External measurements taken on the scalp allow conclusions to be drawn about the underlying nerve cell activity.
A new study carried out by scientists in Tübingen, Germany, shows that the results of measurements using different common methods for the determination of brain activity can largely be directly correlated. Two of these - electroencephalography (EEG) and magnetoencephalography (MEG) - are non-invasive methods in which brain activity is measured on the surface of the head and no surgical intervention is necessary. The third method is the neuronal invasive electrophysiology.
Researchers led by Professor Markus Siegel at the Hertie Institute for Clinical Brain Research at the University of Tübingen (Germany) and colleagues from the Massachusetts Institute of Technology (USA) are now using all three methods in parallel in a single visual experiment. They showed that EEG, MEG and invasive electrophysiology record very similar information during the processing of visual stimuli, such as the color and direction of point movements. In the future, findings from invasive and non-invasive experiments could be better linked.
"In the broader sense, research on human subjects can now also be better compared with research on laboratory animals," said Siegel, head of the study. "However, it is not easy to relate EEG and MEG data to the underlying neuronal circuits," reported Florian Sandhäger, Siegel collaborator and lead author of the study. Both methods measure large electrical and magnetic fields on the surface of the head, which are generated due to brain activity.
They can be used to determine the local sources of signals, but not the activities of individual cells. These can only be clarified with the help of invasive electrophysiology. The wafer-thin microelectrodes measure nerve cell activity directly at the site of events in the brain and thus offer a very high spatial resolution.
Siegel and his team aimed to connect the electric and magnetic fields measured outside the head with the specific nerve cell activity. For this purpose, they developed an experiment in which different colored dot patterns were shown on a screen moving in different directions. First, the scientists examined the brain activity of human subjects while observing these patterns. They used the MEG, while at the same time, they developed a special EEG with which they could measure the comparable brain activity during the task on Rhesus monkeys (Macaca mulatta). In a third step, they carried out the visual experiment with the animals while measuring the nerve cell activity using microelectrodes.
The result: The measured signals contained information on the color and direction of movement of the dot patterns in all three methods. In addition, the scientists identified specific patterns in the MEG and EEG that they could relate to the properties of individual nerve cells in certain brain areas. "Our study helps to place non-invasive measurement techniques in close relation to the underlying cellular mechanisms," explained Siegel and Sandhäger. "This bridge not only contributes to a better understanding of the functioning of the human brain but in the long run may also allow a more accurate interpretation of EEG and MEG measurements in the clinical context.
Sandhaeger et al. (2019): Monkey EEG links neuronal color and motion information across species and scales, eLife, 8: e45645.