The picture that emerges is troubling, at least to anyone who values the subtlety, rather than just the speed, of human thought. People who read text studded with links, the studies show, comprehend less than those who read words printed on pages. 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, updates and other messages understand less than those who are able to concentrate. And people who juggle many tasks are often 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 close 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 and thinking critically. When we're constantly distracted and interrupted, as we tend to be when looking at the screens of our computers and mobile phones, our brains can't forge the strong and expansive neural connections that give distinctiveness and depth to our thinking. Our thoughts become disjointed, our memories weak. The Roman philosopher Seneca may have put it best 2000 years ago: ''To be everywhere is to be nowhere.'' In an article in Science last year, Patricia Greenfield, a developmental psychologist who runs UCLA's Children's Digital Media Centre, reviewed dozens of studies on how different media technologies influence our cognitive abilities. Some of the studies indicated that certain computer tasks, such as playing video games, increase 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. In one experiment at an American university, half a class of students was allowed to use internet-connected laptops during a lecture, while the other had to keep their computers shut. Those who browsed the web performed much worse on a subsequent test of how well they retained the lecture's content. Earlier experiments revealed that as the number of links in an online document goes up, reading comprehension falls, and as more types of information are placed on a screen, we remember less of what we see.

Greenfield concluded that ''every medium develops some cognitive skills at the expense of others''. Our growing use of screen-based media, she said, has strengthened visual-spatial intelligence, which can strengthen the ability to do jobs that involve keeping track of lots of rapidly changing signals, such as piloting a plane or monitoring a patient during surgery. But that has been accompanied by ''new weaknesses in higher-order cognitive processes'', including ''abstract vocabulary, mindfulness, reflection, inductive problem solving, critical thinking, and imagination''. We're becoming, in a word, shallower. Studies of our behaviour online support this conclusion. German researchers found that web browsers usually spend less than 10 seconds looking at a page. Even people doing academic research online tend to ''bounce'' rapidly between different documents, rarely reading more than a page or two, according to a University College London study. Such mental juggling takes a big toll. In a recent experiment at Stanford University, researchers gave various cognitive tests to 49 people who do a lot of media multitasking and 52 people who multitask much less frequently. The heavy multitaskers performed poorly on all the tests. They were more easily distracted, had less control over their attention, and were much less able to distinguish important information from trivia. The researchers were surprised by the results. They expected the intensive multitaskers to have gained some mental advantages. But that wasn't the case. In fact, the multitaskers weren't even good at multitasking. ''Everything distracts them,'' said Clifford Nass, one of the researchers. People who read text studded with links, the studies show, comprehend less than those who read words printed on pages. People who watch busy multimedia presentations remember less than those who take in information in a more sedate and focused manner It would be one thing if the ill-effects went away as soon as we turned off our computers and mobiles. But they don't. The cellular structure of the human brain, scientists have discovered, adapts readily to the tools we use to find, store and share information. By changing our habits of mind, each new technology strengthens certain neural pathways and weakens others. The alterations shape the way we think even when we're not using the technology.

The pioneering neuroscientist Michael Merzenich believes our brains are being ''massively remodelled'' by our ever-intensifying use of the web and related media. In the 1970s and 1980s, Merzenich, now a professor emeritus at the University of California in San Francisco, conducted a famous series of experiments that revealed how extensively and quickly neural circuits change in response to experience. In a conversation late last year, he said that he was profoundly worried about the cognitive consequences of the constant distractions and interruptions the internet bombards us with. The long-term effect on the quality of our intellectual lives, he said, could be ''deadly''. Not all distractions are bad. As most of us know, if we concentrate too intensively on a tough problem, we can get stuck in a mental rut. But if we let the problem sit unattended for a time, we often return to it with a fresh perspective and a burst of creativity. Research by the Dutch psychologist Ap Dijksterhuis indicates that such breaks in our attention give our unconscious mind time to grapple with a problem, bringing to bear information and cognitive processes unavailable to conscious deliberation. We usually make better decisions, his experiments reveal, if we shift our attention away from a mental challenge for a time. But Dijksterhuis's work also shows that our unconscious thought processes don't engage with a problem until we've clearly and consciously defined what the problem is. If we don't have a particular goal in mind, he writes, ''unconscious thought does not occur''.

The constant distractedness that the net encourages - the state of being, to borrow a phrase from T. S. Eliot, ''distracted from distraction by distraction'' - is very different from the kind of temporary, purposeful diversion of our mind that refreshes our thinking. The cacophony of stimuli short-circuits both conscious and unconscious thought, preventing our minds from thinking either deeply or creatively. Our brains turn into simple signal-processing units, shepherding information into consciousness and then back out again. What we seem to be sacrificing in our surfing and searching is our capacity to engage in the quieter, attentive modes of thought that underpin contemplation, reflection and introspection. The web never encourages us to slow down. It keeps us in a state of perpetual mental locomotion. The rise of social networks such as Facebook and Twitter, which pump out streams of brief messages, has only exacerbated the problem. THERE'S nothing wrong with absorbing information quickly and in bits and pieces. We've always skimmed newspapers more than we've read them, and we routinely run our eyes over books and magazines to get the gist of a piece of writing and decide whether it warrants more thorough reading. The ability to scan and browse is as important as the ability to read deeply and think attentively.

What's disturbing is that skimming is becoming our dominant mode of thought. Once a means to an end, a way to identify information for further study, it's becoming an end in itself - our preferred method of both learning and analysis. Dazzled by the net's treasures, we have been blind to the damage we may be doing to our intellectual lives and even our culture. Daily Telegraph, London Loading

The Shallows: How the Internet is Changing the Way We Think, Read and Remember by Nicholas Carr, is published in Australia by Allen&Unwin, $32.99. Carr will be speaking on the topic at The Wheeler Centre in Melbourne on September 23 from 6:15pm - 7:15pm. Seats at the free event can be booked through wheelercentre.com.