28.07.2014
In the human brain electrical oscillations ranging from very low frequencies (< 0.05 Hz) to frequencies above 1 kHz are generated. Activities that are usually measured by electroencephalography (EEG) and magnetoencephalography (MEG) have a frequency range of up to around 100 Hz and originate from postsynaptic potentials. They represent the computational input of the nerve cell. In contrast, ultra-fast oscillations of up to around 1 kHz correspond to the computational output of the neuron in form of action potentials, a process also known as the firing of the neurons. In order to gain an understanding of the neuronal processing of external stimuli and information a detailed knowledge of both, the slow and fast oscillations is therefore necessary.
The detection of ultra-fast oscillations at 1 kHz was possible only invasively by EEG using depth electrodes some 10 years ago. PTB has developed, together with Charité, a low noise EEG amplifier which was able to record such ultra-fast activity non-invasively on the scalp. This was followed by the development of a low-noise magnetic measurement system (MEG) also at PTB aiming for the contactless magnetic detection of the fast oscillations.
We describe first combined EEG/MEG measurements of the high frequency brain signals performed inside an electromagnetically shielded room. We recorded the evoked neuronal response following electrostimulation of the median nerve of five volunteers. The frequency of stimulation was 4.9 Hz with the current set to 1.5 x motor threshold.
The low-noise EEG amplifier has a noise of 4.7 nV/√Hz and the noise figure of the low-noise single-channel MEG system amounts to 0.5 fT/√Hz both determined at 1 kHz. 12000 averages were taken to increase the signal-to-noise ratio.
Both low-noise EEG and MEG systems show three distinct frequency ranges in which both pronounced but also only partial waveform overlap is observed.
range 1: N20 (<100 Hz)
range 2: sigma- bursts ( 450-750 Hz )
range 3: kappa - burst ( 850-1200 Hz)
Time traces of the averaged (N=12000) EEG/MEG epochs. (a) Comparison in range 2: sigma burst. (b) Comparison in range 3: kappa burst
In conclusion, we showed that low-noise EEG/MEG systems are capable of measuring non-invasively somatosensory evoked responses at and above 1 kHz. Both modalities pose no risk to the subject, offer complementary views of the same neuronal activity and their combination might allow an improved source reconstruction of the neuronal generators underlying spike-related high-frequency activities. Together with traditional EEG/MEG a non-invasive tool for investigating neuronal processing appears possible.