Now researchers, reporting in this week's issue of Nature, have found a new nocturnal illusion that could affect a driver's ability to judge the speed of approaching objects. The human retina has four main types of cells, called photoreceptors, that detect light and motion. In bright light, three kinds of cones each discern spatial patterns and color. The rods, on the other hand, are much more sensitive to light but cannot convey color or fine spatial details.
Rods can function with much less light than cones, so they take over when the light dims.
Cones are known to gauge motion reliably, but it wasn't clear how well rods do. The apparent slowpoke was always the bar visible only to the rods. Gegenfurtner's group now plans to use a simulator to test whether this misperception hinders drivers who rely on their peripheral vision to spot objects not illuminated by headlights or streetlights. But some scientists say the main value of the finding is in helping to understand how motion is perceived.