Lise Hobeika
email: lise.hobeika@gmail.com

Marine Taffou

Thibaut Carpentier

Olivier Warusfel

Isabelle Viaud-Delmon

Capturing the dynamics of peripersonal space by integrating expectancy effects and sound propagation properties

Keywords: multisensory integration, audio-tactile coding, looming sound, auditory space, expectancy effects

come   L'archive ouverte pluridisciplinaire

Introduction

The space around the body, called peripersonal space (PPS), is selectively encoded in the brain by a neural network linked to multisensory integration processes [START_REF] Bernasconi | Audiotactile and peripersonal space processing around the trunk in human parietal and temporal cortex: An intracranial EEG study[END_REF][START_REF] Colby | Ventral intraparietal area of the macaque: anatomic location and visual response properties[END_REF][START_REF] Farnè | Auditory peripersonal space in humans[END_REF][START_REF] Gentilucci | Functional organization of inferior area 6 in the macaque monkey -I. Somatotopy and the control of proximal movements[END_REF][START_REF] Graziano | Parieto-frontal interactions, personal space, and defensive behavior[END_REF][START_REF] Graziano | A bimodal map of space: somatosensory receptive fields in the macaque putamen with corresponding visual receptive fields[END_REF][START_REF] Làdavas | Visuo-tactile representation of nearthe-body space[END_REF]. Whereas PPS multisensory encoding has been examined in monkeys and patients with neurophysiological invasive methods, developing a behavioral method to study PPS in healthy humans is not straightforward [START_REF] Cléry | Frontier of self and impact prediction[END_REF]. Following a large literature describing behavioral effects of multisensory integration as a function of the spatial proximity of sensory stimuli to the body [START_REF] Ladavas | Auditory Peripersonal Space in Humans: a Case of Auditory-Tactile Extinction[END_REF][START_REF] Maravita | Multisensory integration and the body schema: Close to hand and within reach[END_REF], Canzoneri and colleagues' developed an audiotactile method to study PPS in humans [START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF]. In this method, participants perform a speeded tactile detection task while an irrelevant sound is looming towards them from the frontal hemifield. The tactile stimulus is delivered at the beginning of the sound, i.e. when the sound source is far from the participant body, or at the end of the sound, i.e. when the sound source is near the participant body, or at intermediate positions in space. The analysis of participants' tactile detection time as a function of the sound source position in space gives information on PPS morphometry.

This method has several assets. First, it is an implicit measure: participants are not aware that the sound position can modulate their reaction times (RTs). Second, the method rests on multisensory RTs facilitation effects with looming stimuli. Thus, the protocol is based on multisensory integration behavioral effects that have been extensively studied [START_REF] Hershenson | Reaction time as a measure of intersensory facilitation[END_REF][START_REF] Spence | Crossmodal links in exogenous covert spatial orienting between touch, audition, and vision[END_REF][START_REF] Sumby | Visual Contribution to Speech Intelligibility in Noise[END_REF] and on moving looming stimuli, which are particularly relevant for the study of PPS. PPS multisensory neurons in monkeys are indeed very reactive to movements, and especially to movements looming towards the body [START_REF] Colby | Ventral intraparietal area of the macaque: anatomic location and visual response properties[END_REF][START_REF] Graziano | A bimodal map of space: somatosensory receptive fields in the macaque putamen with corresponding visual receptive fields[END_REF][START_REF] Rizzolatti | Afferent properties of periarcuate neurons in macaque monkeys. I. Somatosensory responses[END_REF]. The relevance of looming stimuli has also been evidenced in humans [START_REF] Cléry | Impact Prediction by Looming Visual Stimuli Enhances Tactile Detection[END_REF][START_REF] Kandula | Visuo-tactile interactions are dependent on the predictive value of the visual stimulus[END_REF][START_REF] Tajadura-Jiménez | Embodied auditory perception: The emotional impact of approaching and receding sound sources[END_REF][START_REF] Van Der Biest | Watching what's coming near increases tactile sensitivity: An experimental investigation[END_REF]. Lastly, this method measures multisensory facilitation at multiple positions in space, which gives a higher spatial resolution of PPS morphometry. However, the method has some limitations linked to expectancy effects and the involvement of auditory dynamic cues.

In Canzoneri's paradigm, a tactile stimulus is presented during a three second sound in the majority of the experimental trials, and there is no more than one tactile stimulus per sound. Thus, there is a high probability that a tactile event occurs during the duration of the sound. If no tactile event has arrived yet after a two-second duration of sound, the probability that a tactile event occurs in the last second of the sound is even higher. Therefore, the probability of the occurrence of a tactile event evolves during the duration of the sound with the delay from the sound onset. As the distance of the sound from the body also evolves with the delay from sound onset, the temporal expectancy effect is entangled with the changes in sound distance. Consequently, it is not possible to understand the respective role of expectation and sound distance in the behavioral effects observed in the presence of the looming sounds. A study by Kandula and colleagues focused on the contribution of tactile expectancy on RTs in Canzoneri's method [START_REF] Kandula | On the contribution of overt tactile expectations to visuo-tactile interactions within the peripersonal space[END_REF]. They demonstrated that when the probability to receive a tactile stimulation at each trial is high, the large change in expectancy during a trial impacts RTs and masks the multisensory integration effects. It is important to note that expectation is not entirely responsible for the results on PPS obtained with this protocol. If RTs variations were explained only by expectancy effects, the same results should be found for looming and receding sounds which is not the case [START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF][START_REF] Serino | Body part-centered and full body-centered peripersonal space representation[END_REF].

A solution to help disentangling the contribution of expectation from audiotactile integration effects on RTs is to add a baseline, in which there is expectancy but little audiotactile integration. Measuring tactile RTs using a sound fixed in space and located far from participants' body gives a temporal reference for participants to expect a tactile stimulation. In this case, the observed behavioral effects on RTs cannot be related to a modulation of sound distance, hence not to multisensory integration.

Localizing a sound in 3D space is a complex cognitive process that takes into account a wide variety of acoustic parameters. Two of the main acoustic cues for the perception of distances are sound level change and reverberation [START_REF] Shinn-Cunningham | Distance Cues for Virtual Auditory Space[END_REF][START_REF] Zahorik | Auditory Distance Perception in Humans : A Summary of Past and Present Research[END_REF]. Sound intensity globally decreases when sound source distance increases. In open-air condition, the sound intensity follows an inversesquare law, which implies an intensity decrease of 6 dB per doubling of source distance. The sound intensity logarithm (in dB) varies linearly with the logarithm of distances. The sound level is a relative cue for distance perception: it does not allow an estimation of the absolute distance to the source, as a modulation in sound level could be due to a moving sound source or a to decrease of the intensity of the emitted sound. Distance perception is also based on the ratio between the levels of the reverberated and of the direct sound, called direct-to-reverberant sound energy ratio. Sound reverberation is caused by the multiple reflections of the acoustic waves occurring on obstacles and space boundaries, and creates a diffuse surrounding sound field. In contrast with the level of the direct sound, in a closed space, the level of the reverberated sound field does not depend on the distance to the source. Consequently, the direct-to-reverberant sound energy gives an absolute cue on the sound-source distance. Combining these two cues, participants are usually good at comparing the position of two sound sources in space, but are not highly accurate in the evaluation of absolute distances [START_REF] Kolarik | Auditory distance perception in humans : a review of cues , development , neuronal bases , and effects of sensory loss[END_REF][START_REF] Zahorik | Auditory Distance Perception in Humans : A Summary of Past and Present Research[END_REF].

Overall, distance perception is mainly based on direct sound level, which varies logarithmically with sound distances. Studies on source distance perception usually test distances distributed regularly on a logarithmic scale (e.g. [START_REF] Alais | Synchronizing to real events: subjective audiovisual alignment scales with perceived auditory depth and speed of sound[END_REF][START_REF] Fontana | Auditory distance perception in an acoustic pipe[END_REF][START_REF] Zahorik | Loudness constancy with varying sound source distance[END_REF]. However, to our knowledge, PPS studies systematically test distances distributed linearly. With this method, the perceptual differences between distances are highly irregular. For example, there are small acoustic differences when a sound source moves from 100 cm to 90 cm from the observer, but there is a larger one for a movement from 20 cm to 10 cm. Thus, using distances spaced regularly on a logarithmic scale should be more relevant to study the differences in multisensory integration linked to PPS, as it equalizes the perceptual differences between each tested distance. A logarithmic based distances distribution should provide a better sampling of the dynamics of multisensory integration processes in space.

Lastly, we want to obtain an efficient description of the evolution of audiotactile integration in space. Some studies used a sigmoidal fitting to describe RTs evolution with sound source distance in space (e.g. [START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF][START_REF] Ferri | Emotion-inducing approaching sounds shape the boundaries of multisensory peripersonal space[END_REF][START_REF] Taffou | Cynophobic Fear Adaptively Extends Peri-Personal Space[END_REF], some used a linear fitting (e.g. [START_REF] Cardini | Enlarged representation of peripersonal space in pregnancy[END_REF][START_REF] Salomon | Unconscious integration of multisensory bodily inputs in the peripersonal space shapes bodily self-consciousness[END_REF] and others did not perform data fitting (e.g. [START_REF] Hobeika | Social coding of the multisensory space around us[END_REF]Noel et al., 2015b;[START_REF] Serino | Body part-centered and full body-centered peripersonal space representation[END_REF]. As there is no clear consensus on the most suited manner to describe and fit this kind of data, it would be interesting to examine audiotactile integration effects on RTs as a function of the sound source distance when auditory distance and expectation mechanisms are taken into account.

The aim of the present study is to provide a better description of how multisensory integration is modified along PPS, using Canzoneri's and colleagues audiotactile task [START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF]. First, to measure the impact of expectancy effect on RTs during a trial, we added a condition in which the sound source is fixed in space and located at the starting distance of the sound source of the looming sound. Tactile stimulations occurred at the same delays from sound onset in both the fixed sound and the looming sound conditions. Second, we modified the distribution of tested distances in space. As auditory perception in depth strongly involves loudness, which rests on a logarithmic process, it should be more relevant to test audiotactile integration at distances spaced regularly on a logarithmic scale. To test this hypothesis, participants completed two sessions of test: one with distances spaced regularly on a linear scale, and one with distances spaced regularly on a logarithmic scale. Finally, we tried to find a simple fitting representing the evolution of RTs with sound distances. Additionally, we introduced a test at the end of the experiment, in which we assessed participants' emotional response to the looming sound according to its position in depth. This test was based on the Behavioral Assessment Test (BAT), which is widely used in clinical psychology to assess the level of fear of patients in response to a phobic object that is getting closer and closer to them (e.g. [START_REF] Lang | Experimental desensitization of a phobia[END_REF][START_REF] Van Bockstaele | Effects of attention training on self-reported , implicit , physiological and behavioural measures of spider fear[END_REF]. In the present study, the test was an auditory BAT (aBAT) that examined how the emotional value attributed to the sound distance evolves according to a logarithmic and linear repartition of distances. We used the aBAT as an indirect assessment of the perceived position of the sound source in space.

Material and methods

Participants.

Nineteen healthy individuals (12 females, age 24.7 ± 3.6) with normal audition and touch took part in the study. Sample size was decided a priori based on previous studies using similar audiotactile paradigms [START_REF] Serino | Body part-centered and full body-centered peripersonal space representation[END_REF][START_REF] Taffou | Cynophobic Fear Adaptively Extends Peri-Personal Space[END_REF]. All of them were right-handed. As PPS is linked to handedness [START_REF] Hobeika | Anisotropy of lateral peripersonal space is linked to handedness[END_REF], participants' handedness was verified with a questionnaire measuring skilled hand preference. The scores on this questionnaire, called the Flinders Handedness survey (FLANDERS) [START_REF] Nicholls | The flinders handedness survey (FLANDERS): A brief measure of skilled hand preference[END_REF], range from -10 for strong left-handed individuals to +10 for strong right-handed individuals. Participants received a financial compensation of 10€/hour for their participation. They provided a written informed consent prior to the experiment, which was approved by the Institutional Review Board of the French National Institute of Health and Medical Research (INSERM, IRB00003888).

Apparatus.

Participants sat on a chair in a soundproofed room. Both of their hands were palms-down on a table, in contact with their body and aligned with their mid-sagittal plane. A black fabric hid participants' hands. Participants were equipped with Beyer Dynamic DT770 headphones. To control for the visual stimulation and gaze direction, participants were instructed to fix a permanent visual cross located at 150 cm in front of them at eye level.

The auditory stimuli were sounds composed of bursts train (44100 Hz digitization). The bursts train consisted of a succession of Gaussian white noise bursts equalized in intensity (created with Matlab®). Each burst lasted 30ms with 10ms rise and fall times. The time interval between two bursts was 65ms. For the purpose of the experiment, two types of spatialized auditory stimulus were created based on this bursts train sound: static auditory stimuli (fixed sounds) and dynamic auditory stimuli (looming sounds). In order to simulate spatialized auditory sources, the bursts train sound was processed through binaural rendering with the Spat library [START_REF] Carpentier | Twenty years of Ircam Spat: looking back, looking forward[END_REF] running in Max/MSP (6.1.10). This spatialization processor uses an artificial reverberation engine to produce a perceptually plausible virtual acoustic environment. The simulated room effect consisted of the direct sound, early reflections, and a late reverberation tail (reverberation time: 200ms). Table 1 describes the sound attenuation with distance used in this experiment, where direct sound intensity follows an inverse square law. Direct sound and early reflections were binaurally rendered using non-individual head related transfer functions (HRTF) taken from the LISTEN HRTF database (http://recherche.ircam.fr/equipes/salles/listen/). For both types of auditory stimuli, we simulated a sound source located in the frontal hemifield in the right hemispace: the direct sound was spatialized with azimuth -60°, and the early reflection pattern arrived from a cone (with aperture 40°) centered around this direction. The reverberation tail was spatially diffuse. With this procedure, the virtual sound source location can be manipulated by rendering accurate auditory cues such as frequency spectrum, intensity, and inter-aural differences. Extra time was left after the last burst to account for the reverb tail, resulting in sounds lasting 3250ms.

All participants confirmed that they could clearly locate sound sources in the right hemispace. For the fixed sounds, the virtual sound source was static and located at 640cm from participants' head center, at ear level. For the looming sound, the virtual sound source was approaching participants' head center from 640cm to 20cm, at ear level and at a constant speed (210 cm.s -1 ).

The tactile stimulus was a vibratory stimulus delivered by means of a 28mm miniature loudspeaker on the palmar surface of the left index finger of participants. A sinusoid signal was emitted for 20ms at 250 Hz. With these parameters, the vibration of the loudspeaker was perceivable, but the sound was inaudible. A PC running Presentation® software was used to control the presentation of the stimuli and to record the responses. 1

Distances (cm

: Evolution of the direct sound and overall room effect levels (dB) as a function of the distances. The distances specifed

here correspond the distances targeted in the audiotactile experiment.

Design and procedure.

The experiment was composed of two experimental sessions.

Participants completed the two sessions on two different days.

The first session consisted of an audiotactile test. The second session consisted of an audiotactile test followed by three auditory Behavioral Assessment Tests (aBATs). The aim of the audiotactile tests was to evaluate the advantages of our protocol modifications, in terms of expectancy assessment and pertinence of measures, for the study of audiotactile integration behavioral consequences with sound distances. The aim of the aBATs was to indirectly assess the perceived position of the sound sources by examining the emotional value attributed to the sound according to its distance from participants.

Audiotactile tests

At the beginning of the experimental session, participants were asked to place their left index finger on the vibrator and instructed to press a button with their right finger each time they detected a tactile stimulus. For each trial, an auditory stimulus (either a fixed or a looming sound) was presented for 3250ms. The auditory stimulus was preceded by 300ms of silence. A period of silence, with a duration randomly varying between 700 and 1100ms, also occurred after the offset of the sound. In 66.6% of the trials (experimental trials), a tactile stimulus was presented either during the sound or during the period of silence. The remaining 33.3% trials were catch trials with auditory stimulation only. Participants were instructed to ignore the auditory stimuli and to respond as quickly as possible to the tactile stimulation. They were asked to emphasize speed, but to refrain from anticipating. RTs were measured.

Figure 1:Experimental set-up.

Participants performed the audiotactile task by responding to a tactile stimulation on their hand while a task-irrelevant spatialized sound was presented to them in their right hemispace. The sound could be moving towards them from 640 cm to 20cm distance (looming sound), or be static and located at 640cm from the center of the participants' head (fixed sound).

During experimental trials, tactile stimulations were delivered at different delays starting from sound onset. We presented the tactile stimulation 10ms after the burst onset whichever burst among the 33 bursts of the sound was targeted in order to apply the desired delay. When the auditory stimulus presented during the trial was a fixed sound, the distance of the virtual sound source was 640cm when tactile stimulation occurred, regardless of the delay applied between sound onset and tactile stimulation. In contrast, when the auditory stimulus presented during the trial was a looming sound, the distance of the virtual sound source when tactile stimulation occurred depended of the delay applied between sound onset and tactile stimulation -far distances for low temporal delays and near distances for high temporal delays -(see Figure 1).

Two sets of six distances with different spatial distributions in depth were tested in two separated experimental sessions: a set with a logarithmic distribution of distances (640cm, 320cm, 160cm, 80cm, 40cm, 20cm) and a set with a linear distribution of distances 1 (640cm, 520cm, 400cm, 260cm, 140cm, 20cm). The temporal delays used for tactile stimulus delivery and the corresponding distances of the looming sound source at delivery are described in Table 2 for both distances distributions. The evolution of sound level with the approach of the sound source is described on Figure 2, panels a and d. Values are given with reference to the direct sound level at the closest distance, i.e. 20cm. According to the direction of incidence, the sound pressure level delivered to the ears of the participant ranged from 42 to 69dBA and from 45 to 73dBA, for the contralateral and the ipsilateral ears respectively. As depicted by the panels b and e of Figure 2, whereas the difference of sound level between distance conditions are identical when the distances tested are distributed logarithmically, this is not the case when the distance conditions are distributed linearly.

In order to measure RTs in the unimodal tactile condition (without any sound), tactile stimulations were also delivered during the silent periods, preceding or following sound administration, namely at -250ms (Tbefore) and at 3500ms (Tafter) from sound onset (see a temporal description of trials in the logarithmic experimental session and in the linear experimental session in Figure 2c and 2f respectively).

Table 2: Correspondence between the delays of tactile stimulation and sound source distances. Description of the

delays of tactile stimulation from sound onset used in each distances distribution session, and the corresponding distances for bimodal trials, for the looming sound and the fixed sound.

Each participant completed both experimental sessions: one with a logarithmic distances distribution and one with a linear distances distribution. The order of the sessions was counterbalanced between participants. Both sessions were realized in two different days, separated by a maximum of three days. In both experimental sessions, trials were equally divided in 8 blocks of 72 trials, lasting about 5 min each. Each block contained 48 trials with a tactile target, randomly intermingled with 24 catch trials. Thus, in total, there were 384 catch trials without tactile stimulation and tactile RTs were measured in 768 trials corresponding to 24 repetitions in each of 32 conditions: 2 DISTANCES DISTRIBUTION (logarithmic/linear) * 2 SOUND MOUVEMENT (looming/fixed) * 8 DELAY (Tbefore, T1, T2, T3, T4, T5, T6 and Tafter).

Auditory Behavioral Assessment Tests

We compared the emotional appraisal of the sound according to its position in space in three different aBATs: the Logarithmic aBAT, the Linear aBAT and the Fixed aBAT.

Participants were seated on a chair and equipped with Beyer Dynamic DT770 headphones. They were instructed to look at the fixation cross located at 150 cm in front of them, at eye level. During each aBAT, a sound was played continuously through participants' headphones. In the Logarithmic aBAT the sound source was moving and stoped according to logarithmically determined position. In the Linear aBAT, the sound source was moving and stoped according to linearly determined position. In the Fixed aBAT, the sound source was fixed. Each aBAT consisted of six steps. The sound distance at each step corresponded to the sound distance of the corresponding audiotactile tests. In the Logarithmic aBAT, sound source distances at each step were spaced regularly on a logarithmic scale (640cm, 320cm, 160cm, 80cm, 40cm, 20cm); in the Linear aBAT, sound source distances at each step were spaced regularly on a linear scale (640cm, 520cm, 400cm, 260cm, 140cm, 20cm); and in the Fixed aBAT, the sound source was static, located at 640cm at each step. The emotional experience induced by the sound as a function of its distance was assessed with Subjective Units of Distress (SUD; [START_REF] Wolpe | The practice of behavior therapy[END_REF]. SUD is a self-report typically used for measurement of experienced fear or discomfort, which has been shown to correlate with physiological measures of arousal state [START_REF] Thyer | Autonomic correlates of the subjective anxiety scale[END_REF]. At each step of the aBATs, participants had to rate their level of discomfort with SUD on a scale from 0 to 10 -0 corresponding to an absence of discomfort and 10 to the worst discomfort possible -. Between each step, the sound source moved from the previous position to the targeted position at a constant speed during 2 seconds. Then, 500ms after the sound source reached its targeted position, a sound signal was played to indicate participants that they had to rate their level of discomfort. The sound source stays still at the targeted position until the participant answers. The order of presentation of the Logarithmic aBAT and Linear aBAT was counterbalanced between participants; the Fixed aBAT was always presented at the end.

Results

One participant was excluded from analysis due to a high rate of misses in the audiotactile test (23.8% of miss, m±sd of the sample: 2.6 ± 2.0 % of miss). There were eighteen remaining subjects (11 females, age 25.0 ± 3.5). All remaining participants were right-handed, as verified by their FLANDERS score (range: from 6 to 10; M ± SD = 9.6 ± 1.1).

Audiotactile tests

The performances were analyzed in terms of RTs. We considered as a valid answer all RTs between 100 and 1000ms after stimulus onset. For the analyses, RTs were averaged for each subject and for each of the 32 conditions separately (2 DISTANCES DISTRIBUTION * 2 SOUND MOVEMENT * 8 DELAYS).

In the first analysis, we included the factor DISTANCES DISTRIBUTION to verify that the different spatial distributions of the tested sound distances impacted RTs. Indeed, if RTs speed up effects were only due to expectancy effects, there should not be any effect of the factor DISTANCES DISTRIBUTION. Including this factor in the analysis allows appreciating that the spatial distribution of distances impacts RTs, and that analyzing separately the data of the Logarithmic and Linear distances distribution sessions is meaningful. Thus, we first conducted an ANOVA on the mean RTs, with the within-subject factors DISTANCES DISTRIBUTION (two levels: logarithmic, linear), SOUND MOVEMENT (two levels: fixed, looming), and the factor DELAY (eight levels: Tbefore, T1, T2, T3, T4, T5, T6, and Tafter).

Analysis revealed a significant main effect of SOUND MOVEMENT (F (1,17) suggesting that RTs were impacted differently by the sound movement, depending on the delay and the tested distances.

After this verification, 1) we evaluated potential expectancy effects, 2) we evaluated the contribution of multisensory integration effects separately on both distances distribution conditions.

Assessment of the expectancy with the fixed sound condition

We tested whether the RTs in the fixed sound condition can be used as a baseline to measure the tactile expectancy effects. To this aim, we first compared RTs on unimodal trials to bimodal ones in the fixed sound condition, in both distances distributions. We then analyzed if the movement of the sound impacted unimodal tactile detection when it was presented after the sound. We performed these analysis using post-hoc tests in accordance with the significant three-way interaction DISTANCES DISTRIBUTION x MOVEMENT x DELAY described in the previous paragraph. Finally, we fitted the bimodal data of the fixed sound condition as a function of the delay to have a description of the evolution of expectancy effects.

Comparison of unimodal trials and bimodal trials for fixed sound trials. In both distances distribution conditions, RTs occurring at Tbefore were significantly slower than RTs occurring at T2, T3, T4, T5, T6 and Tafter (logarithmic distances distribution: Post-hoc Newman-Keuls' test: p <.001 in all cases; linear distances distribution: Post-hoc Newman-Keuls' test: p <.05 in all cases). RTs occurring at Tafter delay were significantly faster than RTs at Tbefore, T1 and T2 (logarithmic distances distribution: Post-hoc Newman-Keuls' test: p <.001 in all cases; linear distances distribution: Posthoc Newman-Keuls' test: p <.05 in all cases) (see Figure 3). These results indicate that there was a decrease of RTs with the factor delay, likely due to expectancy effects. There was no significant difference between Tbefore and T1, and between T6 and Tafter, suggesting that there was no audiotactile integration with the fixed sound, located far from the body (640cm).

Effect of sound movement on unimodal trials after the sound: for both distances distributions, sound movement did not significantly impact RTs at Tafter (logarithmic distances distribution: Post-hoc Newman-Keuls' test: p= 0.4, Tafter_fixed_sound: 343.8 ± 9.1 ms, Tafter_looming_sound: 335.7 ± 6.7 ms ; linear distances distribution: Post-hoc Newman-Keuls' test: p= 0.3, Tafter_fixed_sound: 346.4 ± 11.6 ms, Tafter_looming_sound: 335.1 ± 10.1 ms). These results suggest that the presence of a fixed or a looming sound had no post-effect on the detection of the unimodal tactile stimulation at Tafter.

Description of the expectancy effects.

In order to describe the effects of expectancy on tactile RTs, we fitted RTs from the bimodal trials in the fixed sound condition with two different functions. We hypothesized that the evolution of tactile RTs as a function of the delay of tactile delivery could follow a linear decrease or a logarithmic decrease, depending on the distances distribution. To test this hypothesis, we plotted participants' RTs in the bimodal trials in the fixed sound condition as a function of the delay of tactile delivery, in the logarithmic and linear distances distribution sessions. Then, we fitted a linear and a logarithmic function to the data, separately for each participant. We used, as the logarithmic function y(x) = a.log(x) + b, where x represented the independent variable (i.e. the delay of tactile stimulation from sound onset), y the dependent variable (i.e. the RTs), a the slope and b the ordinate for log(x) = 0, i.e. for x=1. The linear function we used, is described by y(x) = a.x + b where a is the slope and b is the intercept at x = 0. As both functions contain two parameters, we could compare the quality of the fitting by directly comparing the root mean square errors values (RMSE) for each participant.

For the logarithmic distances distribution, analysis revealed that the logarithmic function was significantly better to describe participants' RTs than the linear function (t(17) = -2.27, p <.05, two-tailed, RMSE logarithmic_fitting = 9.31, RMSE Linear_fitting = 10.48) (see Figure 3). For the linear distances distribution, both functions gave the same performance at describing participants' RTs (p >.05, twotailed, RMSE logarithmic_fiting = 11.1, 95% CI = [9.1, 13.2], RMSE Linear_fitting = 11.2, 95% CI = [9.1, 13.3]).

We then compared the intensity of the expectancy effects for both distances distributions. To this aim, we compared the parameters obtained with the logarithmic decrease fitting for both distances distributions. We found that both parameters a and b were impacted by the distances distribution (parameter a: t(17) = -2.44, p <.05, two-tailed, a Logarithmic_distribution = -24.6, a Linear_distribution = -15.1; parameter b: t(17) = -2.10, p = .05, two-tailed, b Logarithmic_distribution = 541.0, b Linear_distribution = 496.5). Parameters a and b of the logarithmic function were significantly larger in the logarithmic distances distribution than in the linear one. These findings indicate that the evolution of the expectancy effect with time depended on the tested delays of tactile stimulation from sound onset.

Figure 3. Analysis of the expectancy effect. This figure reports the mean tactile RTs (±SEM) as a function of the delay of tactile

stimulation delivery from the trial beginning, in presence of a sound fixed in space (located at 640cm from participants' head center). Tactile stimulation occurred alone in the unimodal trials (Tbefore, Tafter), or occurred in presence of a sound in the bimodal trials (T1, T2, T3, T4, T5 and T6). The shaded region indicated the duration of the sound. RTs are fitted with a decreasing logarithmic function on both figures.

Evaluation of the audiotactile integration effect on RTs in the linear and logarithmic distances distributions

We analyzed in a first ANOVA the effects of distances distribution, sound movement and delay on RTs (see 3.1). Considering that the distances distribution effect significantly interacted with the effects of sound movement and delay on tactile RTs, we analyzed separately the data from the two experimental sessions (logarithmic and linear distances distribution sessions).

Logarithmic distances distribution

Logarithmic distances distribution.

We conducted an ANOVA on the mean RTs of the Logarithmic distances distribution on bimodal trials only, with the within-subject factors SOUND MOVEMENT (two levels: fixed, looming), and the factor DELAY (six levels: T1, T2, T3, T4, T5 and T6). Analysis revealed a significant main effect of SOUND MOVEMENT (F (1,17) = 76.3, p < .001, η p 2 = .818), and a significant main effect of DELAY (F (5,85) = 49.2, p < .001, η p 2 = .743). The analysis also revealed that the two-way interaction SOUND MOVEMENT x DELAY was significant (F (5,85) = 18.2, p < .001, η p 2 = .518), indicating that tactile RTs variation with the temporal delay of tactile delivery from sound onset depended on the movement of the sound (see Figure 4

left figure).

We compared RTs as a function of the presence of the looming or the fixed sound, for each bimodal delay. RTs were significantly faster when the tactile stimulus occurred in presence of a looming sound than in presence of a fixed sound at the delays T3, T4, T5 and T6 (Post-hoc Newman-Keuls' test: p < .001 in all cases). There were no significant difference between RTs in the fixed and looming sound conditions for the delays T1 and T2 (Post-hoc Newman-Keuls' test: p = .61 and p= .15 respectively).

Figure 4. Evaluation of the audiotactile effect on RTs on the linear and logarithmic distances distributions. This figure reports

the mean tactile RTs (±SEM) in presence of the fixed sound (dashed line) or the looming sound (solid line), as a function of the delay of tactile stimulation delivery from the trial beginning. Data of the logarithmic distances distribution session are represented in the left graph, and data for the linear distances distribution session in the right graph. Asterisks indicate significant differences in RTs between fixed sound and looming sound (***p<0.001). On the logarithmic distances distribution, RTs started to be significantly boosted by the proximity of the sound at T3, whereas on the linear distances distribution this significant boost appeared only at T6.

Linear distances distribution.

We conducted an ANOVA on the mean RTs of the Linear distances distribution only, with the within-subject factors SOUND MOVEMENT (two levels: fixed, looming), and the factor DELAY (six levels: T1, T2, T3, T4, T5 and T6). Analysis revealed a significant main effect of SOUND MOVEMENT (F (1,17) = 5.45, p < .05, η p 2 = .243), and a significant main effect of DELAY (F (5,85) = 32.1, p < .001, η p 2 = .654). The analysis also revealed that the twoway interaction SOUND MOVEMENT x DELAY was significant (F (5,85) = 6.81, p < .001, η p 2 = .286), indicating that tactile RTs variation with the temporal delay of tactile delivery from sound onset depended on the movement of the sound (see Figure 4

right figure).

We compared RTs in the presence of the looming sound to RTs in presence of the fixed sound, for each bimodal delays. RTs were significantly faster when the tactile stimulus occurred in presence of a looming sound compared to a fixed sound at the delay T6 (Post-hoc Newman-Keuls' test: p < .001). There were no significant difference between RTs in the fixed and looming sound conditions for the delays T1, T2, T3, T4 and T5 (Post-hoc Newman-Keuls' test: p >.05 in all cases). Description of the spatial dynamic of audiotactile integration behavioral consequences. We wanted to find the best and simplest description of audiotactile integration behavioral impact on tactile RTs as a function of the distance of the auditory source in space, in order to provide better tools to evaluate and investigate the phenomenon. To isolate the impact of audiotactile integration from expectancy effects on RTs, we subtracted RTs in the fixed sound condition from RTs in the looming sound condition for each participant, for each distances distribution, and for each delay. We hypothesized that RTs evolution would follow a logarithmic

Logarithmic distances distribution

Linear distances distribution law when plotted as a function of the sound source distances from the body. Thus, RTs would vary linearly as a function of the logarithm of the sound source distances. To test this hypothesis, we fitted logarithmic and linear functions into RTs data of the logarithmic and linear distances distribution sessions plotted as a function of the distance of the auditory source at tactile delivery time. We used as the logarithmic function: y(x) = a.log(x)+ b, where x represented the independent variable (i.e. the distances or logarithm of distances), y the dependent variable (i.e. the RTs), a the slope and b the ordinate for log(x) = 0, i.e. for x=1. The linear function we used is described by y(x) = a.x + b where a is the slope and b is the intercept at x = 0. As both functions contain two parameters, we can compare the quality of the fitting by directly comparing the root mean square errors values (RMSE).

We fitted RTs data as a function of sound source distances for the logarithmic and linear distances distribution sessions (see Figure 5, a andb). For both distances distributions, analysis revealed that the logarithmic function better described participants' data than linear function (logarithmic distances distribution: t(17) = -2.16, p <.05, two-tailed, RMSE Logaithmic_fitting =12.7, RMSE Linear_fitting = 14.4; linear distances distribution: t(17) = -2.24, p <.05, twotailed, RMSE Logarithmic_fittingl = 17.3, RMSE Linear_fitting = 19.3). Thus, the logarithmic function y(x) = a.log(x) + b seems to be a better description of RTs evolution than the linear function. Moreover, following these results, data from both conditions can be described by a linear relation between RTs and the logarithm of sound source distances as it is illustrated on Figure 5, c andd.

Auditory Behavioral Assessment Tests

We conducted an ANOVA on participants SUDs, with the within-subject factors DISTANCES DISTRIBUTION (three levels: logarithmic, linear, fixed), and STEP (six levels: S1, S2, S3, S4, S5 and S6). The analysis revealed a significant main effect of DISTANCES DISTRIBUTION (F (2,38 = 28.9, p < .001, η p 2 = .604), of STEP (F (5,95 = 67.9, p < .001, η p 2 =

.781) and a significant interaction between DISTANCES DISTRIBUTION x STEP (F (10,190) = 27.6, p < .001, η p 2 = .592), indicating that SUDs were differently impacted by the steps as a function of the sound source distances distribution. c andd). Data of the fixed aBAT are plotted as a function of the steps number, as in this aBAT the sound source distance is fixed at 640cm. Asterisks indicate significant differences in SUDs between distances condition (*p<0.05, **p<0.01, ***p<0.001). SUDs are fitted with a logarithmic function when data are plotted as a function of the distance, and with a linear function when data are plotted as a function of the logarithm of the distance. In the fixed sound condition, there is no significant evolution of SUDs ratings with the steps (e). As illustrated by Figure 6 panel e, SUDs did not significantly vary in the Fixed aBAT, in which the sound source distance was fixed (Post-hoc Newman-Keuls' test: p >.70 in all cases). SUDs at 640cm (sound source starting point) and 20cm (sound source ending point) did not significantly differ between the logarithmic and linear aBATs, (Post-hoc Newman-Keuls' test: p >.40 in both cases). As depicted on Figure 6, panel a and b, whereas SUDs significantly increased at each step of the Logarithmic aBAT (Post-hoc Newman-Keuls' test: p <.05 in all cases), the first increase being between S1 and S2 (corresponding at 640 cm and 320 cm). In the Linear aBAT, SUDs did not significantly vary between the first three steps (Post-hoc Newman-Keuls' test: p > 0.29 in both cases) and started to significantly increase only from S3 on (Post-hoc Newman-Keuls' test: p <.001 in all three cases), the first increase being between S3 and S4 (corresponding at 400 cm and 260 cm).

Finally, we fitted SUDs data from the logarithmic and linear aBATs as a function of the sound source distances with logarithmic and linear functions. We used as the logarithmic function y(x) = a.log(x) + b, where x represented the independent variable (i.e. the distances or logarithm of distances), y the dependent variable (i.e. the SUDs), a the slope and b the ordinate for log(x) = 0, i.e. for x=1. The linear function is described by y(x) = a.x + b where a is the slope and b the intercept at x=0. As both functions contain two parameters, we can compare the quality of the fitting by directly comparing the root mean square errors values (RMSE). For both conditions, analysis revealed that the logarithmic function described better participants' data than the linear function (logarithmic aBAT: t(19) = -4.9, p <.001, RMSE Logarithmic_fitting = 0.58, RMSE Linear_fitting = 1.1; linear aBAT: t(19) = -2.3, p <.05, RMSE Logarithmic_fitting = 0.55, RMSE Linear_fitting = 0.79). Moreover, following these results, data from both conditions can be described by a linear function between SUDs and the logarithm of sound source distances, as illustrated on Figure 6, panels c and d.

Discussion

We aimed at refining the description of the impact of a sound on tactile detection as a function of the sound source distance, using an audiotactile task based on Canzoneri and colleagues' paradigm [START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF]. We demonstrated that a logarithmic distribution of tested sound source distances was more relevant than a linear one to map events occurring in the auditory space. We also showed that using a condition with a fixed sound located far from participants' body provided a good baseline to capture expectancy effects. Finally, we fitted the multisensory effects on RTs according to the sound distance to the body and found that a logarithmic decrease was a good descriptor of its variations.

We used as a baseline condition a tactile detection task, in which participants listened to a sound fixed in space, located far from their body. We also studied tactile detection in unimodal conditions, with tactile stimuli occurring before and after the sound (with respectively Tbefore and Tafter). The analysis of unimodal RTs and fixed sound RTs was highly informative. First, RTs decreased with the delay in the fixed sound condition. Furthermore, RTs at T1 and T6 in the fixed sound condition were respectively at the same level than unimodal RTs occurring before and after the sound. These two observations suggest that, in the fixed sound condition, there is no audiotactile integration effect on RTs and that the observed decrease of RTs with the delay is due to expectation. Subtracting the contribution of expectation to the overall behavioral effect in the looming sound condition gives a more proper measure of the behavioral impact of audiotactile integration. In the looming condition, RTs difference between T1 and T6 is about -100ms in the logarithmic session and -67ms in the linear session. Comparing with RTs difference in the fixed sound condition (-49ms in the logarithmic session and -33ms in the linear session), we conclude that at T6, the RTs speeding up accounts for 50% of audiotactile integration effects and for 50% of expectancy effect. This ratio might be dependent on the proportion of catch trials in the experiment (33% here). Furthermore, as predicted, the evolution of expectation is dependent on the distances distribution [START_REF] Niemi | Foreperiod and Simple Reaction Time[END_REF]. Thus, it is important to evaluate expectancy at each delay, in every new experiment.

The majority of previous studies using similar paradigms did not control for expectancy effects (e.g. [START_REF] Canzoneri | Tool-use reshapes the boundaries of body and peripersonal space representations[END_REF][START_REF] Ferri | Emotion-inducing approaching sounds shape the boundaries of multisensory peripersonal space[END_REF][START_REF] Taffou | Cynophobic Fear Adaptively Extends Peri-Personal Space[END_REF]. We evidenced here that expectation has to been taken into account, as it is responsible for a significant part of the observed behavioral effect. Not assessing the contribution of expectation could be misleading: results attributed to multisensory integration effects could be due to expectancy. Some studies in the literature included a baseline condition: they used unimodal tactile trials, in which tactile stimulations were delivered in the absence of sound. Unimodal tactile trials were usually administered at two different temporal delays, corresponding to the equivalent time of the nearest and the farthest distance used in the audiotactile experiment (for example at T1 and T6 here) (Noel et al., 2015a;[START_REF] Salomon | Unconscious integration of multisensory bodily inputs in the peripersonal space shapes bodily self-consciousness[END_REF][START_REF] Serino | Body part-centered and full body-centered peripersonal space representation[END_REF]. Several issues are related to this method. First, expectancy effects may be different in the absence of sound. The sound beginning and end points give participants a temporal window during which the tactile stimulation can occur, thus impacting participants' anticipation. Second, unimodal trials RTs are indicators of an overall size of expectancy effect but do not evaluate the dynamic changes during the trial. Expectation needs to be evaluated at each delay to reveal the evolution of audiotactile integration impact on RTs according to the sound distance.

Audiotactile integration in the current paradigm depends on the sound dynamic and on the distances distribution. In the logarithmic distances distribution, we observed a speed up effect due to the looming sound presence at the delays T3, T4, T5 and T6. In comparison, the speed up effect appeared only at T6 with the linear distances distribution. This result is not surprising considering that perception of sound source distance is based on a cue varying logarithmically: the largest variation in sound level occurs when the sound is in the nearness of the body. While studies on sound perception in depth are developed using a logarithmic scale of tested distances [START_REF] Alais | Synchronizing to real events: subjective audiovisual alignment scales with perceived auditory depth and speed of sound[END_REF][START_REF] Zahorik | Loudness constancy with varying sound source distance[END_REF], it is remarkable that a paradigm examining the coding of audiotactile stimuli as a function of the distance between the looming sound and the body does not integrate a logarithmic scale of auditory distances. Using a logarithmic distances distribution gives a better resolution to PPS morphometry measure via audiotactile integration paradigms, by using a cognitively relevant mapping of the auditory space.

In the Auditory Behavioral Assessment Tests (aBATs), we collected participants' emotional experience according to their distance to the sound source. SUDs increased as the distance between participants and the sound source decreased. The logarithmic distances and the linear distances aBAT results are highly coherent, as sound source started to create discomfort at comparable distances from participants' body. Taken together, these results indirectly confirmed that participants perceived the different sound positions. Moreover, similarly to tactile RTs, SUDs evolved logarithmically with the sound source distance. As in the audiotactile test, the aBATs results support the idea that a logarithmic distances distribution is more efficient to evaluate behavioral reactions linked to the location of an auditory stimulation in depth. Further, the logarithmic pattern observed in both tests suggests a pertinent, simpler and parsimonious manner to describe and analyze the data obtained: fitting a linear function on the data plotted as a function of the logarithm of the distance.

To capture the dynamic of multisensory integration according to the distance between the auditory stimulus and the body, we fitted RTs data with linear and logarithmic functions. A linear fitting gave a good description of RTs evolution as a function of the logarithm of the distances. Previous studies proposed a sigmoidal function as a representation of RTs evolution depending on the distance of the sound source. A sigmoidal function is a "S"-shaped curve, in which data evolve between a lower and an upper threshold. The inflexion point, i.e. the central point between both thresholds, is considered as measure of PPS size in space [START_REF] Canzoneri | Tool-use reshapes the boundaries of body and peripersonal space representations[END_REF][START_REF] Canzoneri | Dynamic sounds capture the boundaries of peripersonal space representation in humans[END_REF]. In such a framework, PPS has a precise extent in space, implying that behavioral data follow an in-or-out response. With our data, a sigmoidal fitting was not convincing. Moreover, we wanted to use an economical function in term of parameters (a sigmoidal function has four parameters, the linear and logarithmic functions used here have only two parameters). Our analysis suggested that audiotactile integration behavioral effects follow a logarithmic decrease with the distance (thus a linear decrease with the logarithm of distances). Following this result, multisensory integration behavioral impacts according to the sound distance to the body do not display an in-or-out response, but a gradual response which strength decreases with distance until being null. Thus, PPS might not be an inor-out zone. In line with this result, previous studies suggested that, within PPS, the distance between an auditory or visual stimuli continues to influence tactile detection [START_REF] De Haan | Approaching threat modulates visuotactile interactions in peripersonal space[END_REF][START_REF] Hobeika | Anisotropy of lateral peripersonal space is linked to handedness[END_REF] and action preparation [START_REF] Camponogara | When distance matters: Perceptual bias and behavioral response for approaching sounds in peripersonal and extrapersonal space[END_REF] This logarithmic function is also coherent with the theoretical framework proposed by Bufacchi and Iannetti, who described PPS as an action field, in which responses are graded with proximity [START_REF] Bufacchi | An Action Field Theory of Peripersonal Space[END_REF].

In this study, we proposed different modifications of the Canzoneri's audiotactile paradigm to overcome some limitations and improve the power of data analysis. There are still limitations that we did not address. The paradigm is based on the perception of distances. The absolute distance estimation of sound sources is usually a difficult task for nonfamiliar stimuli in absence of reference, in which participants are not accurate [START_REF] Middlebrooks | Sound Localization by Human Listeners[END_REF][START_REF] Zahorik | Auditory Distance Perception in Humans : A Summary of Past and Present Research[END_REF]. However, individuals are accurate in the relative comparison of distances between two sources at different distances. The paradigm rests on looming sounds, and compares the effect of continuously varying sound distances. Due to the variability in absolute distances estimation, the method cannot give results in terms of metrical distances but in terms of distances comparisons. Moreover, the results are highly linked to the range of tested distances [START_REF] Poulton | Range Effects in Experiments on People[END_REF]. Subjects learn the range of stimuli used in the experiment and adapt their behaviors to it. It would be deceptive to give metrical estimation of PPS considering the influence of range effects.

Even if we focused in our work on mastering the auditory aspects of the audiotactile paradigm, the importance of the selection of tactile stimulation type requires to be emphasized. Human skin tactile properties vary widely between different body parts [START_REF] Chouvardas | Tactile displays : Overview and recent advances[END_REF][START_REF] Dargahi | Human tactile perception as a standard for artificial tactile sensing -a review[END_REF]. In the present study, we decided to stimulate finger pads because it is one of the most sensitive body part, with a high density of tactile receptors and a good spatial resolution (Johansson andVallbo, 1979a, 1979b). Our tactile device was a mechanical tactile device: a miniature speaker able to send fast stimulations (20ms in the experiment), and which can send all forms of signal. Electric stimulation can also be used. Electric shocks are detected easily with a good spatial resolution, but they can be painful and non-ecological [START_REF] Chouvardas | Tactile displays : Overview and recent advances[END_REF]. Further refining the audiotactile paradigm implicates identifying the type of tactile stimulation that is the most suited to the experimental context, and to the targeted body part and its tactile receptors.

Conclusion

It is important to master every aspect of the auditory and tactile stimulations to develop a reliable and efficient protocol to study PPS. For auditory stimulations in depth, distances distribution needs to be logarithmic to have a relevant mapping of the space. After controlling for expectancy effects, we found that audiotactile behavioral impacts change logarithmically with sound distance. This finding has important implications for the theoretical aspects of PPS: behavioral responses linked to PPS coding do not follow an in-or-out pattern but a rather gradual pattern. Furthermore, this efficient method to describe audiotactile integration in space could lead to more reliable and precise results for studies aiming at determining the interplay between PPS and multisensory integration.

Figure 2 :

 2 Figure 2: Sound levels and experimental design. (a, b, d, e) Sound level at each burst of the looming sound as a function of the tested distances. Values are given with reference to the direct sound level at the closest distance, i.e. 20cm. Arrows indicate tested distances in the logarithmic distances distribution (a) and in the linear distances distribution (d). As illustrated on graphs a and d, sound level varies exponentially with distance. As expected, sound level varies linearly when distances are regularly spaced on a logarithmic scale (see b), giving a relevant mapping of space. When distances are spaced regularly on a linear scale (see e), the sound level variation is exponential. In this linear distribution, the measures at far distances may be redundant, whereas there is a lack of precision for close distances. (c, f) Description of a trial. For each trial, one tactile stimulation was delivered at one among eight possible delays from sound onset (Tbefore, T1 to T6 and Tafter), corresponding to six possible distances of the sound source from participants' body and to two unimodal trials in the logarithmic distances distribution (c), and in the linear distances distribution (f).

  = 46.7, p < .001, η p 2 = .733), of DELAY (F (7,119) = 48.8, p < .001, η p 2 = .720), and significant two-way interactions between the factors DISTANCES DISTRIBUTION x MOVEMENT (F (1,17) = 36.4, p < .001, η p 2 = .681), DISTANCES DISTRIBUTION x DELAY (F (7,119) = 11.1, p < .001, η p 2 = .394) and MOVEMENT x DELAY (F (7,119) = 12.2, p < .001, η p 2 = .419). Finally, the three-way interaction DISTANCES DISTRIBUTION x MOVEMENT x DELAY was significant (F (7,119) = 3.34, p < .01, η p 2 = .164),

Figure 5

 5 Figure 5 Description of the audiotactile integration consequences on tactile ΔRTs (RTs looming sound -RTs fixed sound) observed in the linear and logarithmic auditory distances distributions sessions. This figure reports the difference of mean tactile RTs (±SEM) between the two sound movement conditions: the fixed sound and the looming sound. Data of the logarithmic distances distribution condition are represented in the left graphs (a and c), and data for the linear distances distribution condition in the right graphs (b and d). The same data are plotted as a function of the sound source distance (upper graphs, a and b) or of the logarithm of the sound source distance (lower graphs, c and d). RTs are fitted with an increasing logarithmic function when data are plotted as a function of the distance, and with a linear function when data are plotted as a function of the logarithm of the distance.

Figure 6 :

 6 Figure 6: Discomfort rating of aBATS. This figure reports mean SUDs (±SEM) at each step of the three aBATs: the Logarithmic distances aBAT (a and c), the Linear distances aBAT (b and d) and the fixed distance aBAT (e). The data of the Logarithmic and Linear aBATs are plotted twice: as a function of the sound source distance (upper graphs, a and b) or as a function of the logarithm of the sound source distance (lower graphs,c and d). Data of the fixed aBAT are plotted as a function of the steps number, as in this aBAT the sound source distance is fixed at 640cm. Asterisks indicate significant differences in SUDs between distances condition (*p<0.05, **p<0.01, ***p<0.001). SUDs are fitted with a logarithmic function when data are plotted as a function of the distance, and with a linear function when data are plotted as a function of the logarithm of the distance. In the fixed sound condition, there is no significant evolution of SUDs ratings with the steps (e).

The linear distances distribution is actually pseudo-linear. The spaces between tested distances are not strictly regular: the space between T1-T2, T2-T3, T4-T5, and T5-T6 is 120cm., whereas the space between T3-T4 is 140cm. This difference is due to experimental design constraints

Acknowledgments

We are grateful to Emmanuel Fléty and Arnaud Recher for their help with the apparatus for tactile stimulation. This work was supported by Sorbonne Universités Investissements d'avenir, Emergence and the French Ministry of Armed Forces (grant number HUM-1-0820).


Maud Gits-Muselli
email: maud.gits-muselli@aphp.fr

Stephane Villiers

Samia Hamane

Béatrice Berçot

Jean-Luc Donay

Blandine Denis

Nicolas Guigue

Alexandre Alanio

Stéphane Bretagne

Stéphane Villiers

Béatrice Bercot

Time to and differential time to blood culture positivity for assessing catheter-related yeast fungaemia: A longitudinal, 7-year study in a single university hospital

Keywords: Blood cultures, central venous catheter, yeast fungaemia, candidaemia, diagnosis, time to positivity, differential time to positivity

INTRODUCTION

Central venous catheters (CVCs) significantly increase the risk of candidaemia, and inversely, candidaemia is often associated with intravascular catheters. [START_REF] Ramage | Our current understanding of fungal biofilms[END_REF] Indeed, the increased incidence in candidaemia has paralleled the increased use in a variety of indwelling catheters.

CVCs are present in at least 80% of cancer patients with candidaemia, for which the source of infection can be clinically related to the catheter. [START_REF] Ben-Ami | Time to blood culture positivity as a marker for catheter-related candidemia[END_REF][START_REF] Lortholary | The risk and clinical outcome of candidemia depending on underlying malignancy[END_REF] As a consequence, most guidelines for candidaemia management recommend removal of existing intravascular catheters, if feasible. [START_REF] Pappas | Executive Summary: Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America[END_REF][START_REF] Maertens | European guidelines for antifungal management in leukemia and hematopoietic stem cell transplant recipients: summary of the ECIL 3--2009 update[END_REF][START_REF] Ullmann | ESCMID* guideline for the diagnosis and management of Candida diseases 2012: adults with haematological malignancies and after haematopoietic stem cell transplantation (HCT)[END_REF][START_REF] Andes | Impact of treatment strategy on outcomes in patients with candidemia and other forms of invasive candidiasis: a patient-level quantitative review of randomized trials[END_REF] Although the recommendation is strong, the evidence is of low-quality, and various authors have recommended a re-consideration of CVC removal on an individual basis. [START_REF] Pappas | Executive Summary: Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America[END_REF][START_REF] Maertens | European guidelines for antifungal management in leukemia and hematopoietic stem cell transplant recipients: summary of the ECIL 3--2009 update[END_REF][START_REF] Ullmann | ESCMID* guideline for the diagnosis and management of Candida diseases 2012: adults with haematological malignancies and after haematopoietic stem cell transplantation (HCT)[END_REF][START_REF] Andes | Impact of treatment strategy on outcomes in patients with candidemia and other forms of invasive candidiasis: a patient-level quantitative review of randomized trials[END_REF][START_REF] Nucci | Early removal of central venous catheter in patients with candidemia does not improve outcome: analysis of 842 patients from 2 randomized clinical trials[END_REF] Practically, one must consider the possible deleterious consequence of CVC removal, especially in patients who require long-term CVC for cancer treatment or parenteral nutrition. This scenario is even more complicated to manage when the catheter's infection is not subsequently confirmed.

To incriminate a CVC as a source of bacteremia, time to positivity (TTP) and differential time to positivity (DTTP) have been extensively studied and are routinely used for decision making. [START_REF] Mermel | Clinical practice guidelines for the diagnosis and management of intravascular catheter-related infection: 2009 Update by the Infectious Diseases Society of America[END_REF][START_REF] Blot | Diagnosis of catheter-related bacteraemia: a prospective comparison of the time to positivity of hub-blood versus peripheral-blood cultures[END_REF][START_REF] Raad | Intravascular catheter-related infections: advances in diagnosis, prevention, and management[END_REF] Few similar analyses have been performed for CVC-related candidaemia but with controversial results and no definitive thresholds. [START_REF] Ben-Ami | Time to blood culture positivity as a marker for catheter-related candidemia[END_REF][START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF][START_REF] Park | Diagnostic usefulness of differential time to positivity for catheter-related candidemia[END_REF] In parallel, TTP has also been evaluated to identify yeast species as soon as the blood culture is positive. [START_REF] Fernández-Cruz | Is it feasible to diagnose catheter-related candidemia without catheter withdrawal?[END_REF][START_REF] Huang | Time to positivity of blood culture can predict different Candida species instead of pathogen concentration in candidemia[END_REF][START_REF] Cobos-Trigueros | Time to positivity and detection of growth in anaerobic blood culture vials predict the presence of Candida glabrata in candidemia: a two-center European cohort study[END_REF][START_REF] Paugam | Longer incubation times for yeast fungemia: importance for presumptive treatment[END_REF] We, therefore, used our comprehensive database on yeast-positive blood culture episodes collected over 7 years in our institution to analyse the usefulness of TTP and DTTP to predict CVC-related yeast fungaemia (CVC-RYF).

MATERIALS AND METHODS

Study design and data collection

We enrolled all ≥15-year-old patients for whom at least one blood culture (BC) bottle was positive for yeast between October 1 st, 2010 and September 30 th, 2017 through the records of the Department of Microbiology of Saint-Louis Hospital. This hospital is a 550-bed tertiary university hospital located in Paris, France, with more than two-thirds of its activity dedicated to haematology or oncology patients. Burn patients were excluded as these were analysed separately because all the removed catheter tips were culture-negative for Candida spp. suggesting that CVC is not the source of candidaemia whereas candidaemia was always concomitant with Candida carriage of the skin. [START_REF] Dudoignon | Outcome and potentially modifiable risk factors for candidemia in critically ill burns patients: A matched cohort study[END_REF] The clinical data collected were age, gender, underlying conditions, hospitalization in

Accepted Article

intensive care unit (ICU) at the time of positive BC, antifungal preexposure defined as administration of systemic antifungal drugs for at least 5 days within 30 days prior to fungaemia (whatever dosage and duration), and outcome at day 30 after the first positive BC as described elsewhere. [START_REF] Bretagne | Predisposing factors and outcome of uncommon yeast species-related fungaemia based on an exhaustive surveillance programme (2002-14)[END_REF] Because of the retrospective design of the study and the multiple regimens of antifungal drugs prescribed in haematology, it was not possible to reliably collect more data on type, dose, and duration of the antifungal drugs.

The type of CVCs, its duration, and the reason for removing it were collected from the Anesthesiology Department. The CVCs were divided into short-term (mainly from ICU) and longterm CVC.

Microbiological methods

Nurses were instructed to draw 10 ml of blood into each aerobe and anaerobe bottles and to send the inoculated bottles to the microbiology laboratory without delay. Bottles were incubated (BioMérieux BacT/ALERT 3D, Marcy l'Etoile, France) for at least 5 days or until positivity.

When the BC was detected as positive, and the Gram stain demonstrated the presence of yeasts, a fresh mount with KOH 15% was done to observe the morphology of the yeast and a subculture was performed in BBL CHROMagar Candida Medium plate (Hardy Laboratories, Santa Monica, CA, USA). Identification was confirmed using the API ID 32C (bioMérieux, Marcy l'Etoile, France) from October 2010 to the end of 2011, then the Biomerieux MALDI-TOF system (VITEK-MS) (bioMérieux, Marcy l'Etoile, France) from January 2013 to September 2017. For the uncommon fungal species [i.e. not for Candida albicans, C. glabrata, C. tropicalis, C. parapsilosis, C. krusei (syn. Pichia kudriavzevii), and Cryptococcus spp.], identification was confirmed by molecular analysis at the National Reference Centre for Invasive Mycoses and Antifungals at Institut Pasteur, Paris, France, as described elsewhere. [START_REF] Bretagne | Predisposing factors and outcome of uncommon yeast species-related fungaemia based on an exhaustive surveillance programme (2002-14)[END_REF] After CVC removal, the catheter tip and/or the chamber of the implantable venous access port system were cultured using a quantitative method. [START_REF] Brun-Buisson | Diagnosis of central venous catheter-related sepsis. Critical level of quantitative tip cultures[END_REF] Briefly, tip and/or chamber were immersed in 1 mL of sterile water and vortexed for 1 min. Then, 50 µL were plated on a blood and chocolate agar Petri dishes. The plate was then incubated at 35°C (±2°C) with 5% (± 1%) CO 2 for 24h. CVC culture was considered negative (CVC-negative) if there was no growth observed after 5 days of culture. A CVC was considered positive (CVC-positive) if either the tip, the chamber, or both grew at least one yeast colony.

Accepted Article

Definition of CVC-related yeast fungaemia

The date of fungaemia was the date of blood sampling. Paired bottles were defined as respectively drawn through CVC and percutaneously within less than 30 min. TTP is routinely recorded by the BacT/Alert system as the time recorded until growth detection after recording the BC bottle in the incubation cabinet. DTTP is defined as the difference in TPP recorded for paired bottles (DTTP = TTP percutaneously -TTP CVC). When multiple positive BC were available for a given patient, only the first was considered for TPP analysis and only the first paired bottles for DTTP, since this was the first information available for the clinicians.

Patients with CVC removed and cultured at the time of fungemia were divided into two groups: (i) CVC-related yeast fungemia (CVC-RYF) when the same yeast species was recovered from at least one BC and from the CVC culture; (ii) non-CVC-related yeast fungemia (NCVC-RYF) when a yeast species was recovered from at least one BC, and the CVC culture was negative or yielded a different species. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] This definition of CVC-RYF was adapted from the definition proposed by Bouza et al. except we considered for TTP calculation the patients with a positive BC drawn through CVC and not only the patient with a positive peripheral BC. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF]

Statistical analysis

Patients with CVC-RYF and NCVC-RYF were compared. Quantitative variables were reported as medians and interquartile ranges (IQR) or means ± standard deviations. Chi 2 , Kruskal Wallis and Mann-Whitney tests were used for quantitative variables (TTP and DTTP). The Area Under a Receiver Operating Characteristic (ROC) Curve (AUC) was calculated to detect the best cut-off values for predicting CVC-RYF using TTP or DDTP values. A p-value <0.05 was considered significant. Prism 7 (GraphPad Software, La Jolla, CA, USA) was used for statistical analyses and graphs.

Ethics statement

The present study is a retrospective analysis of the data collected as part of our usual procedure for standard diagnostics with no additional sampling for the patients and no impact on management. This study was registered with our hospital review committee (reference number: 2018000000077). Because this was a retrospective observational study, it does not require the application for specific informed consent according to French law.

Accepted Article

RESULTS

Selection of patients

Over the 7 years, 249 episodes of fungaemia were recorded in 240 patients. The presence of CVC was noted in 203/240 (85%) patients. The patients with CVC were more often oncology patients (p = 0.01, Supplemental Table S1). We excluded 98 patients because of: lack of CVC culture (n = 65), lack of BC through the CVC (n = 24), co-infection with bacteria (n = 17) or mixed yeasts infection (n = 3) (Figure 1). Finally, a total of 105 patients with CVC removed and cultured were selected for further analysis (Supplemental Table S1) The 98 patients excluded were not statistically different except that they had a less frequent antifungal pre-exposure (p = 0.01) than the selected patients. No recurrent episode for these 105 remaining patients was observed (Supplemental Table S1).

Yeast identification and patient characteristics (Table 1)

The 105 patients were divided into NCVC-RYF (n = 73, 69.5%) and CVC-RYF (n = 32, 30.5%) based on the CVC culture results. The only significant difference between the two groups was the percentage of negative CVC culture when a preexposure to antifungal drugs was recorded [NCVC-RYF 22/73 (30%) vs. CVC-RYF 2/32 (6%); p = 0.007].

The main species involved were C. albicans (46%), C. parapsilosis (19%), C. glabrata (15%), and C. tropicalis (9%), with no statistical difference between NCVC-RYF and CVC-RYF.

Only four non-Candida species were identified (Table 1), representing less than 4% (4/105) of the total. Out of the 32 culture-positive CVCs, 24/32 (75%) grew >10 3 CFU/mL and 8/32 (25%) grew >10 2 CFU/mL without difference in the species distribution or the CVC features (short-term vs. long-term CVC) according to the number of colonies and were therefore not subsequently differentiated.

Most of the CVCs were long-term [82/105; 78%; median duration time: 68 days [IQR: 22-157], and were more frequent in onco-hematology patients. These long-term CVCs were including totally implantable venous access port system (n = 44), tunnelled cuffed catheter (n = 32), and peripheral insertion central catheter (n = 6). The long-term central venous access catheters were not more frequent in CVC-RYFs than in NCVC-RYFs (Table 1). In contrast, the 23 short-term CVCs were more frequent in ICU patients (21/23; 91.3%) with a median duration time of 10 days parapsilosis (p = 0.86). The TTP distribution was not statistically different (Kruskal Wallis test; p = 0.9) between patients who died compared with the patients alive at 30 days (Table 1). The median TTP values were not statistically different (p = 0.9) when comparing NCVC-RYF patients with (median TTP = 32h) or without (median TTP = 31h) pre-exposition with antifungal agent.

Using TPP as a predictor of CVC-RYF, we obtained an AUC of 0.73 [Confidence Interval 95% (CI95%) 0.62 to 0.85, p = 0.0001] when considering all yeast species (Figure 3). A threshold of <10h provided the best positive likelihood ratio (21.6), but the sensitivity was only 28.1% (Table 2). When restricting the analyses to C. albicans (Figure 3), the AUC was improved (0.84 [CI95% 0.69 to 1.00], p = 0.0002). A threshold of <15h was associated with a high positive likelihood ratio (30.5), with a sensitivity of 61% (Supplemental Table S2).

Among the 105 patients included in the study, 65 (62%) had a peripheral positive BC.

When restricted the TTP analysis to these patients, the rate of infected CVC increased from 30.5% (32/105) to 33.8% (22/65), which is not statistically significant, p = 0.6. The median TTP was

Accepted Article

RYF. The median DTTP was significantly higher for CVC-RYF (18.8h [IQR 15.0-22.7]) than for NCVC-RYF (-0.4h [IQR -5.1-4.8]) patients, p <0.0001). Calculation by species was not possible given the low number of positive pairs. The AUC was 0.86 ([CI95% 0.74 to 0.98], p <0.0001) for the diagnosis of CVC-RYF without difference between C. albicans and non-albicans Candida spp. (Figure 4). The sensitivity was 100% in the first 5h, which is translated in a null negative likelihood ratio, although the specificity did not exceed 71% and the positive likelihood ratio was never >5 (Table 3).

DISCUSSION

When analysing 105 patients with CVC withdrawn and cultured, and using a strict definition of infected CVC (i.e., positive or negative CVC culture), a TTP <10h had the best likelihood ratio (21.6), although the sensitivity was only 28.1% for identifying CVC-RYF. A DTTP between 1 and 5h presented 100% specificity but a low positive likelihood ratio (LOR<4).

Our observation stands for a population of oncology patients (86%) with mainly long-term central venous access catheter (78%). Candida albicans represented 46% of the episodes followed by C. parapsilosis (19%). We nevertheless kept the term of fungaemia, acknowledging that only 4 were not Candida species.

Comparison of TTP results with previous reports is difficult because of numerous differences in the studies design, inclusion of children or only adults, different underlying diseases, single or multiple centres recruitment, number of long-term or short-term CVCs included, culture method used for testing CVC tips, variable definitions of CVC infection with probable criteria added, and various Candida spp. considered). Also, the small number of fungaemia and the even smaller number of CVC-RYF add to the heterogeneity. The number of analysed episodes of CVC-RYF rarely reach more than 100 [22 in [START_REF] Fernández-Cruz | Is it feasible to diagnose catheter-related candidemia without catheter withdrawal?[END_REF] , 64 in 2 , 108 in [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] , and 105 in [START_REF] Park | Diagnostic usefulness of differential time to positivity for catheter-related candidemia[END_REF] ] compared to 105 in our study, which precludes subset analyses. Here the method used for CVC testing was different from the one used by other authors. [START_REF] Ben-Ami | Time to blood culture positivity as a marker for catheter-related candidemia[END_REF][START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] Moreover, we analysed the TTP for patients with negative or no peripheral BC in contrast to Bouza et al. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] and Ben-Ami et al. [START_REF] Ben-Ami | Time to blood culture positivity as a marker for catheter-related candidemia[END_REF] who included only patients with at least one positive peripheral BC. When excluding patients with no positive peripheral BC, the TTP decreased to 11.75h [IQR 8.5-20.4] vs. 16.8h [IQR 9.7-28.6] for CVC-RYF. This probably results from the higher fungal load when both peripheral and central bottles are positive. However, to exclude patients without peripheral positive BC does not correspond to onco-haematology patients with long-term CVC for who peripheral BC is not

Accepted Article

obtained because of practical or comfort reasons. Additionally, there is no recommendation to consider peripheral and central yeast fungaemia differently. [START_REF] Pappas | Executive Summary: Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America[END_REF][START_REF] Maertens | European guidelines for antifungal management in leukemia and hematopoietic stem cell transplant recipients: summary of the ECIL 3--2009 update[END_REF][START_REF] Ullmann | ESCMID* guideline for the diagnosis and management of Candida diseases 2012: adults with haematological malignancies and after haematopoietic stem cell transplantation (HCT)[END_REF][START_REF] Andes | Impact of treatment strategy on outcomes in patients with candidemia and other forms of invasive candidiasis: a patient-level quantitative review of randomized trials[END_REF][START_REF] Nucci | Early removal of central venous catheter in patients with candidemia does not improve outcome: analysis of 842 patients from 2 randomized clinical trials[END_REF] Taking all these limitations into account, our best TTP cut-off of 10h is different from other proposals. Ben-Ami et al. suggest a TTP cut-off of 30h (100% sensitive and 51.4% specific) for CVC-related candidaemia with an AUC of 0.76, close to our observation (0.73). [START_REF] Ben-Ami | Time to blood culture positivity as a marker for catheter-related candidemia[END_REF] Bouza et al. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] found TTP to be significantly shorter in peripheral blood culture in adults with catheter-related candidaemia compared to adults with non CVC-related candidaemia (29.8 vs. 36.8h, p <0.03), which we confirmed even if the figures were slightly different (16.8 vs. 29.4h, p = 0.001), and the best TTP cut-off for the prediction of catheter-related candidaemia in adults to be <55h. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] Only two patients in that study had long-term CVCs (tunnelled-CVC). However, even the 10h cut-off that we proposed is too long to be clinically relevant when the goal is withdrawing the CVC as soon as possible. Indeed, the median TTP of the first bottle collected through the CVC was around 17h in the present study, corresponding to a median of two operational days for catheter removal after blood sampling (Figure 2). Waiting further could be deleterious for the patients. [START_REF] Kollef | Septic shock attributed to Candida infection: importance of empiric therapy and source control[END_REF] When restricting to C. albicans, the AUC improved as the positive likelihood ratio for TTP with the threshold of <15h, but this is a formal observation as long as the species is identified at least 24h after BC positivity. Species identification could be achieved much faster using PCR-based identification tests, [START_REF] Buss | Impact of a Multiplex PCR Assay for Bloodstream Infections With and Without Antimicrobial Stewardship Intervention at a Cancer Hospital[END_REF] matrix-assisted laser desorption ionization time-of-flight mass spectrometry, [START_REF] Pulcrano | Rapid and reliable MALDI-TOF mass spectrometry identification of Candida non-albicans isolates from bloodstream infections[END_REF] or peptide nucleic acid fluorescence in situ hybridization. [START_REF] Klingspor | Seven years of clinical experience with the Yeast Traffic Light PNA FISH: Assay performance and possible implications on antifungal therapy[END_REF] TTP can, however, be used for other purposes than diagnosing CVC-RYF. For instance, shorter TTPs was shown associated with mortality. [START_REF] Kim | Clinical impact of time to positivity for Candida species on mortality in patients with candidaemia[END_REF] In contrast, other authors found an association between long TTP and mortality. [START_REF] Nunes | Time to blood culture positivity as a predictor of clinical outcome in patients with Candida albicans bloodstream infection[END_REF] We did not observe any significant difference between the median TTP and the global mortality at day 30 (here 30%), acknowledging that underlying diseases and patient management could prevail in the crude mortality over infected CVC. While it has been previously associated with the number of positive BCs, [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] we did not find such an association. TTP can also be used for presumptive species identification since C. glabrata growth rate is lower than that of other Candida spp. and is known for its decreased susceptibility to azoles. [START_REF] Huang | Time to positivity of blood culture can predict different Candida species instead of pathogen concentration in candidemia[END_REF][START_REF] Cobos-Trigueros | Time to positivity and detection of growth in anaerobic blood culture vials predict the presence of Candida glabrata in candidemia: a two-center European cohort study[END_REF][START_REF] Paugam | Longer incubation times for yeast fungemia: importance for presumptive treatment[END_REF] However, the current recommendations for the initial treatment of candidaemia is candin whatever the species is, [START_REF] Pappas | Executive Summary: Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America[END_REF][START_REF] Maertens | European guidelines for antifungal management in leukemia and hematopoietic stem cell transplant recipients: summary of the ECIL 3--2009 update[END_REF][START_REF] Ullmann | ESCMID* guideline for the diagnosis and management of Candida diseases 2012: adults with haematological malignancies and after haematopoietic stem cell transplantation (HCT)[END_REF] which limits the clinical utility of TPP for identification.

For DTTP, Park et al. found an overall sensitivity and specificity of a DTTP of >2h for diagnosing CVC-RYF of 85% (CI95%: 74% to 93%) and 82% (CI9586% 66% to 92%),

Accepted Article

respectively, when studying 99 pairs, 47 with definite, 14 with probable, and 38 with non CVCrelated candidaemia. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] The authors concluded that a DTTP of >2h was the optimal cut-off, except for CVC infection caused by C. glabrata. Other authors found this threshold accepted for bacteraemia would have a sensitivity of 94% but a specificity of 40%. [START_REF] Raad | Intravascular catheter-related infections: advances in diagnosis, prevention, and management[END_REF] We did not observe major differences between 2 and 5h for the DTTP performance, with specificity not exceeding 71% and positive likelihood ratio <4. An analysis by species was not performed because of the small number of pairs for each Candida species.

A noticeable point is the low yield of positive catheter culture. Only 30% (32/105) of tested CVCs were positive in culture with the method used. [START_REF] Brun-Buisson | Diagnosis of central venous catheter-related sepsis. Critical level of quantitative tip cultures[END_REF] In the literature, the rate of definite infected CVCs is variable: from 20.3% 2 to 44.8% 13 or 56% [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] . Part of these differences could come from the methods used, either semiquantitative (with a threshold of >15 CFU per catheter segment), [START_REF] Maki | A semiquantitative culture method for identifying intravenous-catheter-related infection[END_REF] or quantitative (>10 2 CFU per catheter segment), [START_REF] Brun-Buisson | Diagnosis of central venous catheter-related sepsis. Critical level of quantitative tip cultures[END_REF] although both are equivalent for bacteria [START_REF] Bouza | A prospective, randomized, and comparative study of 3 different methods for the diagnosis of intravascular catheter colonization[END_REF] and knowing that both are usually accepted. 9 Despite these differences, these observations suggest that CVCs could not be the main source of infection, even though they are present in >80% of patients with candidaemia. On the other hand, some authors even suggest that a positive culture tip is not always linked to candidemia, [START_REF] Almeida | Positive tip culture with Candida and negative blood culture: to treat or not to treat? A systematic review with meta-analysis[END_REF] which can interfere with the definition of CVC-RYF. If the CVC is mostly not the source of infection in the cases categorized as CVC-RYF, this could explain why the patients' survival is not impacted by CVC removal. [START_REF] Nucci | Early removal of central venous catheter in patients with candidemia does not improve outcome: analysis of 842 patients from 2 randomized clinical trials[END_REF] Another explanation for the lack of benefit of CVC removal could be that dissemination has already occurred when the CVC is withdrawn. This raises the question of systematic early removal of CVC in the management of candidaemia since some authors did not report differences, [START_REF] Nucci | Early removal of central venous catheter in patients with candidemia does not improve outcome: analysis of 842 patients from 2 randomized clinical trials[END_REF] knowing that patients who cannot tolerate CVC removal have often a worse prognosis. Nevertheless, CVC removal has to be evaluated on an individual basis, knowing that the consensus recommendation is CVC withdrawal if possible as soon as a BC is positive for Candida species. [START_REF] Ullmann | ESCMID* guideline for the diagnosis and management of Candida diseases 2012: adults with haematological malignancies and after haematopoietic stem cell transplantation (HCT)[END_REF][START_REF] Mermel | Clinical practice guidelines for the diagnosis and management of intravascular catheter-related infection: 2009 Update by the Infectious Diseases Society of America[END_REF][START_REF] Mellinghoff | Essentials in Candida bloodstream infection[END_REF] In our study, the removal of long-term CVCs was scheduled by the anaesthesiologist as soon as possible after the BC result in considering the availability of a surgery room for this removal.

The low yield of CVC culture can also be due to the consequence of previous antifungal drugs prescription, and we indeed observed a higher percentage of antifungal drugs preexposure in the group of culture-negative CVC (p = 0.007). This is obviously an important factor to consider when antifungal prophylaxis is increasing in haematology patients. However, the delay between the day of sampling of the first positive BC and CVC removal was short (median 2-3 days) and not statistically different between NCVC-RYF and CVC-RYF suggesting that an antifungal

Accepted Article

treatment between the clinical suspicion and the removal of the CVC may not impact the CVC culture. The low yield of CVC culture could be also due to non optimal culture method. For instance, the length of sonication is rarely controled. In this study, positive CVC culture yielded yeast CFU count above 10 2 CFU/mL and was then strongly suggestive of CVC-RYF. [START_REF] Sherertz | Three-year experience with sonicated vascular catheter cultures in a clinical microbiology laboratory[END_REF][START_REF] Renz | Value of PCR in sonication fluid for the diagnosis of orthopedic hardware-associated infections: Has the molecular era arrived?[END_REF] Lower tresholds could be considered.

We must acknowledge some limitations in our study. The volume of blood collected was not controlled, whereas it is well known that the volume collected from CVC is usually higher than via peripheral venipuncture. [START_REF] Jones | Effect of Clinical Variables on the Volume of Blood Collected for Blood Cultures in an Adult Patient Population[END_REF] The time between blood sampling and BC loading into the incubator was not controlled, knowing that delays tend to shorten TTP. [START_REF] Janapatla | Effect of overnight storage of blood culture bottles on bacterial detection time in the BACTEC 9240 blood culture system[END_REF] The population studied was mostly (86%) oncology patients thereby restricting our findings to this population. One can also argue the choice of analysing the first BC when multiple BCs were available instead of the BC with the shortest TTP. [START_REF] Bouza | Can microbiologists help to assess catheter involvement in candidaemic patients before removal?[END_REF] Our choice was clearly to look for a means to provide, as quickly as possible, information useful for the clinicians and to stick to routine practice.

In conclusion, a TTP <10 h and a DTTP between 2 and 5h could be relevant for incriminating a CVC during yeast fungaemia in oncology patients. However, even the shortest DTTP of 2h as proposed by others [START_REF] Park | Diagnostic usefulness of differential time to positivity for catheter-related candidemia[END_REF] means waiting longer than the already prolonged incubation time required for a BC to become positive for Candida spp. with the potential risk of maintaining an infected CVC. Clearly, rapid molecular-based methods are needed to speed the detection process [START_REF] Clancy | Detecting Infections Rapidly and Easily for Candidemia Trial, Part 2 (DIRECT2): A Prospective, Multicenter Study of the T2Candida Panel[END_REF] and to safely reduce the unnecessary removal of uninfected CVC, acknowledging that these new methods should be evaluated in real-world clinical settings. [START_REF] White | of the International Society of Human and Animal Mycology). Comment on: T2Candida MR as a predictor of outcome in patients with suspected invasive candidiasis starting empirical antifungal treatment: a prospective pilot study[END_REF]

Accepted Article

Table 1: Main characteristics of the 105 patients selected for the central venous catheter (CVC)related yeast fungaemia study. 56 [15-91] 53 [15-82] 63 [18-91] Sex ratio M/F

Patients

Accepted Article

Legends to Figures:

[

  IQR: 6-14]. The motive for removal and culture was always a suspicion of infection. Only 30.5% (32/105) of the tested CVCs were culture-positive, with no statistical difference between long and Accepted Article short-term CVCs (data not shown). The median delay between the date of blood sampling of the first positive BC and the date of CVC removal was 3 days [IQR 2-3] and 2 [IQR 1-3] for NCVC-RYFs and CVC-RYFs respectively (p = 0.08). TTP according to CVC or non-CVC related yeast-fungaemia Considering all species, the median TTP of the first bottle collected from CVC was statistically shorter in the CVC-RYF group (16.8h [IQR 9.7-28.6]) compared to the NCVC-RYF group: median (29.4h [IQR 20.7-41.3], p = 0.0001, Figure 2). When analysing TTP regarding the Candida species, a significant shorter TPP was only observed in C. albicans with a median of 12.9h [10.0-22.7] vs. 32.3h [24.4-41.0]) for CVC-RYF and NCVC-RYF, respectively (p = 0.0001, Figure 2). A trend towards shorter TTPs in the CVC-RYF vs. NCVC-RYF was observed for C. tropicalis (p = 0.09) and C. glabrata (p = 0.17). No difference was observed for C.

  11.75h [IQR 8.5-20.4] for CVC-RYF (vs. 16.8h [IQR 9.7-28.6] for the 105 patients) and 29.3h [IQR 22.7-43.2] (vs. 29.4h [IQR 20.7-41.3] for the 105 patients) for NCVC-RYF (p <0.0001). Out of the 40 patients with no positive peripheral BC recorded, 24 (60%) had a peripheral negative BC, and 16 (40%) did not have any peripheral BC performed. DTTP according to CVC or non-CVC related yeast fungaemia DTTP was available for 52/105 (49.5%) patients, including 32 NCVC-RYF and 20 CVC-

Figure 1 :

 1 Figure 1: Flow Chart of the study proces TTP: time to positivity, DTTP: differential time to positivity. Paired bottles collected within 30 minutes. *Exclusion criteria: lack of CVC Culture, lack of Blood Culture through CVC, coinfection with bacteria, mixed yeasts infection.

Figure 2 :

 2 Figure 2: Comparisons of time to positivity (TTP) between positive (CVC+) and negative (CVC-) central venous catheter for all yeast cultured and according to the four more frequent Candida spp.(*: p = 0.0001). Data presented in scatter dot-plot with the median line and the interquartile range.

Figure 3 :

 3 Figure 3: Receiver operating characteristic (ROC) curve analysis of time to positivity (TTP) for all yeasts (blue line; AUC = 0.73), C. albicans (green line; AUC = 0.84), and non-albicans Candida spp. (pink line; AUC = 0.64).

Figure 4 :

 4 Figure 4: Receiver operating characteristic (ROC) curve analysis of differential time to positivity (DTTP) for all yeasts (blue line; AUC = 0.86), C. albicans (green line; AUC = 0.84), non-albicans Candida spp. (pink line; AUC =0.90).

Acknowledgments

We thank Pr Françoise Dromer for her valuable comments on the manuscript and Raphaël Goudinoux for providing the CVCs type data.

C. glabrata 16 (15) 9 ( 12