Slow motion sets in when the light dims.

When the lights go down, the world may appear to slow, new research suggests. Movement perceived by rods, the cells in the retina that register dim lights, looks slower than the same motion detected by the color-sensitive cones.

Most people see the world through four types of visual-receptor cells. Rods sense brightness alone, while the three types of cones register red, green, and blue wavelengths of light. "Anytime you can see color, you know you're stimulating the cones," says Marty Banks of the University of California, Berkeley.

The researchers, led by Karl R. Gegenfurtner of the Max Planck Institute for Biological Cybernetics in Tubingen, Germany, suspected that the rod- and cone-based visual systems interpret motion differently. For example, Gegenfurtner knew of a color-blind man lacking all three types of cone cells who complained of trouble catching a Frisbee.

In tests on five men with red-green color-blindness, the researchers pitted two types of visual-receptor cells against each other, they report in the April 8 Nature.

About 2 percent of white men can't distinguish green and red because they are missing the cone cells tuned to green light. The researchers manipulated the color and brightness of objects on a computer screen to activate either rods or red-sensitive cones but never the blue-sensitive cones, Gegenfurtner says.

The red-green color-blind subjects compared oscillating patterns, one stimulating the cones and the other, the rods. They judged the speed of the rod-activating pattern to be about 75 percent of the cone-activating pattern's speed.

The rod cells may see the world differently because of the way the retina is wired, says Banks. In very low light, only the rod cells are sensitive enough to see the dimmest objects. To better pick out faint light signals, which could be over-shadowed by random nerve firings, the retina sacrifices resolution. It averages signals from many different rod cells and gathers rod signals for a relatively long time before passing a message further along in the visual system.

In comparison, Banks says, "cones have a direct line through the retina to the cortex." Each cone cell in the retina's center sends an unadulterated signal to the brain. This high-resolution system--like a computer screen--refreshes frequently and responds to movements quickly.

One potential danger of the rod system's slowed perception, Gegenfurtner says, arises during night driving. Although headlights illuminate the road brightly enough for the cones to kick in, objects outside the beams may appear to be moving more slowly than they are.