Within a year, the team will be generating previously undreamt of data and will be the first laboratory in the world to do so, according to their leader Nikos Logothetis, Director of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany.
Logothetis has combined functional Magnetic Resonance Imaging (fMRI), which identifies changes in blood flow to indicate how the brain is performing, with the electrophysiological technique of single neuron recording, which allows him to investigate synaptic and neuronal modifications.
If early promise is fulfilled, he will be able to produce images of isolated brain structures, showing minute local fluctuations in synaptic activity within that structure, while simultaneously imaging the whole brain performing a sensory task.
Last year, Logothetis and his team reported its first successes in combining fMRI with single neuron recording in the macaque, work hailed for establishing that the technique accurately reflects localised neural activity.
Independent researchers also saw that the work opened up the possibility of linking a vast body of information – the triad of data that comprises electrophysiological work, all done in animals, and more recent fMRI recordings in animals and humans.
Linking the data would constitute a breakthrough in imaging terms, says Guy Orban, a neurophysiologist at the Catholic University of Leuven in Belgium. In November, Orban published the first systematic fMRI study of alert monkeys, in collaboration with imaging specialists at the Massachusetts General Hospital in Charlestown.
To begin with they did not have a systematic approach to push things forward, but that is what they have now. This triad is like a new tool which can be used to address the systems level so much better, and show immediately the relevance for humans.
At close hand, Logothetis' laboratory is testament to the development of a whole new imaging technique, one that is a league apart from fMRI on its own. The equipment involves a three-storey, 7.0 tesla magnet with enough space inside to scan a monkey while simultaneously recording from electrodes implanted in its brain. There are also huge engineering and mathematical problems that the researchers had to overcome to make the combined experiments possible.
Magnetic resonance imaging is based on monitoring the responses of hydrogen nuclei in a magnetised object, such as the brain, to perturbations in a magnetic field. Researchers adjust the applied magnetic fields, measure the electromagnetic signals that the nuclei emit and build a detailed image of the whole brain.
They then implant an electrode into a brain that is sitting in this carefully maintained magnetic equilibrium, the tip of which can generate potentials up to 30 volts. The high voltage is a by-product of the need to have a large magnet, and it creates huge problems in a system designed to measure brain potentials in the vicinity of 100 microvolts.
Although interference is huge and destructive, it is also repeatable and predictable. Once you have established what kind of interference you are dealing with it is possible to invert it.
It does not work, however, to wait until the data is in, and then to do the calculations in retrospect, with the help of a computer. Because the unwanted voltages are so many orders of magnitude larger than the neuronal ‘whispers’, the latter are in danger of being swamped and lost. So the researchers measure every single voltage induced in this complex system and continuously ‘neutralise’ those that constitute interference.
One day, says Logothetis, there will be rules and diagrams to follow so that this procedure can be learned. But at the moment, there are only a few people in the world qualified to do it – and they are all in his laboratory.
BioMedNet News
21 March 2002
