Lateral Inhibition Lab
Contrast Illusions:

    Look at the following Contrast Illusions to see how the eye uses contrast information in order to extract information and construct a perception of the world. All of these illusions take advantage of lateral inhibition. In this lab, we will discover why.

Contrasting Squares Illusion  
 1) Look at the Contrasting Squares Illusion (above). The center squares look like two different shades of gray, even though they are the same
 2) Look at the Hermann's grid illusion (below)

Hermann's Grid Illusion
Be sure to review contrast effects and lateral inhibition on pages 165 to 168 of your textbook

McMaster Psychology (note: this site may not work correctly on Macs, use another type of computer)

In this lab, you will use a model of lateral inhibition to simulate the effects seen in the illusions above. Read through the instructions on the site, and play with the model until you are familiar with its operation. The most important thing to know is how to edit the input values. Instructions are given below:

How to Edit the input values: Below each input unit there is an interactive white box. The activity of each input unit can be edited by clicking on the white box beneath it, using the delete and arrow keys, and typing in numeric values. You need to press reset and then cycle several times after you have changed the input states. Note: this demo runs in java and requires the latest version of netscape; unfortunately this version for the macintosh has a bug which causes the values in the input boxes to disappear once they have been edited. However, the demo still works, and you can still see the values appear as the input layer states change.

Before continuing, check your understanding of how the model works with the following questions:
1) What do the input numbers represent?
2) Why are all of the receptor values initially zero?
3) What does pressing "cycle" represent?


Objective: We are going to simulate figure 1B (below):

Contrasting Squares Illusion

Directions:

1. Adjust the weights at the top of the display to the following:
    Excitation: 1.0
    Inhibition: 0.5
    Threshold: 35.0

2. After entering the new numbers be sure to hit RETURN and push the RESET button.

PLEASE NOTE: You need to hit enter after each new input value.  3. Enter the following input values for the Dark/Medium/Dark Pattern:  5    5    5    20    20    20    5    5    5

4. Press the cycle button 20 times, or until the values stabilize, inputting the values for each receptor in the following table:
Reset 0
 0
 0
 0
 0
 0
 0
 0
 0
Cycle 1








Cycle 2








Cycle 3








Cycle 4








Cycle 5









To change the input values for the medium/light/medium exercise you MUST refresh the page!!!!

5. Enter the following input values for the Light/Medium/Light Pattern:  30    30    30    20    20    20    30    30    30

6. Press the cycle button 5 times, inputting the values for each receptor in the following table:
Reset
 0
 0
 0
 0
 0
 0
 0
 0
 0
Cycle 1








Cycle 2








Cycle 3








Cycle 4








Cycle 5









Turn in your answers to the following questions:

1. Explain the results that you notice for each pattern. In particular, how does the brightly illuminated area (represented by the middle three receptors) compare between the two patterns?
2. Use the output pattern for each square to explain the contrasting squares illusion (figure 1A, above).
3. Why does the grey square in the center of a dark area (the right side of the contrasting squares illusion and the first exercise) look brighter than the grey square in the center of a light area (the left side of the contrasting squares illusion and the second exercise)? Extra Credit:

1. If you are interested, you can try to do a similar experiment and explanation for Hermann's grid (figure 2, above). You should simulate a row of receptors in an all light area (a row that goes through an intersection) with a line that goes through a dark square, light area between squares and then another dark square.
2. Find evidence of this illusion in the everyday world! This maybe an accidental occurance or the creator/artist may be taking advantage of this illusion. If possible, take a photo of the illusion. If this is not possible, please describe the illusion in great detail so that we can visualize it.
3. Explain why this happens and why the bright areas appear brighter when the contrasting color is more similar.

What To Turn In:

1. Two completed charts (one for each pattern)
2. Answers to the three questions.