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N -Interval Fourier Analysis (N -FTA) allows for simultaneous spectral assessment of evoked and spontaneous activity in the frequency domain. We applied this method to signals following peripheral electrical nerve stimulation and performed analysis of cortical somatosensory evoked potentials within the 400 to 750 Hz band. For median nerve stimulation, data from eleven volunteers were analyzed. For tibial nerve stimulation, three subjects were investigated. For both stimulation sites, evoked high frequency oscillations (HFOs) components were identified. Furthermore, two kinds of background HFO activity were detected in sham stimulation trials. Spectral component models were applied for quantifying signal properties. Evoked spectral components reflected HFOs being time-locked to the stimulus. The detected spectral components were distributed over the entire investigated spectral band. Their spectral amplitude was close to the limit of the resolution of N -FTA. The experimentally observed spectral amplitude were in quantitative agreement with a model using a Morlet morphology. Within the HFO band, a flat noise floor was observed. Spontaneous physiological background activity contributes significantly to the spectral amplitude. This random activity is the dominant source of interference when extracting evoked HFOs. Within the HFO band, narrow spectral peaks in background activity were detected – both for real and sham stimulation. In the data sampled at 9.6 kHz, such peaks were observed in all recordings. For the 5.0 kHz sampling rate, these peaks were visible in about half of the recordings, and their amplitude was reduced. Based on a mathematical model, these peaks may be generated by organized spontaneous HFO activity producing a stable background wave. • N-FTA allows for spectral analysis of evoked and spontaneous cortical HFO activity. • Evoked HFOs are of low amplitude and contained in a broad spectral band. • Spontaneous near 600 Hz HFOs are spectral peaks of narrow bandwidth. • Evoked and spontaneous HFO cycle length may concur with neuron refractoriness. [ABSTRACT FROM AUTHOR]

Spectral analysis of repeatedly evoked potentials (EPs) is challenging since recordings contain a superposition of evoked signals and spontaneous activity. We developed a novel approach, N -interval Fourier Transform Analysis (N -FTA), which allows for reliable separation and simultaneous assessment of triggered and background spectral components. Median nerve stimulation data from a total of eleven volunteers recorded in two labs with different experimental settings were investigated. Consistently, short latency spectral components were mainly contained in the gamma and high gamma bands. In contrast, spontaneous activity displayed a 1/f spectral profile with distinct alpha and beta peaks. Spontaneous power spectral densities (PSDs) obtained for real and sham stimulation were highly comparable. The low frequency background PSD was more than two orders of magnitude above the spectral short latency peaks. Within the 30 Hz to 90 Hz band, the evoked peaks were -17 dB to -4 dB below the background suggesting that target band filtered short latency deflection might be extracted using less than 100 trials. SEPs following tibial nerve stimulation (3 subjects) displayed a narrower spectral band at about half the bandwidth as compared to median nerve stimulation. Evoked peaks were between 30 Hz and 37 Hz at PSD levels being -10 dB to -4 dB below the background activity. These spectral peaks were related to the short latency response of typical W-morphology. Cortical short latency responses are contained in distinct spectral target bands which are much narrower than the standard bandwidth recommendations for routine recordings. In particular, the high pass corner frequency may be selected about one order of magnitude above the current standard. This might render SEP recordings more robust since it eases the suppression of spontaneous activity and movement artifacts such as eye-blinks. Real-time zero-phase filters are required for translating these findings into improved recording systems. [Display omitted] • N -FTA allows for the simultaneous computation of evoked and spontaneous PSD. • Median nerve short latency responses generated broad evoked spectra in the γ- and high-γ-band. • Highest evoked peaks were approximately 5 dB below the background activity. • Tibial nerve short latency responses gernerated narrow peaks in the γ-band. • Late responses generated evoked PSD of relatively low frequency. [ABSTRACT FROM AUTHOR]