The Roman philosopher Seneca may have put it best 2,000 years ago: "To be everywhere is to be nowhere." Today, the Internet grants us easy access to unprecedented amounts of information. But a growing body of scientific evidence suggests that the Net, with its constant distractions and interruptions, is also turning us into scattered and superficial thinkers.

The picture emerging from the research is deeply troubling, at least to anyone who values the depth, rather than just the velocity, of human thought. People who read text studded with links, the studies show, comprehend less than those who read traditional linear text. People who watch busy multimedia presentations remember less than those who take in information in a more sedate and focused manner. People who are continually distracted by emails, alerts and other messages understand less than those who are able to concentrate. And people who juggle many tasks are less creative and less productive than those who do one thing at a time.

The common thread in these disabilities is the division of attention. The richness of our thoughts, our memories and even our personalities hinges on our ability to focus the mind and sustain concentration. Only when we pay deep attention to a new piece of information are we able to associate it "meaningfully and systematically with knowledge already well established in memory," writes the Nobel Prize-winning neuroscientist Eric Kandel. Such associations are essential to mastering complex concepts.

When we're constantly distracted and interrupted, as we tend to be online, our brains are unable to forge the strong and expansive neural connections that give depth and distinctiveness to our thinking. We become mere signal-processing units, quickly shepherding disjointed bits of information into and then out of short-term memory.

In an article published in Science last year, Patricia Greenfield, a leading developmental psychologist, reviewed dozens of studies on how different media technologies influence our cognitive abilities. Some of the studies indicated that certain computer tasks, like playing video games, can enhance "visual literacy skills," increasing the speed at which people can shift their focus among icons and other images on screens. Other studies, however, found that such rapid shifts in focus, even if performed adeptly, result in less rigorous and "more automatic" thinking.