Non-invasive detection of cortical population spikes: Functional discrimination of pre- vs. postsynaptic components in SEP at 1 kHz

Introduction: Ultrafast EEG signals, featuring frequencies above 500 Hz, can be observed in scalp somatosensory evoked potentials (SEP). Usually, these recordings have a low Signal-to-Noise Ratio (SNR) because weak signals are overlaid by intrinsic noise of much higher amplitude as generated by biological sources and the amplifier. Scalp recordings of SEP show bursts above 400 Hz with submicro-volt amplitudes, which could be extracted from noise up to now only by the use of massive averaging (n > 1000) and digital signal processing. Methods: Here, we investigate SEP up to 1 kHz elicited by median nerve stimulation with rates of either 0.99 Hz or 8.1 Hz. We exploit the increased SNR provided by a custom-made low noise amplifier technology together with an adapted implementation of Canonical Correlation Analyis algorithm, based on the assumption of high level stimulus phase locking across trials. Results: It proved possible in 3/3 subjects to discriminate hf-SEP generators at subcortical and intracortical levels at the single trial level, relying on the different neuronal refractory properties of these components which are phase-locked to the stimulus onset. Discussion: A custom-made low-noise EEG system enables the noninvasive detection of pre- and postsynaptic evoked activity at 1 kHz, i.e., in the frequency range housing spike-related multi-unit activity in intracortical recordings, providing the opportunity of non-invasive monitoring of multi-unit spike activity in man.