If one has to hazard a guess, the science of the early 21st century will be driven by brain research. In 1904, no one had any idea of the epochal contributions to science that were to arrive in 1905. Einstein's special theory of relativity that year, and his general theory ten years later, transformed the vision of space, time and the universe.
Molecular biology and genetics have been the sciences of the late 20th century, most of it techno-science, telling us far more about life than we knew before. It is in the process of ending the idea of "nature" for ever, a revolution to equal Einstein's, but it is doing so by myriad technologies. Brain science will be like that.
The word "neuroscience" was coined around 1960 to group together an ever-expanding package, and was defined by practitioners over the subsequent decade as including "the study of brain development, sensation and perception, learning and memory, movement, sleep, stress, ageing and neurological and psychiatric disorders". The package is still in flux and would now certainly include emotions (a hot topic), language capacities, and brain chemistry and surgery.
Neuropharmacology is the field of applied neuroscience that has had the biggest effects, for better or worse, on more people than any other. Amazing cocktails of mood-altering drugs - antidepressants, neuroleptics, Ritalin and the rest - have modified countless lives. Schizophrenia is the most cruel of widespread mental disorders. We now treat it with drugs. If adequate genetic markers are found for important types of this illness, we shall enter a field of brain genetics. Comparative anatomy teaches us which parts of the brain evolved first. It combines with evolutionary psychology, which tries to explain aspects of human behaviour by their adaptive value.
At the core of the new brain science is an astounding mix of technologies adapted from other sciences. None has been developed with the brain in mind, but they have radically transformed neuroscience.
Until recently, the brain was a black box that usually could be opened only when it ceased to work - that is, in post-mortem autopsy. One could do some research on the side when doing quite invasive surgery, but a controlled experiment on the brain of a living person is morally impossible. Hence, every war in more than a century advanced our knowledge of the brain. Head-injured survivors had strange inabilities to speak or see or feel or remember in ordinary ways. From this we learned that the damaged parts of their brains were essential to the missing functions. Industrial accidents, tumours and disease have also pitched in, but wars delivered a bonanza of phenomena.
Today, as a result of wonderful new non-invasive tools, we do not need injured people for research on the brain. It started in the early 1970s with the Cat scan - computerised axial tomography - which builds up a three-dimensional picture from individual X-rays. Its developers in Britain and the US shared a Nobel prize in 1979. MRI or magnetic resonance imaging and Pet (positron emission tomography) scans followed soon after.
Such machines not only show which bits of a brain are hurt, but can also indicate the parts of a healthy brain that are active, activated, responding to a stimulus, or maybe even thinking.
Many of the inferences are still pretty crude. We deduce that a region is at work when more blood gets there, quickly. This is the continuation of a programme, evolved during the 19th century, of localising functions in particular parts of the brain. The old idea, a sensible first guess, was that specific jobs were done by specific bits of the brain. We now know that it is a lot more complex than this, and that injured regions can have their work taken over elsewhere in the brain. Yet none of that really tells us how the brain works.
However, amazing levels of sophistication are opening up. Typically, they combine different technologies. Here is just one example out of dozens of imaginative initiatives.
Neurons are cells. Information is transmitted in and out of a neuron by proteins that act as gates. Certain proteins in the cells of jellyfish fluoresce green. These have been cloned and mutated to produce colour-coded fluorescence: red, yellow, and so on. What if we splice genetic material from a jellyfish with gating proteins in a mouse, so that the designated mouse neurons fluoresce appropriate colours when the gates are activated? Could this possibly work? It becomes a technical problem.
There have long been microscopes that filter ordinary light and collect only fluorescent light. The same principles can be used in laser microscopes that illuminate samples with the smallest possible amount of light (the two-photon laser microscope). Currently, such techniques work best on the nervous system and brain of the zebra fish, a pretty, rather stupid, but genetically tractable creature.
In the past year or so, several teams have been applying the technology to mice. The aim is to detect and record circuits of individual neurons firing in the brain of a fairly ordinary healthy mouse. Which neurons fire when you prick the animal, which when it is navigating a turn in a maze, which when it notices a tasty morsel or attractive mate? More important, what is the sequence of firings? How are sequences of firings associated with the transmission of information in the brain? Can we figure out how information is coded in patterns and sequences of neuron firings? Not so long ago, we could barely identify regions of the brain used for various functions, such as speech. Now we imagine being able to see the individual neuronal circuits at work. In an article in the 21 July issue of the Journal of Neuroscience, Alison Barth of Carnegie Mellon University and colleagues describe their transgenic mouse, engineered so that individual neurons triggered by stimuli grow. Today, the stimulus is the simplest: sensation from a single whisker. But just wait a few months.
The word "cyborg" was coined by two visionaries working on a Nasa contract in 1960. It is short for cybernetic organism, an organism able to function better with attached or implanted feedback systems. Human cyborgs are all around you, starting with your neighbour who has a pacemaker for the heart. The cochlear implant for impaired hearing falls squarely under applied neuroscience. Bio-engineers in Utah have installed a computer chip in the head of a few fully blind subjects. The chip is connected to the visual cortex of a blind person, who is then able to get a rudimentary sort of vision, grainy but not negligible.
In another project, persons totally paralysed by a stroke and unable to communicate in any way, but with electrodes and chips placed in the head, are able to learn how to direct a cursor on a screen "just by thinking". The spelling out of words is the merest beginning of what may happen with future technologies. For now, the chips are made of sand and the electrodes of metal, but increasingly they will be of synthetic biomaterials. Switch direction, and consider the beginnings of computers built out of living matter.
Where is the money coming from for all this research? You. Brain science prospers because the coffers are wide open. Much of it is motivated by the medical side. The two great illnesses of ageing have been cancer and heart disease. Chemistry for cancer, and engineering for the heart, have made vast strides. As the populations of rich countries get older, a new threat is upon us: dementia. To the public, this means Alzheimer's, the incidence of which, it is predicted, will treble in the next 30 or so years.
People now in the prime of middle age are watching their parents slip into another world, often with gross changes in their former personalities. It now seems almost worse to lose your soul to nothingness than to suffer the ravages of cancer. Hence, there is real money for brain research, money from pharmaceutical companies - though we are still a long way from quick drug fixes. A cynic will notice that the committees funding research are made up of individuals well aware that they are ageing. Young researchers know that to say their research has something to do with memory produces results when they put in for a grant.
Fear helps funding, but there are also great expectations. Brains may not matter much in the cosmic scheme of things. Finding out about the brain is not like solving the riddles of the universe. Brains do, of course, make a vast difference to life on earth, given that humans are the dominant life form. But that is not what drives brain research. We are a narcissistic species, and there is the sense that Brains Are Us. I was appalled recently to pass by a French bookstore and see a new title, Inside the Customer's Head: what neuroscience teaches us about marketing. Yet if you change the title - to "what neuroscience teaches us about the soul", or the emotions, or even religious belief - no eyebrows will be raised. There is a shared belief that as we learn more about the brain we shall learn more about human nature, about ourselves and our kind.
For myself, I want to say "whoa" to speedy inferences that go from neuroscience to who we are. But it does help to explain why brain science is likely to be the science of the next few decades.
Ian Hacking is a professor at the College de France, Paris
