Abstract
The temporal modulation structure of adult-directed speech (ADS) is thought to be encoded by neuronal oscillations in the auditory cortex that fluctuate at different temporal rates. Oscillatory activity is thought to phase-align to amplitude modulations in speech at corresponding rates, thereby supporting parsing of the signal into linguistically relevant units. The temporal modulation structure of infant-directed speech (IDS) is unexplored. Here we compare the amplitude modulation (AM) structure of IDS recorded from mothers speaking, over three occasions, to their 7-, 9-, and 11-month-old infants, and the same mothers speaking ADS. Analysis of the modulation spectrum in each case revealed that modulation energy in the theta band was significantly greater in ADS than in IDS, whereas in the delta band, modulation energy was significantly greater for IDS than ADS. Furthermore, phase alignment between delta- and theta-band AMs was stronger in IDS compared to ADS. This remained the case when IDS and ADS were rate-normalized to control for differences in speech rate. These data indicate stronger rhythmic synchronization and acoustic temporal regularity in IDS compared to ADS, structural acoustic differences that may be important for early language learning.
Highlights
Human speech perception relies in part on neural tracking of the temporal modulation patterns in speech at different timescales simultaneously: “multi-time resolution processing” (Chait, Greenberg, Arai, Simon, & Poeppel, 2015; Ghitza & Greenberg, 2009; Greenberg, 2006; Luo & Poeppel, 2007; Poeppel, 2003)
Rise times in the theta band appear to be important for successful speech encoding, as acoustic “landmarks” in critical band envelopes in the theta range of adult-directed speech (ADS) provide perceptual markers that are critical for speech intelligibility (Doelling, Arnal, Ghitza, & Poeppel, 2014)
There was a significant interaction between Rate and Condition, F(6, 138) = 7.78, p < .0001, showing that the relative amount of modulation energy in the delta, theta and beta/low-gamma bands differed between infant-directed speech (IDS) and ADS
Summary
Human speech perception relies in part on neural tracking of the temporal modulation patterns in speech at different timescales simultaneously: “multi-time resolution processing” (Chait, Greenberg, Arai, Simon, & Poeppel, 2015; Ghitza & Greenberg, 2009; Greenberg, 2006; Luo & Poeppel, 2007; Poeppel, 2003). The S-AMPH modeling demonstrated that English nursery rhymes contained amplitude modulations in three critical temporal rate bands (corresponding to delta, theta, and beta/low gamma, for this speech corpus 0.9–2.5 Hz, 2.5–12 Hz, and 12–40 Hz; see (Leong, 2012; Leong & Goswami, 2015). Amplitude modulations in these bands were hierarchically nested in the nursery rhymes, and phase alignment between the slower amplitude modulation (AM) bands (delta and theta) played a key role in the perception of rhythmic patterning (judging whether the nursery rhymes were trochaic or iambic). We were interested to see whether natural IDS has a different temporal modulation structure to ADS, and whether the strength of phase alignment between the different AM bands (delta, theta, beta/low gamma) might show a different patterning in IDS as compared to ADS
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