(FT: cut-off frequency 5 kHz, peak frequency: 6 kHz, frequency shift 3 kHz LFC: compression ratio 3:2 NFC: cut-off frequency 2 kHz, compression ratio 1.7).įrequency-lowering technology currently exists in the hearing aids of many hearing-aid manufacturers ( Galster, Valentine, Dundas, & Fitz, 2011 Kuk et al., 2006 Kuriger & Lesimple, 2012 McDermott, Baldwin, & Nyfeller, 2010 McDermott & Knight, 2001 Serman, Hannemann, & Kornagel, 2013 Stender & Groth, 2014). Figure 1 shows examples of the input–output functions for FT, LFC, and NFC strategies.Įxamples of the input–output mapping functions for the different frequency lowering schemes frequency transposition (FT), linear frequency compression (LFC), and nonlinear frequency compression (NFC). Typically, the low-frequency components remain unmodified in NFC. In nonlinear frequency compression (NFC), the mapping between the input and output frequencies can be described by f out = f c 1 - 1 / constant * f in 1 / constant (Equation 2) for frequencies above a cut-off f c. Linear frequency compression (LFC) maps all input and output frequencies according to the equation f out = constant* f in (Equation 1).
The mapping curve can be linear or nonlinear. In frequency compression, the high-frequency portion of the spectrum is compressed to fit into a narrower target bandwidth, and there is no spectral superimposition of the original and the compressed signal. In frequency transposition, the high-frequency components of the spectrum are shifted lower and mixed with the original signal. Frequency lowering can be achieved by frequency transposition (FT) and frequency compression (FC) ( Kuk, Keenan, Korhonen, & Lau, 2009). Higher frequencies that are inaudible for the hearing-impaired listener are mapped toward lower frequencies. The proposed tools allowed us to better characterize the CFC patterns emerging during the seizure dynamics, which could pave the way to unveil the underlying neural mechanisms that initiate and propagate the ictal activity.Frequency lowering is one strategy to increase the audibility of acoustic signals for the hearing-impaired listener. In the second one, the PAC was produced by sharp waveforms constituted by non-harmonic high frequency components. In the first one, the PAC was elicited by highly cyclostationary (pseudo-periodic) LFP signals, which were characterized by well-defined harmonic spectral components present in their Fourier spectrum. Importantly, two essentially different PAC patterns produced by non-sinusoidal waveforms were identified. Moreover, the correlation of LFP and CFC for a given recording site across seizures was evaluated in order to quantify the seizure stereotypy.We have found that the ictal activity gives rise to different types of CFC, which were highly stereotyped during the seizure dynamics. Specifically, the Time Locked Index (TLI) and Harmonic Index (HI) were implemented to quantify the presence of harmonics associated to the emergence of CFC. In addition, we have developed specialized tools to characterize the nature of the observed CFC patterns. To quantify the CFC dynamics during the seizure activity we use non-parametric methods: the Phase Locking Value (PLV) and the Modulation Index based on the Kullback-Leibler distance (KLMI). The analysis was performed on LFP obtained from 5 patients undergoing intracerebral electroencephalography (stereo EEG) and 2 patients undergoing subdural electrocorticography (ECoG).
Harmonic frequency free#
Recent works have shown that nested oscillations, associated to the scale free neural activity, and sharp waveforms both produce PAC, however, they reflect two distinct neural mechanisms that are anatomically segregated in the human brain.In this work, we study the CFC dynamics during the seizure activity in patients with focal epilepsy who were candidates for surgery treatment. In PAC, the amplitude of a high frequency band is modulated by the phase of another band with a lower frequency content. PPC represents the phase coherence across frequency bands and/or recording sites, which in general increases between nearby areas of the neural tissue recruited to the ictal event. In particular, PAC and PPC have been observed in local field potentials (LFP) recorded during epileptic seizures. Besides, experimental findings have shown that phase-amplitude (PAC) and phase-phase (PPC) couplings are important variants of CFC linked to physiological and pathological brain states. Cross frequency coupling (CFC) phenomenon has been proposed to be functionally involved in neuronal communication, memory formation and learning.
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