A special greeting to the undergrads at Mount Holyoke.

The McCollough Effect - An On-line Science Exhibit


Take a look at the following grid. It should appear as black stripes on a white background.

Click here and gaze at the two colored grids for a few of minutes. There's no need to stare at a single point on a grid.

If you look at the black-and-white grid again, you should notice a green haze around the horizontal lines, and a magenta haze around the vertical lines. The intensity of this effect varies between individuals. If you don't see this, go gaze at the colored grids for a while longer.

I know what you are thinking: this is a simple afterimage effect. If you think so, walk away from your terminal until you think the after image should be gone. Go home and try it in the morning. Then take a look. Or better, simply rotate the image. Well, maybe that isn't so simple with a CRT, but you could rotate your head.

The Effect

It is called the McCollough Effect, and was originally described by Celeste McCollough in a paper in Science in 1965. It has been the focus of on-going investigation ever since.

The effect typically lasts for hours, or even overnight. The duration can be changed by the consumption of coffee and other psychoactive drugs. One paper found that it is stronger in extroverts than introverts, and might be a reliable test for extroversion.

The precise cause of the effect is unknown, and currently under investigation. It is not a simple case of fatigued neurons: there are neurotransmitters involved and appear to be responsible for the long-lasting nature of the effect. It probably takes place in the V1 processing stage of visual information. This is the first image processing after the signal leaves the retina in the eye. The edge detection circuits somehow become associated with the color. At this stage the processing is monocular: the images from the two eyes have not been combined.

The Lab

You can perform experiments on the McCollough effect right here at your terminal. Here are a few questions you might try to answer. I've supplied some images that might help.

Lab Tools

Here are some lab tools. You can probably think of others: even the brightness knob on your screen may have an effect.

Further Reading.

References are available upon request.


Check out http://www.uq.edu.au/nuq/jack/rivalry.html.

If you have questions or comments, please feel free to send me email.

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