Fast brain signals measured non-invasively for the first time
The brain processes information via slow and fast brain waves. To study the latter, however, electrodes had to be inserted into the brain until now. Researchers from the Charité – Universitätsmedizin Berlin and the Physikalisch-Technische Bundesanstalt (PTB), Berlin Institute, have now made these fast brain signals visible from the outside for the first time – and found an astonishing variability. As the team reports in the scientific journal PNAS*, they used a particularly sensitive magnetic encephalograph for this purpose.
Information processing in the brain is one of the body’s most complex processes; disturbances not infrequently have the effect of serious neurological diseases. Research into signal transmission in the brain is therefore the key to understanding a wide range of diseases – but methodologically it presents scientists with great challenges. In order to be able to observe the nerve cells in their „thought-fast“ work without placing electrodes directly in the brain, two technologies with high time resolution have become established: electroencephalography (EEG) and magnetic encephalography (MEG). Both methods make it possible to visualise brain waves through the skullcap – but reliably only the slow, not the fast ones.
Slow currents – so-called postsynaptic potentials – occur when nerve cells receive signals from other nerve cells. If, on the other hand, they fire themselves and thus pass on information to downstream neurons or even muscles, this causes fast currents lasting only a thousandth of a second: the so-called action potentials. „So far, we have only been able to observe nerve cells receiving information from the outside, but not transmitting it after a single sensory stimulus,“ explains Dr. Gunnar Waterstraat from the Department of Neurology with Experimental Neurology at the Charité Campus Benjamin Franklin. „You could say we were blind in one eye, so to speak.“ A team led by Dr Waterstraat and Dr Rainer Körber from the PTB has now laid the groundwork for that to change. The interdisciplinary research group has succeeded in making MEG technology so sensitive that it can also detect fast brain waves in response to individual sensory stimuli.
The team achieved this by significantly reducing the inherent noise of the MEG device. „The magnetic field sensors in an MEG device are immersed in liquid helium to cool them to -269°C,“ explains Dr Körber. „To do this, the cooling vessel is very elaborately insulated. However, this super insulation consists of foils vapourised with aluminium, which themselves cause magnetic noise and therefore superimpose small magnetic fields, for example from nerve cells. We have now constructed the super insulation of the cooling vessel in such a way that its noise is no longer measurable. In this way, we have succeeded in making the MEG technology ten times more sensitive.“
The research team demonstrated that the new instrument is indeed capable of recording fast brain waves using the example of stimulation of an arm nerve. For this purpose, a nerve on the wrist was electrically stimulated in four healthy volunteers and the MEG sensor was positioned directly above the brain area responsible for processing sensory stimuli from the hand. To exclude sources of interference such as power grids or electronic components, the measurements took place in an electromagnetically shielded measurement chamber at PTB. As the researchers discovered, this made it possible to measure action potentials of a small group of synchronously activated neurons that arose in the cerebral cortex in response to individual stimulation stimuli. „So, for the first time, we non-invasively watched neurons in the brain send information after a touch stimulus,“ Dr Waterstraat points out. „Interestingly, we observed that despite constant stimulation, these fast brain waves were not uniform, but changed from stimulus to stimulus. Moreover, these changes were independent of the slow brain signals. So the information about touching the hand is processed by the brain in an astonishingly variable way, even though all the nerve stimuli were of the same kind.“
The fact that the scientists can now compare individual stimulus responses with each other opens up the possibility for neurological research to investigate previously unanswered questions: What influence do factors such as attention or fatigue have on information processing in the brain? Or the simultaneous occurrence of other stimuli? The highly sensitive MEG system could also contribute to a deeper understanding and better therapy of neurological diseases. For example, epilepsy and Parkinson’s syndrome, among others, are associated with disturbances in rapid brain signals. „With the optimised MEG technology, we now have one more fundamental tool in our neuroscience toolbox to address all these questions non-invasively,“ says Dr Waterstraat.
*Waterstraat G, Körber R et al. Noninvasive neuromagnetic single-trial analysis of human neocortical population spikes. Proc Natl Acad Sci USA 2021. doi: 10.1073/pnas.2017401118