Like nearly all the Guardian's content, what you are about to read was – and this will hardly be a revelation – written using a computer connected to the internet. Obviously, this had no end of benefits, mostly pertaining to the relative ease of my research and the simplicity of contacting the people whose thoughts and opinions you are about to read. Modern communications technology is now so familiar as to seem utterly banal, but set against my clear memories of a time before it arrived, there is still something magical about, say, optimistically sending an email to a scientist in southern California, and then talking to him within an hour.

But then there is the downside. The tool I use to write not only serves as my word processor and digital postbox, but can also double as – among other things – a radio, TV, news-wire portal and shop. Thus, as I put together the following 2,000-ish words, I was entertained in my more idle moments by no end of distractions. I watched YouTube videos of Manic Street Preachers, Yoko Ono, and the Labour leadership candidates. Via Amazon, I bought a £4.99 teach-yourself-to-spell DVD-Rom for my son, which turned out to be rubbish. And at downright stupid hours of the day – 6am, or almost midnight – I once again checked my email on either my phone or computer. Naturally, my inbox was usually either exactly how I had left it, or newly joined by something that could easily have waited – though for some reason, this never seems to register.

Obviously, I am not alone in this affliction. Yesterday, scores of headlines focused on a new report by the media regulator Ofcom, which found that Britons spend more than seven hours a day watching TV, going online, sending texts and reading newspapers, and that web-capable smartphones are now a fixed part of millions of people's lives. Superficially, all this hardly seemed revelatory – but at the lower end of the age range lurked evidence of the world to come. Among 16-to-24-year-olds, television was not nearly as dominant: half their "media time" was devoted to mobile phones and computers – and in turn, two-thirds of that time was spent doing two digital things at once. The younger you are, it seems, the more your media consumption finds you multitasking; I'm a relatively ancient 40, but my habits are increasingly similar.

It often feels as if all this frantic activity creates a constant state of twitchy anxiety, as any addiction usually does. Moreover, having read a freshly published and hotly controversial book about the effect of digital media on the human mind, I may have very good reason to feel scared. Its thesis is simple enough: not only that the modern world's relentless informational overload is killing our capacity for reflection, contemplation, and patience – but that our online habits are also altering the very structure of our brains.

The Shallows is a 250-page book by American writer Nicholas Carr, just published in the US, about to appear in the UK, and already the focus of a noisy debate. Two years ago, Carr wrote an essay for the Atlantic magazine entitled "Is Google making us stupid?" This is the full-length version: an elegantly written cry of anguish about what one admirer calls "the uneducating of Homo sapiens", and a rewiring of neural pathways and networks that may yet deprive the human race of the talents that – ironically enough – drove our journey from caves to PC terminals.

In the book, Carr looks back on such human inventions as the map, the clock and the typewriter, and how much they influenced our essential modes of thought (among the people whose writing was changed by the latter were Friedrich Nietszche and TS Eliot). By the same token, he argues that the internet's "cacophony of stimuli" and "crazy quilt" of information have given rise to "cursory reading, hurried and distracted thinking, and superficial learning" – in contrast to the age of the book, when intelligent humans were encouraged to be contemplative and imaginative.

But here is the really important thing. Carr claims that our burgeoning understanding of how experience rewires our brain's circuits throughout our lives – a matter of what's known as "neuro- plasticity" – seems to point in one very worrying direction. Among the most hair-raising passages in the book is this one: "If, knowing what we know today about the brain's plasticity, you were to set out to invent a medium that would rewire our mental circuits as quickly and thoroughly as possible, you would probably end up designing something that looks and works a lot like the internet."

Surprisingly little research has looked into the internet's effects on the brain, but the work that forms Carr's holy grail was carried out in 2008, by a trio of psychiatrists at UCLA led by Dr Gary Small, himself the co-author of a book titled iBrain: surviving the technological alteration of the modern mind. Under their supervision, 12 experienced web users and 12 digital newcomers used Google, while their brains were scanned. The results, published under the title Your Brain On Google, pointed up a key initial difference between the two groups: in an area of the brain called the dorsolateral prefrontal cortex, which deals with short-term memory and decision-making, the rookies showed hardly any activity, whereas the web veterans were really firing.

Six days later, the novices having been told to spend an hour a day online, the two groups' brains were scanned again – and this time, things got even more interesting: in images of both sets of brains, the pattern of blobs representing mental activity was virtually identical. As Small put it: "After just five days of practice, the exact same neural circuitry in the front part of the brain became active in the internet-naive subjects. Five hours on the internet, and the naive subjects had already rewired their brains."

Small is the director of the Memory and Ageing Research Centre at the University of California, Los Angeles, a specialist in the effects on the brain of the ageing process, and the co-inventor of the first brain-scanning technology to detect the physical evidence of Alzheimer's disease. "Even an old brain can be quite malleable, and responsive to what's going on with technology," he tells me.

He goes on: "It's a basic principle that the brain is very sensitive to any kind of stimulation, and from moment to moment, there is a very complex cascade of neurochemical electrical consequences to every form of stimulation. If you have repeated stimuli, your neural circuits will be excited. But if you neglect other stimuli, other neural circuits will be weakened." This is the nub of Carr's argument: that the online world so taxes the parts of the brain that deal with fleeting and temporary stuff that deep thinking becomes increasingly impossible. As he sees it: "Our ability to learn suffers, and our understanding remains shallow."

Small is only too aware of what too much time spent online can do to other mental processes. Among the young people he calls digital natives (a term first coined by the US writer and educationalist Marc Prensky), he has repeatedly seen a lack of human contact skills – "maintaining eye contact, or noticing non-verbal cues in a conversation". When he can, he does his best somehow to retrain them: "When I go to colleges and talk to students, I have them do one of our face-to-face human contact exercises: 'Turn to someone next to you, preferably someone you don't know, turn off your mobile device.' One person talks and the other one listens, and maintains eye contact. That's very powerful. One pair of kids started dating after they'd done it."

He also fears that texting and instant messaging may already be dampening human creativity, because "we're not thinking outside the box, by ourselves – we're constantly vetting all our new ideas with our friends." He warns that multitasking – surely the internet's essential modus operandi – is "not an efficient way to do things: we make far more errors, and there's a tendency to do things faster, but sloppier." Of late, he has been working with big US corporations – Boeing is the latest example – on how they might get to grips with the effects of online saturation on their younger employees, and reacquaint them with the offline world.

When I ask him how I might stop the internet's more malign effects on my own brain, he sounds slightly more optimistic than Carr: we have the capacity to pull ourselves back from the mental brink – though only if we know what's at stake. "The brain can right itself if we're aware of these issues," he says. "But we have to make decisions as to what we can do about it. Try to balance online time with offline time," he tells me. "What's happening is, we're losing the circadian rhythms we're used to; you go to work, you come home, you spend time talking with your kids."

What about the idea of calming down when you're online? I'm actually pretty good at offline time, but as soon as I'm back at my desk, it's all YouTube and compulsive email checking, and it's rather doing my head in.

"It's hard," he says. "There's a pull. The internet lures us. Our brains become addicted to it. And we have to be aware of that, and not let it control us."

Among the people with walk-on roles in The Shallows is Scott Karp, the editor of a renowned American digital media blog called Publish2, whose reading habits are held up as proof of the fact that plenty of people's brains have long since been rewired by their enthusiastic use of the internet.

Despite a degree from New York University in English and Spanish literature, Carr claims that Karp has given up reading books altogether, perhaps because of what a working life spent online seems to have done to his mental makeup. One of Karp's online posts is quoted as follows: "I was a lit major in college, and used to be a voracious book reader. What happened? What if I do all my reading on the web not so much because the way I read has changed . . . but because the way I THINK has changed?"

As it turns out, Karp has only stopped reading non-fiction. Contrary to Carr's thesis, he says he still has no problem reading novels, and thinks his long-term memory is in as good shape as ever. What he attests to, though, is a radical shift in the way he consumes information, which may or may not have caused his mental circuits to change.

This, he tells me, is all down to his appetite for connecting multiple bits – and, it seems, only bits – of information, rather than digesting big chunks of stuff from single sources, one at a time. "I thrive on that connectedness of information," he says, "so now, I maybe read a given author's argument in much briefer form than a 10,000 word article or a book – and then jump to another author's argument, and follow that train of thought. And sometimes I find that I make leaps in thinking by reading things from different perspectives, and going from lily pad to lily pad."

He assures me he understands any argument's strengths and weaknesses before flitting to the next one, but I'm not so sure. Aren't there thousands of books that have to be read in their entirety before we can really get our head round the author's point of view? The last thumping great book I read was the biography of Barack Obama by David Remnick, the editor of the New Yorker – and the idea of boiling it down to a skimmable extract seems almost offensive. The same applies to, say, any number of books by Marx and Engels, or even (possibly) Ozzy Osbourne's autobiography.

"Absolutely," he says, rather guiltily. "I completely agree with that. And I'm sure that I have come up shallow, if you use Nicholas Carr's argument. But I've only got a finite amount of time."

Whatever, Karp is not fazed by the idea that heavy internet use might be reshaping his brain. "Everything changes our brain," he says. "Everything. That's what the brain does. It's constantly changing and adapting to every experience. It's almost axiomatic to say: 'The internet has changed our brain, and its processes.' Yes, we spend less time concentrating on single sources of information. But when it comes to making value judgements, it becomes difficult to say, 'And we are worse off because of that.'" As we end our conversation, I have a vision of him frantically pinging from blog to website to pdf, and I'm really not so sure.

I get a more convincing antidote to the Carr thesis from Professor Andrew Burn of the University of London's Institute of Education, who has long specialised in the way that children and young people use what far too many people still call "new media", and its effects on their minds. Equating the internet with distraction and shallowness, he tells me, is a fundamental mistake, possibly bound up with Carr's age (he is 50). "He's restricting what he says to the type of activities that the middle-aged blogosphere-addict typically engages in," says Professor Burn. "Is there anything in his book about online role-playing games?"

Not much, I tell him, and he's off. "Carr's argument privileges activities of the skimming and browsing kind. But if you look at research on kids doing online gaming, or exploring virtual worlds such as Second Life, the argument there is about immersion and engagement – and it's even about excessive forms of immersion and engagement that get labelled as addiction. The point is, to play successfully in an online role-playing game, you have to pay an incredible amount of attention to what your team-mates are doing, to the mechanics of the game. You can set up a thesis for The Depths, just as much as The Shallows."

And what of all these worries about the transformation of the human brain? "Temporary synaptic rewiring happens whenever anybody learns anything," he says. "I'm learning a musical instrument at the moment, and I can feel my synapses rewiring themselves, but it's just a biological mechanism. And it seems to me that to say that some neural pathways are good and some are bad – well, how can you possibly say that? It could be a good thing: people are becoming adaptive, and more supple in their search for information." Carr, he reckons, is guilty of a "slippage into an almost evolutionary argument", and he's not having it at all.

He's also not impressed by the way Carr contrasts the allegedly snowballing stupidity of the internet age with the altogether more cerebral phase of human progress when we all read books. "What if the book is Mein Kampf? What if it's Jeffrey Archer? Or Barbara Cartland? Am I not better off playing a well-constructed online game, or reading Aristotle's poetics online? I really don't see why books should particularly promote worthwhile thought, unless they're worthwhile books. And the same applies to what's on the internet."

This all sounds both comforting and convincing, until I return to The Shallows, and a particularly sobering sentence on page 222 (contrary to Carr's darker predictions, I easily made it to the end). "We are welcoming the frenziedness into our souls," he writes. There's something chilling about those words, and even 20 stupid minutes on YouTube and an impulse buy from Amazon cannot quite remove them from my brain.

• The Shallows: How the Internet is Changing the Way We Think, Read and Remember will be published next month in the UK by Atlantic Books, price £17.99.