A neuromagnetic view through the skull

When information processing in the brain experiences disruptions, this often results in serious neurological diseases. Investigating signal transmission inside the brain is therefore the key to a better understanding of diseases such as Parkinson’s or epilepsy. In this context, two noninvasive methods have established themselves: electroencephalography (EEG) and magnetoencephalography (MEG). Both methods detect the slow brain currents reliably, but not the fast ones.

Slow brain currents (postsynaptic potentials) occur when neurons receive signals from other neurons. However, if a neuron fires an impulse to other downstream neurons or to muscles, this generates fast brain currents with a duration of merely a thousandth of a second (so-called action potentials). Such fast brain currents that occur in response to individual sensory stimuli have now been made visible in the investigations conducted jointly by PTB and the Charité.

This was made possible by considerably reducing the intrinsic noise of the sensing system used. The magnetic field sensors – superconducting quantum interference devices (SQUIDs) – are immersed in liquid helium to cool them down to –269 °C. The insulation of the cooling vessel used for this purpose is very elaborate and consists of so-called superinsulation which is made from aluminized foils. Even though aluminum is not ferromagnetic, electrons moving inside the metal generate magnetic noise which interferes with small magnetic fields of neurons, for instance. In this new approach, the superinsulation of the cooling vessel was implemented in such a way that its intrinsic noise was negligible. This enabled an increase in the sensitivity of the sensor technology by a factor of ten.

Measurements carried out after stimulating an arm nerve in healthy subjects confirmed that MEG is able to measure the fast stimulus responses. What was surprising, however, was that the fast brain currents were not uniform despite the constant stimulation. The brain currents varied from one stimulus to the next, independent of the slow brain currents. This suggests that the brain processes the information pertaining to the touch of a hand with surprising variability although all neural stimuli are of the same type.

The results open up new answers to fundamental neurological issues such as the influence of attention or tiredness on the processing of information by the brain. The new sensing system may also contribute to a deeper understanding and the better treatment of neurological diseases.