What is so mesmerizing about pointillist paintings like Seurat's Sunday Afternoon at La Grande Jatte? At first, we're impressed by the technical virtuosity of the work. It's an immense painting that Greta and I visited many times when we were in college in Chicago (and now, whenever we return for a visit):

As you can see even in this reduced image, the painting is composed of tiny dots. But what you may not notice is that the dots in a given region of the painting aren't all the same color. Take a look at this detail:

The leaves in the trees range from red to yellow to green to blue, and the water includes blues, purples, greens, and yellows.

When you visit the painting in person, you might get the impression that the colors actually change as you approach or move away from the work. Of course, they can't really be changing. But vision scientist Pascal Mamassian believes that what you experience as you view a pointillist work might be similar to an effect described by Patrick Monnier and Steven K. Shevell. Take a look at this image:

The orange/pink rings in the center of each large ring appear to all be different colors. But actually they are the same. Click on the image to see an animated figure that removes the other colors. What's particularly remarkable about this illusion is that in rings a and c, the color immediately next to the central ring is the same. The only difference between the figures is the frequency at which the rings change color. The same effect occurs between rings b and d.

The central rings in c and d appear to be much closer to each other in color than the central rings in a and b. Monnier and Shevell had several volunteers look at figures like this, comparing the multi-colored rings to single rings on white background. They adjusted the figures on white until they appeared to match the rings in the figure. The responses were consistent: everyone with normal color vision appears to experience the illusion.

Monnier and Shevell believe this illusion is due to an "antagonism" that occurs in the brain in response to just some the cones used in the eye to detect color: S-cones, which respond to the shortest "blue" wavelengths of light. That's because the differences between what was perceived and the actual color seen can be completely explained by the S-cone response: no differences in M-cone (green) or L-cone (red) responses were observed.

The antagonism only occurs at certain frequencies -- and this could lead to colors in the Seurat painting appearing to change when our distance to the painting makes the dots appear about the same width as the rings in Monnier and Shevell's study: about 0.15 degrees of visual angle.

P MAMASSIAN (2008). Ambiguities and conventions in the perception of visual art Vision Research, 48 (20), 2143-2153 DOI: 10.1016/j.visres.2008.06.010