The dreams of Mary Shelley, author of "Frankenstein," involved a pale student kneeling beside a corpse that was jerking back to life. Paul McCartney's contained the melody of "Yesterday," while director James Cameron's inspired the "Terminator" films.

With their eerie mixture of the familiar and the bizarre, it is easy to look for meaning in these nightly wanderings. But why do our brains take these journeys, and why do they contain such outlandish twists and turns?

Armchair psychoanalysts are still disputing Sigmund Freud's attempts to interpret dreams, but neuroscientists and psychologists have recently made big strides in understanding the way the brain builds our dreams. They've also found startling hints that our use of technology may be permanently changing the nature of dreams.

Some of the best attempts to catalog dreams asked participants to jot them down as soon as they woke up or had volunteers sleep in a lab where they were awakened and immediately questioned. Such experiments have shown that our dreams tend to be silent movies: Just half contain traces of sounds. It is even more unusual to enjoy a meal or feel damp grass beneath your feet while asleep: Taste, smell and touch appear only very rarely in dreams.

Similar studies have tried to pin down factors that might influence what we dream about, with little effect.

More recently, scientists have been looking at the brain's activity during sleep for clues to the making of dreams. Of particular interest is the idea that sleep helps to cement our memories for future recall. After first recording an event in the hippocampus — which can be thought of as memory's printing press — the brain transfers its contents to the cortex, where it files the recollection for long-term storage.

Making memories

Mark Blagrove and his team from Britain's Swansea University have found that memories enter our dreams in two separate stages. They first float into our consciousness on the night after the event itself, which might reflect the initial recording of the memory. Then, they reappear five to seven days later, which may be a sign of consolidation.

The sleeping brain also allows us to see associations between different events in life. This might dredge up old memories and plant them in our dreams, which in turn might explain why we often dream of people and places that we haven't seen or visited for months or even years.

Technicolor dreams

Our dreams are more than a collection of characters and objects, though. They come in many different styles — from trivial and disordered to intense visions — and our emotional undercurrents seem to be a guiding force.

Ernest Hartmann, a professor of psychiatry at Tufts University in Medford, Mass., studied the dream diaries of people who had recently suffered a painful personal loss. He found that they were more likely to have particularly vivid dreams that focus on a single central image rather than a meandering narrative. These dreams also were more memorable than those from more placid times.

Perhaps the intense images are an indication of how difficult it is to integrate a traumatic event with the rest of our autobiography, Hartmann says. "I think it makes a new trauma less traumatic," Hartmann says.

Then there's the impact of technology. Some research suggests that as TV shifted from black-and-white to color, it may have caused a similar shift in dreams. Eva Murzyn at Britain's University of Derby has found that people who take part in the "World of Warcraft" online role-playing game incorporate its user interface into their dream adventures.

Jayne Gackenbach at Grant MacEwan University in Edmonton, Alberta, has found that online gamers report a greater sense of control over their dreams, with the feeling that they are active participants inside a virtual reality. She points out that gamers are more likely to try to fight back when they dream of being pursued by an enemy, for instance. This seems to make their dreams less scary and more exciting.

"They say things like 'This was a nightmare, but it was awesome.' They are invigorated by it," Gackenbach says.