# Light Contrast Illusions

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What Are Photoreceptors?

What Is Lateral Inhibition?

How Does The Hermann Grid Work?

How Do Mach Bands Work?

What Are Light Contrasts?

How Does White's Illusion Work?

When looking at the image to the left, can you determine what colors the two gray squares are? Is the left square a 2, 3, or 4? Is the right square a 4 or 5? Surely, the square on the right has to be darker than the square on the left. Right?

Actually, both the right and left squares are same color as number 4. What makes them look like two different colors is light contrast, a phenomenon closely related to Lateral Inhibition.

To understand light contrast you must first understand the difference between lightness and intensity. Although both are measures of the strength of light reflecting off an object, intensity is a physical and constant measure while lightness is a perceptual and subjective measure. The level of an object's lightness depends on the amount of light reflected from the surface of that object. The level of an object's intensity remains constant no matter how much light is is reflected from the surface.

With ON-Center receptive fields, if light is detected solely by the light photoreceptors in the center portion of a receptive field (e.g., field A to the left), the response of that field increases proportionately to any increase in light level. However, if light is also detected by the dark photoreceptor surround, the two regions conflict with one another and it takes a greater amount of light for the field to respond.

The greater the amount of light shined on the surround the more conflicted the center and surround of the receptive field becomes. Because of this, a receptive field could view a single color with the same intensity differently depending on the amount of light detected by the field's surrounding photoreceptors. Similar to the Mach Bands illusion, Lateral inhibition can cause objects identical in intensity to appear different in lightness. This is the illusory phenomenon of light contrast. Returning to the example above, by looking only at the squares without the surrounding colors, you will see that they are both identical in lightness.

To the left, is another example. Each of the squares on the left side behind the darker background appears lighter than the squares on the right side behind the lighter background. In fact, each of the squares are the same color. You can see for this yourself by scrolling over the image. Without the gradient background, you can clearly see that each of the squares are in fact, the same color.

Notice also that the squares in the middle appear to blend into the background. This further shows that the squares are the same color. Each of the squares are a medium gray color and as the background shifts from black to white, the medium gray area in the middle that matches the squares, blends together.

Another example of this phenomenon is on each page of this site. The vertical stripe that seems to get darker as it progresses down the page is actually one color. Because of the shift in the background color from red to green, the greenish brown color of the stripe (a color located in between red and green on a color wheel) appears as if it gets darker as it approaches the lighter color (green) of the gradient background and appears lighter in comparison to be darker of the gradient colors (red).

Another example of this phenomenon is White's Illusion.

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