Neural processing of hyper-and hypo-articulated vowels in Infant-Directed Speech

When addressing infants, adults use a speech register known as infant-directed speech (IDS). Compared to adult-directed speech (ADS), IDS has a number of distinctive acoustic and linguistic features. Vowel hyperarticulation, the expansion of the acoustic space between the corner vowels /i,u,a/, is one feature specifically proposed to facilitate language acquisition processes. Interestingly, the presence of vowel hyperarticulation in IDS appears to be dependent on the infant’s communicative and linguistic needs. Mothers do not hyperarticulate vowels in IDS to infants with hearing loss (Lam & Kitamura, 2010) or infants at-risk for dyslexia (Kalashnikova et al., 2018), indicating that infants’ ability to hear and process speech can influence speakers’ IDS to them. Given the important role of vowel hyperarticulation in early language acquisition, it is of interest to investigate the effects of IDS with hypo-articulated vowels on infants’ early linguistic processing.This study investigated whether there is a neurophysiological difference in the processing of hyper- and hypo-articulated vowels in IDS by comparing the electroencephalographic (EEG) signatures of typical IDS with hyperarticulated vowels (hyper-IDS), IDS that lacks vowel hyperarticulation (hypo-IDS), and ADS in 9 month-old-infants (N = 12). Event-related potentials (ERPs) were recorded while infants listened to familiar words in hyper-IDS, hypo-IDS, and ADS registers. If hyper-IDS facilitates infants’ early lexical processing, we expected it to elicit a different pattern in brain potentials compared to hypo-IDS and ADS.Regarding electrophysiological measures, mean amplitudes were calculated in the 250-500ms time window measured from word onset, since ERP amplitudes in this window have been proposed to reflect increased semantic processing (Kidd et al., 2018; Zangl & Mills, 2007). A Speech (hyper-IDS, hypo-IDS, ADS) x Antpost (frontal, central, parietal, occipital) x Laterality (left, right) ANOVA yielded a main effect of Speech (F(2, 22) = 7.054, p = .004, p2 = .391) and Speech x Antpost x Laterality interaction (F(6, 66) = 2.771, p = .018, p2 = .201), meaning that the factor Speech interacted with topographical factors. Hypo-IDS was found to elicit a broadly distributed negativity compared to hyper-IDS and ADS respectively, which gave rise to more positive amplitudes in the same time window. Taken together, these results suggest a decrease in lexical processing for hypo-IDS.These findings indicate different brain responses to IDS with and without vowel hyperarticulation in early language processing, supporting the assumption that vowel hyperarticulation in IDS influences infants’ early linguistic processing. Given that early brain potentials occurring at 200-500ms have previously been suggested to reflect increased lexical or semantic processing, these findings indicate that infants are sensitive to the specific acoustic qualities of IDS, which facilitate semantic processing (Junge et al., 2014). Importantly, the current findings demonstrate that IDS with hyperarticulated vowels provides infants with a rich linguistic signal. When hyperarticulation is absent, lexical processing appears to be impeded. This is an especially important insight with regard to infants with hearing loss who do not have access to this feature in IDS, but who may instead rely more on other perceptual advantages offered by this register