Electromagnetic recordings reveal latency differences in speech and tone processing in humans

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Speech Processing

Background:

• Understanding how the human brain processes auditory information, particularly distinguishing between speech and non-speech sounds, is crucial.
• Previous research has explored brain responses to various acoustic stimuli, but direct comparisons between tones and speech with matched spectral properties are limited.

Purpose of the Study:

• To investigate and compare human auditory electromagnetic brain responses (using MEG and EEG) to spectrally matched sinusoidal tones and speech stimuli.
• To determine if differences in amplitude or latency of specific brain responses (N1-P2, N1m-P2m) can differentiate speech from tone processing.
• To examine the effect of glottal excitation variations in speech on these brain responses.

Main Methods:

• Whole-head magnetoencephalography (MEG) and scalp electroencephalography (EEG) were employed to record brain activity.
• Sinusoidal tones and Finnish vowel /a/ stimuli with controlled glottal excitations were presented.
• Stimulus frequency and intensity were adjusted to match the spectra of speech sounds.
• Analysis focused on the electric N1-P2 and magnetic N1m-P2m response complexes, including amplitude and latency.

Main Results:

• Both tone and speech stimuli elicited prominent N1-P2 (electric) and N1m-P2m (magnetic) responses.
• Speech sounds showed larger N1 and P2 amplitudes compared to tones, though this difference was less pronounced in MEG.
• Crucially, both N1(m) and P2(m) latencies were consistently shorter for tones than for vowels.
• The source of the N1m response was located in the auditory cortex for both stimulus types, with no significant location differences based on stimulus type.
• Variations in glottal excitation of vowels did not affect response amplitudes or latencies.
• The anterior-posterior positioning of N1m and P2m sources showed hemispheric differences.

Conclusions:

• The latency of auditory evoked potentials (N1(m) and P2(m)) is a reliable neural marker for differentiating speech from tone processing in the human brain.
• While amplitude differences exist, they are less consistent across measurement modalities (EEG vs. MEG) for distinguishing these sound types.
• The auditory cortex processes both tones and speech, with specific temporal characteristics of neural responses playing a key role in speech perception.