Chapter 1. Center-Surround Illusions

1.1 Introduction

Cognitive Tool Kit
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Center-Surround Illusions

Seeing white means there is a lot of light, seeing black means there is very little light; the shades of gray fall between those opposites. This description suggests that how bright something looks, as in the image below, depends upon how much light comes to your eye from that object. Remember the center-surround receptive field experiment. Is that always, 100%, true? While pondering that question, look at where the two arrows point in the image below. Then continue on to the experiment to examine this question more fully. (Note: If you have not done the Center-Surround experiment yet, you should go back and do that before proceeding to the present experiment. The concepts presented in the Center-Surround experiment are crucial for an understanding of what is presented here.)

1.2 Experiment Setup

1.3 Instructions

Instructions

Your task is to match the two center squares. At the beginning of each trial, the brightness of the center square on the right will be any of a large number of random brightnesses. Below the white surrounding square will be a + button; clicking it will make the center square brighter. Conversely, clicking the – button will make the center square darker. When you are done adjusting the brightness to make the right center square match the brightness of the one on the left, click the Match button.

Key What Response Means
+ or = (same key) Increase intensity of right center square.
- Decrease intensity of right center square.

1.4 Experiment

Begin Experiment

1.5 Results

Results

1.6 Debriefing

Debriefing

Recall the center surround receptive fields experiment. All the information that comes to our brain has to go through these receptive fields. Recall how they responded. When there is no light in the receptive field, the cell fires at a very slow firing rate. When the cell is on-center, and the center area is full of light but the surround has no light, it fires at a very high firing rate. Finally, when the entire receptive field is full of light, the cell again fires at a very slow firing rate.

Thus, these receptive fields respond more to light filling only part of the receptive field than to the total amount of light.

From these results, it has been concluded that center-surround receptive fields are more sensitive to the changes in light levels in the receptive field and not to the absolute level of light. In other words, these receptive fields do not respond to the amount of light, but instead compare light levels from nearby areas of the retina. This finding has been used to explain several visual illusions, such as the Mach Bands shown below.

The dark bar at the yellow arrow and the bright bar at the red arrow do not actually exist (Cornsweet, 1970). They are caused by the reaction of the receptive fields at the edges where these bars occur. Another illusion explained by these center-surround receptive fields is the well-known Hermann grid shown below.

The intersections of the white lines look dark and reflect the greater inhibition of cells positioned at the intersection than those positioned along the lines away from the intersection.

So how does this explain the results of the Simultaneous Contrast experiment you just conducted? Each central gray area has a surrounding area; the gray area on the left is surrounded by a dark region and the central area on the right by a light region, as seen below.

Center-surround receptive fields compare areas. The dark outer area on the left does not suppress our response to the light coming from the square inside it, allowing it to look bright. However, the bright outer area on the right does suppress the response to the central area on the left, making it look darker than it is. This illusion seems to be the result of how these center-surround receptive fields function.

References:

Cornsweet, T. N. (1970). Visual Perception. New York: Academic Press.

Enroth-Cugell, C. & Robson, J. (1966). The contrast sensitivity of retinal ganglion cells of the cat. Journal of Physiology, 187, 517-566.

Kuffler, S. W. (1953). Discharge patterns and functional organization of mammalian retina. Journal of Neurophysiology, 16, 37-68.

1.7 Quiz

Quiz

Question 1.1

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1
Incorrect.
correct
The independent variable is the value that is changed by the experimenter. In this case, the intensity of the surround was manipulated.

Question 1.2

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1
Correct.
Incorrect.
The dependent variable is the value that the experimenter collects to indicate how you performed in the experiment. In this case, we recorded the intensity setting of the right center square when you pressed the match button. Thus, the correct answer is the setting of the right center square.

Question 1.3

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1
Correct.
Incorrect.
Since all of the squares can be seen at the same time and we see a difference in brightness that is not real, this phenomenon has been termed simultaneous contrast.

Question 1.4

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Correct.
Incorrect.
Generally, a bright surrounding region makes the center square appear darker than it is.

Question 1.5

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Correct.
Incorrect.
These receptive fields are more sensitive to contrast than to light levels. Thus, it makes sense that they make objects seem to have more contrast than they actually do.