The tracking of temporal information in speech is frequently used to study speech encoding in dynamic brain activity. Often, studies use traditional, generic frequency bands in their analysis (for example delta [1 – 4 Hz] or theta [4 – 8 Hz] bands). However, there are large inter-individual differences in speech rate. For example, audiobooks are typically narrated with 150 words per minute (2.5 Hz), while the world’s fastest speaker can talk at 637 words per minute (10.6 Hz). We therefore reasoned that speech tracking analyses should take into account the specific regularities (e.g. speech rate) of the stimuli. This is exactly what we did in this study: We extracted the time-scales for phrases, words, syllables and phonemes in our sentences and based our analyses on these stimulus-specific bands.
Previous studies also mainly used continuous speech to analyse speech tracking. This is a fantastic, “real-world” paradigm, but it lacks the possibility to directly analyse comprehension. We therefore played single sentences to our participants and asked, after each sentence, to indicate which out of four words they had heard in the sentence. This way, we obtained a single-trial comprehension measure. We also recorded participants’ magnetoencephalography (MEG) and did our analyses on source projections of brain activity.
We show two different speech tracking effects that help participants to comprehend speech and both act concurrently at time-scales within the traditional delta band: First, the left middle temporal cortex (MTG) tracks speech at the word time-scale, which is probably useful for word segmentation and mapping the sound-to-meaning. And second, the left premotor cortex (PM) tracks speech at the phrasal time-scale, likely indicating the use of temporal predictions during speech perception.
Previous studies have shown that the motor system is involved in predicting the timing of upcoming stimuli by using its beta rhythm. We therefore hypothesised that a cross-frequency coupling between beta-power and delta-phase at the phrasal time-scale could drive the effect in the motor system. This is indeed what we found and this was also directly relevant for comprehension.
By using stimulus-specific frequency bands and single-trial comprehension, we show specific functional and perceptually relevant speech tracking processes along the auditory-motor pathway. In particular, we provide new insights regarding the function and relevance of the motor system for speech perception.
If you would like to read the full manuscript, you can find a preprint on bioRxiv here.