The origins and development of speech envelope tracking during the first months of life

Abstract

When humans listen to speech, their neural activity tracks the slow amplitude fluctuations of the speech signal over time, known as the speech envelope. Studies suggest that the quality of this tracking is related to the quality of speech comprehension. However, a critical unanswered question is how envelope tracking arises and what role it plays in language development. Relatedly, its causal role in comprehension remains unclear, as some studies have found it to be present even for unintelligible speech. Using electroencephalography, we investigated whether the neural activity of newborns and 6-month-olds is able to track the speech envelope of familiar and unfamiliar languages in order to explore the developmental origins and functional role of envelope tracking. Our results show that amplitude and phase tracking take place at birth for familiar and unfamiliar languages alike, i.e. independently of prenatal experience. However, by 6 months language familiarity modulates the ability to track the amplitude of the speech envelope, while phase tracking continues to be universal. Our findings support the hypothesis that amplitude and phase tracking could represent two different neural mechanisms of oscillatory synchronisation and may thus play different roles in speech perception.

Keywords: EEG; Infants; Newborns; Speech envelope tracking; Speech perception.

Fig. 1

EEG experimental setup and design. (a) Newborn with EEG cap. (b) Location of recorded channels according to the international 10-20 system. (c) Experiment block design. ISI: Interstimulus interval, IBI: Interblock interval.

Fig. 2

Amplitude envelope tracking of a French sentence in a newborn participant. The black curves represent the speech envelope of a French sentence and the blue curves the cortical activity measured per channel. For visualization purposes the EEG data was shifted backward in time using the time lag between the neural response and the speech signal. The Spearman correlation between the two signals is indicated by “ρ”. The vertical line at 250 ms indicates the beginning of the envelope-following period. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 3

Amplitude tracking results. (a, b) Topographic distribution of channels with significant (different from zero and from chance permutations) amplitude tracking results for each language (purple for French, green for Spanish, orange for English) for the (a) group of 47 newborns, and (b) group of 25 six-month-olds. (c, d) Amplitude tracking results for the three languages at channel Fz, as an illustration. The yellow boxes represent the distributions of the results from the measured EEG data, and the blue boxes represent the distributions of the results from the permuted data, for the (c) newborn group, and (d) 6-month-old group. n.s.: non-significant, * p < 0.05, ** p < 0.01, *** p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 4

Phase tracking results. (a, b) Topographic distribution of channels with significant phase tracking results for each language for the (a) newborn group, and (b) 6-month-old group. (c, d) Phase tracking results for the three languages at channel Fz, as an illustration, for the (c) newborn group, and (d) 6-month-old group. Plotting conventions as before.