 vision considered normal. Legal blindness occurs when the best corrected vision is 20/200 (or the visual field is 20 degrees or less). Visual acuity can be assessed in a number of ways. The most common means of assessing visual acuity is the Snellen Chart. Another approach is the use of Landolt C rings, which don't require any knowledge of the alphabet. Another test with the same benefit is the random E (or tumbling E) test. In either test, the patient simply points in one of four directions (up, down, left, right) to indicate the position of the opening in the C (Landolt C rings) or the direction of the prongs of the E (tumbling E test).

Probably the most common visual disorders affect visual acuity by causing images to focus in front of or behind the retina. These disorders are myopia (nearsightedness) and hypermetropia or hypermetropia (farsightedness, also hyperopia). Use Peter Kaiser's chapter on Visual Acuity to learn more about these common disorders.

For comparison purposes, assume that the picture below is a scene as it would be perceived by a person with normal vision. Later in the laboratory, you will see the same image as it might be perceived by people with various visual disorders.

Photo courtesy of the National Eye Institute, National Institutes of Health

a. Cornea. The most common disorder of the cornea is astigmatism, which is fairly common. Because it is the most exposed part of the eye, abrasions of the cornea are also quite common. You can learn more about the cornea and disorders of the cornea at the National Eye Institute site, The Cornea Research Foundation, and/or the Family Practice Notebook site. Because of the lack of blood vessels (little concern about rejection), corneal transplants are quite successful.

b. Anterior Chamber. As you know, the anterior chamber is filled with aqueous humor, which continually flows into the chamber. However, if the aqueous humor flows into the anterior chamber (see animation on this page and this video) and cannot exit, pressure in the eye is elevated--a disorder called glaucoma. The blockage can occur in a number of ways, but the most common blockage results in primary open-angle glaucoma. Angle-closure (closed-angle) glaucoma is less common. The pressure actually causes its damage at the optic disk, and the initial damage is typically to peripheral vision (see picture below). In fact, glaucoma refers to the damage to the optic nerve (cupping) at the optic disk, which can occur for a variety of reasons (Mayo Clinic, Angeles Vision Clinic). Awareness of glaucoma as a serious visual disorder even among younger adults was elevated by the early retirement of the Hall-of-Fame ballplayer Kirby Puckett (1960-2006) of the Minnesota Twins.

Photo courtesy of the National Eye Institute, National Institutes of Health

c. Lens.The lens is important for bringing the image into focus on the retina through accommodation. A common disorder among people over 40 years of age is presbyopia, which is an inability to accommodate (PubMed Health). Another lens disorder, one version of which becomes more prevalent among even older people, is cataracts, which is a clouding of the lens (Mayo Clinic).

Photo courtesy of the National Eye Institute, National Institutes of Health

d. Retina. Unfortunately, a whole host of disorders can afflict the retina. For our purposes, we'll focus on two disorders. As seen in the picture below, age-related macular degeneration (AMD) creates a loss of vision at the point of focus (MD Foundation, National Eye Institute). The macula is the area immediately around the fovea (and in some cases the terms are use synonymously). Thus, this disorder has a profound effect on visual function.

Photo courtesy of the National Eye Institute, National Institutes of Health

Retinitis pigmentosa is an hereditary disorder that affects the rods. As such, scotopic vision would deteriorate. Peripheral vision is also lost, as seen in the picture below.

Photo courtesy of the National Eye Institute, National Institutes of Health

Because the ophthalmological community has a strong presence on the web, you can learn more about a wide range of visual disorders by checking out these sites. Keep in mind that many of the sites are the web presence of organizations that seek to promote a particular procedure (e.g., LASIK surgery). Thus, you should probably be a bit skeptical about treatment claims that are made. You'll find some really gross illustrations if you stray into grand rounds at some sites, but you should find some of the information quite interesting. You may want to use one of these sites as starting points:

• American Optometric Association
• National Eye Institute
• Cleveland Clinic
• The Mayo Clinic (MayoClinic.com, MayoClinic.org)
• All About Vision
• The Merck Manual (Eye Disorders)

4. Visual Pathways from the Eyes through the Brain

Use the figure below to trace the pathways that process visual information from the eyes [Hint: Labeled A :-) ] to the visual cortex. Identify each structure on your worksheet and give a brief description of what happens at each structure.

To learn about the pathways and structures, you need to explore a few different web sites. First, go to Molavi's tutorials to work through the one on Basic Visual Pathways and then the one on Central Visual Pathways. When going through the Basic Visual Pathways tutorial, you can ignore the information on Meyer's Loop if you wish. However, place your answers to the first three lesion questions on your worksheet. (Don't peek at the answers before responding!) Next, work your way through George Mather's Visual Physiology site.

OK, now you have some further information about the primary visual cortex and secondary visual cortex. One good source for the primary visual cortex is provided by Matthew Schmolesky at the Webvision site. On your worksheets, list information you have learned about each of several different areas of the visual cortex. Try to write down information that goes beyond that found in your textbook.

5. The M, P, and K Pathways and the What and Where Pathways

As you've worked through the previous exercises, you will surely have seen reference to the M (Magno or Magnocellular) and P (Parvo or Parvocellular) pathways and possibly the K (Koniocellular) pathway. (And you will surely have read about these pathways in your textbook!) Just as a brief summary, and to expand a bit on the prior information, determine and then list on your worksheet the differences among the three pathways.

At some point in the processing of visual information, it seems likely that these pathways will no longer be distinct. (You may have noted Mather's comment to that effect when talking about the M and P pathways and V2.) In the early stages, however, the three pathways are quite distinct (though the K pathway is a bit difficult to find). How have researchers learned about the differing functions of the two pathways? You should be able to articulate a way to illustrate the differences in the two pathways, though you'd need to think in terms of non-human research. On your worksheet, provide a brief outline of how you would conduct research to illustrate the differences in function between the M, P, and K pathways.

You should also be able to find information about the structure and function of the What and Where pathways. Briefly distinguish between these two pathways.

6. Receptive Fields

In some of the earlier exercises, you should have noted mention of receptive fields. In general, you should think of a receptive field as a map of the portion of the retina that affects the activity of a particular neuron. Stimulation of the appropriate area of the retina can either increase or decrease the rate of firing of a neuron (from its normal rate of firing, called its maintained activity). Mapping receptive fields requires the use of single-cell recording, so the research is done with non-human organisms (particularly cats and monkeys). The basic message of all the research to date is that the closer to the eye (e.g., optic nerve, LGN) the simpler the receptive field and the further from the eye (e.g., secondary visual cortex) the more complex the receptive field. Receptive fields serve to define the various visual cells (simple cortical cell, complex cortical cell, etc.). To provide you with background on receptive fields, first look at Peter Kaiser's explanation of receptive fields. Next look at John Krantz's explanation of receptive fields.

At the early stages of the visual system (precortical), neurons have circular receptive fields. An inner circle will either cause excitation or inhibition of the neuron. That inner circular area will have an antagonistic surround, which is a surround that causes the opposite action in the neuron. Thus, these neurons are said to have receptive fields that are either on-center, off-surround or off-center, on-surround. As you will see later, when we discuss computational approaches to vision, these receptive fields are extraordinarily useful in the detection of edges!

In the visual cortex, neurons have more complex receptive fields. As you worked through Molavi's tutorial on Central Visual Pathways (F. Receptive Fields in V1), you should have noted her explanation of how input from neurons in the LGN can be combined to produce the more complex receptive fields found in the visual cortex.