Multichannel single trial MEG of cortical population spikes – SPIKE MEG

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Evolution has shaped the human brain as a “single-trial” processor reacting fast and reliably to environmental events, and the cerebral processing of information is one of the body’s most complex processes. The demand to observe the brain’s operation ‘at the speed of thought’, but without the need to place electrodes inside the brain, inspired two temporally high-resolution techniques: electroencephalography (EEG) and magnetoencephalography (MEG) which rely on measuring the electromagnetic field originating from ionic neuronal currents. These can be categorized as slow and fast currents having different microscopic origins: Slow currents—known as postsynaptic potentials—occur when impulse signals, fired by one nerve cell, are received by another. The firing of such impulses (which transmit information to downstream neurons or muscles) produces fast currents which last for just a millisecond – these 'spikes' are known as action potentials. However, while results for slow currents from EEG and MEG are reliable, those for fast currents in general are not. The Physikalisch-Technische Bundesanstalt (PTB) is a spearhead in the development of MEG technology, and its recently constructed ultralow-noise magnetometer enabled the first non-invasive single-trial characterization of evoked human cortical population spikes, revealing amplitudes highly variable between trials and intertrial correlated response latencies. These unique recordings opened up the prospect that low-noise MEG might enable integral non-invasive measurements of neuronal currents at both low and high frequencies. However, these pilot measurements were performed with a single-channel MEG device, precluding analysis of individual neuronal sources in the orchestrated network activity of the cortical network. The proposed project will build on this momentum and design and construct a novel ultralow-noise multichannel MEG system that will enable spatially resolved non-invasive measurements of both slow synaptic currents and fast spiking activity at the same time. We will also make use of the complementary nature of EEG and MEG which have different spatial sensitivities to neuronal source characteristics by integrating our previous experience on low-noise EEG. Neurologists from Charité-Universitätsmedizin Berlin team up with physicists and engineers from PTB to answer questions that conventionally call for invasive microelectrode recordings. We will examine the hypothesis that neuronal oscillations (slow currents) in the somatosensory cortex affects local spiking behavior (fast currents) and that spiking in one part of the somatosensory cortex inhibits concurrent spiking activity in neighboring regions. The iterative interaction and tight integration of technologic and physiologic expertise in this project is key to the development of a novel ultralow-noise MEG/EEG device that will be transformative for human non-invasive neurophysiology.