Abstract
Natural sensory signals have nonlinear structures dynamically composed of the carrier frequencies and the variation of the amplitude (i.e., envelope). How the human brain processes the envelope information is still poorly understood, largely due to the conventional analysis failing to quantify it directly. Here, we used a recently developed method, Holo-Hilbert spectral analysis, and steady-state visually evoked potential collected using electroencephalography (EEG) recordings to investigate how the human visual system processes the envelope of amplitude-modulated signals, in this case with a 14 Hz carrier and a 2 Hz envelope. The EEG results demonstrated that in addition to the fundamental stimulus frequencies, 4 Hz amplitude modulation residing in 14 Hz carrier and a broad range of carrier frequencies covering from 8 to 32 Hz modulated by 2 Hz amplitude modulation are also found in the two-dimensional frequency spectrum, which have not yet been recognized before. The envelope of the stimulus is also found to dominantly modulate the response to the incoming signal. The findings thus reveal that the electrophysiological response to amplitude-modulated stimuli is more complex than could be revealed by, for example, Fourier analysis. This highlights the dynamics of neural processes in the visual system.
Highlights
Natural sensory signals have nonlinear structures dynamically composed of the carrier frequencies and the variation of the amplitude
Holo-Hilbert spectral analysis is mainly based on the Empirical Mode Decomposition (EMD) to resolve the identification of intrinsic amplitude modulations by representing the data in multiple dimensions
The masking EMD, which was applied in Holo-Hilbert Spectral Analysis (HHSA) in this study, should successfully decompose the multi-component signal into single modes, as known as intrinsic mode functions (IMFs), which naturally retain the physical meaning of the signal without suffering mode-mixing
Summary
Natural sensory signals have nonlinear structures dynamically composed of the carrier frequencies and the variation of the amplitude (i.e., envelope). The brain processes natural sensory stimuli in a dynamic, non-stationary and nonlinear way[1,2,3] Such naturalistic stimuli are often a combination of several sinusoidal oscillations with various frequencies generating a signal with an embedded envelope that modulates the amplitude of the carrier frequency. HHSA, which is a nonlinear analysis tool based on the Empirical Mode Decomposition (EMD), provides a fully informational and high-dimensional frequency representation of data resulting from non-stationary and nonlinear processes That is, both the carrier frequencies (fc) and the amplitude modulation frequencies (fam) in the signal can be examined simultaneously in the Holo-Hilbert spectrum (HHS)[14] (see the illustration of HHSA in Supplementary Fig. S1). Experiment 2 aimed to further understand the role of the envelope in interocular interactions, with how AM signals presented to one eye affects a uniform sine-wave signal presented to the other eye
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