Swiss and American researchers have drawn up the first high-resolution map of how some of the most important fibres in the human brain communicate.
It provides a draft of the connections in the cerebral cortex - the outer layer of the brain responsible for activities such as reasoning and planning - and fills in some of the important blanks concerning this organ.
The scientists used a variation of magnetic resonance imaging (MRI), known as diffusion spectrum imaging, to study the brains of five healthy volunteers.
This technique allowed them to determine both the density and the orientation of brain connections. After computer analysis of the images, the researchers were able to determine the zones of the cortex where activity was the strongest.
"Earlier [brain] atlases are maps using information from post-mortem studies carried out over centuries," lead author Patric Hagmann told swissinfo. "But we have no information about connectivity and individual living subjects."
Most of what had been learned about neural connections had until now been gleaned through animal studies.
This latest research, published in the free-access online journal PloS Biology, allowed the scientists to discover a central zone through which most of these activities transit. A comparison of the results with a measure of brain activity using a functional MRI on each volunteer confirmed the findings.
This activation of this zone had previously been considered to be the result of wandering thoughts and self-awareness.
"We can measure a significant correlation between brain anatomy and brain dynamics," said study co-author Olaf Sporns of Indiana University in a statement. "This means that if we know how the brain is connected we can predict what the brain will do."
The actual result is something electricians are more than familiar with.
"This is really a wiring diagram of the brain," said Hagmann, a radiologist at Lausanne's university hospital. "There are plenty of studies about brain functions, but until now we weren't able to study the wiring."
That's because standard MRI studies were only able to identify troughs and peaks of neural activity during decision-making processes, for example. But this gave almost no information about the underlying network of the brain.
The research team, which also includes scientists from Lausanne's Federal Institute of Technology and Harvard University, considers this wiring diagram as the first step towards developing large-scale models of the brain.
Hagmann and Sporns plan to map brains as they develop and age, as well as how connections change because of disease and injury.
Hagmann told swissinfo that many diseases involve for example multiple subtle alterations of the brain. The sum of these alterations might have very striking effects, such as in the case of Alzheimer's disease and schizophrenia.
"Small alterations won't have an effect, but if they are in critical areas of the brain and you add them up, you might get symptoms of disease," he added.
"These pathologies will be the targets for future studies because we need an exhaustive map of the brain to understand the effect of the disconnection on the global functioning of the brain."
swissinfo, Scott Capper
Nuclear magnetic resonance imaging (MRI) is primarily a medical imaging technique most commonly used to visualise the structure and function of the body.
MRI is especially useful when it comes to differentiating between the different soft tissues of the body, and is often employed for neurological, musculoskeletal, cardiovascular, and cancer imaging.
The technique involves a powerful magnetic field that aligns and rotates hydrogen atoms found in water in the body.
The signal they emit can be manipulated so as to reconstruct an image of the body.
Diffusion MRI uses the properties of water molecules in the body. For example, they are more likely to move along the axis of a neural fibre, giving its direction.
Functional MRI measures blood flow to determine when an area of the brain is activated for example, but does not show the underlying network.
At least SFr630 million ($617 million) is spent in Switzerland every year on researching psychiatric and neurological illnesses, according to a Zurich University study published on July 1.
98% of brain research is funded by private industry.
Pro person SFr86 of private money is spent on brain disease research every year. The European average is SFr14.
The study says that research into psychiatric illness is underfunded, receiving just one-third of all resources, although this type of illness generates two-thirds of all costs associated with brain disease.