Human identity, the idea that defines each and every one of us, could be facing an unprecedented crisis. It is a crisis that would threaten long-held notions of who we are, what we do and how we behave. It goes right to the heart - or the head - of us all. This crisis could reshape how we interact with each other, alter what makes us happy, and modify our capacity for reaching our full potential as individuals. And it's caused by one simple fact: the human brain, that most sensitive of organs, is under threat from the modern world.

Unless we wake up to the damage that the gadget-filled, pharmaceutically-enhanced 21st century is doing to our brains, we could be sleepwalking towards a future in which neuro-chip technology blurs the line between living and non-living machines, and between our bodies and the outside world. It would be a world where such devices could enhance our muscle power, or our senses, beyond the norm, and where we all take a daily cocktail of drugs to control our moods and performance.

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Already, an electronic chip is being developed that could allow a paralysed patient to move a robotic limb just by thinking about it. As for drug manipulated moods, they're already with us - although so far only to a medically prescribed extent. Increasing numbers of people already take Prozac for depression, Paxil as an antidote for shyness, and give Ritalin to children to improve their concentration. But what if there were still more pills to enhance or "correct" a range of other specific mental functions?

What would such aspirations to be "perfect" or "better" do to our notions of identity, and what would it do to those who could not get their hands on the pills? Would some finally have become more equal than others, as George Orwell always feared? Of course, there are benefits from technical progress - but there are great dangers as well, and I believe that we are seeing some of those today. I'm a neuroscientist and my day-to-day research at Oxford University strives for an ever greater understanding - and therefore maybe, one day, a cure - for Alzheimer's disease.

But one vital fact I have learnt is that the brain is not the unchanging organ that we might imagine. It not only goes on developing, changing and, in some tragic cases, eventually deteriorating with age, it is also substantially shaped by what we do to it and by the experience of daily life. When I say "shaped", I'm not talking figuratively or metaphorically; I'm talking literally. At a microcellular level, the infinitely complex network of nerve cells that make up the constituent parts of the brain actually change in response to certain experiences and stimuli.