Physics in Neuroscience
In recent years, physics has played a major role in the rapid advance of cognitive neuroscience. The most widely used methods of mapping brain function, using magnetic resonance imaging (fMRI), were developed in the early 1990s by USA-based physicists working at labs in Boston, Minneapolis, Murray Hill and Washington. Professor Turner was himself one of these pioneers. Methods for interpreting measurements of magnetic fields generated by brain activity (MEG) have also seen major recent improvement in the hands of physicists. The physics of electromagnetic fields, nuclear spins, and the biophysics of blood and brain tissue have thus been combined to provide noninvasive methods that can resolve brain activity within a millimeter in space, or milliseconds in time. These methods allow the exploration of profound neuropsychological questions regarding the organization of human brain function, and even the relationship between mind and brain. With a much improved understanding of normal brain function, it is hoped that far more empirically based treatments for a wide range of neurological and psychiatric conditions can be developed.
For this reason, the Department of Neurophysics has been established at the Max-Planck-Institute for Cognitive and Human Sciences in Leipzig. In order to push forward the frontiers of neuroimaging techniques, it is necessary to integrate the efforts of physicists skilled in MRI technique development, neuroanatomists, image analysts, engineers and neuropsychologists. This Department has unlimited access to a state-of-the-art high field MRI scanner suitable for human use, working at 7 tesla, more than double the field strength of the most powerful clinical MRI scanners used today. Researchers can also use the other superb facilities of the Institute, including an excellent MEG scanner and two 3 T MRI scanners (see NMR web-page).
Brain Mapping, both Functional and Anatomical
The most urgent challenge is to answer the question: what next can MRI methods teach us regarding the structure and function of the human brain? Because of MRI's remarkable safety record, MRI techniques can be easily explored with volunteer human subjects. High field strength allows the use of very high spatial resolution, so that details of structures less than half a millimeter in size can be distinguished. The organization of the brain's white matter, consisting of the major long nerve fibres connecting different brain regions, can also be investigated with much greater accuracy. Changes in MRI image intensity caused by local brain activity can be measured with greater sensitivity and spatial precision.
Thus the potential exists for a richer and more detailed scientific understanding of the association of brain function with particular brain structures. Until now, imaging neuroscience has relied on the quite loose association between location in space, cortical folding, and histological analyses of cell structure in cadaver brain, such as that of Brodmann (1909), to find approximate answers to the important question: what type of neuronal territory has what type of function? But now we are discovering that many areas of grey matter have a distinctive appearance in MRI scanning, allowing us to characterize with great precision the neural substrate for particular brain processes. The goal of mapping many of these areas, to form a natural parcellation of grey matter for each individual, is perhaps within sight. Furthermore, the changes in brain structure, such as the thickness of grey matter, that are known to occur with repeated experience or practice, can be explored much more easily than at lower magnetic field strengths.
To make the best use of these powerful capabilities requires novel MRI techniques, novel hardware, and very careful attention to the safety of human subjects. At normal magnetic field strengths, MRI industry maintains routine and highly effective safety standards. Exposure to high steady magnetic fields has shown no harmful effects, on the basis of test results with hundreds of human subjects in many laboratories across the world, and indeed there is no physical reason to anticipate such effects. But there is a safety issue at high magnetic field, for which the MRI industry has yet to develop standard procedures. To produce magnetic resonance images requires excitation of the magnetization of the spins of protons in water molecules in tissue. This is performed by delivering pulses of radio-frequency electromagnetic energy, which can in principle heat the tissue. The careful safeguards set in place by MRI scanner manufacturers must be supplemented at high field strength by electromagnetic field calculations providing detailed limits to safe scanner operation. Such calculations can also guide strategies for the most efficient scanning techniques, and the Department thus includes experts in radio-frequency modelling and hardware, to explore this possibility.