domain: spi phys chim

text:
Z Huvelin
email: zhao.huvelin@onera.fr

C Gouroglian

N Horézan

S Naka

Role of refractory elements in near-alpha titanium alloys on high temperature mechanical properties

The effect of Tungsten (W), Tantalum (Ta) and simultaneous addition of Germanium (Ge) and Silicon (Si) on the microstructure evolution, tensile and creep properties of the near-alpha alloy Ti-5.7Al-3.9Sn-3.7Zr-0.7Nb-0.5Mo-0.35Si-0.05C have been investigated at high temperatures up to 650°C. Microstructural characterizations following solution treatment at 1050°C for 2 hours with oil quenching and aging treatment at 700°C for 2 hours followed by air cooling, highlighted that the additions of refractory elements such as W and Ta led to a decrease of both the volume fraction of the primary alpha phase (a p ) and its average size. Tensile tests performed up to 650°C revealed a significant improvement in tensile strength with additions of W and Ta, even though a decrease of ductility has been also detected. Creep tests carried out at 600°C under a constant stress of 200 MPa pointed out that, refractory elements, Ge and Si have a beneficial effect on both primary and steady-state creep strain rates.

Introduction

Near-alpha titanium alloys are promising candidates for aerospace applications owing to their high specific strength, good corrosion resistance and high temperature resistance [1]. The latest high temperature titanium alloys, such as IMI834 [2], Ti1100 [3], BT36 [4] or Ti600 [5], have been developed for high temperature application up to 600°C as compressor disk and blade for gas turbines of advanced jet engines, but in-service these alloys cannot be used above 550°C. Over the past decades numerous research works have been carried out to find suitable strengthening method to improve high temperature properties up to or even higher than 600°C. Alloying is one of the fundamental methods to improve the high temperature mechanical properties of titanium alloys. Conventional near-alpha Ti alloys are strengthened by fine alpha2 phase with DO 19 structure and intermetallic silicides [7][8]. Up to now, numerous works were focused on the influence of alpha stabilizing alloying elements such as Ga, Sn, Hf and Zr on the high temperature mechanical properties of the near-alphaα titanium alloys [9][10]12]. These studies demonstrated that the solid solution strengthening and the formation of α 2 phase are effective in improving high temperature strength, but degrade the ductility of the material [10][11] probably due to the lower symmetry of its DO 19 crystal structure. The effect of silicon addition was also extensively studied to increase high temperature tensile and creep strength of near-α and α+β titanium alloys [9,[13][START_REF]Titanium '92, Scien,ce and Technology[END_REF]. This is achieved by solid solution strengthening and precipitation strengthening. However, it has been reported that silicide precipitation also decreases the ductility [13,[START_REF] Ramachandra | [END_REF][16] because fracture occurs due to the linkage of the cracks that are initiated in the regions where the slip bands intersect with silicide particles [17]. Recently, Kitashhima et al. [6] evaluated the effect of Ge addition on the tensile properties, impact toughness and fracture behavior of a near-alpha titanium alloy because it has several similar aspects to Si. For example, Ge and Si are completely soluble, Ge can form Ti 5 Ge 3 germanide having the same crystal structure as Ti 5 Si 3 silicide. Moreover the solubility of Ge in the alpha phase is higher than that of Si [23]. Results of their studies suggest a significant increase in YS and UTS at room temperature with Ge additions. Moreover, the ductility of the alloys increases with Ge content, especially at high temperatures. However, hardly any literature deals with the effects of refractory elements such as W or Ta on high temperatures properties of near-alpha titanium alloys although these elements are well known to be effective for high temperature tensile and creep strength in titanium aluminide system [18] due to their low rate of diffusion and high melting point. In the present study, the effect of refractory elements additions in a Ti-834 type alloy on microstructure evolution, high temperature tensile strength and creep properties was investigated.

Materials and experiments

Ti-834M alloy with modified composition was made from high purity Ti-sponge and alloying elements, melted by vacuum arc-furnace. Subsequently, the 30 cm 3 ingot was then hot rolled to 13*13 mm 2 square cross-section bar at a temperature in the alpha+beta domain. A second square cross-section bar Ti-834R, was re-melted from an ingot with a standard chemical composition of Ti-834 alloy and then hot rolled in the same conditions. Finally, a forged slice in Ti-834D with a diameter of 217 mm and thickness of 13 mm was provided by Timet.

The bimodal microstructure of the alloy in as-received state is presented in Figure 1b. The alloy designation and chemical composition are shown in Table 1. curves and also by solution treating of small samples at temperature ranging 1000°C to 1065°C, followed by oil cooling. Metallographic observations of the as-quenched samples were done to determine the beta-transus temperature at and above which the final microstructure contained no coarse primary alpha phase. beta-transus temperatures of the 834M and 834D alloys measured by DTA and metallographic technique are listed in Table 2. All alloys were subjected to solution treatment at 1015°C (30°C below to beta-transus) for 2h followed by oil quench and to a subsequent ageing heat treatment of 2h at 700°C followed by air cooling. Microstructural investigations of as-received and heat treated specimens were performed on a scanning electron microscope (SEM) Zeiss DMS 982. The phase fractions of the primary alpha phase were determined by quantitative analysis of SEM micrographs. 1045 ± 5°C 1045°C < T β < 1055°C Specimens for tensile and creep tests were machined from the solution treated and aged bars and slice. The tensile and creep specimens had gauge dimensions of 7 mm (diameter) and 50 mm (length). Tensile tests were performed in air at a temperature up to 650°C using an Instron 5582 testing machine at a strain rate of 3.6 x 10 -4 s -1 . The creep tests were conducted until rupture of the specimens at 600°C under a constant stress of 200MPa in air. The tensile fracture modes of the tensile specimens were investigated by SEM.

Results and discussions

Microstructure characterizations

Figure 1 shows the microstructures, which are perpendicular to the rolling direction and forging direction of 834M and 834D alloys, respectively. The alloys exhibit a bimodal microstructure with primary alpha phase (a p ) and beta/alpha lamellar structures (b t ), wherein the dark and bright phases are alpha phase and beta phase, respectively. Due to the rolling process, alpha grains in 834M alloy have an elongated morphology (Figure 1a) whereas in the forged 834 slice, alpha grains are equiaxed (Figure 1b). The volume fraction of primary alpha phase was estimated at 34% and 29% in 834M and 834 alloys, respectively. This microstructural feature is closely related to the beta-transus temperature. b) Figure 2 shows the microstructures of heat-treated alloys. It can be seen in 834M and 834D alloys a bimodal microstructure with relatively fine secondary alpha (a s ) lamella in b t . Thus, the heat treatment erased partially the initial texture of the hot rolled 834M alloy. The image analysis from SEM micrographs shows a higher volume fraction of primary alpha phase in 834D alloy, which was estimated at 26.5%, than in 834M alloy with a volume fraction of 18.2%. The average size of the equiaxed alpha grains was estimated at 8.8 µm in 834M alloy against 17.4 µm in the 834D alloy. This result can be explained by the addition of beta stabilizing elements such as Ta or/and W, which are known to have low diffusivity. Thus these elements appear control the diffusional growth process in 834M alloy. Furthermore, according to Kitashima et al. [6], an addition of Ge can also decrease the volume fraction of primary alpha phase. Figure 3 shows the EDS-WDS elemental mapping in heat-treated 834M alloy. This indicates that the beta stabilizing elements such as Ta and W, are preferentially located in the retained b t phase, whereas, Si, Ge and Zr are concentrated in the small precipitates, located at interfaces between a p and b t . These precipitates are pointed out by the white arrows on the Figure 3. These precipitates were too fines to be resolved at SEM scale. To accurately identify the precipitates nature, their volume fraction and morphology, it would have required an extensive TEM study, which is scheduled for later.

Tensile tests

Figure 4 displays the tensile stress-strain curves of 834M, and 834D alloys at 600°C, and the tensile stress-strain curve of the 834M specimen tested at 650°C. At 600°C, 0.2% yield strength (YS), ultimate tensile strength (UTS) and elongation were 544 MPa, 683 MPa and 21%, respectively for 834M specimen. About the 834D, it displays 0.2% YS of 456, UTS of 554 and elongation of 18%. Thus, the alloy with additions of refractory elements W and Ta presents an increase of 0.2% YS by almost 19%, of UTS by 23% and of elongation by 16%. Further, despite the increase of the testing temperature up to 650°C, the 834M alloy displays even higher 0.2% yield strength (YS) and ultimate tensile strength (UTS) than the 834D sample, which tested at 600°C (Figure 4). Surprisingly, the elongation of the 834M alloy has decreased with increasing tensile test temperature from 600°C to 650°C. Thereby, the additions of refractory elements such as W and/or Ta increased significantly the high temperatures tensile strength even though the ductility of the material seems to be affected. These results can be attributed to the solid solution strengthening by refractory elements.

Figure 4 -Nominal stress-strain curves of 834M alloy at 600 and 650°C and of 834D alloy at 600°C

The fractographies of high temperatures tensile-tested samples are provided in Figure 5. The 834D alloy exhibits a combination of fine dimples and coarse elongated dimples (Figure 5a andb), which indicate that the fracture model of this specimen is ductile fracture primarily. Whereas, the 834M alloy subjected to ductile fracture and transgranular cleavage fractures which are inferred from the fractographies including some dimples and fracture facets (Figure 5c andd). Thus, the additions of refractory elements lead to the formation of faceted fractures in the regions of colony microstructure, which could explain the decrease of the elongation with the increase of tensile testing temperature of the 834M alloy.

Creep tests

As a reminder, the 834R alloy is a piece of the 834D alloy which was hot rolled in the same conditions than 834M alloy. It was also submitted to the same solution and ageing treatments of 1015°C/2h/oil quenching + 700°C/2h/air cooling. Constant load creep tests were conducted on the 834M and 834R alloys at 600°C under a stress of 200MPa. Figure 6 shows the creep curves of both 834M and 834R samples. These curves indicate clearly beneficial effect of refractory elements additions on the creep strain resistance since the 834M alloy displays a steady-state creep rate of 10 -8.s -1 which is 6 times lower than the creep rate of the 834R alloy. Such result can be related to the amount of a s phase. Indeed, extensive studies [START_REF] Bania | Titanium Science and Technology[END_REF][START_REF] Seagle | [END_REF][21] demonstrated that both primary and secondary creep rate increase with the amount of a p phase in the microstructure. Nevertheless, they do postulate that the observed primary alpha effect may be secondary and that the refinement of the transformed beta microstructure resulting from heat treatment closer to the beta-transus may be responsible for the enhanced creep resistance. Our creep results are in accordance with earlier findings of [21] since the 834M alloy containing a higher volume fraction of α s (Figure 2) exhibits the slower steady-state creep rate but also primary creep strain (Figure 6a andb). Another hypothesis related to the creep deformation mechanism can be put forward. According to Hayes et al. [22], the creep deformation is controlled by dislocation motion and that deformation is clearly dominated by deformation within the alpha phase, especially the diffusion rate of trace elements such as Ni can accelerate notably the creep rate by accelerating the rate of lattice self-diffusion therefore accelerates the rate of dislocation climb. W and Ta are commonly considered to be slow diffusers due to their large atomic seize thus, their additions can slow down the rate of diffusion in alpha phase, especially as W and Ta are preferentially partitioned in a s phase. Concerning the effect of Ge and/or Si additions on the creep response of the material, a detailed work on high temperature deformation mechanisms is underway in order to clarify their role, and the results will be published elsewhere.

Conclusions

The current study on the effect of refractory elements combined to Ge and Si additions on the microstructure evolution, high temperature tensile properties and creep resistance of the near-alpha titanium alloy Ti-834 led to following conclusions:

(1) Additions of refractory elements such as W and Ta to Ti-834 alloy resulted in a decrease in the volume fraction of a p and also in its average size. This effect has been attributed to the beta stabilizing characteristic of W and Ta. (2) A pronounced increase in the 0.2% YS and UTS at 600°C and 650°C with the additions of refractory elements and Ge and Si was observed. This has been attributed to the effect of solid solution strengthening by W and Ta additions. However, a decrease of ductility has been detected in the 834M alloy. This should be correlated to the formation of faceted fractures in the regions of colony microstructure.

(3) The creep resistance is also improved by the combined additions of refractory elements (W and Ta) and light elements (Ge and Si). Not only the steady-state creep rate was reduced by 6 times but also the primary creep rate was improved. This performance can be directly related to the amount of a s due to the additions of refractory elements but also to the slow rate of diffusion characteristic of these refractory elements.

Figure 1 -

 1 Figure 1 -Microstructure in as-received state of a) 834M alloy hot-rolled bar and b) 834D alloy forged slice

Figure 2 -

 2 Figure 2 -Backscattered electron micrographs of heat-treated a) 834M and b) 834D alloys (1015°C/2h/oil quenching and 700°C/2h/air cooling)

Figure 3 -

 3 Figure 3 -EDS-WDS elemental mapping micrographs of the 834M alloy

Figure 5 -

 5 Figure 5 -Fractographies of 600°C tensile tested 834D a) and b), 834M c) and d)

Figure 6 -

 6 Figure 6 -a) Steady-state creep curves, b) primary creep curves of 834M and 834R under a constant load stress of 200MPa, at 600°C

Table 1 -

 1 Alloys designation and chemical composition

	Alloy designation	Al	Sn	Zr	Mo	Nb	(Ta+W+Ge)	(Si+C)
	834M	5.8	3.95	3.5	-	-	3.7	0.15
	834D/R	5.7	3.97	3.68	0.52	0.68	-	0.4
	beta-transus temperatures of 834M and 834D alloys were determined by Differential Thermal Analysis (DTA)

Table 2 -

 2 beta-transus temperature of the 834M alloy and 834D

	beta-transus	DTA	Metallographic Technique
	834M	1048 ± 5°C	1040°C < T β < 1050°C
	834D/R


Ronan Fablet
email: rfablet@ifremer.fr

Jean-Marie Augustin
email: augustin@ifremer.fr

VARIATIONAL MULTI-WAVELET RESTORATION OF NOISY IMAGES

This paper presents a new technique for noise removal in images. It benefits both from the recent advances in waveletbased and variational denoising. Whereas wavelet-based analysis tends to strongly depend on the selected wavelet basis, we propose to combine and fuse several mono-wavelet analysis within a variational framework. The associated energy function involves M-estimator in order to guarantee the robustness to outliers and to preserve image structures (edges, ridges,...). An experimental evaluation for a Gaussian additive noise validates the proposed approach and an application to speckle removal in sonar sea-bed images highlights the interest of this approach for real images.

PROBLEM STATEMENT AND RELATED WORK

Noise removal in images is a common issue for a wide range of applications: multimedia, teledetection, underwater imaging, medical imaging,.... Two main categories of techniques have been recently proposed to cope with image restoration. On the one hand, wavelet-based approaches [START_REF] Donoho | De-noising by soft-thresholding[END_REF][START_REF] Pizurica | A joint inter-and intrascale statistical model for bayesian wavelet based image denoising[END_REF][START_REF] Sendur | Bivariate shrinkage functions for wavelet-based image denoising[END_REF][START_REF] Sintes | Interferometric side scan sonar signal denoised by wavelets[END_REF] cope with this issue through the shrinkage of wavelet coefficients. Despite good performances, these approaches tend to produce local artifacts for high level noise. On the other hand, variational methods [START_REF] Rudin | Nonlinear total variation based noise removal algorithms[END_REF][START_REF] Samson | A variational model for image classification and restoration[END_REF] based on non-quadratic criteria (e.g., the total variation) have been introduced to state image denoising as a minimization issue. This energy-based setting usually involves two terms: a regularity term defined on the image gradient and a data-driven term measuring the distance between the noisy image and the denoised one.

Therefore, it seems appealing to benefit from both categories of approaches. For instance, in [START_REF] Durand | Reconstruction of wavelet coefficients using total variation minimization[END_REF], wavelet shrinkage was stated in a variational setting through the minimization of a total variation criterion. In this paper, we give the emphasis to a multi-wavelet approach while stating the image restoration within a variational framework. Since waveletbased denoising performances are known to strongly depend on the chosen wavelet basis, we aim at combining several wavelet analysis. To this end, the proposed variational setting provides a well-formalized fusion criterion. The subsequent is organized as follows. In Section 2, we briefly review mono-wavelet image denoising. The proposed variational multi-wavelet approach is described in Section 3. Section 4 present experimental results on synthetic images and an application to speckle removal in underwater sonar images.

WAVELET SHRINKAGE

Wavelet shrinkage for noise removal was first introduced by Donoho etal [START_REF] Donoho | De-noising by soft-thresholding[END_REF]. The detail coefficients of a wavelet decomposition are indicators of the presence of signal ruptures: the greater the more relevant for image reconstruction. Consequently, the restoration of noisy images can be viewed as the shrinkage of these wavelet coefficients. Initially, Donoho etal. [START_REF] Donoho | De-noising by soft-thresholding[END_REF] have proposed the hard-and softshrinkage schemes. For instance, given an image I and the associated wavelet decomposition W I, the hard-shrinkage of the detail coefficients {W I k,l }, where k refers to the scale level and l to the type of detail coefficients (horizontal, vertical or diagonal), resorts to:

∀p, W I k,l (p) =    0, if W I k,l (p) < δ W I k,l (p), otherwise (1)
The threshold δ is set w.r.t. the noise characteristics: for instance, for a Gaussian additive noise of standard deviation σ, δ is typically set to 3σ. The filtered image Ĩ simply results from the reconstruction using the modified wavelet coefficients W I:

Ĩ = W I N,0 Φ N,0 + N k=1 3 l=1 W I k,l Φ k,l (2)
where Φ k,l is the wavelet basis function at resolution k for the quarter l (l = 0, low frequency, l = 1, 2, 3; horizontal, vertical and diagonal high frequency).

The shrinkage rules can also be deduced from a Bayesian setting [START_REF] Pizurica | A joint inter-and intrascale statistical model for bayesian wavelet based image denoising[END_REF][START_REF] Sendur | Bivariate shrinkage functions for wavelet-based image denoising[END_REF] when a priori information on the distribution of the wavelet coefficients is considered. For instance, in [START_REF] Sendur | Bivariate shrinkage functions for wavelet-based image denoising[END_REF], a bivariate shrinkage scheme exploiting the inter-scale dependencies is proposed:

∀p, W I k,l (p) = α(k, l, p) - √ 3σ N /σ + α(k, l, p) W I k,l (p) (3) with α(k, l, p) = W I 2 k,l (p) + W I 2 k+1,l (p + )
, where p + is the father of the point p in the wavelet decomposition. σ N is the standard deviation of the Gaussian noise and σ the deviation of the a priori model for inter-scale dependencies. This soft shrinkage scheme was proven experimentally to outperform Donoho's scheme [START_REF] Sendur | Bivariate shrinkage functions for wavelet-based image denoising[END_REF]. In the subsequent, we will use this scheme to perform a multi-wavelet analysis.

VARIATIONAL MULTI-WAVELET DENOISING

The key contribution of this paper is to introduce a wellformalized framework to achieve a multi-wavelet denoising. As illustrated in Fig. 1, the performance of waveletbased denoising techniques strongly depend on the choice of the wavelet basis. Besides, whereas a variety of wavelet basis (e.g., daubechies, curvelets, ridgelets,...) has been introduced [START_REF] Mallat | A wavelet tour of signal processing[END_REF][START_REF] Starck | The curvelet transform for image denoising[END_REF], real images usually involve different types of structures. Hence, selecting only one wavelet basis might not be optimal.

Rather than selecting an optimal wavelet basis, we propose to exploit the decompositions obtained from several wavelet basis. Image restoration then comes to fuse these analysis within a variational framework detailed below.

Robust energy setting

Given a noisy image I and a chosen set of wavelet basis B, we first apply the bivariate shrinkage for each wavelet basis in B and compute the resulting filtered outputs { Ĩb } b∈B . The fusion step is then stated as the following variational issue:

min b∈B ρ b ( Ĩ(p) -Ĩb (p)) + ρ 2 (∇I(p)) (4)
where ρ i a M-estimator (e.g., Welsch M-estimator ρ(x) = 1exp(-x 2 /σ 2 ) with σ a scale parameter) and γ a weight balancing the importance of the two energy terms. The first term can be referred to as the data-driven term: it states that the restored image Ĩ should be close to the mono-wavelet result { Ĩb }. The use of the M-estimator ρ 1 guarantees the robustness to outliers such as local artifacts. The second term set on the image gradient is aimed at imposing some regularity constraints on the output image. Whereas quadratic criteria are known to oversmooth meaningful image structures (edges, ridges,...), the M-estimator ρ 2 provides a mean to preserve these structures in the filtered image, while smoothing uniform areas.

The proposed variational setting can be regarded as a way to drive the fusion of several mono-wavelet analysis by a regularity constraint.

Minimization issue

Rather than exploiting the Euler-Lagrange partial derivative equation derived from Eq.4, we adopt a discrete Markovian setting. The energy in minimization (4) then resorts to:

J(I) = p b∈B ρ b (I(p) -Ĩb (p)) + (p,q)∈V ρ 2 (I(p) -I(q)) (5)
where ∈ V is the 4-neighborhood.

The minimization of function J(I) is then solved for using an iterated reweighted least-square scheme [START_REF] Huber | Robust Statistics[END_REF]. It iterates two steps. First, given the current estimate Ĩ, the following robust weights are computed:

∀p, ∀b ∈ B, w b (p) = ψ 1 ( Ĩ(p) -Ĩb (p)) (6)
∀(p, q) ∈ V, w 2 (p, q) = ψ 2 ( Ĩ(p) -Ĩ(q)) (7)
where ψ i is the influence function associated to ρ i (i.e., ψ i (r) = 1/r • dρ/dr(r)). The second step comes to minimize the reweighted least-square function J(I) given by:

J(I) = p b∈B w 1 (p) I(p) -Ĩb (p) 2 + (p,q)∈V w 2 (p, q) I(p) -I(q) 2 (8)
This minimization is carried out using the ICM algorithm. The initialization is provided by the mean over the monowavelet outputs:

Ĩ0 = 1 |B| b∈B Ĩb
In practice, we use Welsch M-estimator ρ i (x) = 1-exp(-x 2 /σ 2 i ). The scale parameters ({σ b }, σ 2 ) are initially set according to the median value of the residuals:

σ b = 1.57med(| Ĩ(p) -Ĩb (p)|), σ 2 = 1.57med | Ĩ(p) -Ĩ(q)|, (p, q) ∈ V ;
and then updated w.r.t. a geometrical law at each iteration.

Coarse-to-fine strategy

In order to improve the convergence of the proposed scheme, we take advantage from the multiresolution structure of the wavelet decompositions to design a coarse-to-fine strategy.

At a given level resolution k * , we first reconstruct the mono-wavelet outputs { Ĩb k * } b∈B from the shrinked wavelet coefficients { W I b } b∈B using only resolution levels greater than k * :

Ĩb k * = W I b N,0 Φ N,0 + N k=k * 3 l=1 W I b k,l Φ k,l (9)
Minimization ( 4) is carried out w.r.t. { Ĩb k } b∈B to determine the filtered image Ĩk * at resolution k * . Given Ĩk * , we compute the associated wavelet decompositions {W Ĩb k * } b∈B , which are used to update the wavelet decompositions

{ W I b } b∈B at level k * k: ∀b, ∀l, W I b k * ,l = (W Ĩb k * ) k * ,l (10)
This procedure is iterated at resolution k * -1 using the updated wavelet decompositions { W I b } b∈B . The output at the finest resolution provides the filtered image.

EXPERIMENTS

Quantitative analysis on synthetic images

To validate the proposed approach, different experiments have been carried out both for synthetic and real images.

In both cases, we use the same parameter setting: wavelet decomposition with 4 levels, a set of 10 wavelet basis (haar, dauchechies 4, 6 and 8, coiflets 1, 2 and 3, and symlets 4, 5 and 6), γ = 10. In a first step, we have performed a quantitative analysis for two synthetic images, reported in Fig. 1, w.r.t. to different an additive Gaussian noise with a variance from 10 to 90. It demonstrates that, in any case, the multi-wavelet denoising outperforms all the mono-wavelet ones. In addition, the results also stress the improvements brought by combining all mono-wavelet filtering though the proposed variational fusion scheme. The average gain resulting from the fusion scheme is of 0.75dB w.r.t. the best mono-wavelet denoising and 2dB w.r.t. the worst one.

Application to speckle removal in seabed sonar images

We apply the proposed multi-wavelet variational denoising to speckle removal in seabed sonar images. Fig. 3 depicts an example of sonar image. Such images are highly corrupted by a multiplicative speckle noise. In order to resort to an additive noise, we first process the logarithm of the sonar image. The resulting noise can be Fig. 1. Comparison of the denoising results for an additive Gaussian with a variance from 10 to 100 applied to "lena" : we plot the evolution of the pSNR as a function of the standard deviation of the additive Gaussian noise for the worst (WorstMonoW) and best (BestMonoW) mono-wavelet denoising result and the result of the variational multi-wavelet method (VarMultiW).

roughly approximated as a Gaussian noise. We then apply the proposed denoising scheme and reconstruct the filtered image using the exponential function.

Fig. 3 reports an example of speckle removal for a seabed sonar image depicting a wreck lying on sand. We provide a comparison of the proposed multi-wavelet variational denoising and of the Frost scheme [START_REF] Frost | A model for radar images and its application to adaptive digital filtering of multiplicative noise[END_REF]. The Frost scheme is a locally adaptive linear filter. The zoom on details of the wreck clearly stress the improvements brought by our approach. Local details such as masts are better separated from noise using the variational approach.

CONCLUSION AND FUTURE WORK

In this paper, we have coped with the restoration of noisy images. We have emphasized the potential improvements brought by combining several wavelet analysis w.r.t. different types of wavelet basis or different parametrization of a given wavelet basis. This fusion issue is stated in a robust variational framework. The use of robust estimators allow us to cope with the presence of outliers in the mono-wavelet denoising outputs and to preserve relevant image structures such as edges. The experiments carried out for synthetic images corrupted by an additive Gaussian noise quantitatively validate the improvement brought by our scheme compared to mono-wavelet schemes. Besides, the application to speckle removal in sea-bed sonar images demonstrate its efficiency to cope with image restoration in complex real cases.

In future work, we will investigate the adaption of our variational multi-wavelet scheme to local noise characteristics. Besides, concerning the application to the analysis of sonar images, it might be relevant to better account for the characteristics of shadow areas.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Comparison of the multi-wavelet variational denoising to the best mono-wavelet denoising for "lena" corrupted by aGassian additive noise with a standard deviation set to 40. We display a zoom on the area around the eyes. noisy image (speckle noise) locally adaptive Frost denoising multi-wavelet variational denoising


F Rakotomanana

L Jolivet

R V Randremanana

J P Rudant

Most studies of anopheline mosquito larval ecology have been done in rural settings. However, latest data based on two cross-sectional surveys in Antananarivo, the capitol of Madagascar, shown low rate of malaria cases among febrile episodes but autochthonous malaria cases exist. Anopheles funestus constitutes the main vector of malaria in the highlands of Madagascar. This paper described the determination of their potential breeding site using remotely sensed data. A supervised classification by the classical method of maximum likelihood was used for enhanced thematic mapper image of Landsat 7. Overall accuracy of the classification was 86% and kappa index was 0.835. Determination of landscape change by subtraction of images acquired on January and July was carried out for the Advanced Synthetic Aperture Image Precision images of Envisat. Increased backscatter coefficient between the two periods made possible to raise ambiguity between rice fields and other vegetation. That may improve the determination of potential anopheles breeding sites.

BACKGROUND

In most of sub-Saharan Africa countries, malaria transmission is generally intense in rural settings. In contrast, malaria transmission occurs at significantly lower level in urban areas where population density is high, and where the number of mosquito breeding sites is reduced [START_REF] Robert | Malaria transmission in urban sub-Saharan Africa[END_REF]. Central Highlands of Madagascar are known as a malaria epidemic-prone area. Several malaria cases have been reported during the last epidemic of the 80's. In addition to a policy of early drug therapy, a vector control programme was launched by the "Ministère de la Santé, du Planning Familial et de la Protection Sociale". It was based on indoor insecticide spraying. Anopheles funestus constitutes the main vector of malaria in the Central Highlands of Madagascar. Rice fields are known as their potential breeding sites [START_REF] Jambou | Malaria in Antananarivo: evaluation of a post-epidemic situation[END_REF][START_REF] Jeanne | Malaria and schistosomiasis: 2 examples using systems of geographical information and teledetection in Madagascar[END_REF][START_REF] Laventure | Rice: source of life and death on the plateaux of Madagascar[END_REF].

The latest data based on two cross-sectional surveys in 43 health centres in Antananarivo showed low rate of malaria cases. In February 2003 (rainy season) and July 2003 (dry season), 1.9% positive cases and 1.5% positive cases were respectively detected. Only two cases of 15, both P. falciparum, were likely to have been autochthonous (0.26%). In July, three cases were considered to be of local origin (0.4%) [START_REF] Rabarijaona | Paludisme sur les hautes terres centrales de Madagascar: Stratégie de lutte[END_REF].

Antananarivo, like most of urban areas was considered as a malaria transmission free zone. However, the latest cross-sectional surveys had shown that autochthonous malaria cases exist. The issue is to know if these cases could induce an epidemic. Antananarivo landscape is characterized by both urban and agricultural area with vast rice field plains. Rapid change of urban area involved updating the map of potential mosquito breeding sites using remotely sensed data. It constitutes a relevant parameter to assess the malaria epidemic risk. This study aimed to determine the contribution of radar remote sensing to detect these mosquitobreeding sites.

MATERIALS AND METHODS

The study area includes the urban area of Antananarivo and the immediate neighbourhoods, 18°48'S and 47°24'E in the North West 19°00'S and 47°42'E in the South East. The landscape is characterized by both urban and agricultural area with vast rice field plains in the north and west.

An Enhanced Thematic Mapper Plus (ETM +) image of Landsat 7, acquired on May 2000 and two Advanced Synthetic Aperture Image Mode Precision (ASA_IMP_1P) images of Envisat, acquired on January and July 2004, were used for the determination of the mosquito potential breeding sites (Tab. 1).

A supervised classification of the ETM+ image by the classical method of maximum likelihood was used to map rice fields. The classification accuracy was estimated by calculating the Kappa coefficient. Adaptive filters were applied to improve the quality of ASA_IMP_1P images. Landscape change between January and July was compared by the Topographic maps and GPS location were used to complete visual interpretation of the images and for supervised classification of the image ETM+ Landsat7. There were used especially for identifying the training sites, the test sites location and for the ground truth validation. Geographical coordinates of these sites were positioned using handheld Global Positionning System (GPS).

Digital Elevation Model (DEM) was acquired by the way of Epidemioproject). It was used to assert from their location the rice fields in the plain and the vegetation located on the crests.

RESULTS

Composite image, with near infrared (band 4) in red channel the mid-infrared (band 5) in the channel of green and red (band 3) in the channel of blue, shows vegetation type and condition characterized by colour hues (Fig 2).

Antananarivo Figure 2. Infrared false colour composite of the image ETM+ Landsat7

The supervised classification by the method of maximum likelihood showed confusion between rice fields and chlorophyllous trees. Similar confusion was observed between lakes and flooded rice fields. Overall accuracy of the classification was 86%. Kappa coefficient was 0.835. Determination of landscape change was carried out by subtraction images of Envisat after normalization (Fig 3 and4). Rice fields present higher response in July than in January. Low backscatter coefficients characterized the flooded rice field in January. Young rice shoot in the seedbeds, cut plants after the harvest and rough ground of rice fields in ploughing were characterized by high backscatter coefficient in July. Turbid water body in January, rainy season, present higher backscatter coefficient. Change between the two periods made possible to raise ambiguity between rice fields and other vegetation (Fig 5).

It allowed removing commission error observed with optical data. However, the results cannot map rice fields only from other potential breeding sites like marsh and other wetland.

CONCLUSION

In health application, remote sensing is commonly used to identify vector habitats e.g. vector borne diseases. Kaya et al. reported that the combined use of remote sensing and GIS provides a strong tool for monitoring environmental conditions that are conducive to malaria, and for mapping the disease risk to human population [START_REF] Kaya | The use of Radar RS for identifying Environmental factors associated with malaria risk in Coastal Kenya in International Geoscience And RS Symposium[END_REF]. Results shows that supervised classification of optical data by the method of maximum likelihood may present confusion between rice fields and other vegetation so such study showed interest in using high spatial resolution combining passive and active data acquisition [START_REF] Rakotomanana | Using high spatial resolution data for the identification of paddy fields, larval potential habitats of anopheles vector of malaria in the Central Highlands of Madagascar in Envisat & ERS Symposium[END_REF][START_REF] Thomas | Cartographie des rizières d'une zone des Hautes Terres Centrales Malgaches pour la détermination des zones à risque du paludisme. Complémentarité SPOT et radar[END_REF]. Uneven aspect of the highlands of Madagascar constitutes the limitation of using radar remotely sensed data. In so far as rice fields located at the bottom of small valleys or in terrace, following hill round, using radar data is not really suitable for reliable extraction of rice fields. That was not the case of the plain of Antananarivo where the signal change of rice fields was easily detectable even if their intensity varied, due to the lag of cultivation calendar and the problem of water management. The contribution of radar imagery allows (i) data acquisition during wet season which eases the comparison of different periods [START_REF] Jolivet | Télédétection et Paludisme urbain: Apport des images radar dans la cartographie des gîtes potentiels d'Anophèles dans la région d'Antananarivo[END_REF][START_REF] Ribbes | Utilisation des données radar pour le suivi des rizières: cartographie, suivi de croissance et estimation du rendement[END_REF] and (ii) erasing confusion between larval habitats and other vegetation such as reeds, trees. Hence radar remote sensing is the obvious choice as the most appropriate data source for multidate acquisition. This work showed the interest of using high spatial resolution image combining passive and active data acquisition. The results contribute to the malaria vector control in epidemiological application.

Figure 1 .

 1 Figure 1. Location of study zone

• • L

 L La ak ke e • •B Bu ui il lt t A Ar re ea a • •R Ri ic ce e f fi ie el ld ds s • •U Ur rb ba an n a ar re ea a

REMOTE SENSING AND URBAN MALARIA: RADAR ENVISAT CONTRIBUTION FOR THE DETERMINATION OF POTENTIAL ANOPHELES BREEDING SITE IN ANTANANARIVO (MADAGASCAR)

_____________________________________________________ Proc. 'Envisat Symposium 2007', Montreux, Switzerland 23-27 April 2007 (ESA SP-636, July 2007) way of visual interpretation and backscatter coefficient analysis.

Table 1 .

 1 Data specification

	ETM + Landsat 7
	Date	2000/05/12
	Preprocessing level	1G
	Band	1-7, P
	ASAR Envisat
	Date	2004/01/19
		2004/07/12
	Product type	ASAR_IMP_1P
	Polarisation	VV
	Band	C
	Digital

Elevation Model (EO Epidemio Project)

	Date	2004
	Product type	ASAR_IM4
		ASAR_IM5
	Topographic maps
	Date	1970
	Scale	1:10 000
		1:50 000

Acknowledgments: We are grateful for the financial and collaborative support from the Action Concertée InterPasteurienne (ACIP), the European Spacial agency by the way of Project CAT 1-2320 and Earth Observation in Epidemiology, the Ecole Nationale des Sciences Géographique (ENSG) and Institut Pasteur de Madagascar.

Figure 4. Image of July 2004 (ASA_IMP_1P Envisat)


Céline Hidalgo
email: celine.hidalgo@univ-amu.fr

Jacques Pesnot-Lerousseau

Stéphane Patrick Marquis

Stéphane Roman

Noël Nguyen

Daniele Schön

Rhythmic training improves temporal conversational abilities in children with hearing loss

Keywords: Cochlear Implants, Intervention, Speech, Neuroimaging, Communication Cochlear implant, rehabilitation, speech, EEG, temporal predictions

Rhythmic training improves temporal conversational abilities in children with hearing loss

Introduction

The most natural and common usage of speech is in conversation. Interactions between interlocutors heavily rely on anticipatory processes in order to optimize communicative exchanges and minimize interruptions [START_REF] Pickering | An integrated theory of language production and comprehension[END_REF][START_REF] Levinson | Timing in turn-taking and its implications for processing models of language[END_REF]. In fact, as it is the case in any joint action implying two or more individuals, a major key of the success of an optimal communicative exchange is the ability for each interlocutor to temporally predict her/his partner's actions [START_REF] Sebanz | Prediction in Joint Action: What, When, and Where[END_REF][START_REF] Kourtis | Predictive representation of other people's actions in joint action planning: An EEG study[END_REF]. For instance, anticipating the end of the speech turn on the basis of the speech rate allows turn-taking at an optimal pace [START_REF] Wilson | An oscillator model of the timing of turn-taking[END_REF]. An anticipation of turn-ends allows avoiding speech overlap of the interlocutors as well as over-long silences that may break the fluency of the exchange. Moreover, it may also allow a better perception of the grouping and prominence of speech that may in turn facilitate speech spectral processing [START_REF] Cummins | Rhythmic constraints on stress timing in English[END_REF][START_REF] Port | Meter and speech[END_REF][START_REF] Peelle | Neural Oscillations Carry Speech Rhythm through to Comprehension[END_REF]. This ability to readily take turns is possibly facilitated by the fact that interlocutors tend to adopt a common rhythm, based on shared syllable timing [START_REF] Street | Speech convergence and speech evaluation in fact-finding interviews[END_REF][START_REF] Himberg | Word-by-word entrainment of speech rhythm during joint story building[END_REF]. This phenomenon is considered as taking place automatically and is often referred to as between-speaker rhythmic convergence.

Recent studies have focused on how these anticipatory skills develop during infancy. It has been shown that 5-month-old babies already engage in turn-taking with their mothers, with few overlaps and short turn-taking windows [START_REF] Hilbrink | Early developmental changes in the timing of turn-taking: a longitudinal study of mother-infant interaction[END_REF].

Children with a prelingual hearing loss (HL), whether equipped with cochlear implant or conventional prostheses, are particularly vulnerable in terms of temporal prediction abilities because of the early auditory deprivation. This has a direct impact upon speech perception abilities, especially in acoustically-degraded environmental conditions, as well as on the mastery of certain communication skills. More precisely, children with HL have difficulties in imitating the speech rate of their interlocutor (i.e. temporal convergence) and this goes along with reduced speech intelligibility [START_REF] Freeman | Speech rate, rate-matching, and intelligibility in earlyimplanted cochlear implant users[END_REF]. These children are also less sensitive than normal-hearing children to the temporal regularity of stress distribution in speech in an interactive verbal task [START_REF] Hidalgo | Speak on time! Effects of a musical rhythmic training on children with hearing loss[END_REF]. Adolescents with CI both use different strategies for repairing communication failures and are less sensitive to prosodic cues relevant to spoken discourse understanding [START_REF] Holt | The Use of Prosodic Cues in Sentence Processing by Prelingually Deaf Users of Cochlear Implants[END_REF][START_REF] Holt | Discourse Strategies and the Production of Prosody by Prelingually Deaf Adolescent Cochlear Implant Users[END_REF] compared to normal-hearing peers.

In a rehabilitation perspective, it seems essential to take into account the alteration of this type of communication skills in hearing-impaired children in order to improve the quality of their verbal interactions as well as the development of their social relations [START_REF] Hoffman | Comparisons of Social Competence in Young Children With and Without Hearing Loss: A Dynamic Systems Framework[END_REF][START_REF] Manson | Convergence of speech rate in conversation predicts cooperation[END_REF][START_REF] Wong | Psychosocial Development in 5-Year-Old Children With Hearing Loss Using Hearing Aids or Cochlear Implants[END_REF]). An intervention aimed at improving temporal prediction abilities based on rhythm training seems particularly relevant.

Indeed, in a sentence repetition task, presenting a musical rhythm before each sentence improves hearing-impaired children's accuracy, even more so when the rhythm of the musical prime matches that of the sentence [START_REF] Cason | Rhythmic priming enhances speech production abilities: Evidence from prelingually deaf children[END_REF]. Moreover, we [START_REF] Hidalgo | Speak on time! Effects of a musical rhythmic training on children with hearing loss[END_REF] recently showed that following 30 minutes of rhythmic training (compared to a control condition), children with HL show an enhanced capacity to accommodate to a temporally predictable flow of turn exchanges in an interactional setting. These results fit well with the literature showing the beneficial effect of musical expertise, particularly in the rhythmical domain, on sensitivity to speech rhythm [START_REF] Marie | Musicians and the metric structure of words[END_REF][START_REF] Moritz | Links between early rhythm skills, musical training, and phonological awareness[END_REF][START_REF] Magne | Speech rhythm sensitivity and musical aptitude: ERPs and individual differences[END_REF], and that of rhythmic stimulation on speech processing [START_REF] Cason | Bridging music and speech rhythm: rhythmic priming and audio-motor training affect speech perception[END_REF][START_REF] Cason | Rhythmic priming enhances the phonological processing of speech[END_REF][START_REF] Chern | New evidence of a rhythmic priming effect that enhances grammaticality judgments in children[END_REF], both in normal people and people with a variety of language Rhythmic training improves temporal conversational abilities in children with hearing loss 5 disorders [START_REF] Kotz | The basal ganglia are receptive to rhythmic compensation during auditory syntactic processing: ERP patient data[END_REF][START_REF] Przybylski | Rhythmic auditory stimulation influences syntactic processing in children with developmental language disorders[END_REF][START_REF] Cason | Rhythmic priming enhances speech production abilities: Evidence from prelingually deaf children[END_REF][START_REF] Flaugnacco | Music Training Increases Phonological Awareness and Reading Skills in Developmental Dyslexia: A Randomized Control Trial[END_REF][START_REF] Bedoin | Temporally Regular Musical Primes Facilitate Subsequent Syntax Processing in Children with Specific Language Impairment[END_REF][START_REF] Schön | Short-and long-term rhythmic interventions: perspectives for language rehabilitation[END_REF].

In the light of the above-mentioned studies illustrating the beneficial effect of rhythmic stimulation on temporal predictions and the processing of auditory information in both adults and children (for a review, see Haegens & Zion Golumbic, 2017), we conducted a combined EEG and behavioral study to measure children's temporal adaptation capacities and to examine the influence of musical rhythmic training on speech perception and production abilities in children with HL.

More precisely, we ran two experiments. In Experiment 1, children with normal hearing (NH) were asked to perform a picture-naming task in alternation with a virtual partner (adapted from [START_REF] Hidalgo | Speak on time! Effects of a musical rhythmic training on children with hearing loss[END_REF]. The speech rate of the virtual partner could be slow or fast and the temporal structure of turns could be regular or irregular. We performed acoustic analyses of speech production to determine to what extent children converged towards the virtual partner's (VP) speech rate as well as to measure accuracy and consistency of word stress placement across regular and irregular turns. Event-related potentials were used to assess the sensitivity to turn regularity.

In Experiment 2, children with HL, wearing either cochlear implants (CIs) or hearing aids (HAs), did the picture-naming task twice: once following 30 minutes of rhythmic musical training, and once following 30 minutes of auditory training. We hypothesized that the rhythmic training would improve the ability of children with HL to predict conversational dynamics which in turn would facilitate speech perception and production.

Overall, the aim of this study was twofold. Firstly, we used a turn-taking task to assess in normal hearing (NH) children the speech rate convergence ability as well as the accommodation ability. to adapt to temporal deviancy in the regularity of the turns.

Experiment 1

Material and Method

Participants

Twenty-two children with normal hearing (NH) (13 girls) were recruited from an elementary school in Marseille (France). They were aged from 6 to 10 years (mean = 102.6 months, SD = 18.5 months) and were all French native speakers without any known visual, speech, cognitive or hearing disorder. This experiment was approved by the Ethics Committee Sud Méditerranée I (n°ID RCB: 2015-A01490-49). All parents signed a consent form.

Stimuli

For the alternation naming task, we selected 46 pictures from the French BD2I database [START_REF] Cannard | Picture naming in 3-to 8-yearold French children: methodological considerations for name agreement[END_REF]. These pictures were named correctly by over 90% of a normative sample of 3 to 8-year-old children (mean score = 98.8%, SD=1.6%).

46 picture names (disyllabic words) were recorded in a soundproof booth by a male native French speaker (henceforth, the virtual partner, VP). Four words were also recorded by an 8year-old male native French speaker to be used as an example of the alternating procedure at the beginning of each block.

We performed an automatic word segmentation of the VP's recording using SPPAS (Bigi, 2015, 1.8.0 version) and PRAAT (Boersma & Weenink, 2012). All words were then Rhythmic training improves temporal conversational abilities in children with hearing loss 7 manipulated to obtain a fast (80% of initial word duration) and a slow (120%) version for each of them, using a wavelet-based method for audio dilation and compression [START_REF] Pallone | A wavelet based method for audio-video synchronization in broadcasting applications[END_REF]. For both the fast and slow versions, p-centers, taken as rhythmic perceptual anchor points [START_REF] Morton | Perceptual Centers (P-centers)[END_REF], were semi-automatically located on the vowel of the second, stressed syllable, as the 2/3rd of the vowel energy rise [START_REF] Cummins | Rhythmic constraints on stress timing in English[END_REF].

Procedure

Before the beginning of the picture-naming task, children were familiarized with the pictures and the experimental procedure to make sure that the words produced for each picture both were the expected ones and were pronounced in an intelligible manner. Then, children were equipped with a 21-channel electrode cap (International 10/20 system sites) and were seated in a Faraday booth in front of a computer screen. They were asked to name the pictures displayed on the screen, in alternation with a virtual partner (Figure 1).

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The experiment consisted of two separate blocks, one with a slow and one with a fast VP speech rate. Each block started with 4 pairs of pictures that were named by the adult virtual partner in perfect alternation with the voice of a pre-recorded 8-year-old boy in order to establish, implicitly, a regular pace of alternation. Next, 132 pictures were presented successively on the screen and the child was instructed to name the pictures in alternation with the VP. Care was taken to avoid semantic or phonological priming between successive words. A given word pair (VP-child) never appeared twice. The child was praised and encouraged before and between the two blocks. A short pause was given between blocks. The duration of each block was around 5 minutes. This resulted in a 2 by 2 factorial design manipulating the Regularity of alternation (regular or irregular) and Speech Rate (slow or fast) across blocks. The order of block presentation (fast versus slow) was counterbalanced across participants.

Stimuli delivery was programmed using the software Presentation (Neurobehavioral Systems 18.0). Pictures were displayed on a computer (DELL Precision T1700, screen resolution 1366

x 768). The voice of the VP was delivered through a 2040 YAMAHA amplifier and two NS 1020 Studio YAMAHA loudspeakers. The output sound volume was adjusted at a comfortable level for each child. The child's speech was recorded with a lapel microphone (Sennheiser) linked to a ZOOM H4n numeric recorder at a 44 kHz sampling rate.

For every child, we also assessed rhythmic and neuropsychological abilities. Rhythmic assessment included tapping along a 90 pulses/minute metronome (henceforth paced tapping) test, tapping in syncopation along a 55 pulses/minute metronome (henceforth syncopated tapping) and reproduction of 10 rhythms (3 to 8 sounds each) (henceforth rhythmic patterns reproduction). Children tapped onto a cardboard box containing a ZOOM H4n digital recorder. Neuropsychological tests were issued from NEPSY and WISC IV. These tests included digits direct and indirect span test and an auditory attention test requiring to place a token in a box each time the child heard "red" among other words.

Behavioral data processing and statistical analysis

Children's convergence abilities were evaluated by comparing word duration (in ms) across the two speech rates (fast and slow).

Children's accommodation capacities were determined by the consistency of the children's pcenter placement across trials and the accuracy (temporal asynchrony) relative to the VP's Rhythmic training improves temporal conversational abilities in children with hearing loss 9 stress placement. First, we extracted the temporal interval (in ms) between the p-centers of the VP and the p-centers of children (Inter-Word-Interval, IWI). Then we computed circular statistics (CircStats, [START_REF] Berens | CircStat: A MATLAB toolbox for circular statistics[END_REF] on the IWI in order to express the children's p-center placement as a vector in a circular space relative to the expected moment in time (half of the inter-trial interval of the virtual partner) (statistics also used in [START_REF] Hidalgo | Speak on time! Effects of a musical rhythmic training on children with hearing loss[END_REF][START_REF] Himberg | Word-by-word entrainment of speech rhythm during joint story building[END_REF]. P-center placement consistency was measured by the vector length, and accuracy by the vector angle. Words produced with an error, a hesitation or simultaneously with the virtual partner were excluded from all analyses, resulting in rejection of approximately 30% of the children's productions.

We used R (R Core Team, 2013) and the lme4 package (Bates, Maechler & Bolker, 2013) to fit linear mixed models on single trial data for both word duration and vector angle.

All models were built with participants as random effect. Statistical significance of the fixed effects was assessed by model comparison on AIC, thus arbitrating between complexity and explanatory power of the models. Normality and homoscedasticity of the residuals were systematically visually inspected. Reported p-values are Satterthwhaite approximations obtained with the lmerTest package (Kuznetsova, Brockhoff & Christensen, 2015). As consistency is always positive and not normally distributed, we applied a logit transform before model fitting [START_REF] Falk | Non-verbal sensorimotor timing deficits in children and adolescents who stutter[END_REF]. Furthermore, random effects were dropped for consistency, as this summary statistics (estimated by vector length) gives only one value per participant and condition.

In the paced and syncopated tapping tasks, tapping times were extracted using Adobe Audition (2.0) and circular analyses were used to compute the children's consistency and accuracy.

In the rhythmic patterns reproduction, the anonymized children's performances were scored from 1 to 9 (1: no reproduction, 9: perfect performance) by one judge (the last author) with Correlations were computed using Spearman's rank, reported with 95% confidence intervals, between rhythmic measures (paced tapping, and rhythmic patterns reproduction), behavioural and electrophysiological responses during the turn-taking task.

Electrophysiological data processing and statistical analysis

EEG signal processing was done using EEGLAB [START_REF] Delorme | EEGLAB: an open source toolbox for analysis of singletrial EEG dynamics including independent component analysis[END_REF]) and custom Matlab scripts. Continuous data were filtered using a high-pass filter (cutoff frequency 1 Hz) and major artifacts rejected. Independent Component Analysis (ICA) was used to remove physiological artifacts such as eye blinks and muscular activity. For the Event-Related Potentials (ERPs) analysis, data were segmented into 900ms epochs starting 100ms before the presentation of the VP's words onsets. Further rejection of remaining artifacts was done on the basis of visual inspection of each epoch and was always lower than 10% of the total number of epochs in a given condition.

We conducted two types of statistical analysis. The first analysis was run on the peak of the MMN using a classical pairwise comparison. First, a temporal window for peak search was defined using the average of all trials. Then, for regular and irregular conditions, the peak amplitude was computed as the mean value ±40ms around the individual peak. A Region of Interest (ROI) was defined on the basis of the topography of the average of all conditions, using a hierarchical clustering technique. The MMN-like ROI comprised 'P3', 'P4', 'O1', 'O2', 'P7', 'P8', and 'Pz' (see Figure 4A). The second analysis consisted of a cluster-based analysis in the time domain on the same ROI to analyze the differences between regular and irregular conditions. Significance was assessed using non-parametric pairwise two-tailed permutation tests that provide corrected p-values for multiple comparisons. For each

Results

Behavioral results. The speech rate of the VP affected children's word duration (β = -33.37, SE = 3.76, t = -8.85, p < .001). More precisely, children's words lasted longer in the Slow compared to the Fast condition (see Figure 2A).

The analyses of stress placement focused on consistency, i.e. whether the child placed the stress consistently in the same time window across trials, and accuracy, i.e. how close the stress was placed with respect to the expected time point.

The regularity of alternation influenced children's consistency and accuracy in stress placement. Children placed word p-centers more consistently in the Regular compared to the Irregular condition (β = 0.74, SE = 0.28, t = 2.667, p <. 01) (see Figure 2B). They also placed word stresses close to the expected time point in the Regular trials while they were late in Irregular trials (β = -0.81, SE = 0.03, t = -24.09, p <.001, Figure 2B).

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Electrophysiological results. Results showed a MMN-like effect following Irregular trials (ES = -3.7µV, t (1,19) = -5.5834, p < 0.001). This effect was significant across a long-lasting time cluster, from 124 to 492ms after stimulus onset (p < 0.01). Correlation analysis on the peak amplitude of the MMN-like effect and the vector length showed that children who were more

Material and methods

Participants

Sixteen children with HL (7 girls) wearing bilateral or unilateral cochlear implants (CIs) and/or Hearing Aids (HAs) (see Table 1 for details) were recruited, mostly via the ENT paediatric department at the Timone Hospital in Marseilles. All but two were enrolled in a mainstream primary school. They were aged from 6 to 10 years (mean = 102 months, SD = 15.6 months) and were all native speakers of French without any reported visual or associated speech and cognitive impairment. Data of two participants were excluded because of poor EEG data quality. This experiment was approved by the Ethics Committee Sud Méditerranée I (n°ID RCB: 2015-A01490-49).

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PLEASE INSERT

Stimuli and procedure

The stimuli, task and procedure were identical to Experiment 1 with the exception that children with HL performed the task twice: once after 30 minutes of a speech-therapy like auditory training, and once after 30 minutes of a session of active rhythm. The two sessions were always separated by at least one week and their order was counterbalanced across children.

The rhythmic training comprised the following exercises: Follow the beat (balance the arms and walk with a metronome). Structure the beat into meter (listen and tap the strong beat with the feet and feel and then tap other beats clapping hands with therapist). Learn a new rhythm (listen to music and find a repetitive rhythm, move to it and tap using claves). Follow metric changes (listen to music and change body movement type with metrical changes). Body tapping (in absence of an external stimulus clap hands, tap chest, stop and keep internal beat).

Beat boxing (produce rhythmic patterns with mouth, alone and with therapist).

The auditory training comprised the following exercises: Timbre recognition across categories (recognize sounds belonging to different categories: animals, musical instruments, environmental sounds). Timbre recognition within categories (recognize sounds belonging to the same category: animals, musical instruments, environmental sounds). Sound sequences recognition across categories (recognize sequences of increasing length comprising sounds belonging to different categories). Sound sequences recognition within categories (recognize sequences of increasing length comprising sounds belonging to a same category). The level of difficulty in these exercises was manipulated by reducing the timbral difference between sounds. EEG data processing was also identical to Experiment 1 with the only exception that a second ICA was run on EEG epoched data to isolate and remove the potential artefacts generated by the cochlear implant or hearing aids (Campos Viola et al., 2012). Spatial ROI for MMN-like effect was the same as the NH children. Statistical analyses were also the same, with the only exception that the threshold for cluster statistic in time was set to 1.77, i.e. p-value ≤ 0.05 for a t-test with 13 degrees of freedom.

Processing and statistical analyses

In order to reduce the number of comparisons we only computed correlations on the correlated variables found in the NH group analysis, i.e. those with r > .40. Data from the Rhythmic and the Auditory sessions were averaged. The aim of this study was twofold. Firstly, we measured the speech rate convergence and temporal accommodation abilities in 6 to 10-year-old children during a turn-taking task with a virtual partner, and to bridge these abilities to the sensitivity to temporal deviancy as measured by an MMN-like effect. Secondly, we assessed whether a single 30-minute session of rhythmic training can enhance temporal prediction in children with hearing loss and, in turn, affect speech temporal skills that are relevant for a fluent conversation.

Results

Behavioral

Results show that both children with NH and children with HL converge towards the speech rate of a virtual partner. Moreover, in both children groups, stress placement is more consistent and accurate following regular VP turns compared to irregular turns. Most importantly, the rhythmic training session improves the accuracy of children with HL in placing the stress at irregular turns. In other words, rhythmic training seems to endow children with greater flexibility in adapting to a sudden change in the temporal context. Finally, both groups (NH-HL) show an MMN-like response to irregular turns of the virtual partner. This response is larger in children with HL following rhythmic training compared to the same children following auditory training. We will first discuss children convergence abilities by analysing the effects of the virtual partner's speech rate on children speech, then the effect of alternation regularity on children's temporal accommodation abilities and, in a second step, we will discuss the effects of rhythmic training on temporal accommodation capacities of children with HL.

Speech rate convergence with a virtual partner

One innovative aspect of this experimental design is that it allows, by manipulating the virtual partner's speech rate, to estimate the speech rate convergence, namely the natural tendency to minimize the speech rate difference between partners in a conversation [START_REF] Manson | Convergence of speech rate in conversation predicts cooperation[END_REF]Schultz, O 'brien, Phillips, & Mcfarland, 2016). Our results confirm previous evidence of this phenomenon in a real communication setup (R. L. [START_REF] Street | Speech convergence among talkative and reticent three year-olds[END_REF]; R. L. [START_REF] Street | Social and linguistic factors influencing adaptation in children's speech[END_REF] extending it to an interaction with a virtual non-adaptive partner and with single words. Importantly, we demonstrate speech rate convergence in a population of children with HL. While Freeman & Pisoni, (2017) found a less consistent convergence in cochlear implant users compared to normal hearing children, this could be due to important differences in the setup, which include using a natural and thus variable interlocutor (Freeman & Pisoni) versus a constant speech rate in the present study, and the use of a sentence repetition task versus a much easier object naming task. Thus, our experimental design, using an alternating naming task, seems particularly suitable to assess convergence abilities even in populations wherein these abilities may be impaired.

Temporal accommodation abilities and speech-turn regularity

The consistency of accent placement in regular turns for the two groups (NH-HL) corroborates our previous results [START_REF] Hidalgo | Speak on time! Effects of a musical rhythmic training on children with hearing loss[END_REF]. In that study, children had to name words whose accent patterns (number of syllables) could be identical or not to those of the virtual partner with which the children interacted; the turns of speech were thus more or less regular. Overall, children were consistent in placing word accents, especially when the words they had to produce had the same accent pattern as the words produced by the virtual partner.

In the present study, children in both groups (NH and HL) were more accurate during regular than irregular turns in placing the stress of the word at the « expected » moment, that is precisely in between two turns of the virtual partner. Interestingly, during regular turns, children tended to place the stress slightly before this expected moment. This is reminiscent of the negative asynchrony described in sensori-motor tapping tasks and has been interpreted in terms of anticipatory behaviour [START_REF] Repp | Sensorimotor synchronization: A review of the tapping literature[END_REF] that can be absent in other species [START_REF] Zarco | Subsecond Timing in Primates: Comparison of Interval Production Between Human Subjects and Rhesus Monkeys[END_REF][START_REF] Patel | The Evolutionary Biology of Musical Rhythm: Was Darwin Wrong?[END_REF][START_REF] Merchant | Are non-human primates capable of rhythmic entrainment? Evidence for the gradual audiomotor evolution hypothesis[END_REF]. By contrast, ERP results indicate that the auditory system of normal-hearing children was sensitive to the deviation from temporal expectations in speech turns, similarly to temporal deviations in rhythmic musical sequences [START_REF] Vuust | To musicians, the message is in the meter pre-attentive neuronal responses to incongruent rhythm are left-lateralized in musicians[END_REF]. Interestingly, because the VP was following a very slow pace (2000ms between words), temporal expectation possibly emerged from the subdivision of this slow regular pace by the child turns (~1000ms). This is supported by the correlation between the temporal accommodation capacities of the child and the amplitude of the MMN-like effect. Interestingly, the MMN-like measure was issued from a temporal cluster lasting up to ~500ms which may imply a role of attentive and conscious processing. In other words, the better the child consciously perceives a change in the regularity of the alternation, the better s/he can predict and adapt speech turns. This correlation of performance of normal hearing children and children with hearing loss between perception and production abilities suggests that anticipation and adaptation capacities during verbal interaction are tightly related to the sensitivity of the auditory system in processing the temporal structure of speech. The importance for speech accommodation of directing attention and adapting to

Effect of the rhythmic training session on speech perception and temporal accommodation abilities in children with HL

Children did not show a greater convergence on virtual partner speech rate after the rhythmic compared to the auditory training possibly because our design, with an artificial partner who keeps the same speech rate along the blocks, renders the speech rate very easy to reproduce. However, following only 30 minutes of rhythmic training, children with HL improved their performance in the turn-taking task. More precisely, they became more accurate in temporally placing the stress of the word following an irregular turn, wherein the virtual partner anticipates the turn. This is the condition that requires the greatest flexibility to quickly adapt to the anticipated turn. This result, coupled to the greater and longer-lasting effect on the ERPs to deviant turns, suggests that the rhythmic training has engendered a more robust and prompt processing of temporal deviations. This is in line with previous findings showing that musicians are more consistent and accurate than non-musicians in several types of temporal tasks, and are also more sensitive to temporal deviations occurring in a regular auditory sequence [START_REF] Drake | Reproduction of musical rhythms by children, adult musicians, and adult nonmusicians[END_REF][START_REF] Rammsayer | Temporal Information Processing in Musicians and Nonmusicians[END_REF][START_REF] Repp | Sensorimotor synchronization and perception of timing: Effects of music training and task experience[END_REF][START_REF] Yee | Sensitivity to event timing in regular and irregular sequences: Influences of musical skill[END_REF][START_REF] Jones | Sensitivity to time change: The role of context and skill[END_REF]. Moreover, musical expertise improves temporal processing not only in music but also in speech at different temporal scales [START_REF] Cameron | Enhanced timing abilities in percussionists generalize to rhythms without a musical beat[END_REF][START_REF] Geiser | Refinement of metre perceptiontraining increases hierarchical metre processing[END_REF][START_REF] Marie | Musicians and the metric structure of words[END_REF] [START_REF] Timm | Temporal Feature Perception in Cochlear Implant Users[END_REF]. Moreover, children with cochlear implant that practice regular singing show enhanced P3a-like responses to several sound feature modifications compared to children with cochlear implant that do not practice regular singing [START_REF] Torppa | Interplay between singing and cortical processing of music : A longitudinal study in children with cochlear implants Interplay between singing and cortical processing of music : A longitudinal study in children with cochlear implants[END_REF]. Similarly, in the present study, music training mostly affected the amplitude of the MMN-like component, which has been described as involved in attentional processes [START_REF] Gray | P300 as an index of attention to self-relevant stimuli[END_REF]). Thus, it seems that the rhythmic session has enhanced the children's ability to make accurate temporal predictions on the occurrence of regular turns. Then, small deviations from regularity may not only be automatically detected by the auditory system, but also actively processed to adapt speech behavior. This effect is not visible for the same children following 30 minutes of auditory training. This enhanced prediction is possibly mediated by a greater involvement of the motor and premotor cortex in musicians [START_REF] Krause | Functional network interactions during sensorimotor synchronization in musicians and non-musicians[END_REF] that in turn affects auditory perception [START_REF] Merchant | Finding the beat: a neural perspective across humans and non-human primates[END_REF]. In other words, the greater implication of the motor system in auditory perception improves the precision of temporal prediction of auditory events [START_REF] Morillon | Motor contributions to the temporal precision of auditory attention[END_REF][START_REF] Morillon | Motor origin of temporal predictions in auditory attention[END_REF]. Thus, active rhythmic training, involving body movement and sensorimotor synchronization, has possibly increased the involvement of motor system in temporal prediction, facilitating the detection of irregular turns and temporal accommodation. Moreover, the active rhythmic training proposed here stimulated not only the synchronization abilities on an external rhythm but also the interpersonal coordination (for instance via rhythm repetition and rhythm exchanges). This has possibly facilitated the realization of the picture naming alternation task as a joint action Rhythmic training improves temporal conversational abilities in children with hearing loss 21 to be realized in coordination with the partner. Interestingly, joint action planning compared with individual action planning can result in amplitude modulations of the P300 [START_REF] Kourtis | Predictive representation of other people's actions in joint action planning: An EEG study[END_REF].

To conclude, while the regularity in speech turns in our paradigm was exaggerated compared to natural conversation, one can imagine that the same mechanism is at work in both natural and controlled exchanges. Thus, rhythmic training could be a powerful tool to facilitate temporally accurate speech interaction, which in turn will improve speech coordination in conversation [START_REF] Konvalinka | Follow you, follow me: Continuous mutual prediction and adaptation in joint tapping[END_REF], and boost engagement in prosocial and cooperative attitudes [START_REF] Cirelli | Fourteen-month-old infants use interpersonal synchrony as a cue to direct helpfulness[END_REF][START_REF] Valdesolo | The rhythm of joint action: Synchrony promotes cooperative ability[END_REF].

  Rhythmic training improves temporal conversational abilities in children with hearing loss 4

  Rhythmic training improves temporal conversational abilities in children with hearing loss 6 Convergence was assessed by estimating changes of the speech rate in the fast compared to the slow condition. Accommodation was assessed by estimating changes in alternation in the irregular compared to the regular condition. Secondly, we tested the hypothesis that a single session of rhythmic training, by improving temporal prediction, may facilitate perception, convergence and accommodation to the temporal deviancy in a conversational context in children with HL.

  Rhythmic training improves temporal conversational abilities in children with hearing loss 8 While, as stated above, most utterances of the VP were presented at a regular temporal interval of 2000ms, ~20% of the words of the VP were anticipated at 1600ms (in both blocks) and pseudo-randomly distributed across trials.

  Rhythmic training improves temporal conversational abilities in children with hearing loss 10 expertise in music using a blind procedure. The average score across trials was used for further analyses.

  Rhythmic training improves temporal conversational abilities in children with hearing loss 11 permutation (N = 8,191), time clusters were defined on the basis of temporal adjacency by regrouping samples whose T-statistic was larger than 1.73, i.e. p-value ≤ 0.05 for a t-test with 19 degrees of freedom. Cluster-level statistics were then calculated by taking the sum of the T-values within the cluster. Only temporal clusters with corrected p-values ≤ 0.05 are reported.

  temporal deviancy of the virtual partner, i.e. with a large MMN-like effect, were also more consistent in the turn-taking task, i.e. with a high vector length (Regular r = .58, CI[.85, .14]; Irregular r = .44, CI[.73,.003]). MMN-like amplitude (MMN-like effect size in the significant cluster) was also positively correlated with the accuracy in stress placement during the Irregular intervals (r = .53; CI[.09,.85]). Finally, we also found a positive correlation between children consistency in the tapping task and the turn-taking task during Regular intervals (r = .50 CI[.07, .75]). The reproduction of rhythmic patterns did not show clear correlations with any of the other variables.Experiment 2

  Rhythmic training improves temporal conversational abilities in children with hearing loss 14 Behavioral analyses were identical to Experiment 1. We tested the effects of speech rate, regularity and training as well as all the interactions.

  results. As for NH children, the speech rate of the virtual partner affected the speech rate of children with HL, in both conditions of stimulation (auditory Training and rhythmic Training). More precisely, children's word durations were shorter in the fast condition compared to the slow condition after both Auditory training (β = -39.75, SE = 3.86, t = -10.27, p < .001) and Rhythmic training (β = -42.24, SE = 3.76, t = -11.22, p < .001). The interaction term did not improve the fit of the model (β = 2.22, SE = 5.67, t = 3.92, p = .695) (see Figure 3A and 3C). ==================== PLEASE INSERT FIGURE 3 AROUND HERE ==================== As for NH children, the regularity of alternation influenced stress placement consistency and accuracy of children with HL. Children with HL placed the stress more consistently in the Rhythmic training improves temporal conversational abilities in children with hearing loss 15 regular compared to the irregular trials (F(2.53) = 5.75, p = .005) (Figure 3B and 3D). They also placed word stresses closer to the expected time point in the regular trials compared to irregular trials (β = -0.66, SE = 0.03, t = -10.27, p <.001) (Figure 3B and 3D). While the interaction between speech rate and regularity did not improve the model significantly (p = 0.5), the interaction between training type and regularity of alternation significantly improved the fit of the model for accuracy data: the perturbation induced by irregular trials on accuracy of stress placement was reduced following Rhythmic training (β =-0.66, SE = 0.03, t = -17.61, p < .001) compared to Auditory training (β = -0.79, SE = 0.03, t = -21.39, p < .001 see Figure 3B). Electrophysiological results. Analyses on the peak showed a MMN-like effect following both Rhythmic Training and Auditory Training (t(1, 47)= -6.4654, p < .0001). The MMN peak amplitude effect did not differ across the two conditions of stimulation (t (1,13) = 1.1778, p = 0.26). By contrast, clusters analysis revealed a long lasting MMN-like effect from 239 to 469ms following Rhythmic Training (p <.05) (see figure 4B) while no cluster reached significance following Auditory Training (p > 0.4) (see Figure 4C).Correlation analyses revealed that children who were more sensitive to the temporal deviancy of the virtual partner (MMN-like effect size in the significant cluster) were also more accurate in the stress placement in the irregular turns (r = .42 CI [-.03, .65]).Rhythmic training improves temporal conversational abilities in children with hearing loss 16

  Rhythmic training improves temporal conversational abilities in children with hearing loss 18 during irregular turns, children were late in placing the word stress. During these (irregular) turns, the distance between the consecutive accents of the PV was suddenly shortened, which implied for the child a local readjustment of the internal temporal model of the alternation. According to the Adaptation and Anticipation Model (ADAM) model (van der Steen & Keller, 2013), interacting individuals generate temporal predictions on both their own and their partner's actions. This relies on an anticipation mechanism based on the temporal regularities of the partner's behavior and an adaptation mechanism allowing to adjust to local temporal (phase) variations. During the irregular turns, children need both the anticipation module to estimate the duration of the next interval and the adaptation module to make local corrections to adjust to the temporal deviation of the VP.

  Rhythmic training improves temporal conversational abilities in children with hearing loss 19 temporal deviations is particularly emphasized in deaf children after the rhythmic stimulation session.

  ; Chobert J, Marie C, François C, Schön D, 2011; Elmer, Meyer, & Rhythmic training improves temporal conversational abilities in children with hearing loss 20 Jäncke, 2012; Sares, Foster, Allen, & Hyde, 2018). When we turn to populations with hearing loss, cochlear implant users undergoing musical training show a greater sensitivity to fine temporal modification of sounds, as reflected in larger ERP responses compared to cochlear implant users without musical training

Figure 2 .

 2 Figure 2. Behavioral results for NH children. A. Convergence results. Effect of the virtual

Figure 3 .

 3 Figure 3. Behavioral results for HI children. A. Convergence results following a 30-min

Figure 4 .

 4 Figure 4. Electrophysiological (EEG) results. Grand average of Event Related Potentials for

F

Figure 1 .Figure 2 .Figure 3 .

 123 Figure 1. Task and procedure. A. Schematic representation of the task. The virtual partner pronounces isolated words at regular (blue, 2000 ms) or irregular (red, 1600 ms) intervals. The child names the objects presented on the screen in alternation with the virtual partner. The speech rate of the virtual partner can be fast (light blue, top panel) or slow (dark blue, bottom panel). The vertical arrows indicate the p-center placement of the words. IWI stands for Inter Words Intervals, namely the distance between the virtual partner p-center and the child p-center. WD stands for Words Duration, namely the duration of the words pronounced by the child. B. Schematic representation of the procedure. Normal hearing (NH) children ran the task once and children with hearing loss (HI) ran the task twice (order was counterbalanced). For all figures, the ear represents children with normal hearing, the maracas represent children with hearing loss after rhythmic training, and the loudspeaker represents children with hearing loss after auditory training. 340x209mm (300 x 300 DPI)

Figure 4 .

 4 Figure 4. Electrophysiological (EEG) results. Grand average of Event Related Potentials for regular and irregular trials. Time zero is the onset of the word pronounced by the virtual partner. Shadowed areas represent significant time clusters (p<.05). A. NH children. B. HI children after rhythmic training. C. HI children after auditory training. 340x209mm (300 x 300 DPI)

  TABLE 1 AROUND HERE Rhythmic training improves temporal conversational abilities in children with hearing loss

	F o r
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	13

Table 1 . Demographic data of children with HL

 1 Rhythmic training improves temporal conversational abilities in children with hearing loss 24 wearing a hearing device before implantation). Thresholds are given for each device (children wearing a cochlear implant without contralateral hearing aid like Child n°2, did not have any usable acoustic hearing in the unaided ear). Task and procedure. A. Schematic representation of the task. The virtual partner pronounces isolated words at regular (blue, 2000 ms) or irregular (red, 1600 ms) intervals.The child names the objects presented on the screen in alternation with the virtual partner.The speech rate of the virtual partner can be fast (light blue, top panel) or slow (dark blue, bottom panel). The vertical arrows indicate the p-center placement of the words. IWI stands for Inter Words Intervals, namely the distance between the virtual partner p-center and the child p-center. WD stands for Words Duration, namely the duration of the words pronounced by the child. B. Schematic representation of the procedure. Normal hearing (NH) children ran the task once and children with hearing loss (HI) ran the task twice (order was counterbalanced). For all figures, the ear represents children with normal hearing, the maracas represent children with hearing loss after rhythmic training, and the loudspeaker represents children with hearing loss after auditory training.

	F o r
	P e
	e r
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	w

. HA: hearing aid; CI: cochlear implant (in case of bilateral implantation, n°1 means the duration of use of the 1st implant and n°2 the duration of use of the second contralateral implant, HA in parenthesis means the onset of

Journal of Speech, Language, and Hearing Research

Acknowledgments

Research supported by grants of the Fondation Agir pour l'Audition (APA RD-2016-9), ANR-16-CE28-0012 RALP, ANR-16-CONV-0002 (ILCB), ANR-11-LABX-0036 (BLRI) and the Excellence Initiative of Aix-Marseille University (A*MIDEX). We wish to thank Romane Pradels and Laura Leone for their contribution in speech data analysis.