Pauline Rasset
email: pauline.rasset@unicaen.fr

Jessica Mange
email: jessica.mange@unicaen.fr

Benoît Montalan
email: benoit.montalan@univ-rouen.fr

Look me in the eyes! A pre-registered eye-tracking study investigating the visual attention and affective reactions to faces with a visible difference

Keywords: Facial disfigurement, Eye-tracking, Visual attention, Affective states

This research aims to determine how disfigurement alters visual attention paid to faces and to examine whether such a potential modified pattern of visual attention to faces with visible difference were associated, in turn, with perceiver's stigmatizing affective reactions. A pilot study (N = 38) and a pre-registered experimental eye-tracking study (N = 89) were conducted. First, the visual explorations of faces with and without disfigurement were compared. The association of these visual explorations with affective reactions were investigated next. Findings suggest that disfigurement impacts visual attention toward faces; attention is not merely attracted to the disfigured area but it is also diverted particularly from the eye area. Disfigurement also eases disgust-related, surprise-related, anxietyrelated, and, to a lesser extent, hostility-related affective states. Exploratory interaction effects between attention to the eyes and to the disfigured part of the face revealed a hybrid effect on disgust-related affect and an increase in surprise-related affect when participants fixated more upon the disfigured area and fixated less upon the eyes. Thus, perceiver's attention is captured by disfigurement and also diverted from face internal features which seems to play a role in the affective reactions elicited.

Introduction

Living with a facial disfigurement may be a devastating social experience leading to psychopathologies (e.g., depression, anxiety; [START_REF] Van Loey | Psychopathology and psychological problems in patients with burn scars[END_REF]. Beyond exceptional although detrimental outcomes (e.g., bullying, hate crimes; Changing Face, 2021) these stigmatized people have to overcome challenges in most of their social encounters (e.g., naked stares, curiosity, teasing; [START_REF] Macgregor | Facial disfigurement: Problems and management of social interaction and implications for mental health[END_REF]. Many people living with a visible difference suffer from their condition because they have to deal with the gaze of others as much as with the expression of others' emotions which are often negative (e.g., anxiety, disgust) but can also be "positive" (e.g., sympathy) and still remind those affected of their stigmatized condition [START_REF] Bonanno | Mapping out the social experience of cancer patients with facial disfigurement[END_REF][START_REF] Macgregor | Facial disfigurement: Problems and management of social interaction and implications for mental health[END_REF]. This study aims to further explore the way people with no disfigurement pay attention and emotionally react to faces with facial disfigurement.

Investigating how faces with visible difference are looked upon

Faces form a unique category of visual objects, which play important roles (e.g., biological functions, communication, transmission of social signals; for a review see [START_REF] Hugenberg | Faces are central to social cognition[END_REF].

Little is known about the impact of disfigurement on face perception whilst authors argue that this visible stigma could be particularly difficult to deal with (see [START_REF] Bruce | Face perception[END_REF][START_REF] Hugenberg | The face of humanity[END_REF].

Whether it is because of their emotional, social and biological importance, faces are conceptualized as the most important stimuli for social interactions [START_REF] Hugenberg | Faces are central to social cognition[END_REF]. The processing of faces is supported by specialized distributed networks within the brain (e.g., fusiform face area; [START_REF] Kanwisher | Domain specificity in face perception[END_REF], faces are detected quickly and capture people's attention compared to other objects [START_REF] Langton | Attention capture by faces[END_REF][START_REF] Lewis | Face detection: Mapping human performance[END_REF][START_REF] Ro | Changing faces: A detection advantage in the flicker paradigm[END_REF][START_REF] Theeuwes | Faces capture attention: Evidence from inhibition of return[END_REF]. This face expertise has been attributed to the ability to process faces configurally [START_REF] Maurer | The many faces of configural processing[END_REF]. This typical process of normal face perception matters as it may contribute to perceiving the humanness on a human face [START_REF] Hugenberg | The face of humanity[END_REF].

In order to explore visual attention paid to faces, researchers have been using eye-tracking technologies, which enabled them to know where and how people look at faces [START_REF] Bruce | Face perception[END_REF].

Based on the assumption that people gaze more often and longer on more important sources of information (see [START_REF] Rahal | Understanding cognitive and affective mechanisms in social psychology through eye-tracking[END_REF], different cognitive processes can be investigated through eye tracking measures, notably information weighting or processing depth, with total dwell time (i.e., total time spent fixating on a specific location) or time to first fixations. Using eye-tracking technology, the importance of internal expressive features when viewing a face to get useful information (i.e., eyes, nose, mouth) was demonstrated [START_REF] Min | Visual attention analysis and prediction on human faces[END_REF]. Specifically, Vo and colleagues (2012) demonstrated that participants stared at the eyes more when watching a photograph of a face making eye-contact, the mouth when the target was speaking and the nose when the target was moving fast. Eyes have received special interest as they are supposed to be a rich source of information (e.g., regarding mental states; [START_REF] Baron-Cohen | Is there a "language of the eyes"? Evidence from normal adults, and adults with autism or asperger syndrome[END_REF] and not only as a result of their saliency (Birmingham et al., 2009a).

Despite a great deal of research on the perception of regular faces and although disfigurement stigma is supposed to interfere in faces processing [START_REF] Hugenberg | Faces are central to social cognition[END_REF], little is known about the perception of faces with disfigurement. Nevertheless, some studies have used eye-trackers to investigate attention distracted by the disfigured part of a face. For example, some research works have highlighted how attention is captured by a crooked nose [START_REF] Godoy | The straight truth: Measuring observer attention to the crooked nose[END_REF], a scar on the cheek [START_REF] Madera | Discrimination against facially stigmatized applicants in interviews: An eye-tracking and face-to-face investigation[END_REF], cleft lips [START_REF] Dindaroğlu | Visual perception of faces with unilateral and bilateral cleft lip and palate: An eye-tracking study[END_REF], malocclusion in general [START_REF] Wang | Objective method for evaluating orthodontic treatment from the lay perspective: An eye-tracking study[END_REF], nasal deformities [START_REF] Van Schijndel | Eye tracker based study: Perception of faces with a cleft lip and nose deformity[END_REF], and facial paralysis [START_REF] Ishii | The social distraction of facial paralysis: Objective measurement of social attention using eye-tracking[END_REF]; for a review see [START_REF] Asaad | Eye-tracking technology in plastic and reconstructive surgery: A systematic review[END_REF]. Ishii and colleagues (2009) demonstrated that facial deformities do not solely attract attention but they also engender a different pattern of visual attention. Yet, as they used scan paths that are difficult to interpret in terms of underlying cognitive processes [START_REF] Rahal | Understanding cognitive and affective mechanisms in social psychology through eye-tracking[END_REF]) and as they did not investigate the impact of this change on the stigmatizing reactions, their findings would benefit from further investigation.

Affective reactions to faces with visible difference

Affective reactions have been the most studied public stigma reactions to facial disfigurement (see A. [START_REF] Stone | Emotional responses to disfigured faces: The influences of perceived anonymity, empathy, and disgust sensitivity[END_REF]. People living with facial disfigurement testify to strong emotional reactions [START_REF] Macgregor | Facial disfigurement: Problems and management of social interaction and implications for mental health[END_REF]. More particularly, based on the testimonies of people with disfigurement (including his own), Partridge (1998) described a set of affective states that the perceivers may express, when communication goes wrong between people with and without facial disfigurement. He summarized these affective states in the acronym "SCARED (SCARED meaning "Sorry or Shocked, Curious or Confused, Anxious, Repelled, Embarrassed, Distressed").

From the perspective of the perceivers, various affective states have been evidenced, including negative emotions (i.e., disgust, fear, anger, shame, guilt, sadness, pity), but neutral and positive emotions too (i.e., curiosity, surprise, sympathy, compassion;[START_REF] Blascovich | Perceiver threat in social interactions with stigmatized others[END_REF][START_REF] Shanmugarajah | The role of disgust emotions in the observer response to facial disfigurement[END_REF][START_REF] Stone | Emotional responses to disfigured faces: The influences of perceived anonymity, empathy, and disgust sensitivity[END_REF]. A specific feature lies in the expression of negative emotions. Indeed, [START_REF] Stone | Emotional responses to disfigured faces: The influences of perceived anonymity, empathy, and disgust sensitivity[END_REF] noticed that negative emotions were evidenced but only in a condition of high anonymity, which underlines how studies examining affective reactions to facial disfigurement have to carefully consider self-presentation biases to limit their impact on the report of affective states.

Further, research mixing qualitative and quantitative studies was designed in order to better understand the affective dimension in the public stigma of facial disfigurement [START_REF] Rasset | Réactions psychologiques suite à la perception d′un visage présentant une défiguration (projet de thèse VISHU)[END_REF][START_REF] Rasset | Un)veiling distinct feelings: A typology of affective reactions while meeting people with facial disfigurement for the first time[END_REF]. In an initial study, Rasset and colleagues (2020) asked participants to provide all the affective reactions they could feel while meeting an individual with a facial disfigurement. The researchers used a self-presentation paradigm (i.e., a paradigm varying instructions to present oneself in a good vs. bad light; [START_REF] Jellison | A self-presentation approach to the fundamental attribution error: The norm of internality[END_REF] to ease socially undesirable affective states. In a second study, the affective states have been reduced to a six-factor typology comprising sympathy-related, surprise-related, neutrality-related, anxiety-related, disgust-related, and hostility-related affective states [START_REF] Rasset | Un)veiling distinct feelings: A typology of affective reactions while meeting people with facial disfigurement for the first time[END_REF]. On the whole, ambivalent affective reactions are evidenced towards individuals presenting a facial disfigurement.

Visual attention and stigmatizing affective reactions to visible differences

Concerning stigmatizing cognitive reactions, research has suggested a causal link such that the way people look upon disfigurement influences the way they react to it. For instance, [START_REF] Madera | Discrimination against facially stigmatized applicants in interviews: An eye-tracking and face-to-face investigation[END_REF] compared the attention paid to faces in an interview context to explore the outcomes of this attentional bias. Their study revealed that the attention distracted by disfigurement impaired the memory, which lowered the rating of the applicant with facial disfigurement.

Yet, concerning stigmatizing affective reactions, the impact of visual attention is not well understood. To our knowledge, only one study investigated the relationships between visual attention and the stigmatizing affective reactions. In an eye-tracking study, [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF] notably revealed that 1) disfigurement elicited negative emotions and 2) heightened attention on the disfigured part of the face was related to the intensity of the negative emotional experience reported. Their findings are supported by previous research showing that attention and emotion are related (for a review, see Kaspar & König, 2012). For instance, there is an attentional bias towards disgust-inducing words [START_REF] Charash | Attention bias for disgust[END_REF], and visual attentional capture of surprising stimuli (for a review, see [START_REF] Horstmann | The surprise-attention link: A review[END_REF]. In contrast, there is a paucity of research showing that attention may be influenced by emotional arousal (for a review, see [START_REF] Fenske | Affective influences of selective attention[END_REF]. As an exception to the above, researchers have demonstrated that when asked to focus on targets, people devaluate the stimuli they had to ignore, showing that the orientation of the attention influences the preferences [START_REF] Raymond | Emotional devaluation of distracting patterns and faces: A consequence of attentional inhibition during visual search?[END_REF]. Accordingly, the change in the visual attention towards faces with disfigurement could heighten affective reactions, especially the attentional capture of facial disfigurement could foster its devaluation. [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF] work unveiled possible links between visual attention towards disfigurement and stigmatizing affective reactions. Nevertheless, since they considered the disfigured part of the face in an isolated manner, they did not investigate the changes in the way the perceivers explore faces as a whole. Faces with disfigurement actually elicit heightened attention on the disfigured area but probably also weaken attention on other usually important parts (i.e., internal expressive features: eyes, nose, mouth). Affective states may thus positively correlate with attention on the disfigured part, negatively correlate with attention on the important central features, especially the eyes [START_REF] Kawakami | An eye for the I: Preferential attention to the eyes of ingroup members[END_REF], or a combination of both. In line with this assumption, when expanding upon their reexamination of previous data, [START_REF] Madera | To look or not to look: Acknowledging facial stigmas in the interview to reduce discrimination[END_REF] discussed a possible interruption in "normal face processing" to explain the impact of facial disfigurement on stigmatizing reactions. Thus, stigmatizing affective reactions to facial disfigurement may be related to a global change in the way people look at faces bearing a disfigurement rather than to the attentional capture triggered by facial disfigurement.

The aim of this research was thus to show that the perception of a facial disfigurement changes the regular perception of human faces, which is related to the stigmatizing affective reactions to faces with facial disfigurement.

Overview of studies and hypotheses

Both pilot and pre-registered experimental eye-tracking studies were conducted to analyze gaze toward faces with disfigurement and its impact on affective states. In the pilot study, we compared the impact of faces with and without disfigurement on visual attention. We hypothesized that facial disfigurement would change visual attention towards faces. Specifically, we assumed that more visual attention would be given to the disfigurement location when facing someone with a visible facial difference and the opposite pattern would arise for facial central features (eyes, nose, lips) and especially for the eyes [START_REF] Kawakami | An eye for the I: Preferential attention to the eyes of ingroup members[END_REF]. In the pre-registered study, we compared the impact of faces with and without disfigurement on affective states that they trigger. We expected that faces with visible difference would elicit more disgust-related, anxiety-related, hostility-related, surprise-related, and less sympathy-related and neutrality-related affective states (see A. [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF]. Further, we assumed that visual attention plays a mediation role in the relationship between disfigurement and the perception of affective states. Specifically, we expected not only quicker and longer visual attention to the disfigured part of the face but also slower and shorter visual attention to other important parts of the face (i.e., eyes, nose, lips) would mediate the effect of faces with facial disfigurement on affective states.

Pilot study

The pilot study examined effects of the presence of a disfigurement on visual attention to faces.

The participants' visual fixations were recorded while they were looking at different faces with and without disfigurement.

Method

Participants

Forty-eight undergraduate students volunteered to take part in the study without financial compensation. Seven participants were withdrawn due to calibration errors and three were withdrawn since they reported having a disfigurement. The remaining thirty-eight participants comprised 28 females (mean age = 20.30 years; SD = 2.21). Sample size in the present article was based on [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF], who used eye tracking to examine associations between attention in the area of disfigurement and emotions. A post hoc power analysis was conducted using the G*Power software package [START_REF] Buchner | G* Power 3.1 manual[END_REF] based on the smallest effect size (of a hypothesized effect) found in the study. The sample size of 38 was used as a basis for the statistical power analysis. For a medium effect size (d = .70) and standard parameters of α = .05, the post hoc power analysis indicated a power to detect obtained effects 1ß = .99, for a t test for difference between two dependent means.

Stimuli

The faces used in this experiment have been selected based on a pretest. A database containing faces was designed to assess the impact of disfigurement on visual attention. Our aim was to create a database of at least 16 faces. Indeed, we sought to show our participants faces with and without disfigurement (2 types of faces), when controlling for sex of the face (2 sexes) and side of the disfigurement (2: left or right). We therefore intended to generate a database comprising at least 2*2*2 = 8 faces, each one of which could be presented with and without a disfigurement (2 sets = 16 faces).

Therefore, we took pictures of 21 men and women in closely cropped portraits (head and top of shoulders). All had given their permission for their picture to be used and modified for research purposes. All targets were asked to display a neutral facial expression and to look straight at the camera.

The images were cropped to 300 X 300 pixels. The original faces have been modified using Adobe Photoshop (Adobe Systems, San Jose, California) to put a burning scar-like disfigurement on either their left or right cheek. We obtained a total of 42 faces 1 . The same face could thus be either presented with or without disfigurement.

In order to obtain a uniform sample of pictures, we pilot-tested the photographs on dimensions inferred automatically from face perception (see [START_REF] Saminaden | Afterimages of savages: Implicit associations between primitives, animals and children[END_REF] and on plausibility with a separate sample of 41 students (mean age = 19.46 years, SD = 1.41, F = 33). We divided our initial database of 42 faces in two counterbalanced sets. In this way, in each set, participants could see faces, the half of which presented a disfigurement, while the other half did not; half of them belonged to women, half of them belonged to men. For faces with disfigurement: half of the faces presented a disfigurement pattern on the left cheek, while half presented a disfigurement on the right one. Such precautions were taken to control for any effect of sex, specificity or lateralization of face perception.

Participants were asked to estimate the age, rated attractiveness (1 "not attractive at all", 7 "very attractive"), gender (1 "not feminine at all", 7 "very feminine"; 1 "not masculine at all", 7 "very masculine"), positive versus negative expressions (1 "very negative expression", 4 "neutral expression", 7 "very positive expression") and about the extent to which they believed that each photograph had been modified (1 "not at all", 7 "very much"). Since our goal was to make comparisons between the same faces with and without disfigurement, we decided to select the faces based on their realness and not on their other characteristics. On the 21 faces presenting a disfigurement, we decided to keep only those that were evaluated as being less modified than the average of the photographs (i.e., those with negative Z-score, each mean score being under the threshold of 4). On the initial number of photographs, 20 remained (i.e., 10 faces in their original and disfigured condition). These photographs were evaluated as being globally equivalent on all other criteria.

Material and Measures

We decided to use an eye-tracking technology, which is an unobtrusive and direct technique that allows researchers to not rely exclusively on self-report measures which are biased by social desirability when it comes to judging people with facial disfigurement [START_REF] Rahal | Understanding cognitive and affective mechanisms in social psychology through eye-tracking[END_REF]; A. Stone 1 These pictures have been created by a graphic designer who made several proposals of disfigurement for each face. A small study (N = 12) was conducted in order to select the most plausible disfigurement for each face.

& Potton, 2014). Eye movements were recorded using a Tobii X3-120 Eye-tracker (SR Research Ltd.), with a sampling rate of 120 hz (it returns a location every 8.33 ms). To classify these samples (raw data) and constitute either a fixation or a saccade (there is no motion so we assume there is no smooth pursuit), we applied an algorithm based on a velocity threshold called "I-VT Fixation Filter" (I-VT stands for Identification -Velocity Threshold). The default values chosen by Tobii in the I-VT implementation are as follows: VT = 30°/s, Maximum Angle between fixations = 0.5°, and the Minimum duration of fixation = 60 ms2 (Tobii Technology, 2012). Experimental recordings were preceded by a nine-point calibration procedure implemented in the Tobii Studio software. Participants were seated at a typical viewing distance of about 65 cm (head restraint was not used, since the Tobii system does not require it).

Eight Areas of Interest (AOIs) were manually drawn for the hairline, forehead, eyes, nose, target cheek, other cheek, lips and chin on each of the face displays. AOIs were hand drawn based on Fincher (2019). All features were defined as separate nonoverlapping AOIs (see Figure 1).

The following eye-tracking variables were selected 3 : Total dwell time (i.e., the sum of all dwell times in the one and same AOI over a trial). For each AOI, the mean total dwell times (in seconds) were calculated by averaging the total dwell times of all stimuli, distinguishing disfigured and original faces. Time to first fixation (i.e., the time period from entering the AOI until the first fixation is made).

For each AOI, the mean time to first fixation was calculated by determining, relative to the onset of the trial, how long it took for participants to initially fixate in an AOI with lower values representing a faster orientation of the gaze. If an AOI was not fixated in the 5 s window, we gave it a value of 5 for the time to first fixation analyses. The mean time to first fixation for each AOI was calculated separately for disfigured and original faces. These eye-tracking variables have been selected due to the different inferences that they allow with regard to attention, and could thus reinforce the assumption of a change in visual attention if evidenced. Indeed, beyond being common measures in the field (for a review, see [START_REF] Asaad | Eye-tracking technology in plastic and reconstructive surgery: A systematic review[END_REF], the total dwell time can be regarded as an indicator of processing depth and effort, whereas time to first fixation is an indicator of information weighting [START_REF] Rahal | Understanding cognitive and affective mechanisms in social psychology through eye-tracking[END_REF].

Procedure

Participants were recruited directly on the campus by the experimenter. They volunteered to participate and provided an informed consent. All participants were naïve to the hypotheses of the experiment. All participants were tested individually in a focus room. After the eye-tracking calibration, they were left alone until the end of the procedure.

During the training phase, participants viewed four faces (2 men and 2 women) of celebrities to familiarize them with the task. Then, during the experimental phase, they were presented 10 faces. As we did with the photographs selection of the pretest, we divided our initial database of 20 faces in two counterbalanced sets. In this way, participants in each set could see faces, half of which presented a disfigurement, while the other half did not; half of them were women faces, while the other half were men faces. For faces with disfigurement: half of the disfigurement were placed on the left cheek, half of them were placed on the right one. Such precautions were taken to control for any effect of sex, specificity or lateralization of face perception.

Each trial consisted of the following sequence: a fixation cross in the center of the screen and displayed for 2 seconds, then the face displayed for 5 seconds. So as to sustain their attention without biasing their visual exploration towards any specific feature, we asked the participants to rate the extent to which each person was considered as being fully human by people in general 4 . Each trial lasted approximately 10 seconds. The length and location of the participants' visual fixations were recorded for each AOI while they viewed each face. After providing socio-demographic information, participants were fully debriefed and thanked.

Statistical analysis

A mixed within-participant design was employed. The within-participant factors were type of face (original vs. disfigured) and Area of Interest (AO: 8 areas). Statistical analyses were performed using Jasp software (Jasp Team, 2020). A 2 (Type of face: original, disfigured) X 8 (AOI: hairline, forehead, eyes, nose, target cheek, other cheek, lips, chin) repeated measures ANOVAs were run on each eye tracking variables. Greenhouse-Geisser corrections were used to correct for analyses that violated the assumption of sphericity.

Results

There was a main effect of Type of face on the total dwell time, a main effect of AOI on the total dwell time and an interaction between Type of face and AOI (see Table 1). Simple effects tests revealed thatas expectedthe target cheek was fixated upon for longer durations when the face included a disfigurement than when it was presented in its original condition (see Figure 2). The opposite pattern emerged for eyes, nose, other cheek and lips but no effect was evidenced for hairline, forehead, nor chin. A bigger effect size was observed for eyes compared to nose, other cheek and lips.

There was no effect of Type of face on the time to first fixation (see Table 1). There was a main effect of AOI on time to first fixation and an interaction between the Type of face and AOI. Simple effects tests revealedas expected -that the targeted cheek received a quicker look when the face was presented with a disfigurement than when it was presented its original condition (see Figure 3). The opposite pattern emerged for eyes, nose, other cheek, and lips but no effect was evidenced for hairline, forehead, nor chin. A bigger effect size was observed for lips compared to eyes, nose and other cheek.

Table 1. Means (Standard Deviations) of eye-tracking variables for each AOI depending on the type of face evaluated in

Pilot Study. Results of repeated measures ANOVA are also included.

Note. Values for total dwell time and time to first fixation are in seconds. Contrary to total dwell time, lower scores indicate more attention for time to first fixation. Time to first fixation for the eyes and nose has to be considered carefully because for some faces the participants were asked to focus of these areas at the onset of each image.

Discussion

As expected, participants gazed more quickly and longer at the cheek when it included a disfigurement. Interestingly, people also gazed more slowly and for less time at the eyes, nose, cheek without disfigurement and lips, whereas no effect was evidenced for hairline, forehead, or chin. The patterns of gaze in the pilot study replicated former studies showing that facial disfigurement affects attentional allocation [START_REF] Boutsen | Differential impact of disfiguring facial features on overt and covert attention[END_REF]). Yet, changes in visual attention were not limited to the location of the disfigurement but rather expanded to the internal expressive features of the face. After the disfigured area, the effect was stronger for the eyes as previously evidenced for different pathologies in other studies (e.g., [START_REF] Meyer-Marcotty | Visual Face perception of adults with unilateral cleft lip and palate in comparison to controls-An eyetracking study[END_REF]. Following [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF], we decided to go beyond the attention paid to the disfigurement and further explore the affective reactions to photographs showing people with disfigurement potentially associated with the attentional bias evidenced in this pilot study. Specifically, we explored the link between the visual attention paid to the internal features of the face (eyes, nose, mouth, target cheek and other cheek) and affective states.

Pre-registered study

The pre-registered study examined the mediating role of visual attention paid to faces with and without disfigurement on stigmatizing affective reactions. More specifically, this study was designed to replicate the impact of facial disfigurement on the allocation of visual attention towards the internal features of the face found in the pilot study and to determine if the formerly evidenced changes were related to affective reactions to facial disfigurement. The participants' visual fixations were recorded while they viewed faces with and without a disfigurement at which time they had to estimate the affective reactions that the faces they were seeing might trigger in others.

Method

Participants

Ninety-nine participants volunteered to take part in the study without any financial compensation. Seven participants were withdrawn due to calibration errors and three were withdrawn since they reported having a disfigurement. The remaining 89 participants comprised 52 women and 37 men (M = 19.83 years; SD = 1.79). G*Power [START_REF] Buchner | G* Power 3.1 manual[END_REF] was used to calculate a priori sample size, using a power of 95% for a large effect size of f = 0.40 (A. [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF]. This study was preregistered at https://osf.io/ybvpa/?view_only=5b77a73587094a9f9c22881c72f6289a.

Material

Since our experimental design required each face to be presented 18 times (i.e., one presentation for each affective state), using too many stimuli would have lengthened the duration of the experimental procedure and potentially exhausted the participants. Therefore, following formerly used procedures (A.

Stone & Potton, 2019), we selected four faces in our picture database (two men and two women in both disfigured and original conditions). To avoid a possible intergroup bias based on age (our target sample was made up of university student, i.e., young people of college-age), we decided to choose faces in the upper middle age range of our database constituted from our pre-test (M = 38.67, SD = 14.32). Faces were in the middle of the expression range (M = 4.10, SD = 1.21). Perception of gender was homogeneous (for the female faces: M = 6.62, SD = 0.16; for the male faces: M = 6.75, SD = 0.23).

Faces were perceived as similar on attractiveness (M = 3.40, SD = 0.52). Only five of the eight AOI drawn in the pilot study were used in the pre-registered study (eyes, nose, mouth, target cheek and other cheek), retaining the exact same size. Eye movements were recorded using the same Tobii X120 Eyetracker.

Measures

The same eye-tracking variables were selected (total dwell time and time to first fixation). As disfigurement did not impact visual attention toward hairline, forehead, and chin in the pilot study, only five AOIs were kept to gain statistical power in the pre-registered study (i.e., eyes, nose, target cheek, other cheek, and lips).

Participants were asked to report their perception of 18 different emotions which the faces may trigger (see [START_REF] Rasset | Un)veiling distinct feelings: A typology of affective reactions while meeting people with facial disfigurement for the first time[END_REF]): sympathy-related (i.e., sympathy, benevolence, tenderness; Cronbach's α = .83), anxiety-related (i.e., fear, anxiety, apprehension; α = .90), surpriserelated (i.e., surprise, astonishment, curiosity; α = .79), hostility-related (i.e., contempt, hate, hostility; α = .83), disgust-related (i.e., disgust, repulsion, nausea; α = .87) and neutrality-related (i.e., neutrality, calm, indifference; α = .70). Participants had to report "to which extent most people that see the person on the photograph may experience" the written emotion on a numeric scale of 1-7 (corresponding to: 1-"totally disagree", 4 -"neither agree nor disagree", 7 -"totally agree"). A hetero-perception formulation was chosen to reduce social desirability which may bias the participants' responses (for a similar procedure, see [START_REF] Louvet | Social judgment of people with a disability in the workplace[END_REF]. As people infer how other people would react based on their own reactions [START_REF] Nickerson | How we know-and sometimes misjudge-what others know: Imputing own knowledge to others[END_REF], this strategy was used to encourage participants to report their emotional reactions without trying to emphasize desirable ones at the expense of less desirable ones.

Procedure

The procedure was globally the same as that of the pilot study except for the instructions given to the participants. Indeed, each trial consisted of the following sequence: a fixation cross in the center of the screen displayed for 2 seconds, then a face displayed for 5 seconds. Then the participants were asked to report the extent to which they estimated that people facing this person may feel one of the above-mentioned affective states. Each face was presented 18 times in fixed random order, once for each affective state, bringing the total of trials to 72. Each trial lasted approximately 10 seconds. The length and location of the participants' visual fixations were recorded for each AOI while participants were viewing each face. After providing socio-demographic information, participants were fully debriefed and thanked.

Statistical analysis

Several analyses were run. First, the hypothesized impact of disfigurement on visual attention was tested. For that purpose, a 2 (Type of face: original, wih disfigurement) X 5 (AOI: eyes, nose, target cheek, other cheek, lips) repeated measures ANOVAs was run on each eye tracking variables. Greenhouse-Geisser corrections were used to correct the analyses that violated the assumption of sphericity. Second, the hypothesized impact of disfigurement on affective states was tested. For that purpose, a 2 (Type of face: original, with disfigurement) X 6 (Type of emotion: sympathy-related, surprise-related, neutrality-related, anxiety-related, disgust-related, hostility-related) repeated measures ANOVA was run. Finally, the relationship between affective states and visual attention was explored through regression analyses. There was no effect of Type of face on the total dwell time (see Table 2). There was a main effect of AOI on total dwell time and an interaction between Type of face and AOI. Simple effects tests revealed thatas expectedthe targeted cheek was fixated for longer durations when the face included a disfigurement than in its original condition (see Figure 2). The opposite pattern emerged for eyes, nose, other cheek and lips. Similarly to the pilot study, a bigger effect size was observed for eyes compared to nose, other cheek and lips showing the stable negative impact disfigurement has on visual attention towards the eye area. Note. Values for total dwell time and time to first fixation are in seconds. Contrary to total dwell time, lower scores indicate more attention for time to first fixation. Time to first fixation for the eyes and nose has to be considered carefully because for some faces the participants were asked to focus of these areas at the onset of each image.

Results

Impact of disfigurement on visual attention

There was a main effect of Type of face on the time to first fixation, a main effect of AOI on the time to first fixation, and an interaction between Type of face and AOI (see Table 2). Simple effects tests revealed thatas expected -the targeted cheek was gazed at more quickly when the face presented a disfigurement than when it was presented in its original condition (see Figure 3). The opposite pattern emerged for eyes, other cheek, and lips, but no effect emerged for the nose.

Impact of disfigurement on affective states

There was no effect of Type of face on affective state perception. There was a main effect of Type of emotion and an interaction between Type of face and Type of emotion (see Table 3 for details of all analyses and descriptive statistics). Simple effects tests revealed thatas expectedsurprise, disgust, anxiety, andto a lesser extent -hostility were more strongly endorsed when the face included a disfigurement than when the face was presented in its original condition. The opposite expected pattern emerged for neutrality, but no difference was evidenced for sympathy.

Affective state

Sympathyrelated

Surpriserelated

Neutralityrelated

Anxietyrelated

Disgustrelated

Hostility

Relationship between visual attention and affective states

First, in order to examine correlations between emotion perception and the indicators of visual attention, scores for all measures were computed by considering the difference score of disfigured faces minus original faces means (for a similar procedure, see A. [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF]. Higher (positive)

scores systematically indicate greater emotion perception / attention to faces with disfigurement whereas lower (negative) scores refer to greater emotion perception / attention to original faces. Only small correlations emerged between emotion perception and visual attention to the targeted cheek, indicating that the longer participants concentrated on the disfigurement, the less they expressed neutrality and the more they expressed surprise when viewing a disfigured face vs. original face (see Table 4). We decided not to run the pre-registered mediational analysis because of a lack of power. Specifically, our correlations were globally not high enough and our sample size (N = 89) was too small to obtain a satisfactory statistical power, even for a mediational analysis computed on the basis of our strongest correlation (i.e., r = .40; 1 -β = .16; [START_REF] Schoemann | Determining power and sample size for simple and complex mediation models[END_REF].

AOI

Sympathyrelated

Surpriserelated

Neutralityrelated

Anxietyrelated

Disgustrelated

Hostilityrelated M (SD)

Exploratory analyses

Linear regressions with interaction analyses were conducted in order to go beyond the main effect of visual attention on each AOI in isolation. Indeed, we hypothesized that the impact of heightened attention to the disfigured part of the face had on affective reactions could depend on the other parts that were also attentionally neglected, such as the eyes. Since we recorded measures on five AOIs, numerous combinations were possible. As we were interested in the impact of disfigurement and as we had shown that attention was mainly diverted from the eye region, we decided to investigate this specific interaction and its associations with affective reactions. We used the scores computed for the correlational analyses.

Considering the surprise-related affective state, results showed a significant effect of total dwell time difference scores on the target cheek region (see Table 5 for details). The association indicated that the more participants stared at the disfigured part of the face, the more they reported surprise-related affective states watching disfigured faces. A marginally significant interaction effect also occurred revealing that this effect was amplified when participants stared less at the eyes of the disfigured faces.

Considering disgust-related state affect, an interaction occurred, revealing a hybrid effect. The more participants stared at the disfigured part of the face, the more they reported disgust-related affective states though the effect was present only when they stared less at the eyes of those faces5 .

Discussion

The pre-registered study replicates the impact of disfigurement on visual attention towards faces evidenced in the pilot study. Disfigurement had a negative impact on attention that is usually allocated to the eye regions. Moreover, disfigurement impacted perceptions of disgust-related, surprise-related, anxiety-related, and, to a lesser extent, hostility-related affective states. In our pre-registered study, disfigurement did not impact sympathy-related affective states. Indeed, the participants did not expect significantly less sympathy for faces with a disfigurement than for faces without a disfigurement. This result was unexpected since we tried to lessen potential social desirability biases. Participants did not report more sympathy either, maybe because they already displayed high scores of sympathy for the other faces. Put differently, individuals expressed less neutrality, more surprise, disgust, anxiety, and hostility and as much sympathy when viewing faces with a disfigurement. These results support the idea that physical stigma often elicits ambivalent reactions (see [START_REF] Dovidio | Stigma: Introduction and overview[END_REF][START_REF] Weiner | An attributional analysis of reactions to stigmas[END_REF].

Like [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF], we found small correlations between attention paid to the disfigured part of the face and affective states of surprise and neutrality (a negative correlation for the latter), but we did not find any link with the other affective states. Although disfigurement influences both attentional and affective reactions, we cannot affirm that both reactions are directly correlated. One may argue that, contrary to [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF], we did not manipulate disfigurements severity.

Nevertheless, as affective reactions towards disfigurement were clearly elicited, we doubt that these correlations would occur only for severe disfigurement and not for a mild one. Moreover, because of the hetero-perception instruction, one may argue that participants expressed affective states that did not truly reflect how they actually felt. Yet, this instruction has been successfully used by other authors (e.g., (Louvet et al., 2009b) and even if participants were not expressing their own affective states, these reactions still reflected social stigma. That is, participants may have expressed stigma at a social rather than individual level (i.e., through cultural representations) which is still of high interest for our research purpose.

General Discussion

The aim of this research was to investigate cognitive and affective reactions towards people living with a visible difference. Specifically, in order to broaden the results obtained by [START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF], we wanted to explore how these processes might relate to each other. We hypothesized that visibly different faces should engender a different pattern of visual attention, which should be related, in turn, with affective experience. In two studies, we have demonstrated that disfigurement impacts the way individuals explore faces. Disfigurement also impacts perceptions of disgust-related, surpriserelated, anxiety-related, neutrality-related, and, to a lesser extent, hostility-related affective states.

Nevertheless, a few weak direct correlations among these cognitive and affective reactions were evidenced. Both the impact of disfigurement on the visual exploration of faces, and the impact of a change in visual attention towards faces with disfigurement will be discussed.

First, the impact of disfigurement on the visual exploration of faces has to be considered carefully. While showing modifications in the time to first fixation and total dwell time on disfigured faces, we highlighted changes in information weighting, in information depth and efforts made to process faces with disfigurement [START_REF] Rahal | Understanding cognitive and affective mechanisms in social psychology through eye-tracking[END_REF]. Attention is not merely attracted to the disfigured area but it is also diverted from the other areas, notably the eyes. We have known for a long time that individuals gaze preferentially at the eye area (e.g., [START_REF] Janik | Eyes as the center of focus in the visual examination of human faces[END_REF]. For example, people gaze longer at the eye area of their ingroup members vs. outgroup members (e.g., [START_REF] Kawakami | An eye for the I: Preferential attention to the eyes of ingroup members[END_REF]. The eyes play key roles [START_REF] Itier | Attention to eyes in face perception[END_REF]. For example, from others' gaze direction, we infer the relative importance of objects in the environment (see [START_REF] Hugenberg | Faces are central to social cognition[END_REF]. At a neural level, research has shown that eyes are particularly relevant for the configural processing of faces [START_REF] Itier | Early face processing specificity: It's in the eyes![END_REF]. As facial disfigurement not only attracted attention to the disfigured area but also diverted it from other facial features, including the eyes, it may have violated the typical configuration of features, jeopardizing face perception as a whole [START_REF] Schwarzer | Gaze behavior in analytical and holistic face processing[END_REF]. It is noteworthy that failure in processing a face in a face-like manner is also called "perceptual dehumanization", which has detrimental outcomes for the targets [START_REF] Bernard | Objectifying objectification: When and why people are cognitively reduced to their parts akin to objects[END_REF][START_REF] Fincher | Social antecedents and perceptual consequences of how we look at others[END_REF][START_REF] Haslam | Dehumanization and infrahumanization[END_REF]. As well, researchers using fMRI have found evidence of a possible neural mechanism underlying dehumanization of people with facial disfigurement [START_REF] Hartung | Behavioural and neural responses to facial disfigurement[END_REF]Workman et al., 2020). Thus, further research should explore the dehumanization process of people living with a visible difference as a result of a loss in eye-contact or a change in face processing as this may -at least partly -explain the antisocial behaviors that people living with a visible difference sometimes have to endure (Changing Face, 2021).

Second, the impact of this change in visual attention towards faces with disfigurement on the affection reactions they elicit is unclear. Based on previous work, we hypothesized that visual attention would mediate the relationship between disfigurement and stigmatizing affective reactions, as it does between disfigurement and other stigmatizing reactions [START_REF] Madera | Discrimination against facially stigmatized applicants in interviews: An eye-tracking and face-to-face investigation[END_REF]. The results of this research rather show that interactions between attention paid to different parts of the face and disfigurement may influence affective reactions. In two studies we showed that disfigurement did not merely attract visual attention to its location on a face, but also diverted gaze from internal features, including the eyes. In our exploratory analyses, we investigated a moderating role of visual attention to the eyes on the impact of visual attention to the disfigured area on affective reactions towards the face.

We showed that the interaction of attention towards these two elements might be critical. For example, it seems that faces with disfigurement elicit disgust only when people stare at the disfigurement without looking at the eyes. As we did not experimentally manipulate gaze and as we did not plan this analysis, the role of this moderation is tentative. This loss in eye-to-eye contact hypothesis should thus be better investigated in future studies. More generally, and in line with the weak correlations observed, the direction of the effect also has to be discussed. A higher number of studies show the reverse effect of the one we hypothesized in this research, i.e., that emotions can influence visual attention (for a review, see Kaspar & König, 2012). For instance, unconscious negative priming influences the observer's gaze pattern [START_REF] Hirschberger | Looking away from death: Defensive attention as a form of terror management[END_REF]. It is possible that viewing the first face with disfigurement influenced the emotional reactions that, in turn, influenced the subsequent visual patterns. Nevertheless, in real settings, people start by seeing the disfigurement before reacting to it. Thus, in line with the two-factors theory of emotions [START_REF] Schachter | Cognitive, social, and physiological determinants of emotional state[END_REF], it is also possible that disfigurement acted as a stimulus fostering an undifferentiated physiological arousal, and that the emotion elicited in turn influenced gaze behavior. However, experimental investigations of these alternative assumptions are needed to arbitrate between these various possibilities.

Potential limitations and future directions

There are variables that were not assessed in this study that may have had an influence, including those related to facial disfigurement, the perceiver, and the context. First, characteristics of the facial disfigurement may play a role. For instance, the perceived severity of the disfigurement and its location on the face can have an influence on the affective reactions [START_REF] Shanmugarajah | The role of disgust emotions in the observer response to facial disfigurement[END_REF][START_REF] Stone | Emotional responses to disfigured faces and disgust sensitivity: An eye-tracking study[END_REF]. As the results of this research highlight the major role of attention to the eyes, the impact of a disfigurement located on the eyes area warrants consideration. One could think that such disfigurements hinder the strategies developed by the perceiver aimed at reducing the impact of the disfigurement. Further studies are thus needed to highlight this issue. As our studies compared faces in their original setting vs. disfigured faces, some affective states may have been elicited by the mere presence of a mark on the targeted face (e.g., curiosity). Although previous research has shown that the salience of disfigurement alone does not fully explain the impact on perceivers' reactions, future research would benefit from showing disfigurement-specific effects on affective states in comparison to visible non-stigmatizing cues [START_REF] Boutsen | Differential impact of disfiguring facial features on overt and covert attention[END_REF]. Because we used a relatively small number of stimuli, it will be necessary to replicate this work with other images in order to attest the stability of the effect.

Second, considering the perceiver, several dispositional variables may be influential. As outlined earlier, social desirability is known to bias responses to questionnaires about the emotions felt while perceiving people with a facial disfigurement (A. [START_REF] Stone | Emotional responses to disfigured faces: The influences of perceived anonymity, empathy, and disgust sensitivity[END_REF]. We tried to circumvent this bias by using a hetero-perception formulation, which has limitations. Further research is needed to highlight these affective states with measures upon which people do not have much control, such as physiological indexes (e.g., cardiovascular reactivity; [START_REF] Blascovich | Perceiver threat in social interactions with stigmatized others[END_REF]. Although promising, the lack of specificity of these measures requires researchers to combine them in order to show a single affective response, which is even more problematic for stimuli that elicit varied affective states.

However, the impact of social desirability bias highlights the role of motivations in the acknowledgment of stigmatizing affective reactions and raises questions about the impact of variables such as attitudes toward disfigurement, which should be evaluated in future research. Some disorders may also have an influence on visual attentions and stigmatizing affective reactions, e.g., neurodevelopmental disorder such as the autism spectrum disorder [START_REF] Duan | Visual attention analysis and prediction on human faces for children with autism spectrum disorder[END_REF] and might be incorporated within future work.

Finally, the context may also be influential. We have shown that the eyes were less deeply processed when a disfigurement was added to a face, but our material was exclusively made up of photographs. Since the facial feature capturing the perceivers' interest varies according to the context, future studies would benefit from replicating this experiment while using dynamic situations for example such as videos (see [START_REF] Vo | Do the eyes really have it? Dynamic allocation of attention when viewing moving faces[END_REF]. Future studies investigating real interactions are also needed (e.g., with wearable eye-trackers).

Implications

By deepening our understanding of public stigma, this research has broad implications. First, interfacing with their social environment may be quite challenging for people with a facial disfigurement and a better understanding of reactions of their perceivers can help manage interactions. For that purpose, social skills training programs have been developed notably in order to ease social interactions (see A. [START_REF] Clarke | Psychosocial aspects of facial disfigurement: Problems, management and the role of a lay-led organization[END_REF]; see also the work done by charity Changing Faces). Second, to anticipate one's own reactions is also crucial for people who may interact with people bearing a facial disfigurement and who do not want to discriminate against them. Acknowledging one's reactions, understanding the impact they may have on people with facial disfigurement in order to regulate them, can be crucial to ease the relationships, especially in situations of integration of people with disfigurement. Finally, this research also provides evidence that training fostering eye-contact deserves further attention, as this training could ease social interactions for both people with FD and people without FD.

Conclusion

Among the stigmatizing reactions that people living with facial disfigurement have to endure, two that are especially difficult to deal with are stares and manifestations of affective reactions. A better understanding of these reactions is thus needed in order to improve the participation in the social life of visibly different people. Our findings stress the issue of a global change in the way people with visibly different faces are perceived. In this regard, the effect of an eye-to-eye contact loss may be especially detrimental. Further considerations are thus needed in order to know if an eye-to-eye contact retrieval could benefit social interactions with visibly different people.

Figure 1 .

 1 Figure 1. Panel left illustrates a schematic of the regions of interest used in analysis. Panel right illustrates Heatmaps of faces with(out) disfigurement. Heatmaps have been generated based on fixation counts. See the online article for the color version

Figure 2 .

 2 Figure 2. Mean total dwell time occurring in each AOI in both studies, with standard deviations.

Figure 3 .

 3 Figure 3. Mean time to first fixation occurring in each AOI in both studies, with standard deviations.

Table 2 .

 2 Means (Standard Deviations) of eye-tracking variables for each AOI depending on the type of face evaluated in Preregistered Study. Results of repeated measures ANOVA are also included.

	AOI	Eyes	Nose	Target Cheek	Other cheek	Lips	All
	Type of face
	Total dwell Time. Main effect of Type of Face: F(1, 88) = 0.44, p = .51 , η 2 G < .001
	Original	1.96 (0.82)	0.97 (0.57)	0.22 (0.20)	0.28 (0.26)	0.31 (0.18)	0.74 (0.82)	Main effect of AOI
	Disfigured	1.72 (0.77)	0.86 (0.52)	0.71 (0.38)	0.21 (0.18)	0.25 (0.15)	0.75 (0.72)	F(1.53, 134.36) = 167.13 p < .001
	All	1.84 (0.80)	0.91 (0.54)	0.47 (0.39)	0.24 (0.23)	0.28 (0.17)	0.75 (0.77)	η 2 G = .63
	Simple effet test	F = 40.74 p < .001 d = 0.68	F = 12.63 p < .001 d = 0.38	F = 325.58 p < .001 d = 1.91	F = 18.84 p < .001 d = 0.46	F = 31.32 p < .001 d = 0.59		Interaction effect F(2.37, 208..42) = 98.12 p < .001 η 2 G = .07
	Time to First Fixation. Main effect of Type of Face: F(1, 88) = 60.64, p < .001 , η 2 G = .01
	Original	1.24 (0.90)	1.08 (0.79)	4.02 (0.73)	3.81 (0.84)	3.56 (0.72)	2.74 (1 .52)	Main effect of AOI
	Disfigured	1.34 (0.89)	1.11 (0.78)	2.65 (0.81)	4.04 (0.67)	3.78 (0.68)	2.53 (1.43)	F(1.93, 169.84) = 319.54 p < .001
	All	1.29 (0.90)	1.09 (0.78)	3.33 (1.02)	3.92 (0.77)	3.67 (0.70)	2.66 (1.48)	η 2 G = .71
	Simple effet test	F = 5.47 p = .02 d = 0.25	F = 0.21 p = .64 d = 0.05	F = 456.72 p < .001 d = 2.26	F = 16.39 p < .001 d = 0.43	F = 19.59 p < .001 d = 0.47		Interaction effect F(3.40, 299.42) = 147.86 p < .001 η 2 G = .13

Table 3 .

 3 Means (Standard Deviations) of affective states measures depending on the type of face evaluated in Pre-registered Study. Results of repeated measures ANOVA are also included.

Table 4 .

 4 Means

(Standard Deviations) and correlations (Bravais-Pearson's r)

between and of eye-tracking variables difference scores for each AOI and affective states difference scores in Pre-registered Study.

  Note. Higher (lower) values indicate either positive (negative) correlations between eye-tracking variables and affective states for disfigured faces. *p < .05, **p < .01, ***p < .01.

	Total dwell time
	Eyes	.10	-.08	.23*	-.04	-.02	-.03	-0.23 (0.34)
	Nose	.01	.06	-.26	-.02	.11	.03	-0.11 (0.28)
	Target cheek	.07	.30***	-.35***	.13	.15	.03	0.49 (0.25)
	Other Cheek	-.23*	-.13	.10	.06	-.04	-.05	-0.07 (0.15)
	Lips	-.16	-.13	.10	.14	-.04	.00	-0.06 (0.10)
	Time to first fixation
	Eyes	.13	-.02	-.14	-.08	.01	-.15	0.10 (0.39)
	Nose	-.04	.14	.11	-.04	-.04	-.04	0.03 (0.54)
	Target cheek	.07	-.30***	.35***	-.06	-.15	-.09	-1.37 (0.60)
	Other Cheek	.09	.02	-.06	-.06	.07	.11	0.23 (0.53)
	Lips	.19	.15	-.21*	-.09	.04	-.11	0.21 (0.46)
	M (SD)	0.02 (1.34)	2.07 (1.73) -1.46 (1.34) 1.14 (1.33)	1.74 (1.44)	0.28 (1.20)

Table 5 .

 5 Linear regression coefficient table, total dwell time difference scores on AOI as independent variables, and heightened affective states difference scores as dependent variable inPre-registered Study.

		Model Summary	t	p	β	95% CI
	Disgust	R 2 = .07, F(3,85) = 2.12, p = .10
	Intercept		11.09	<.001***
	Target cheek		0.95	.35	0.10	-0.11 ; 0.32
	Eyes		-0.19	.85	-0.02	-0.23 ; 0.19
	Target cheek x Eyes		-2.07	.04*	-0.29	-0.56 ; -0.01
	Surprise	R 2 = .12, F(3,85) = 3.83, p < .05
	Intercept		11.37	<.001***
	Target cheek		2.41	.02*	0.25	0.04 ; 0.46
	Eyes		-0.55	.58	-0.06	-0.26 ; 0.14
	Target cheek x Eyes		-1.67	.10 †	-0.22	-0.49 ; -0.04
	Note. Higher (lower) values indicate either positive (negative) correlations between eye-tracking variables and affective states
	for disfigured faces. †p < .10, *p < .05, ***p < .01.

One may argue that another requirement could be that saccades occur at least 100 ms after image onset, since fixations with shorter latencies may reflect saccades or anticipatory eye movements (e.g.,[START_REF] Gao | Eyes on the bodies: An eye tracking study on deployment of visual attention among females with body dissatisfaction[END_REF]). Yet, since 1) we were interested in comparing the amount of attention between each condition (with and without disfigurement) but not between the areas, 2) this deletion was not planned,

3) this deletion is not routinely made in the field (or not specified), we decided to keep the first 100 ms. Nevertheless, we tested this additional requirement on our data and observed no change. Additional analyses are available upon request.[START_REF] Wang | Objective method for evaluating orthodontic treatment from the lay perspective: An eye-tracking study[END_REF] Initially, we also conducted our analyses on the eye-tracking indicator "proportion of fixations" -i.e., the occurrence with which an AOI is looked at. As this indicator was highly correlated with the "total dwell time", we decided to only present the results of the total dwell time in this article. Results with proportions of fixation are available as supplementary materials on https://osf.io/ybvpa/?view_only=5b77a73587094a9f9c22881c72f6289a.

Although there was no hypothesis formulated regarding this measure, we planned exploratory analyses that could not be run following a problem in data recording.

To go further in these exploratory analyses, we have also investigated the moderating role of the other AOIs, which were not significant.

Acknowledgments

The authors thank Anne Lacherez for realizing the stimuli, Julie Brisson for providing both the experimental device and her know-how to be able to use it, Celine Quint for the proofreading, and Antoine Luu for his help with the software. This work was supported by the Normandy Region [grant RIN 100%, 2018] and the Foundation Gueules Cassées 2020].

Ethics

Ethical approval was obtained from the local institutional review committee of the laboratory of CRFDP -University of Rouen Normandy (N° 2019-06-A). Every participant gave their informed consent to participate in the studies.

Open Practices

All data and materials have been made publicly available via the Open Science Framework and can be accessed at this link: https:// osf.io/ybvpa/?view_only= 5b77a73587094a9f9c22881c72f6289a. All stimuli that the communicators generated for the experiments and reported in this article are available


Gaoyu Liu

# Yakui Deng

# Bin Ni

Giao T M Nguyen

Cédric Vancaeyzeele

Annie Brûlet

Frédéric Vidal

Cédric Plesse
email: cedric.plesse@cyu.fr

Min-Hui Li
email: min-hui.li@chimieparistech.psl.eu

Electroactive Bi-functional Liquid Crystal Elastomer Actuators

Keywords: liquid crystal elastomer (LCE), electroactive actuator, bending, contraction, conducting polymer

Liquid crystal elastomers (LCEs) with promising applications in the field of actuators and soft robotics have been reported. However, most of them are activated by external heating or light illumination. The examples of electroactive LCEs are still limited; moreover, they are monofunctional with one type of deformation (bending or contraction). We report here on a trilayer electroactive LCE (eLCE) by intimate combination of LCE and ionic electroactive polymer device (i-EAD). This eLCE is bi-functional and can perform either bending or contractile deformations by the control of the low-voltage stimulation. By applying a voltage of ±2 V at 0.1 Hz, the redox behavior and associated ionic motion provide a bending strain difference of 0.80%. Besides, by applying a voltage of ±6 V at 10 Hz, the ionic current-induced Joule heating triggers the muscle-like linear contraction with 20% strain for eLCE without load.

With load eLCE can lift a weight of 270 times of eLCE-actuator weight, while keeping 20% strain and affording 5.38 kJ•m -3 work capacity. This approach of combining two smart polymer technologies (LCE and i-EAD) in a single device is promising for the development of smart materials with multiple degrees of freedom in soft robotics, electronic devices, and sensors.

Introduction

In the last decades, the stimuli-responsive smart materials that actively undergoes a predetermined change in their geometry or dimensions upon external stimulations are the focus of the research of many scientists and engineers. [1] Among them, liquid crystal elastomers (LCEs), especially the uniformly ordered nematic LCEs, possess fascinating features combining the entropic elasticity of elastomers and the ability to undergo a reversible and alignment-dependent shape-change behavior of liquid crystal polymers (LCPs). [2][3][4][5][6][7] Thus, LCEs show promising potentials as smart actuators, for example, those contracting/expanding linearly like mammalian muscles. [8] LCEs are thermo-active because they are based on thermotropic liquid crystals. [9][10][11][12] When photo-responsive units like azobenzene are incorporated in the LCE chemical structures, LCEs exhibit photo-mechanical properties. [13][14][15][16][17][18][19][20][21] Accordingly, temperature and light are the most used stimuli in actuators and sensors made of LCEs. However, in the world of actuators, electrical energy is the most convenient and the most in demand stimuli. Indeed, the nature does use electrical impulses between nerves and muscles/skins for actuation and sensing with extraordinary efficacy, and electrical stimulation is also more widely utilizable as driving forces in industrial devices. Efforts have been made to achieve electroactive LCEs (eLCEs). The first example of eLCE is made of ferroelectric smectic LCE film of a thickness of 100 nm, where 4% strain has been achieved under an electric field of 1.5 MV m -1 by electroclinic effect of ferroelectric LC. [22] Then, most of eLCEs were electrothermal systems developed from nematic LCEs by introducing Joule effect, [START_REF] Finkelmann | Smart Structures and Materials: Electroactive Polymer Actuators and Devices[END_REF][START_REF] Agrawal | [END_REF][25][26][27][28][29][30][31][32][33][START_REF] Greco | Bending actuation of a composite liquid crystal elastomer via direct Joule heating[END_REF][START_REF] Greco | [END_REF][36][37][38] or Peltier effect [39] . These eLCEs achieved the strain (typically 10%-100%) and stress of normal nematic LCEs via electrothermal effect under low voltage (typically < 10 2 V). The most studied electrothermal nematic eLCEs use Joule heating and can be divided into two categories, one is electronic eLCEs where the Joule effect is introduced by highly conducting materials like graphite, [START_REF] Finkelmann | Smart Structures and Materials: Electroactive Polymer Actuators and Devices[END_REF] carbon black, [START_REF] Agrawal | [END_REF]25] carbon nanotubes, [26] metal wires, [27][28][29][30] liquid metals [31][32][33] ; another one is ionic eLCEs where LCE is combined with ionic electroactive polymer device (i-EAD) [START_REF] Greco | Bending actuation of a composite liquid crystal elastomer via direct Joule heating[END_REF][START_REF] Greco | [END_REF][36][37][38] .

The i-EAD is also an interesting smart material by itself due to its low driving voltage (in the range of few volts) and the ability to operate in open-air and to convert either electrical stimulation into reversible large bending deformations (actuator) or mechanical stimulation into electrical signal (sensor). Typical tri-layer i-EAD [START_REF] Martinez | Electromechanically Active Polymers: A Concise Reference[END_REF][START_REF] Rohtlaid | [END_REF][42] consists of an ionic conducting membrane sandwiched by two electronically conducting polymers (ECPs) as electrodes; the most popular ECP is poly (3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT-PSS). [43][44][45][46][47] Indeed, upon electrical stimulation at low voltage and low frequency, ECP can be electrochemically oxidized or reduced in the presence of an electrolyte. To insure the overall electroneutrality of ECP, the expulsion or insertion of ions takes place within the ECP, leading to the shrinkage or swelling of the ECP layer. With the tri-layer configuration, the opposite volume variations occurring at the cathode and the anode result in a large bending movement of the i-EAD. However, large muscle-like linear contraction/elongation is sparsely reported for this type of i-EAD, and the few examples exhibited low strokes (only about 0.5% contraction ratio). [START_REF] Rohtlaid | [END_REF]48] Therefore, the development of ionic eLCEs, by implementing the electrical stimulus and achieving large muscle-like linear contractions appears clearly as a promising approach to develop multifunctional smart materials with unprecedented behaviours and motions.

Nevertheless, a few tentative works in this direction [START_REF] Greco | Bending actuation of a composite liquid crystal elastomer via direct Joule heating[END_REF][START_REF] Greco | [END_REF][36][37][38] only showed limited effects. Domenici et al. [START_REF] Greco | Bending actuation of a composite liquid crystal elastomer via direct Joule heating[END_REF][START_REF] Greco | [END_REF] proposed a bi-layer system, where a conductive PEDOT:PSS thin layer was deposited on the surface of a well-aligned (monodomain) nematic LCE. They only observed bending or microwinkling deformation under electrical stimuli. Jakli et al. [36,37] used a tri-layer system, where a nematic LCE membrane containing ionic liquids was sandwiched by two conductive PEDOT:PSS thin layer. Again, only bending deformation was observed under electrical stimuli, though an improvement in electroactive bending of i-EAD was obtained by the LC alignment.

Other reports of LCEs containing ionic liquids [37,38] just showed increased performance of mechanoelectrical response in sensing mode. In brief, no electroactive linear actuation was achieved in the reported systems, [START_REF] Greco | Bending actuation of a composite liquid crystal elastomer via direct Joule heating[END_REF][START_REF] Greco | [END_REF][36][37][38] probably because the change of conformation of LC polymer chain was not obtained by the electronical stimulation.

In this paper, we report on the first system of bi-functional ionic eLCEs that can perform both bending deformation and linear contraction/elongation, all under the control of electrical signals (Figure 1). A tri-layer system composed of carefully designed ionically conducting LCE membrane sandwiched between two layers of PEDOT:PSS has been adopted in our ionic eLCEs.

By applying a low voltage at low frequency, the redox behavior and associated ionic motion of the i-EAD component provide bending deformation of the device. On the other hand, the application of a low voltage at higher frequency generates an ionic current-induced Joule heating that triggers the muscle-like contractile response of the eLCE membrane; linear contraction/elongation deformations of the device are finally achieved. This approach of combining two synergetic smart polymer technologies, i.e., LCEs and i-EAD, in a single device, is promising for the development of smart materials with multiple degrees of freedom, which are much sought after in the field of soft robotics.

Results and Discussion

Ion conducting LCE membrane: preparation and characterization

Figure 1a shows the trilayer ionic eLCE composed of an ion conducting LCE membrane in the center and two PEDOT:PSS-based ECP electrodes attached to the both surfaces of LCE membrane. The central LCE membrane was first prepared by a two-stage polymerization/crosslinking [12] of the LC monomer RM82 in the presence of an ionic liquid.

The complete composition of LCE membrane is shown in Figure 1b and its preparation in Scheme S1 in supporting information (SI) (also see SI for detailed preparation procedure). The RM82 is a commercially available LC monomer and chosen because of its relatively low nematic to isotropic phase transition temperature (TNI = 65 °C). [36] Poly(ethylene glycol)diacrylate (PEGDA) was used as co-monomer of RM82 due to its affinity with the ionic liquid electrolyte chosen here, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI). 1,4-butanediol bis(thioglycolate) (DT), trimethylolpropane tris(3mercoptopropianate) (TT) and triethylamine (TEA) were chain extender, crosslinker and base catalyst, respectively, in the first stage polymerization/crosslinking by thiol-acrylate Michael addition click reaction. The total acrylate groups and thiol groups are not equimolar; the acrylates were in excess so that the second polymerization/crosslinking based on acrylate would be carried out later. A slightly crosslinked polydomain LCE was obtained after the thiol-acrylate Michael addition, where mesogens aligned in the same direction only in local domains and the director of each domain (n) was random. Then, a mechanical stretch of this polydomain LCE was performed to get mesogens orientated uniformly along the stretching direction. Finally, a second polymerization/crosslinking through free radical thiol-ene addition of remaining thiol groups of the first stage and free radical polymerization of acrylates leftover was conducted under UV light in the presence of Irgacure 369, to complete the network formation and lock the LC orientation. A moderately crosslinked monodomain LCE was thus obtained, which features global alignment and shows large anisotropy in various macroscopic properties.

The key components in the ionically conducting LCE membrane are LC monomer RM82 and PEG-containing monomer PEGDA, because the linear actuation depends on the LC part, while the part from PEGDA has affinity with ionic liquid to ensure the ionic mobility. The balance between these two components is of paramount importance for the performance of LCE membrane. Therefore, a series of LCE films (named as LCE1, LCE2, LCE3, LCE4 and LCE5) with the molar ratio RM82:PEGDA = 6:1, 4:1, 2:1, 1:1 and 1:4, respectively, have been prepared (see Table S1 and synthetical detail in SI). All these films have been carefully characterized.

The polymer network formation was confirmed using Fourier transform infrared (FT-IR) spectroscopy by observing the disappearance of acrylate C=C band at 1623 cm -1 in final aligned LCE (Figure S1).

To find the optimal molar ratio of RM82:PEGDA for ion conducting LCE, both contraction/elongation deformation and ionic conductivity of these films have been evaluated. S2-4 show thermal responsive contraction and elongation of the samples LCE1-5 upon heating-cooling cycle. The contraction ratio is defined as (L0 -L)/L0, where L0 is the original length of the film at 25 °C and L the film length at any given temperature. The maximal contraction ratio at T = 100 °C after nematic-isotropic transition was measured as 28%, 24% and 10% for LCE1, LCE2 and LCE3, respectively. As expected, the contraction ratio decreases with the RM82 molar ratio's decreasing, until no contraction for LCE4 and LCE5 with the molar ratio RM82:PEGDA equal to or below 1:1. Then, polarized optical microscopy (POM) and two-dimensional wide angle X-ray scattering (2D-WAXS) were employed to examine the sample orientation. The light transmission of the sample LCE1, 2 or 3 placed with alignment direction (n) parallel to polarizer (p) was clearly different from the sample placed with n tilted at 45° relative to p (Figure 2b and Figure S2-S4). 2D-WAXS allowed to quantify the alignment and its evolution as a function of temperature as shown in Figure 2c for LCE2 (see Figure S5 for more patterns at room temperature and Video S1 for patterns of LCE2 upon heating and cooling). The nematic order parameter S was calculated from the angular profile of crescent-like signals along the alignment direction [49] (see SI for details). For example, for LCE2 S = 0.48 at 25 °C (nematic), while S = 0.18 at 130 °C (isotropic). The order parameter S as a function of temperature (Figure S6) exhibits a crossover located around 80 °C that corresponds to the nematic-isotropic transition (TNI). However, some anisotropic order still existed after TNI at high temperature even after the film shrunk completely. This observation can be explained by the crosslinked nature of the material that may prevent full relaxation toward the complete disordered state. As expected, the order parameter at 25 °C decreased with the nematic molecule (RM82) molar proportion, with S = 0.54  0.2 for LCE1, S = 0.48  0.2 for LCE2, S = 0.35  0.2 for LCE3 and S = 0.33  0.2 for LCE4 (see Figure S5). This result is coherent with the tendency of the decrease of contraction ratio. In conclusion, from the point of view of contraction/elongation performance, LCE1, LCE2 and LCE3 with contraction ratio of 28%, 24% and 10%, respectively, are the valuable candidates for the construction of ionic eLCEs.

Figure 2a and Figures

Then, ionic conductivity (𝜎) of LCE1, LCE2 and LCE3 were measured by electrochemical impendence spectroscopy (EIS) in the temperature range of 20-70 °C (see SI for detail). [50] At 20 °C, 𝜎 (LCE1) was measured as 5.3×10 -5 S/cm , 𝜎 (LCE2) as 1.0×10 -4 S/cm and 𝜎 (LCE3) as 4.2×10 -4 S/cm. As expected, the 𝜎 value increased with the increase of polar component PEGDA. The ionic conductivity was also improved upon heating as shown in Figure 2d. At 70 °C, 𝜎 (LCE1) was measured as 4.0×10 -4 S/cm, 𝜎 (LCE2) as 4.4×10 -4 S/cm and 𝜎 (LCE3) as 2.2×10 -3 S/cm. The small angle X-ray scattering (SAXS) experiment on LCE2 effectively showed the phase segregation between LC-rich domains and PEG-rich domains, where the PEG-rich domains ensure the ionic conduction (see Figure S7-S9 in SI for data and discussion).

Analysing both contraction/elongation deformation and ionic conductivity of LCE1, 2 and 3 films, LCE2 showed a good balance of these two properties: linear contraction ratio of 24% that is close to the value of natural muscle, and the ionic conductivity 𝜎 at the order of 10 -4 S/cm that is a reasonable value for electroactive LCE. [36,38] Therefore, LCE2 was selected as the central membrane of trilayer ionic eLCEs, and further thermal mechanical characterization was made on LCE2.

The thermal stability of LCE2 was first examined by thermogravimetric analysis (TGA) (see Figure S10), showing degradation onset temperature with 5% of mass loss (T 5% ) of 330 °C and thus a good thermal stability. The quasi-static stress-strain curve of LCE2 film at 25 °C was measured using the universal tensile machine by stretching the films along the nematic orientation direction. The Young's modulus and breaking strength of LCE2 film reached 1.22 MPa and 0.84 MPa, respectively (Figure S11). The thermal mechanical properties of LCE2 were then studied by dynamic mechanical analysis (DMA). Figure 2e shows the typical viscoelastic behavior of the film at temperature ramp mode. The α relaxation temperature (Tα) of LCE2 is -4.8 °C corresponding to the loss factor tan δ peak. The storage modulus E' decreased upon heating and tended to a rubbery plateau at T > 25 °C. However, a careful analysis on the E' curve of LCE2 revealed a slightly re-increasing tendency of E' after T = 80 °C (indicated by the black vertical dotted line) that corresponds to the nematic-isotropic phase transition (TNI, see WAXS results above-discussed and Figure S6). This re-increasing tendency of E' was attributed to the contraction deformation occurred at TNI. To confirm this assignment, a heating-cooling-heating cycle with supplemental heating run was performed to check the reproducibility of this E' re-increasing tendency. If it was not reproducible, it could be attributed to an irreversible secondary crosslinking at high temperature. Figure S12 effectively shows the reproducibility of this E' re-increasing tendency and confirmed its origin of LCE thermal contraction at nematic-isotropic transition. To evaluate the LCE thermal actuating behavior, LCE2 film was exposed to temperature ramps from 20 °C to 140 °C and from 140 °C to 20 °C (TNI  80 °C), and the deformation strain was measured in iso-stress mode (stress = 0.02 N) by DMA. The obtained strain curves as a function of time are presented in Figure 2f. Reversible strains from 0% to -33% during the heating-cooling cycle indicate the LCE2 film contracts and elongates reversibly with sweeping temperature. All above thermal mechanical characterizations confirm the thermal actuation performance of LCE despite the presence of the ionic liquid EMIM TFSI. Moreover, for comparison the pure LCE without ionic liquids was also prepared by the same two-stage polymerization/crosslinking method. This pure LCE film was tested by DMA at iso-stress mode. Figure S13 shows the slope of strain vs time 10 (0.07% S -1 ) of both LCE2 without and with ionic liquids (Figure S13a-b) are nearly the same.

However, the maximum reversible strain was 45% for the LCE2 without ionic liquids during the heating-cooling cycle, while it was 33% for the LCE2 with ionic liquids. Therefore, the existence of ionic liquids does not affect the thermal actuation speed but affects the contraction ratio of LCE due to their dilution effect in the LCE sample.

Trilayer ionic eLCE

The ionically conducting LCE membrane (LCE2) was then combined with PEDOT:PSS polymer electrodes (Figure 1c) for the preparation of trilayer ionic eLCE (see Scheme S2).

PEDOT:PSS was chosen as ECP electrode due to its commercial availability, high conductivity, and ability to be casted directly on ionic membranes. [44] Into the pristine PEDOT:PSS, PEGDA and PEGMA bearing flexible short chains and dimethyl sulfoxide (DMSO) were added to improve the ionic mobility and electrical conductivity. [42] The photo-initiator Irgacure 2959 was also added in order to initiate polymerization/crosslinking of the (meth)acrylate end-groups of PEGDA and PEGMA to get electrode with better mechanical properties (such as elasticity).

Concretely, PEDOT:PSS and additives were first deposited on the top surface of LCE2 followed by the water evaporation at 50 °C for 1 hour. The sample was let to return to room temperature and exposed to UV light for photo-polymerization/crosslinking to obtain the first electrode. Then, the sample was turned over and the procedure was repeated for the other surface of LCE2 to obtain the second electrode (see Scheme S2 and SI for elaboration procedure). [51] To measure the thickness of electrode, the same mixture of PEDOT:PSS electrode was deposited on a glass slide with the same area as the LCE2. The thickness of the PEDOT:PSS electrode was then measured accurately as 0.008 mm by profilometer DEKTAK 150. Its electronic conductivity (𝜎 𝑒 ) was 319.14 S/cm (see SI for detail). [42] The good thermal stability of the eLCE was assessed by TGA with degradation onset temperature T5% at 280 °C (Figure S10). The deformation strain of trilayer eLCE in iso-stress mode was measured by DMA to verify if the eLCE kept the thermally induced linear actuation.

Figure S14 showed that during the first heating, 33% contraction strain was obtained. In the subsequent first cooling, the eLCE could recover 90% of its initial length. In the following heating/cooling cycle, the contraction/elongation is nearly reversible with deformation strain of 23%. The first 10% irreversible deformation is probably caused by the presence of PEDOT:PSS electrodes. Nevertheless, the PEDOT:PSS electrodes were globally compatible with the thermal contraction/elongation deformation of LCE. The electroactive redox process of the trilayer eLCE film was then analyzed by cyclic voltammetry (CV). As shown in Figure S15, CV curves indicated the occurrence of expected redox process in PEDOT:PSS electrodes within voltage scan between 3 V and -3 V, [52] with anodic peak and cathodic peak potentials Epa = 1.20 V and Epc = -1.10 V. Therefore, in the following electro-actuation studies, we start with the voltage of 2 V.

The electroactive bending deformation of the trilayer eLCE was first investigated under a square wave potential of ±2 V at different frequency from 0.0125 to 20 Hz. Figure 3a gives a schematic illustration of the principle of electroactive bending. When applying a positive potential on the top electrode (anode), the PEDOT in the electrode is oxidized to PEDOT x+ . As PSS acts as an ion selective layer, it allows only cation motion within PEDOT:PSS electrode.

To keep charge neutrality, cations will be expelled from oxidized electrode leading to a shrinkage of the top electrode. On the bottom electrode (cathode), inverse phenomenon occurs; PEDOT y+ in the electrode is reduced to PEDOT. To keep charge neutrality, cations will be inserted into the cathode causing its expansion. The top-shrinkage and bottom-expansion led finally to a bending deformation toward top side (Figure 3a). [START_REF] Martinez | Electromechanically Active Polymers: A Concise Reference[END_REF] When applying a negative potential on the top electrode (now cathode), an inverse bending deformation toward bottom side was obtained (see Figure 3b and also Video S2 for deformation movie). Figure 3b shows the photographic images of trilayer eLCE bending deformation under potential of ±2 V at 0.1 Hz. The precise mechanical bending responses were recorded by a laser displacement sensor (Figure 3c), and the bending deformation was quantified by the bending strain difference (Δε) between the two electrodes (Figure 3d and see "electrochemomechanical characterizations" in SI ). As shown in Figure S16, for the trilayer eLCE under electrical stimulation of ±2 V the bending strain difference Δε of 1.3% was achieved at 0.0125 Hz, the lowest frequency tested.

A value of Δε of 0.80% was obtained at 0.1 Hz (Figure 3e). The corresponding currents in the system were also measured as a function of time (Figure 3f). The value of Δε decreased rapidly with the increase of frequency with Δε of 0.14% at 1 Hz, and it became insignificant when the frequency was above 5 Hz. This result is expected since the bending actuation is based on ion diffusion that is slow by nature. Therefore, lower frequency (longer time) is favorable to reach higher strain difference Δε. On the other hand, the ionic conductivity of the central LCE membrane (a LC ionogel) remains limited (10 -4 S/cm) here, while ionic conductivities of ionogels can be increased above 10 -3 S/cm with proper design. [53] Therefore, there is room for improvement with the bending strain in the future. The bending deformation with 𝛥𝜀 = 0.80% can be repeated at least 80 times (only 10 times shown here). (f) The corresponding currents measured upon bending deformation.

Since the bending deformation became insignificant when the frequency was above 5 Hz, the frequency of 10 Hz was then chosen to test the electroactive contraction deformation of trilayer eLCE without bending behavior. The alternating ionic current is susceptible to produce Joule effect due to the movement of ionic species in the eLCE device, which can heat the LCE until nematic to isotropic transition. Keeping the frequency constant at 10 Hz, the square potential of ±2 V, ±3 V, ±4 V, ±5 V and ±6 V were applied, respectively, for the contraction test of trilayer eLCE. The contraction was observed only with ±6 V in the time scale of 10 mins. It is comprehensible because the Joule heat is proportional to the voltage square, the conductivity, and the time. A minimal voltage is necessary for a certain conductivity value to increase the speed of ionic motion in the LCE membrane and to improve the Joule effect for heating. [54] A more elaborate test was then made by applying the field (±6 V, 10 Hz) on the trilayer eLCE film until 200 seconds, and meanwhile measuring the currents, temperatures, and contractions of the sample. Figure S17 shows the currents as a function of time, the maximum current being reached around 160 s. However, a maximum of 20% contraction was observed at about 100 seconds, while T = 134 °C was recorded by thermal camera. In order to avoid unnecessary overheat of the sample, a square wave potential of ±6 V at 10 Hz frequency with a duration of around 100 s or until the maximal contraction was adopted in the following for a pristine trilayer eLCE film.

Figure 4 shows the eLCE sample deformations without load (Figure 4a) and with load (4.70 mN) (Figure 4b) under 3 cycles of power on/off, currents (Figure 4c) and contraction ratio (Figure 4d) as a function of time being measured together with temperatures of sample at starting and maximal contraction states. After each power-on of about 100 s, the sample was left power-off for 20 mins of pause to let it return to room temperature. A maximum of 20% contraction at 134 °C was recorded for the free film without load during the 1 st cycle and a nearly complete recovery of its initial length was obtained after cooling down to 23 °C (Figure 4a). However, the maximum contraction ratio was decreased to 16% and 8% for the 2 nd cycle and 3 rd cycle. The power-on duration to reach the maximal contraction was 74 s, 93 s and 157 s for the 1 st , 2 nd and 3 rd cycle, respectively. For the 3 rd cycle, the extension of power-on time didn't lead to more contraction. Moreover, during the 3 rd power-on the maximal sample temperature stagnated at 42 °C, while the current didn't increase after 180 s power-on (Figure S18). This is probably caused by the drop of the conductivity of eLCE, which could be attributed to damage or delamination PEDOT:PSS layers.

Then, a load was attached to the trilayer eLCE film along the orientation (length) direction to examine the ability of eLCE to perform mechanical work under electrical field of ±6 V and 10 Hz (Figure 4b). The own weight of the mobile part of eLCE film (the part playing the role of actuator) is about 0.0175 g (taking density as 1 g•cm -3 ), while the total load of the hung standard weight and the clamp is of 4.80 g. During the 1 st power-on, the eLCE film could lift the weight of 4.80 g (274 times of the actuator weight) up to 2 mm height (∆h) (see also Video S3 in SI).

The work done by the eLCE was 9.40×10 -5 J with a work capacity of 5.38 kJ•m -3 (see SI for details). The eLCE mechanical properties under the electroactive stimulation (±6 V and 10 Hz) during three cycles are shown in Table S2. The eLCE lifted the load to a smaller height ∆h, i.e., 1.5 mm and 1 mm, respectively, for the 2 nd and 3 rd cycle. The contraction ratios of 20%, 15% and 10% for the loaded eLCE during three cycles were similar to those for the unloaded free eLCE. The current and contraction ratio as a function of the time were also similar in both loaded and unloaded cases. The work and work capacity decreased also gradually from 1 st , 2 nd to 3 rd power cycle.

As far as we are aware, this is the first example of bi-functional electroactive LCE that can perform either bending or contractile deformation under low voltages of low or high frequency (comparison in Table S3). The contractile deformation of eLCE can be repeated 3 times with strain of 20% -10% and lift up the load 274 times of the eLCE actuator weight. The bending deformation with typically 0.80% of bending strain difference can be repeated at least 80 times (10 times shown in Figure 3c) without fatigue. The performance of eLCE could be improved in the future by increasing its ionic conductivity and the connection between PEDOT:PSS layers and LCE layer. Moreover, decreasing the thickness of the ion conducting LCE film could also be a promising approach. First, it could significantly improve the bending strain difference and bending rate due to geometrical consideration and the decrease of ionic resistance. Secondly, it could also improve the contractile deformation rate of the eLCE by promoting much faster heating and accelerating the cooling of the device by heat dissipation.

Conclusion

In this work, we have elaborated eLCE actuators that can perform selectively either bending deformation or linear contractile deformation under electrical stimulation of low voltages, thanks to synergistic functionalities of ionic EAD and ion conducting LCE. The approach relied on the combination and interaction of two smart polymer technologies, i.e., LCEs and ionic EADs, in a single device made of an ion-conducting membrane based on aligned LC elastomers and two optimized conducting polymer electrodes as ionic EADs. By applying low voltage at low frequency (±2 V at 0.1 Hz), the redox behavior and associated ionic motion of the ionic EAD components provided bending deformation of eLCE up to a bending strain difference of 0.80%. On the other hand, by applying low voltage at higher frequency (±6 V and 10 Hz), the ionic current-induced Joule heating triggered the muscle-like contractile response of the ionically conducting LCE component, which made a linear contraction ratio of 20% of eLCE without load and afford a work capacity of 5.38 kJ•m -3 with a load of 4.80 g (274 times of the weight of eLCE actuator) while keeping 20% strain. The possibility of this ionic eLCE actuator to perform simultaneously bending and contractile deformation is under investigation. Efforts are also being made to improve the anti-fatigue features of eLCE. The multifunctional ionic eLCE actuator developed here represents a promising approach to develop future smart materials with multiple biomimetic degrees of freedom like the emblematic octopus tentacle, long-time desired for soft robotics.

Experimental Section/Methods

Experimental details are given in the Supporting Information.

Figure 1 .

 1 Figure 1. (a) Bi-functional trilayer eLCE film: the yellow central layer represents the ionically

Figure 2 .

 2 Figure 2. Characterization of the LCE2 film (RM82:PEGDA = 4:1). (a) Reversible

Figure 3 .

 3 Figure 3. The mechanical response of trilayer eLCE (PEDOT:PSS)/LCE2/(PEDOT:PSS)

Figure 4 .

 4 Figure 4. Linear electroactuations of trilayer eLCE film under square wave potential (±6 V, 10

Acknowledgements

Gaoyu Liu and Yakui Deng contributed equally to this work. This work is financially supported by the French National Research Agency (ANR-22-CE06-0023, AS-LCE). The China Scholarship Council (CSC) is gratefully acknowledged by Gaoyu Liu, Yakui Deng and Bin Ni for funding their PhD scholarships.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.