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Teaching About Lateral Inhibition in the Retina
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by Wesley P. Jordan
St. Mary's College of Maryland
St. Mary's City, Maryland
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| | Introduction
Perception is an active brain process. One common example in introductory texts is Mach bands, the intensification of contrast around a boundary between light and dark. This activity demonstrates lateral inhibition, the neural mechanism responsible for Mach bands. The activity is based on research in the retina of the horseshoe crab (Limulus) but is suitable to describe lateral inhibition in the mammalian retina as well.
Perhaps more than any other sense, humans rely on vision to interpret the world around them. We all know how hard it is to find our way to the bathroom in the middle of the night. As you approach the open bathroom door, the door frame seems only slightly darker than the open space, but even in very dim illumination you can make out the difference. If you were to take a picture of this opening with a camera, you might get nothing more than a solid black picture. Although we may first think that the eye is a camera that faithfully captures images from the external world, this is not so. When light is absorbed by photoreceptors in the retina, the receptors send electrical signals to other retinal cells that accentuate the subtle differences in brightness around the door. The actual physical difference in illumination between the door frame and open door is very slight, but the perception of the difference is magnified by the actions of these retinal cells.
The process by which edges are highlighted is called lateral inhibition. First studied in the eye of the horseshoe crab, Limulus, we now know that lateral inhibition is a mechanism found throughout the animal kingdom. In the horseshoe crab, photoreceptors in the retina are connected such that when stimulated by light each receptor inhibits its immediate neighbors. Three rules describe the rate of activity in each cell -- rules established by the neural connections within the retina:
Rule 1: Each cell has a certain amount of spontaneous activity in the dark.
Rule 2: Any cell that is exposed to light gets extra energy from absorbing a photon -- energy reflected in an increased level of activity.
Rule 3: Each cell in the light has an inhibitory effect on its immediate neighbors.
This activity establishes an analogy between cellular activity and actors on a stage. All actors (stars and supporting cast alike) begin with a certain amount of money, representing the spontaneous level of activity in photoreceptors in the dark. The stars of the production, those actors in the "limelight," receive additional money. The rising career of a star actor has an inhibitory effect on actors not so favored. Thus actors in the limelight receive money from each of their immediate neighbors on stage, analogous to the inhibitory effect each active photoreceptor has on neighboring cells.
Method and Results
Begin by discussing the importance of identifying edges and other visual discontinuities in the world. Students are very good at generating examples. Develop the concept that visual perception is an active process. If the class has experience with visual illusions, you can use these phenomena as examples of how perception is different than the "reality" of the visual world. Show a demonstration of Mach bands.1 Point out the perception of increased dark and light bands around the light-dark borders. Identify lateral inhibition as the mechanism underlying the enhanced perception of edges, and write the rules of retinal interaction (above) on the blackboard.
Select at least seven student volunteers to line up in front of the class. Each student represents a photoreceptor in the retina. Explain that each photoreceptor has a spontaneous rate of neural activity in the dark. When exposed to light, the activity level increases. The activity in each photoreceptor is represented with money. The more money a person (photoreceptor) has, the higher the rate of activity in that cell.
Give each student two quarters (50 cents) to represent the spontaneous level of activity in the dark (rule 1). Select at least three adjacent students in the middle of the line to be the star actors and shine the limelight on them. Applying rule 2, give each of these students another four quarters ($1). The money comes from you as the outside source of energy (light).
Now have the students apply rule 3. Each student in the limelight gets one quarter (25 cents) from each neighbor. Students in the dark get nothing.
Have the students count their money and display the results on the blackboard. If there are nine students and the middle five are stimulated with light, the data would be:
| Light |
-- |
-- |
On |
On |
On |
On |
On |
-- |
-- |
| Position |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
| Money |
$0.50 |
$0.25 |
$1.75 |
$1.50 |
$1.50 |
$1.50 |
$1.75 |
$0.25 |
$0.50 |
Draw a graph of the amount of money (y-axis) versus the ordinal position of each student in background (a Mach band).
Amount of Money Versus Ordinal Position
Discussion
Make sure the class understands the sources of activity in the retinal cells. Some activity arises from basic metabolic processes (spontaneous activity) and some from an outside source (the absorption of photons). In addition, the neural connections between cells produce a mutual or lateral inhibition of neighboring cells. This inhibition is "hardwired" and cannot be changed. The intensity of the inhibition is positively correlated with the level of activity in the cell. Thus each cell always inhibits its neighbors somewhat; however, the amount of inhibition produced by spontaneous activity is so small compared with that produced when a cell is stimulated by light as to be negligible.2
Discuss with the class the functional significance of lateral inhibition. This automatic process enhances the contrast along borders of light and dark, making them easier to identify. In nature, discontinuities of illumination (edges) often correspond to important changes in the landscape that identify objects.
Mach bands do not exist in the real world. A physicist's light meter would detect only a regular stair-step increase in light intensity at each border. The perception that the dark side of the edge is darker and the light side lighter is created in the retina by cellular activity that is different in pattern than what a light meter detects. Perception is an active process that is determined, in part, by the neural circuitry within the sensory pathways.
Notes
1. A number of Web sites provide good-quality images. Print them directly onto an overhead transparency; the photocopying process distorts the luminance of the image. To find an image, search for "Mach bands" or try:
 http://www.nist.gov/lispix/imlab/illusions/machband.html
2. You can make the demonstration slightly more realistic by having each cell in the dark collect a "tax" of one cent from each neighbor while retaining the quarter tax collected by each cell in the light. However, my experience has been that this complication makes it more difficult for students to understand the basic principles. In addition, the increase in inhibition produced by a cell in the light is thousands of times greater than that produced by the spontaneous activity of a cell in the dark, making the 25:1 ratio of this modification unrealistic along a quantitative dimension.
Wesley P. Jordan is a professor of psychology and the dean of admissions and financial aid at St. Mary's College of Maryland in St. Mary's City, Maryland. His research is in behavioral neuroscience, with a focus on how the brain changes as a result of learning and how it stores memories.
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