title: Non-invasive single-trial EEG detection of evoked human neocortical population spikes.
abstract: OBJECTIVE
  Human high-frequency (>400 Hz) components of somatosensory evoked potentials (hf-SEPs), which can be recorded non-invasively at the scalp, are generated by cortical population spikes, as inferred from microelectrode recordings in non-human primates. It is a critical limitation to broader neurophysiological study of hf-SEPs in that hundreds of responses have to be averaged to detect hf-SEPs reliably. Here, we establish a framework for detecting human hf-SEPs non-invasively in single trials.
  
  
  METHODS
  Spatio-temporal features were extracted from band-pass filtered (400-900 Hz) hf-SEPs by bilinear Common Spatio-Temporal Patterns (bCSTP) and then classified by a weighted Extreme Learning Machine (w-ELM). The effect of varying signal-to-noise ratio (SNR), number of trials, and degree of w-ELM re-weighting was characterized using surrogate data. For practical demonstration of the algorithm, median nerve hf-SEPs were recorded inside a shielded room in four subjects, spanning the hf-SEP signal-to-noise ratio characteristic for a larger population, utilizing a custom-built 29-channel low-noise EEG amplifier.
  
  
  RESULTS
  Using surrogate data, the SNR proved to be pivotal to detect hf-SEPs in single trials efficiently, with the trade-off between sensitivity and specificity of the algorithm being obtained by the w-ELM re-weighting parameter. In practice, human hf-SEPs were detected non-invasively in single trials with a sensitivity of up to 99% and a specificity of up to 97% in two subjects, even without any recourse to knowledge of stimulus timing. Matching with the results of the surrogate data analysis, these rates dropped to 62-79% sensitivity and 18-31% specificity in two subjects with lower SNR.
  
  
  CONCLUSIONS
  Otherwise buried in background noise, human high-frequency EEG components can be extracted from low-noise recordings. Specifically, refined supervised filter optimization and classification enables the reliable detection of single-trial hf-SEPs, representing non-invasive correlates of cortical population spikes.
  
  
  CONCLUSIONS
  While low-frequency EEG reflects summed postsynaptic potentials, and thereby neuronal input, we suggest that high-frequency EEG (>400 Hz) can provide non-invasive access to the unaveraged output of neuronal computation, i.e., single-trial population spike activity evoked in the responsive neuronal ensemble.
authors.affiliation: Neurophysics Group, Department of Neurology, Campus Benjamin Franklin, Charite - University Medicine Berlin, Hindenburgdamm 30, 12203 Berlin, Germany; Bernstein Focus: Neurotechnology Berlin, Germany. Electronic address: gunnar.waterstraat@charite.de.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Waterstraat
authors.affiliation: Bernstein Focus: Neurotechnology Berlin, Germany; Physikalisch-Technische Bundesanstalt, Institute Berlin, Abbestr. 2-12, 10587 Berlin, Germany. Electronic address: martin.burghoff@ptb.de.
authors.forename: Martin
authors.initials: M
authors.lastname: Burghoff
authors.affiliation: Neurophysics Group, Department of Neurology, Campus Benjamin Franklin, Charite - University Medicine Berlin, Hindenburgdamm 30, 12203 Berlin, Germany; Bernstein Focus: Neurotechnology Berlin, Germany. Electronic address: tommaso.fedele@charite.de.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Fedele
authors.affiliation: Neurophysics Group, Department of Neurology, Campus Benjamin Franklin, Charite - University Medicine Berlin, Hindenburgdamm 30, 12203 Berlin, Germany; Bernstein Center for Computational Neuroscience Berlin, Germany. Electronic address: vadim.nikulin@charite.de.
authors.forename: Vadim
authors.initials: V
authors.lastname: Nikulin
authors.affiliation: Bernstein Focus: Neurotechnology Berlin, Germany; Physikalisch-Technische Bundesanstalt, Institute Berlin, Abbestr. 2-12, 10587 Berlin, Germany.
authors.forename: Hans Jürgen
authors.initials: HJ
authors.lastname: Scheer
authors.affiliation: Neurophysics Group, Department of Neurology, Campus Benjamin Franklin, Charite - University Medicine Berlin, Hindenburgdamm 30, 12203 Berlin, Germany; Bernstein Focus: Neurotechnology Berlin, Germany; Bernstein Center for Computational Neuroscience Berlin, Germany. Electronic address: gabriel.curio@charite.de.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Curio
mesh_terms: D000328:Adult; D000465:Algorithms; D001931:Brain Mapping / Q000379:methods*; D004569:Electroencephalography / Q000379:methods*; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D006801:Humans; D008297:Male; D019579:Neocortex / Q000502:physiology*
keywords: Evoked potentials; High-frequency oscillations; Single-trial detection; Somatosensory system