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    • Neuroimaging studies also provided evidence

    2018-10-25

    Neuroimaging studies also provided evidence for a dissociation between lexical/grammatical processing and sound-based ace inhibitor to time compressed speech, the two processes involving different neural pathways. In an fMRI study, Peelle et al. (2004) presented syntactically simple or complex sentences compressed to 80%, 65%, or 50% of their normal duration. Time-compressed sentences produced activation in the anterior cingulate, the striatum, the premotor cortex, and portions of temporal cortex, regardless of syntactic complexity. Others studies found that some brain regions, e.g. the Heschl’s gyrus (Nourski et al., 2009) and the neighboring sectors of the superior temporal gyrus (Vagharchakian et al., 2012), showed a pattern of activity that followed the temporal envelop of compressed speech, even when linguistic comprehension broke down, e.g. at 20% compression rate. Other brain areas, such as the anterior part of the superior temporal sulcus, by contrast, showed a constant response, not locked to the compression rate of the speech signal for levels of compression that were intelligible (40%, 60%, 80% and 100% compression), but ceased to respond for compression levels that were no longer understandable, i.e. 20% (Vagharchakian et al., 2012). These studies investigated adults and older children, i.e. participants who are proficient speakers of a language and have considerable experience with speech and language processing in general. Thus, the developmental origins and the existence of a critical period for this ability remain unexplored. It is unclear if adaptation to time-compressed speech can occur independently of any experience with speech (at least with broadcast speech, transmitted through the air, as experienced postnatally). Several hypotheses may be considered. First, this ability might rely on top-down linguistic knowledge of the lexicon and/or the grammar (morphology, syntax, semantics), which helps listeners discover linguistically relevant constants in the time-altered speech stream. In this case, newborns and young infants should fail to adapt to time-compressed speech. This hypothesis is relatively unlikely, since adults and older children can adapt to compressed speech in an unknown language, as long as that is rhythmically similar to their mother tongue. Second, one may assume that the adaptation ability depends on experience with spoken language in general (not specifically with the native language). Such a hypothesis predicts that adults and children can adapt to time-compressed speech, as has been observed, but that newborns, who only have experience with speech as heard in utero, which is very different from regular speech transmitted through the air, would fail. Third, it may be the case that little or no experience is needed for the adaptation ability to occur, so the degraded, low-pass filtered speech signal experienced prenatally, which only preserves the prosodic properties of the native language (Gerhardt et al., 1992; Querleu et al., 1988), may be sufficient. In this case, newborns may adapt to time-compressed speech successfully. Here, we show that this is indeed the case, suggesting that adaptation occurs at the auditory/phonological and not at the lexical/grammatical levels. This finding brings the first developmental evidence to the hypothesis that adaptation to time-compressed speech is an auditory/phonological phenomenon. Specifically, using near-infrared spectroscopy (NIRS) we tested the ability of the newborn brain to discriminate and to adapt to speech at three levels of compression: (i) normal, uncompressed speech, i.e. 100% of its original duration, (ii) speech compressed at a level comprehensible for adults, i.e. 60% of its initial duration, and (iii) speech compressed at a level that is no longer comprehensible for adults, i.e. 30% of its original duration. NIRS is a powerful and easy-to-use neuroimaging method, well suited to test young infants (Gervain et al., 2011; Rossi et al., 2012). It uses the absorbance properties of oxygenated and deoxygenated hemoglobin to assess the metabolic correlates of brain activity, i.e. the hemodynamic response function (HRF).