Neural cartography: how to draw a map of the brain

As natural explorers, we like to make maps of the world and the discoveries we make. A map of the human brain remains a major challenge for neuroscientists today, but the Human Connectome Project could be a significant step forward. This large cross-Atlantic project (which includes the University of Oxford) is working towards mapping the brain’s network of “wires” – a kind of road map to understand the ins and outs of the brain. The teams involved hope that identifying these complex networks will help us understand how we function and how we behave. So far, this work has generated some fantastic images of the intricate circuitry in our brain – in fact, British band Muse couldn’t help but stick one of them on the cover of their recent album.

How can the Human Connectome Project build such a complex three-dimensional map? Magnetic resonance imaging (MRI) is a useful tool for studying the brain, without having to cut open someone’s head. The MRI machine creates a strong magnetic field and can assemble three-dimensional images of the brain because of the magnetic properties of molecules found in our heads. There are many different types of MRI, with fMRI being a popular tool to look at the function of different brain areas. Traditionally, fMRI has been used in a ‘modular’ fashion – that is, to ask what the function of region X is. 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 relying on other areas in order to recognise faces? These questions push for a greater understanding of how areas are connected and the networks that lie within the brain.

To address these questions, we can look towards another MRI technique called diffusion tensor imaging (DTI). Whilst fMRI looks at activity in the grey matter (neuron cells bodies that make up the brain’s hardware), DTI looks at white matter, or the bundles of wires that connect the different bits of hardware in our brain. DTI tracks how water molecules move around in the brain and can be used to identify the bundles of wires that water will diffuse along. To do this, it first calculates which direction water is diffusing in for each of the three-dimensional pixels, or ‘voxels’, that make up an image of the brain generated by the MRI machine. Next, it uses this network of directions to connect the dots and track where the wires are in the brain.

This neural cartography is not only a useful tool to draw a map of the human circuitry – it can also be valuable in preparing for surgery, as demonstrated recently by a team of neurosurgeons in California. In order to avoid healthy tissue damage when removing tumours, neurosurgeons used DTI images of their patients to map out the vitals circuits around the tumour. They used this method in a patient who had a tumour lodged in his occipital lobe, the region of the brain dedicated to vision. Using DTI, they were able to remove the tumour without affecting the wires carrying information from the eyes to the occipital lobe.

DTI can be a powerful technique to map the brain and its connections, in both a research and medical context. Here in Oxford and in the United States, the Human Connectome Project aims to collect DTI images from a very large cohort of healthy adult volunteers. Using this data, they will develop sophisticated analysis methods to make sense of the diffusion information and build a complex three-dimensional map of the brain’s most important circuits. By releasing their large map to the general neuroscientist population, the project hopes that the availability of good data will lead to further discoveries about the brain wires which enable us to interact with our world.