Delta/theta band EEG differentially tracks low and high frequency speech-derived envelopes

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Speech Processing

Background:

• Speech representation in the brain is often studied by measuring neural activity alignment to the speech envelope.
• Broadband speech envelopes are commonly used for quantifying speech tracking, combining acoustic fluctuations across all spectral ranges.

Purpose of the Study:

• To investigate how spectrally-limited speech envelopes are encoded in the human brain using electroencephalography (EEG).
• To determine if using broadband speech envelopes distorts the understanding of speech encoding.
• To explore the differential tracking of low and high frequency speech envelopes.

Main Methods:

• EEG recordings were used to measure brain activity.
• Spectrally-limited speech-derived envelopes from low (0.2-0.83 kHz) and high (2.66-8 kHz) frequency bands were presented using noise carriers.
• Neural tracking was analyzed in the 1-6 Hz EEG bands.

Main Results:

• EEG tracking in the 1-6 Hz bands differentially encoded low and high frequency speech envelopes, irrespective of carrier frequency.
• This differential encoding was sensitive to attentional manipulations.
• Low and high frequency speech envelopes correspond to distinct phonemic features.

Conclusions:

• The broadband speech envelope can distort the analysis of neural speech encoding.
• Distinct spectral components of the speech envelope are processed by separate neural mechanisms within the same EEG frequency bands.
• Speech tracking involves context-dependent emphasis on different spectral ranges, suggesting functionally distinct processes are easily confounded by broadband analysis.