Neuronal spiking is commonly recorded by invasive sharp microelectrodes, whereas standard noninvasive macroapproaches (e.g., electroencephalography [EEG] and magnetoencephalography [MEG]) predominantly represent mass postsynaptic potentials. A notable exception are low-amplitude high-frequency (∼600 Hz) somatosensory EEG/MEG responses that can represent population spikes when averaged over hundreds of trials to raise the signal-to-noise ratio. Here, a recent leap in MEG technology-featuring a factor 10 reduction in white noise level compared with standard systems-is leveraged to establish an effective single-trial portrayal of evoked cortical population spike bursts in healthy human subjects. This time-resolved approach proved instrumental in revealing a significant trial-to-trial variability of burst amplitudes as well as time-correlated (∼10 s) fluctuations of burst response latencies. Thus, ultralow-noise MEG enables noninvasive single-trial analyses of human cortical population spikes concurrent with low-frequency mass postsynaptic activity and thereby could comprehensively characterize cortical processing, potentially also in diseases not amenable to invasive microelectrode recordings.
Relation between hfSERs and low-frequency evoked responses; exemplary data of subject S1. Trials were sorted according to the rms amplitude of single-trial hfSERs. This procedure clearly differentiated between weak (5th percentile) and strong (95th percentile) hfSERs while the noise level remained approximately constant. Notably, concurrently evoked low-frequency components of single-trial responses were approximately equal for weak and strong hfSERs (Top). This was corroborated by forming percentile-specific subaverages (Bottom) of low-frequency responses, which were equal despite a clear gradual recruitment of hfSER subaverages underlining the effectiveness of the single-trial sorting.