Simultaneous Color Contrast

Background:

While the perception of a color is primarily determined by the physical properties of the light that reaches the retina (wavelength primarily determines hue, amplitude primarily determines brightness and purity primarily determines saturation), other factors influence the perception of color. One of the factors that play a role in the perception of colors is other adjacent colors. Two identical color patches laid on top of two differently colored backgrounds may not appear to be the same color:

The green circles shown above usually do not appear to be the same color -- most people perceive the green circle on the right as darker than the green circle on the left. Physically, they are identical to each other. The perceived color of the circles is influenced by the color of the background -- the dark (V = 0.26) green background makes the green circle on the left seem lighter, while the much brighter (V = 0.87) pink background makes the green circle on the right seem darker. That is what Chevreul was saying in the quotation -- the perceptual system attempts to make the colors of the circle and the background as different as possible.

An astute reader will notice a similarity between the simultaneous color contrast and the enhancement of edges (changes in contrast) performed by the retinal ganglion cells. The retinal ganglion cells not only pick out edges, they perceptually enhance the edge -- they make the perceived difference in contrast greater than it physically is. This is shown with Mach bands:

Each of the six bands of light have a uniform physical intensity -- all parts of the leftmost, black band have the same physical brightness. Likewise, all parts of the second, very dark gray band, have the same physical brightness which is just a little brighter than the black, first band. Psychologically, each band is not uniformly light. The part of the edge in the darker band next to the lighter band appears slightly darker than the rest of the darker band. Likewise, the part of the edge in the lighter band next to the darker band appears slightly lighter than the rest of the lighter band. That is, the change in contrast (the edge) is perceived as greater than it physically is. The edge (the change in contrast) is perceptually enhanced.

One explanation for this edge enhancement is based on the retinal ganglion cell's receptive field. Recall that the receptive field of retinal ganglion cells consist of two concentric parts -- the center and the surround -- which work antagonistically. If the center is excited by light, then the surround is inhibited by light. When the amount of excitation is sufficiently greater than the amount of inhibition, the retinal ganglion cell will fire -- it will send a message toward the cortex. The following diagram shows the receptive fields from four retinal ganglion cells. For the upper two receptive fields, which neuron responds more frequently -- the one with the receptive field on the left or the one on the right? For the lower two receptive fields, which neuron responds more frequenlty -- the one with the receptive field on the left or the one on the right?

For the upper two receptive fields, the neuron that the left-hand receptive field belongs to is responding at baseline. Anytime a center-surround receptive field is uniformly illuminated, the neuron will respond at baseline because the amount of excitation from the center and the amount of inhibition from the surround will be the same and will cancel each other. The upper, right receptive field will respond below baseline. There is no excitation from the center (the entire center is in the dark band) and some inhibition from the surround (part of the surround is in the bright band on the right.) Because perceived brightness correlates with the rate of neural activity, the darker part of the edge must appear darker than the middle of the dark band.

For the lower two receptive fields, the neuron that the right-hand receptive field belongs to is responding at baseline. Anytime a center-surround receptive field is uniformly illuminated, the neuron will respond at baseline because the amount of excitation from the center and the amount of inhibition from the surround will be the same and will cancel each other. The lower, left receptive field will respond above baseline. There is some excitation from the center (the entire center is in the lighter band) but not as much inhibition from the surround (part of the surround is in the dark band on the left.) Because perceived brightness correlates with the rate of neural activity, the lighter part of the edge must appear lighter than the middle of the light band. The edge (change in contrast) has been perceptually enhanced by the retinal ganglion cells.

How does the edge enhancement of the retinal ganglion cells relate to simultaneous color contrast? The color circle forms an edge (a change in contrast) with its background. In the sample above, the green circle on the left is physically brighter than its dark green background. This creates a change in contrast -- an edge. The edge enhancement of the retinal ganglion cells should produce a Mach band like effect at the edge. Likewise, the green circle on the right is physically darker than its pink background. This creates a change in contrast -- an edge. The edge enhancement of the retinal ganglion cells should again produce a Mach band like effect at the edge. The difference in colors is enhanced; to borrow Chevreul's words, "they appear as dissimilar as possible."

Simultaneous color contrast demonstrates that you need to know more than the physical properties of a light in order to determine the perception of the light. Other factors, such as surrounding colors, also influence the perceived color of the light.

The Activity:

In this activity you will experience the simultaneous color contrast effect. Two color patches (the circles) are presented below. The two patches have the same hue and saturation but might differ in their brightness. Use the slider to adjust the intensity of the right-hand color patch until it is perceived to be the same color as the left-hand color patch. When you think the colors of the patches are the same, you can check your perception in three different ways. You can click on the "Show intensity of left-hand circle" button to see its intensity and compare that intensity to the value to the left of the intensity slider. If your perception is veridical -- if it truthfully represents the physical properties of the light coming from the screen -- the two intensities should be identical. Second, you can click on the "Show intensity under mouse" button and then move your mouse to the center of each of the two color patches. The two intensities should be the same. Finally, and perhaps most telling in how much simultaneous color contrast influences our perception of colors, you can hide the backgrounds by clicking on the "Hide background" button. With the backgrounds gone, the two color patches should appear identical if your perception is veridical. Most likely, the two color patches will appear quite diferent when you remove the background.

To try answer set of colors, click on the "New colors" button.

Intensity of circle on the right: 0.50