Human beings are natural explorers. We like to discover strange new places and make maps of the worlds we find. However, one particular world that we have yet to fully discover lies nestled within our own heads. The human brain is a hub of electrical and chemical activity where millions of cells talk to each other via a complex network of inter-connected fibres. But, since we don’t speak “neuron”, how can we understand how the brain works?
One way to do this is by making a map of the human brain. Without having to cut into someone’s head, the best way to build such a map is to use magnetic resonance imaging (or MRI). When you lie inside an MRI scanner, the scanner’s strong magnetic field interacts with the magnetic properties of the different molecules within your head. The scanner can use this interaction to create three-dimensional images of your brain. There are many different types of MRI images that it can create. Perhaps the most popular is functional MRI (or fMRI). FMRI is used to find out which parts of the brain are important for different functions, like seeing or hearing. It works by detecting where oxygen is present in the blood — if there is more oxygenated blood in a particular brain area when doing a certain task, we can assume that this area is important for that task. For example, fMRI has shown that a brain region called the fusiform face area helps us to recognise faces. But how is the fusiform face area getting its information? And is it communicating with other areas?
One way to answer these questions is to look at how different brain areas are inter-connected using another type of MRI, called diffusion-weighted imaging. Whilst fMRI looks at activity in the cell bodies that make up the brain’s hardware (called grey matter), diffusion-weighted imaging looks at the bundles of wires that connect the different bits of grey matter in our brain. These bundles are called white matter. Diffusion-weighted imaging tracks the motion of water molecules in the brain’s white matter, and identifies the bundles of wires that water will diffuse along. A team of neuroscientists from the UK and USA called the “Human Connectome Project” have collected these types of MRI images from hundreds of healthy adult volunteers. They are using this data to create a kind of road map of the brain’s inter-connected fibres and pathways. They hope that identifying the brain’s networks will help us understand how we function and live our lives. So far, they have generated some fantastic images of the intricate circuitry within our brains — in fact, British band Muse couldn’t help but stick one of them on the cover of their album The 2nd Law. More recently, the Human Connectome Project used their complex brain map to show that the way some brain areas speak to each other could help explain certain human behaviours. They found that a particular set of areas in the brain — areas that normally talk to each other when you are resting, seeing and hearing, moving and paying attention — communicated differently in people with mostly ‘positive’ behaviour and lifestyle traits (like high life satisfaction, good vocabulary, good memory) compared to people with mostly ‘negative’ traits (like aggression, poor sleep, alcohol and drug use).
This kind of “neural cartography” using MRI is also valuable in medicine. A couple of years ago, neurosurgeons in California wanted to find a way to avoid damaging healthy tissue when they removed brain tumours. What they did was collect diffusion-weighted images from their patients and use them to map out the vital white matter circuits around the tumour. One patient for example had a tumour lodged in his occipital lobe; the region at the back of the brain important for sight. Using diffusion-weighted imaging, the neurosurgery team were able to remove the tumour without damaging the fibres that carry information from the eyes to the brain.
MRI can be a powerful tool to map the brain and its circuits of inter-connected fibres. With the help of large projects like the Human Connectome Project, we can begin to build a map of these circuits. This map will bring us one step closer to understanding the complex and intricate world of neurons. In particular, how millions of brain cells talk to each other and how this communication ultimately dictates who we are as human beings.