Lee Gomes of the Wall Street Journal writes that recent research shows that a brain rewards itself with a squirt of natural opiates when it comes across new information that requires interpretation. That's why, he concludes, people stay on the Web for long periods of time.

What is it about a Web site that might make it literally irresistible? Clues are offered by research conducted by Irving Biederman, a neuroscientist at the University of Southern California, who is interested in the evolutionary and biological basis of the human need for information.

Dr. Biederman first showed a collection of photographs to volunteer test subjects, and found they said they preferred certain kinds of pictures (monkeys in a tree or a group of houses along a river) over others (an empty parking lot or a pile of old paint cans).

The preferred pictures had certain common features, including a good vantage on a landscape and an element of mystery. In one way or another, said Dr. Biederman, they all presented new information that somehow needed to be interpreted.

When he hooked up volunteers to a brain-scanning machine, the preferred pictures were shown to generate much more brain activity than the unpreferred shots. While researchers don't yet know what exactly these brain scans signify, a likely possibility involves increased production of the brain's pleasure-enhancing neurotransmitters called opioids.