How Come?: Seeing spots that aren't really there

Stare at a lightbulb and then turn it out. You should see spots, floating in the darkness. These visual echoes are known as afterimages - or ghost images.

Afterimages appear when our eyes' light sensors get overwhelmed by a signal. The membrane of light-sensing cells at the back of the eye is known as the retina. About 95 percent of retinal cells are rods, cells that allow us to see in dim light. The rest are cones, which allow us to see colors. Cones work best in bright light, which is why colors are so hard to tell apart in the dark.

Struck by a burst of incoming light, the retina's cells send signals through the optic nerve connecting the eye to the brain. But look at a lightbulb for many seconds, and the cells become desensitized, rather like your nose becoming used to a strong odor.

As retina cells tire and we glance away from a lightbulb, we see an afterimage, popping up everywhere we look. Afterimages may be both positive and negative. So we may see an image of a glowing lightbulb, followed by a (negative) image of a dark lightbulb. Negative afterimages usually last longer. But with our eyes closed, or in the dark, we may see a long-lasting positive image.

Scientists can explain negative afterimages. A glowing bulb appears as a bulb-shaped dark spot because the retina's cells are desensitized to light where the brightest light struck them. But no one is sure why we see positive images. Researchers think the bright images appear as the brain processes strong signals previously sent by the retina. The brighter the light, the more likely we will see positive afterimages, and the longer they will last. (Even a brief glance at the sun will makes us see bright spots, which should warn us that staring at the sun can cause permanent vision loss.)

Afterimages can also appear in false colors, a result of "cone fatigue." Cone cells are specialized; some respond to red light, others to blue, and others to green. (White light is made of a rainbow of colors. So add red, blue and green together, and we see white.)

Try staring at a bright red shirt for more than 30 seconds. Then glance around your room. You should notice two aftereffects: First, all red objects will seem washed out, since overstimulated red cone cells have temporarily stopped responding. Second, you'll see a floating, shirt-shaped blob - in blue-green.

Why is the afterimage of your crimson shirt a lovely aqua? When you look at, say, a white wall, your eyes will be receiving the full spectrum of light, including red, blue and green. Your eyes' tuckered-out red cones won't process the reds. But the perky blue and green cones will send plenty of "blue!" and "green!" messages to the brain. The result, until your red cones reboot: a ghostly, greenish-blue shirt, hovering in your bedroom.

Try the interactive Lilac Chaser afterimage generator at michaelbach.de/ot/col_lilac

Chaser/index.html.