br Materials and methods br Results br Discussion We hypothe

Materials and methods
Results
Discussion We hypothesize that the relative weakness of the canonical response to the normal and the 60% compression rates is due to habituation to the stimuli across time. This habituation can be quite fast, and may happen already within the first block. While habituation across blocks is a commonly used measure in NIRS (e.g. Benavides-Varela et al., 2012; Bouchon et al., 2015), habituation within a stimulation block is hard to detect with NIRS. Existing fMRI data on adults show that terbinafine hydrochloride to time-compressed speech occurs within the first 16 sentences (Adank and Devlin, 2010), and this number is already reached by the first alternating/non-alternating block pair. Newborns therefore might have been habituated to 60% compressed speech during the first pair of blocks, leading to reduced amplitudes between the hemodynamic responses evoked by normal and 60% compressed speech blocks. This might explain why these responses are not significantly different from baseline. Such repetition suppression effects are often taken to be neural signatures of the existence of a stable representation of the signal (see Nordt et al., 2016, for a review). Given newborn infants’ prenatal experience with their native language as well as their broad-based, universal speech perception abilities, it is not implausible to assume that they may build a stable representation of normal and moderately time-compressed speech fast and efficiently. Our experimental design was not set up to specifically test the time course of adaptation itself. A more detailed description of this process is nevertheless theoretically relevant, and will require further research. Second, in our design, different block types contained different numbers of utterances. This choice was made to equate for the absolute amount of stimulation across block types, as this is known to influence the hemodynamic response, especially in young infants (Minagawa-Kawai et al., 2013). However, one might argue that the differences we observed across conditions might be due to this difference in the number of utterances, and not to compression rate itself. In particular, it may be the case that the inverted response observed in the fastest speech rate may be a deactivation or neural habituation effect due to the higher number of repetitions in that condition. Indeed, considerable redundancy in the stimuli has been shown to give rise to habituation effects in the NIRS response of newborns (Bouchon et al., 2015). While we did not explicitly test this alternative, there is indirect evidence in our data that this explanation is unlikely. While the non-alternating 30% compression blocks did indeed contain the greatest number of utterances (Fig. 2), alternating blocks using this compression rate also had a much higher number of utterances than non-alternating normal blocks or alternating blocks with 60% compression rate. Yet, they did not evoke an inverted response. Furthermore, the number of utterances in the different block types ranged from 6 through 7, 8 and 9 to 11. This should have resulted in graded NIRS responses for the different block types if the number of utterances played a role, which was not the case. More generally, and returning to our research question about the origin of the mechanisms underlying adaptation to compressed speech, we had evoked several potential hypotheses: adaptation might happen at the level of (i) knowledge about the lexicon and/or grammar of the native language, (ii) processing of broadcast speech or (iii) general auditory processing requiring no experience with broadcast speech. Indeed, in the adult literature, the drop of performance at high compression ratios has been explained by several models: (i) the saturation of the lexical buffer, which gets filled up at speech rates faster than the inherent speed of linguistic read-off and processing (Vagharchakian et al., 2012), (ii) an impossibility to map phonological representations to articulatory motor plans (Adank and Devlin, 2010; Peelle et al., 2004), or (iii) a failure to find the subphonemic and rhythmic landmarks in the compressed speech signal (Pallier et al., 1998; Sebastián-Gallés et al., 2000).