D. Jong, For absolute initial segments the hyperarticulation account predicts that all phonemically distinctive contrasts will be directly affected by stress, not just sonority contrasts 1995: 493) Under this assumption, prosodic strengthening thus relates to communication by enhancing the distinctiveness of speech sounds and by maximizing phonological contrasts. But, are all distinctive properties of a given segment enhanced in strong positions? If not, which features and contrasts are enhanced? Our study on the full oral French vowel system is particularly instructive for these issues In this dense system, the 10 vowels contrast in pairs according to height, backness, and rounding. Let's consider first the feature [±round]. The phonetic contrast between front rounded and unrounded vowels is affected by prosodic position in terms of F3 and F3?F2 values and dispersion, lip width and lip aperture e, ?/ have a larger constriction at the lips accompanied by a higher F3 (for at least 3 speakers) and a larger F3?F2 distance who observed an increase of F3 for all unrounded vowels at higher prosodic boundaries All together these variations in IP-initial position could be interpreted as an enhancement of the [?round] specifications of these vowels. However, for the rounded vowels, not all properties that make the vowels [+ round] are enhanced. Articulatorily, rounded vowels tend to have a larger rather than a smaller lip constriction in IP-initial positions and no change is observed in lip protrusion, the most visible lip feature (Benoît et al., 1994) and an important parameter for modifying the length of the labial cavity Acoustically however, the variations at play concur to enhance spectral properties that are distinctive for this category (a lowering of F3 of /y/ for S1, S3, S4 with a narrowing of F3?F2 for S1 and S4, and a lowering of F3 for This is shown especially for /y/ and/or for two of the four speakers (S1, S4) A similar lowering of F3 as well as a narrowing of F3?F2 was found for In the absence of a reduction of the labial constriction and a lengthening of the labial cavity, this lowering of F3 remains to be explained. It is possible that complementary manoeuvres at the tongue and/or larynx level concur to expand the cavities that can be responsible for a lowering of F3. In our last analysis we showed that in both acoustic and articulatory terms, the phonetic contrast between [+round] and [?round] vowels is maximized in IP-initial position for the closed and mid-closed vowel pairs. Our lip aperture and width results match one of the scenarios depicted by (chart (c) of their Fig. 1): the two contrastive categories are affected by strengthening in a similar way; both undergo an increase in the contrastive measure (here, lip aperture and width) but because the increase is much larger for one of the categories (here, unrounded vowels), the consequence is an enhancement of the articulatory contrast in the strengthened position. From an acoustico-perceptual point of view, the effect of boundary induced variations also results in a maximization of the distinction between the acoustic targets. As far as height and backness are concerned, we observed an effect of boundary on F1, F2 and F2?F1. First, as already stated, the vowels /e, ?, ø, a, ?, o, u/ are found to be more peripheral in the acoustic space defined by F1 and F2, and therefore are potentially more distinct within this larger acoustic space. This enlargement of the acoustic space in IP-initial position replicates the results found byRecall that in these studies however vowels are not always domain-initial but can be in the IP-initial CV syllable.) Within the four aperture degrees in the system, open and mid-open French vowels seem to pattern together in terms of boundary conditioned variations. A systematic increase of F1 is observed for which could be interpreted as an enhancement of their [+low] specification. However, an increase of F1 is also found for the [?low] vowels /ø/ for the 4 speakers, while for the other [?low] vowels no clear trend emerge (with an increase or decrease of F1 for These last results could again be explained by the limited articulatory (and acoustic) freedom in such a dense system for the targets getting closer to the palate. The maximization of acoustic contrast between front and back vowels is also clear in IP-initial position. The raising of F2 for, reference to Lindblom IP-initial position These acoustic results echo the effects found by FougeronS1, S4)S1, S4)S1, S3)S1) can be interpreted as an enhancement of the [?back] specification of these vowels, while the narrowing of the F2?F1 distance for all back vowels, and the diminution of F2 for Cho et al. since no effect on F4?F3 appears in our data. In conclusion, our results brings additional evidence in favour of an out-put driven motivation of initial strengthening. 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