Neuroimaging refers to a set of brain-scanning techniques that allow us to observe the levels of activity in a participant's brain, usually while they are performing some psychological task, such as listening to words.

Prior to neuroimaging, the only way that we could learn about the human brain was by cataloguing the set of impairments that a group of patients would exhibit when they suffered damage to a given brain region. A surprising amount of progress was made using this method, but it was hampered by several limitations. Most forms of brain damage involve large areas, very likely including regions in addition to the small part of the brain the researcher is interested in. There is also the common problem of brain plasticity - the ability of the central nervous system to re-wire itself and co-opt intact regions with a similar function to take on a larger role.

However, with the advent of neuroimaging a couple of decades ago, neuroscience was revolutionised. For the first time, we could centre in on the function of any part of the brain we were interested in studying, and apply a host of cognitive tests to participants in the scanner to ascertain which functions were associated with a relatively small area and which were not.

Although there are many different scanning techniques available, the two most common forms of neuroimaging are Positron Emission Tomography (or PET), and functional Magnetic Resonance Imaging (or fMRI). Both rely on 20th century advances in physics to capture three dimensional images of brain activity.

PET scanning normally involves injecting a slightly radioactive form of water (with a very short half-life) into the blood-stream of the subject, which enters the brain circulation a minute or two later. Those areas of the brain that are more involved in the current task will use more blood, and therefore will be slightly more radioactive than regions irrelevant to the task.

fMRI, on the other hand, is completely non-invasive. This technique relies on the magnetic properties of the nuclear constituents of our blood. When parts of our brain are more active, in order to gain more energy, they draw oxygen from the blood supply, which in turn changes the magnetic characteristics of the blood. This minute magnetic difference can then be detected by the fMRI scanner.

fMRI, a more recent scanning technique than PET, is predominantly used at present, due to its non-invasive nature, superior picture resolution (a few millimetres cubed per pixel, normally) and ability to take very fast images of the whole brain (of the order of a few seconds per image).

As an example of a typical study, say a participant is viewing pictures of places while in the scanner. As a result, the brain's place area (in the temporal lobes) will increase in blood flow (picked up by the PET scanner) or will use more energy, in the form of taking oxygen from the blood (picked up by the fMRI scanner). Using this simple principle of linking increases in brain activity to specific psychological processes, we can learn vital details about the functional mapping of the brain.

The overall goal of providing a detailed "cognitive map" of the human brain has very clearly taken a leap forward as a result of neuroimaging. In every psychological area, including perception, our emotions and memories, and language and learning, we are applying brain-scanning techniques to discover which parts of the brain support these processes.

While such work undoubtedly helps us learn about how the normal brain carries out all the mental tasks it does, there is huge scope for using neuroimaging techniques to learn more about various psychiatric and neurological disorders. Considerable work has used neuroimaging on a host of patient groups, including those with schizophrenia, anxiety, depression, autism and Parkinson's disease. By comparing the brain activity of those patients with normal participants on a set of specific tasks, much can be learnt about how their neurophysiology differs from the population. This difference, it is presumed, at least partially explains their debilitating symptoms. In addition, neuroimaging can be used on patients both on and off medication to discover exactly what effect the drugs are having on the brain.

We can even use neuroimaging techniques on patients too ill to appear responsive, such as those in a coma, to discover what residual cognitive function they may have - and possibly make predictions about their recovery. For instance, such patients have been shown pictures of familiar faces while in the PET scanner, and the brain area known to process faces has correspondingly increased in activation, indicating at least some intact higher-order functioning.

It is clear that neuroimaging is a very powerful tool in helping us to understand both normal and abnormal brain-function. However, it is also worth noting just how much of an approximation these techniques produce. Brain activity changes not every second, but every millisecond, while a cube a few millimetres wide doesn't capture a single neuron but many thousands. The hope, though, is that with the continuing optimisation of current techniques, and the emergence of new types of scanners, we will measure the brain in more and more detail, causing a further acceleration of our understanding of the most complex organ in the world.

Many research laboratories need volunteers for their brain scanning studies. If you are interested in finding out about taking part you can locate local UK Centres by typing 'Brain Imaging UK' into any search engine. If you live in the Cambridge area you can go directly to the volunteer recruitment section of the Medical Research Council Cognition and Brain Sciences site (you need to be local to the area to take part).