Assigning a social status from face adornments: an fMRI study

Keywords: Human evolution, symbols, social cognition, face perception, Neuroarchaelogy, fMRI

The human face has been culturally modified for at least 150,000 years using practices like painting, tattooing and scarification to convey symbolic meanings and individual identity. The present study used functional magnetic resonance imaging to explore the brain networks involved in attributing social status from face decorations. Results showed the fusiform gyrus, orbitofrontal cortex, and salience network were involved in social encoding, categorization, and evaluation. The hippocampus and parahippocampus were activated due to the memory and associative skills required for the task, while the inferior frontal gyrus likely interpreted face ornaments as symbols. Resting-state functional connectivity analysis clarified the interaction between these regions. The study highlights the importance of these neural interactions in the symbolic interpretation of social markers on the human face, which were likely active in early Homo species and intensified with Homo sapiens populations as more complex technologies were developed to culturalize the human face.

Introduction

The use of technologies to change the appearance of our bodies to communicate information about our identity and social role dates back hundreds of thousands of years. Body painting, tattooing, scarification, wearing of ornaments, mutilations, hairstyles, and clothing are amongst the best-known practices for performing these functions in traditional societies [START_REF] Barth | Ethnic Groups and Boundaries. The Social organization of culture difference[END_REF][START_REF] Dubin | North American Indian Jewelry and Adornment[END_REF][START_REF] Hodder | The Meanings of Things. Material Culture and Symbolic Expression[END_REF][START_REF] Klumpp | Aesthetics, expertise, and ethnicity: Okiek and Maasai perspectives on personal ornament[END_REF][START_REF] Sanders | Ethnic Boundaries and Identity in Plural Societies[END_REF] .

Personal ornaments, in particular, play a crucial role in communicating ethnic affiliation, reinforcing the sense of belonging to the group and its cohesion, establishing boundaries with neighboring groups, and conveying information on linguistic, ideological, and religious membership [START_REF] Dubin | The history of beads: from 30,000 B.C. to the present[END_REF][START_REF] Hatton | Ostrich eggshell beads from Ga-Mohana Hill North Rockshelter, southern Kalahari, and the implications for understanding social networks during Marine Isotope Stage 2[END_REF][START_REF] Kuhn | Paleolithic Ornaments: Implications for Cognition, Demography and Identity[END_REF][START_REF] Kuper | Costume and Identity[END_REF][START_REF] Nițu | Mobility and social identity in the Mid Upper Paleolithic: New personal ornaments from Poiana Cireșului (Piatra Neamț, Romania)[END_REF][START_REF] Pitarch Martí | The earliest evidence of coloured ornaments in China: The ochred ostrich eggshell beads from Shuidonggou Locality 2[END_REF][START_REF]Beads and bead makers: gender, material culture, and meaning[END_REF][START_REF] Vanhaeren | Aurignacian ethno-linguistic geography of Europe revealed by personal ornaments[END_REF][START_REF] Wright | An Early Upper Palaeolithic decorated bone tubular rod from Pod Hradem Cave, Czech Republic[END_REF] . Ornaments can also provide information about social status, gender, marital situation, and the number of children the wearer has had. Special ornaments and body paints may be put on at rites of passage occurring at the individual birth, during initiation ceremonies, marriage, healing, or death [START_REF] Beckwith | African ceremonies[END_REF][START_REF] Carey | Beads and Beadwork of West and Central Africa[END_REF][START_REF] Carter | Elite Dress and Regional Identity: Chimú-Inka Perforated Ornaments from Samanco, Nepeña Valley, Coastal Peru[END_REF][START_REF] Ogundiran | Of Small Things Remembered: Beads, Cowries, and Cultural Translations of the Atlantic Experience in Yorubaland[END_REF][START_REF] Twala | Beads as regulating the social life of the Zulu and Swazi[END_REF] .

The earliest use of red ochre goes back to 500 ka in Africa [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF][START_REF] Dapschauskas | The Emergence of Habitual Ochre Use in Africa and its Significance for The Development of Ritual Behavior During The Middle Stone Age[END_REF][START_REF] Rosso | The first uses of colour: what do we know?[END_REF][START_REF] Watts | Early Evidence for Brilliant Ritualized Display: Specularite Use in the Northern Cape (South Africa) between ∼500 and ∼300 Ka[END_REF] , 380 ka in Europe [START_REF] Rosso | The first uses of colour: what do we know?[END_REF][START_REF] De Lumley | Les crayons d'ocre du site acheuléen de Terra Amata[END_REF][START_REF] Roebroeks | Use of red ochre by early Neandertals[END_REF] , and 73 ka in Asia [START_REF] Rosso | The first uses of colour: what do we know?[END_REF][START_REF] Errico | L'émergence des comportements symboliques en Afrique et en Asie[END_REF][START_REF] Langley | Symbolic expression in Pleistocene Sahul, Sunda, and Wallacea[END_REF] . Dapschauskas and colleagues (2022) identified three phases of ochre use in the African Middle Stone Age: an initial phase from 500 ka to 330 ka, an "emergent" phase from 330 ka to 160 ka, and a "habitual" phase from 160 ka to 40 ka. The latter phase, when a third of archaeological sites contain ochre, is interpreted by these authors as the manifestation of intensifying ritual activity in early populations of Homo sapiens. This view is consistent with the results of studies indicating that in this last and the previous phase, certain types of mineral pigments were transported over long distances [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF][START_REF] Errico | Pigments from the Middle Palaeolithic levels of Es-Skhul (Mount Carmel, Israel)[END_REF][START_REF] Dayet | Ochre Provenance and Procurement Strategies During The Middle Stone Age at Diepkloof Rock Shelter, South Africa: Ochre during the MSA at Diepkloof rock shelter[END_REF] , that certain shades of red were particularly sought after [START_REF] Hovers | An Early Case of Color Symbolism: Ochre Use by Modern Humans in Qafzeh Cave[END_REF][START_REF] Watts | Red ochre, body painting, and language: interpreting the Blombos ochre[END_REF][START_REF] Watts | The pigments from Pinnacle Point Cave 13B, Western Cape, South Africa[END_REF] , that ochre was modified by heating to change its color [START_REF] Godfrey-Smith | Past thermal history of goethite and hematite fragments from Qafzeh Cave deduced from thermal activation characteristics of the 110°C TL peak of enclosed quartz grains[END_REF][START_REF] Salomon | Selection and heating of colouring materials in the Mousterian level of Es-Skhul (c. 100 000 years BP, Mount Carmel, Israel)[END_REF] but see [START_REF] Wadley | Post-depositional heating may cause over-representation of red-coloured ochre in stone age sites[END_REF] , and that in some cases very small quantities of pigments were produced [START_REF] Rosso | Patterns of change and continuity in ochre use during the late Middle Stone Age of the Horn of Africa: The Porc-Epic Cave record[END_REF] , a behavior more consistent with a symbolic than a utilitarian function.

The wearing of personal ornaments, many of which are deliberately covered with ochre, is attested since at least 142 ka in North Africa, 80 ka in Southern Africa, and 120 ka in the Near East [START_REF] Mayer | Shells and ochre in Middle Paleolithic Qafzeh Cave, Israel: indications for modern behavior[END_REF][START_REF] Mayer | On holes and strings: Earliest displays of human adornment in the Middle Palaeolithic[END_REF][START_REF] Bouzouggar | 000-year-old shell beads from North Africa and implications for the origins of modern human behavior[END_REF][START_REF] Errico | Nassarius kraussianus shell beads from Blombos Cave: evidence for symbolic behaviour in the Middle Stone Age[END_REF][START_REF] Errico | Additional evidence on the use of personal ornaments in the Middle Paleolithic of North Africa[END_REF][START_REF] Sehasseh | Early Middle Stone Age personal ornaments from Bizmoune Cave, Essaouira, Morocco[END_REF][START_REF] Shipton | 78,000-year-old record of Middle and Later Stone Age innovation in an East African tropical forest[END_REF][START_REF] Steele | Personal ornaments in early prehistory a review of shells as personal ornamentation during the African Middle Stone Age[END_REF][START_REF] Vanhaeren | Middle Paleolithic Shell Beads in Israel and Algeria[END_REF] . Because the understanding by others of the meaning attached to ornaments and body paints presupposes the existence of shared codes, archaeological objects which have fulfilled these functions are often considered reliable evidence for the emergence of language and symbolic material cultures in our genus [START_REF] Errico | Nassarius kraussianus shell beads from Blombos Cave: evidence for symbolic behaviour in the Middle Stone Age[END_REF][START_REF] Errico | Additional evidence on the use of personal ornaments in the Middle Paleolithic of North Africa[END_REF][START_REF] Errico | From the origin of language to the diversification of languages[END_REF][START_REF] Davidson | The Archaeology of Perception : traces of depiction and language[END_REF][START_REF] Marshack | Some Implications of the Paleolithic Symbolic Evidence For the Origin of Language[END_REF][START_REF] Vanhaeren | Thinking strings: Additional evidence for personal ornament use in the Middle Stone Age at Blombos Cave, South Africa[END_REF] . In this regard, wearing body adornments can be considered an archaeological indicator of modern social cognition.

Although body symbols played a key role in all human societies and appeared very early in human history, the cerebral regions mobilized by their perception and interpretation remain unknown. Numerous studies have focused on the brain substrates of the emotional aspects of social cognition and perspective-taking (Theory of Mind). They have emphasized the role of the medial prefrontal cortex, the temporoparietal junction, and the temporal poles [START_REF] Adolphs | The Social Brain: Neural Basis of Social Knowledge[END_REF][START_REF] Frith | The social brain?[END_REF] . However, one study showed that social status recognition was minimally disrupted following ventromedial lesions, suggesting that the network involved in this function would be distinct from that dealing with the emotional aspects of social cognition [START_REF] Karafin | Dominance Attributions Following Damage to the Ventromedial Prefrontal Cortex[END_REF] . Neuroimaging studies have confirmed that the perception of social hierarchies relies on the intraparietal sulcus, the dorsolateral and orbital frontal cortex, and the lateral occipital and occipitotemporal cortex [START_REF] Chiao | Neural basis of social status hierarchy across species[END_REF][START_REF] Chiao | Neural representations of social status hierarchy in human inferior parietal cortex[END_REF][START_REF] Marsh | Dominance and Submission: The Ventrolateral Prefrontal Cortex and Responses to Status Cues[END_REF][START_REF] Zink | Know Your Place: Neural Processing of Social Hierarchy in Humans[END_REF] . However, the identification of social markers does not necessarily imply a ranking.

Body ornaments and facial paintings may convey information on social roles disconnected from a social hierarchy. Although body paintings and the wearing of beads to express social roles are attested in the earliest Homo sapiens and probably in Neanderthals [START_REF] Jaubert | Spiritual and symbolic activities of Neanderthals[END_REF][START_REF] Vanhaeren | Les objets de parure[END_REF][START_REF] Zilhão | Symbolic use of marine shells and mineral pigments by Iberian Neandertals[END_REF] , very little is known about the brain networks involved in processing such information, the possible processes that led to a complexification of these behaviors, as well as their timeline. In the present study, participants were asked to assign social roles or statuses to faces adorned with paintings, beads, or both. At the same time, their brain activity was monitored using functional magnetic resonance imaging (fMRI). Participants were given no guidance on the meaning of the face decorations and had to create their arbitrary social code. The attribution of a social status mobilizes implicit and explicit processes. Implicit processes are rapid, require little cognitive effort, and can occur without awareness. Explicit processes are cognitively demanding, slow, and deliberative [START_REF] Forbes | The Role of the Human Prefrontal Cortex in Social Cognition and Moral Judgment[END_REF] . To isolate explicit processes, we included, using the same stimuli, a perceptual task (1-back) that does not explicitly require a social role attribution. Brain activity during this task was compared to that performed during explicit social status attributions. In addition, at rest, the functional connectivity of the brain regions involved was analyzed to provide information on the interaction of the brain areas implicated in the social status attribution task. Our results identify, for the first time, the brain networks engaged in attributing social status from different arrangements of paintings and ornaments on the human face, the way they work in synergy, and provide sound bases on which build an evolutionary scenario for the gradual integration of these brain areas during the evolution of our genus.

Materials and methods

Ethics statements

The 'Nord-Ouest III' local Ethics Committee approved the study on 10/14/2021 (N° IDRCB: 2021-A01817-34). All the participants signed informed consent before the MRI acquisition.

Participants

Thirty-five healthy adults (age range 18-29 years, mean age 22 ± 2 years (SD), 18 women, four left-handed) with no neurological history were included. One participant was excluded from the analysis because of a brain abnormality discovered during MRI acquisition.

Experimental design

The functional acquisition was organized in a single session consisting of six runs during which participants had to perform a selection task (first three runs), then a 1-back task (last three runs). After receiving instructions for these tasks, participants completed a short training run outside the MRI.

Stimuli

The set of stimuli included pictures of faces (up to below the shoulders) of 34 unknown people in the same range of age (17 women, 17 men, around 30 years old) wearing ornaments and adopting a neutral expression. Each face was ornamented with either spherical wooden beads, red paintings, or a combination of both (Figure 1). Ornaments included earrings with one or three beads, necklaces with one or two chains of beads, a diadem consisting of a chain of beads, and a single large spherical bead in the middle of the forehead. Red paintings included one or three vertical lines on the chin; a dot or a horizontal line on the forehead; oblique lines on the cheeks; and a large horizontal band including the eyes. Associations of paintings and beads were designed to make both types of ornamentation gradually more invasive on the face. In all, twelve types of facial ornamentation have been designed and implemented.

Selection task (event-related paradigm)

These three runs followed a slow event-related design, i.e., the change in the BOLD signal was collected for each stimulus presentation, and the time between each presentation allowed the signal to return to its baseline level. The order of presentation was randomized. The stimuli corresponded to a triplet of photos of three different persons of the same gender (male or female), one wearing ornaments, one with paintings, and one with both (Figure 2). Within a triplet, the richness of the ornaments was comparable between the pictures to avoid biases in the choice. There were three levels of richness between the triplets (Figure 2). The selection task was implemented as follows (Figure 3): a question was displayed during 0.75 s. The question could concern either the displayed persons' social role (Social status condition) or the type of ornamentation they displayed (Ornament check condition).

Then, a new triplet of pictures was shown for 4 s. The participant had to choose, by pressing the corresponding button of a response box as soon as they made their decision, the person who best fitted the proposed social status, e.g., "Shaman" (Social status condition) or the person who corresponded to the ornament type proposition, e.g., "Painted cheeks" (Ornament check condition). Each question was asked twice for each gender. The list of questions is displayed in Table 1.

The Ornament check condition was designed as a control condition, bearing the same pictures as the social status condition. It required attention to the ornaments without implementing social cognition processes. Then, a fixation cross was displayed, and a square appeared after a variable delay (3.5 s ± 1 s). Participants had to click the "1" button on the response box when the square appeared. This constituted the baseline, allowing the BOLD signal to return to its baseline level between events. Each event lasted 12.75 s. Over the three runs, participants saw 80 stimuli, 40 in the social status condition and 40 in the ornament check condition. Each run lasted 5 min and 51 s each and included 27 events (except run C, which included one less event) for a total duration of 5 min 38 s). Stimuli were presented in random order within each run. Immediately after the MRI acquisition, the experimenter asked the participant the criteria on which they based their social role attribution in the status condition.

1-back task (block design)

In the 1-back task, participants viewed a succession of ornamented faces (displayed for 1 s each, with an interstimulus interval of 983 ms). The participants had to report the repetition of two faces (Face condition) or two types of ornamentation (Ornament condition, including three modalities: paintings, beads, or both simultaneously) by pressing the "1" button on the response box. This repetition criterion was displayed during 750 ms at the beginning of each block. Fifteen stimuli belonging to the same category of ornamentation were presented within the same block (i.e., within a block, there were no images belonging to different categories).

There were three repetitions per block. Each of the three runs lasted 4 min and 15 s and included six experimental blocks of 30.6 s interspersed with seven fixation blocks of 10.2 s.

Each run had four blocks of ornament condition and two blocks of face condition. The presentation order of the 1-back runs was randomized.

MRI acquisition

Neuroimaging data acquisition was performed using a Siemens Prisma 3 Tesla MRI scanner.

Structural images were acquired using a high-resolution T1-weighted 3D sequence (TR = 2000 ms, TE = 2.03 ms; flip angle = 8°; 192 slices and isotropic voxel volume of 1 mm 3 ).

Functional images were obtained using a whole-brain T2*-weighted echo planar image acquisition (T2*-EPI Multiband x6, sequence parameters: TR = 850 ms; TE = 35 ms; flip angle = 56°; 66 axial slices and isotropic voxel size of 2.4 mm 3 ). The first sequence lasted 8 min and recorded participants' brain activity during resting state (i.e., when they let their thoughts flow freely, without having a task to perform or falling asleep). This acquisition was used to perform a resting-state functional connectivity analysis. Then, functional images were acquired when the participants performed tasks based on stimuli perception. This was done during six runs (three for each task: selection and 1-back). The presentation of the experiment was programmed in E-prime software 3.0 (Psychology Software Tools, Pittsburgh, PA, USA).

The stimuli were displayed on a 27″ screen. Participants saw the stimuli through the back of the magnet tunnel via a mirror mounted on the head antenna.

Data analysis 2.5.1. Behavioral analysis

For the selection task, we evaluated the effects of condition (Social status or Ornament check), participant gender, and stimulus gender on reaction time using a linear mixed-effects model, adjusting for random effects at the participant level. A three-factor interaction term between condition, participant gender, and stimulus gender (and their lower-order terms) was defined as fixed-effect predictors and reaction time as the dependent variable. The significance of fixed effects was assessed through ANOVA components.

Functional neuroimaging analysis

T1-weighted scans were normalized via a specific template (T1-80TVS) corresponding to the MNI space using SPM12. The 192 EPI-BOLD scans were realigned in each run using a rigid transformation to correct the participant's motion during the fMRI sessions. Then, the EPI-BOLD scans were rigidly registered structurally to the T1-weighted scan. All registration matrices were combined to warp the EPI-BOLD functional scans to standard space with trilinear interpolation. Once in standard space, a 5-mm-wavelength Gaussian filter was applied.

In the first level analysis, a generalized linear model (GLM, statistical parametric mapping (SPM 12), http://www.fil.ion.ucl.ac.uk/spm/) was performed for each participant to process the task-related fMRI data, with the effects of interest (tasks) modeled by boxcar functions corresponding to events or blocks, convolved with the standard hemodynamic SPM temporal response function. We then calculated the effect of individual contrast maps corresponding to each experimental condition. Note that eight non-interest regressors were included in the GLM analysis: time series for white matter, CSF (average time series of voxels belonging to each tissue class), the six motion parameters, and linear temporal drift.

Group analysis (second-level analysis) of fMRI data was conducted using JMP® Ornament check] contrast than in the [Ornament minus Face] contrast.

Resting-state analysis

The task-based functional analysis was complemented with a resting-state functional connectivity analysis using the CONN v 20.b toolbox software [START_REF] Whitfield-Gabrieli | A Functional Connectivity Toolbox for Correlated and Anticorrelated Brain Networks[END_REF] , which runs under MATLAB 2021a.

Functional imaging data were pre-processed using the CONN default pre-processing pipeline for volume-based analyses. The steps for functional data comprise realignment and unwarping for subject motion estimation and correction (12 parameters). Next, centering to (0,0,0) coordinates and ART-based outlier detection identification was applied. Segmentation and normalization to MNI space were applied next. Structural data were translated to (0,0,0) center coordinates, segmented (gray/white/CSF), and normalized to MNI space. In the denoising step, we applied band-pass filtering (0.01-0.1 Hz) after regression of realignment parameters (12), white and gray matter, and CSF confounds. Then, we applied linear detrending and despiking after regression. For the ROI to ROI functional connectivity analyses, we used AICHA atlas [START_REF] Joliot | AICHA: An atlas of intrinsic connectivity of homotopic areas[END_REF] . We considered the 95 hROIs activated in the [Status minus Ornament] contrast. For group-level results, we calculated ROI-to-ROI connectivity correlations, threshold with a unilateral t-test, and FDR-corrected p < 0.05.

Results

Behavioral results

Participants responded faster in the ornament check condition (mean response time ± SD: 1.3s ± 0.5s) than in the social status condition (mean response time ± SD: 2s ± 0.8s): F(1,32) = 227.8 p < 0.0001. Participant gender and stimulus gender had no significant effects (either main or interactions).

Post-MRI debriefing of the selection task

Twenty-two participants reported that they considered ornamentation a more important criterion than phenotype in assigning a social role/status. A few reported they sometimes paid attention to facial features, for example, in cases of indecision or for specific roles such as father/mother. Eleven participants reported paying more attention to facial characteristics than to ornamentation. Two participants stated that the most important criterion for them (facial features or ornamentation) varied according to the questions. All participants reported that they never answered randomly, except in rare exceptions. Participants generally reported having an attribution strategy in place that they maintained throughout the experiment. For example, some participants associated the absence of beads with a mobile role, such as scout or hunter. For the same role, there was not necessarily a consensus among participants. For example, some participants attributed warrior status to faces wearing only beads, while others attributed this status to faces bearing only paintings.

Neuroimaging results

Social status minus Ornament check (event-related paradigm)

The [Social status minus Ornament check] contrast revealed a set of 95 cortical and subcortical regions that were more activated when participants assigned social status to adorned faces than when they assessed the type of ornamentation ( Table 2, Figure 4).

In the occipital lobe, these regions included the lateral occipital cortex and the fusiform gyrus (including the Fusiform Face Area, FFA). We used the Neurosynth platform [START_REF] Yarkoni | Largescale automated synthesis of human functional neuroimaging data[END_REF] to synthesize the activations reported in the literature during face perception and ensure their consistency with our results. We conducted a meta-analysis including 125 studies that contained the term "neutral face" in their abstracts, i.e., pictures of faces adopting a neutral expression. It evidenced the involvement of a right fusiform region (MNI coordinates of the activation peak:

38, -42, -16). This matched the location of the G_Fusiform-4-R in our AICHA atlas (MNI coordinates of the center of mass: 44, -46, -18). The FFA occupied a large portion of this functional region of the AICHA atlas.

The activations extended to the parahippocampal gyrus on the medial side of the temporal lobe. In the parietal lobe, the intraparietal sulcus was activated bilaterally. In the frontal lobe, activations included the middle and inferior frontal gyri on the lateral side and the anterior part of the supplementary motor area medially. Activations also concerned several paralimbic and limbic cortex regions, such as the anterior insula, the anterior cingulate, the posterior cingulate and adjacent precuneus, the orbitofrontal cortex, the temporal poles, and the hippocampus. The subcortical structures, the head of the caudate nucleus, and the thalamus, especially in its mediodorsal part, were also involved. Numbers indicate the z value of the axial slice in MNI space.

Ornament minus Face (1-back paradigm)

The [Ornament minus Face] contrast revealed a set of 81 cortical and subcortical regions, which were more activated when participants checked the repetition of ornamentation than when they looked for the repetition of faces. These regions were mostly located in the lateral and inferior occipital cortices and the fusiform gyrus, extending to the inferior temporal gyrus. Participants also activated the intraparietal sulcus, the anterior insula, and some frontal regions, such as the superior frontal sulcus, inferior frontal sulcus, the supplementary area, and the middle frontal gyrus.

Among these regions, 37 hROIs were significantly less activated in the [Ornament minus Face] contrast than in the [Social status minus Ornament check] contrast (Table 2). The color scale (green to red) reflects the t-value on each connection averaged across subjects.

Resting-state functional connectivity

The resting-state functional connectivity analysis revealed 348 positive connections significant across subjects (p < 0.05 FDR, univariate t-test) between the 37 hROIs. T-values varied from 2 to 25 (Figure 5). These 37 hROIs can be divided into two groups based on their resting-state functional connectivity. A network connected the precuneus and temporal lobe regions, including the hippocampus, the parahippocampal cortex, the temporal pole, and a part of the fusiform gyrus. A second network connected mainly frontal regions, including the inferior frontal sulcus and gyrus, the orbitofrontal cortex, the dorsal anterior cingulate cortex, the supplementary motor area, and the anterior insula.

The G-Fusiform-4-R and the Temporal_Pole_Sup-1-R regions were connected to 22 and 27 hROIS, respectively. The G-Fusiform-4-R and the Temporal_Pole_Sup-1-R regions were strongly connected to their group and many regions of the other group (see supplementary materials for detailed results). The S_Orbital-2 was connected to 26 hROIs.

Discussion

This study aimed to identify the brain regions involved in attributing social status from the visual analysis of adorned faces. Adorning one's body to transmit social information represents a symbolic behavior that appeared at least 150,000 years ago and probably much earlier. Therefore, we can assume that the networks revealed in the present study were, at least to a degree, functional in the earliest Homo sapiens and contemporary or earlier hominins displaying such behaviors.

These regions can be categorized into four groups: 1. occipitotemporal regions of the ventral visual pathway, including lateral occipital regions, fusiform gyrus, parahippocampal gyrus extending to the hippocampus, and the temporal poles; 2. regions belonging to the salience network such as the anterior insula and the anterior cingulate cortex; 3. the intraparietal sulcus; and 4. the ventral and dorsal regions of the lateral prefrontal cortex and the orbitofrontal cortex.

Some of these regions were also activated in the 1-back task, indicating that they are not specific to an explicit social attribution but may be involved in an implicit social appraisal. This is the case for most visual regions (except Fusiform-4-R and Fusiform-2-L), the intraparietal sulcus, and most of the thalamus and retrosplenial regions. In contrast, activity in the inferior and orbital frontal areas, hippocampal and parahippocampal regions, the temporal poles, and the salience network, including the anterior cingulate and parts of the anterior insula, remained significant when activity in these regions during the 1-back task was subtracted.

Visual Ventral pathway and medial temporal regions

Lateral and ventral occipital regions were more activated by the social status attribution than by the assessment of decoration type. This suggests that deeper visual processing is required to attribute a social status. Most of these occipitotemporal regions were also activated during the 1-back task and were thus not specifically involved in assigning a social status to adorned faces. However, two hROIs were significantly more activated during social status attribution than in the 1-back task, namely G_Fusiform-2-L and G_Fusiform-4-R. The latter is particularly interesting since it includes the so-called fusiform face area (FFA), which is sensitive to face perception [START_REF] Kanwisher | The fusiform face area: a module in human extrastriate cortex specialized for face perception[END_REF][START_REF] Kanwisher | The fusiform face area: a cortical region specialized for the perception of faces[END_REF][START_REF] Mccarthy | Face-Specific Processing in the Human Fusiform Gyrus[END_REF] and lateralized in the right hemisphere [START_REF] Rossion | Defining face perception areas in the human brain: A large-scale factorial fMRI face localizer analysis[END_REF] . Thus, although all conditions included face perception and none required specific attention to faces, FFA appeared more solicited by social status assignment. It has been shown that FFA is sensitive to physical characteristics and their possible social correlates [START_REF] Contreras | Multivoxel Patterns in Fusiform Face Area Differentiate Faces by Sex and Race[END_REF][START_REF] Freeman | The Neural Basis of Categorical Face Perception: Graded Representations of Face Gender in Fusiform and Orbitofrontal Cortices[END_REF] . More recently, a study showed that FFA processes characteristics such as social traits, gender, and high-level visual features of faces [START_REF] Tsantani | FFA and OFA Encode Distinct Types of Face Identity Information[END_REF] and might thus initiate the social processing of faces. The results of the present study suggest that, in the context of social role attribution, FFA can process nonphysiognomic features. This is consistent with the fact that the FFA promotes holistic rather than local processing [START_REF] Andrews | Internal and External Features of the Face Are Represented Holistically in Face-Selective Regions of Visual Cortex[END_REF][START_REF] Kok | Holistic processing only? The role of the right fusiform face area in radiological expertise[END_REF][START_REF] Zhang | The Fusiform Face Area Is Engaged in Holistic, Not Parts-Based, Representation of Faces[END_REF] . Ornamented faces may have been perceived as a whole in the social attribution task, while attention was focused on details during the assessment of decoration type and the 1-back task. In other words, attributing social status involves a more complex process relying on a set of components, such as the types of decoration, their association, their location on the face, and the face itself.

In summary, the activation of FFA in our social status assignment task could reflect the implementation of preliminary social categorization processes based on a holistic analysis of ornamented faces, which is further achieved in other regions of the brain, particularly the orbitofrontal cortex.

In the anterior extension of the ventral visual pathway, we found that the hippocampus and parahippocampal gyrus were more activated by the social status attribution than by the ornament type assignment and significantly more activated when compared to the 1-back task, reflecting their specificity to social status attribution. The hippocampus reflects episodic memorization processes strongly involved in social cognition [START_REF] Laurita | The Hippocampus and Social Cognition[END_REF][START_REF] Montagrin | The social hippocampus: M ONTAGRIN ET AL[END_REF] . The parahippocampus appears to play, among others, a pivotal role in contextual associative processing [START_REF] Aminoff | The Parahippocampal Cortex Mediates Spatial and Nonspatial Associations[END_REF][START_REF] Li | The Parahippocampal Cortex Mediates Contextual Associative Memory: Evidence from an fMRI Study[END_REF] , i.e., in binding elements composing stimuli. It provides a unified context for further processing (see [START_REF] Aminoff | The role of the parahippocampal cortex in cognition[END_REF] for a review). In the framework of the present study, participants arbitrarily associated face decorations with social status. After the fMRI sessions, they reported that once they had established an ornament/status association strategy, they stuck to it throughout the sessions, with exceptional random responses. Contextual associations were thus an essential aspect of the processes involved in the status assignment task. The activation of the parahippocampal cortex reasonably reflects the implementation of these processes. It has been suggested that the anterior part of the parahippocampus preferentially processes non-spatial contextual associations, and the posterior part, comprising the parahippocampal Place Area (PPA), spatial associations [START_REF] Aminoff | The Parahippocampal Cortex Mediates Spatial and Nonspatial Associations[END_REF][START_REF] Bar | Cortical analysis of visual context[END_REF] . In the present study, the activation of the anterior parahippocampus is consistent with the non-spatial nature of the associations.

The activation of the medial temporal gyrus might be linked to one of the temporal poles.

Several studies have documented the involvement of the temporal pole in social cognition, and this region is considered part of the social brain network [START_REF] Balgova | The role of the ventrolateral anterior temporal lobes in social cognition[END_REF][START_REF] Bechara | The neurology of social cognition[END_REF][START_REF] Frith | The social brain: allowing humans to boldly go where no other species has been[END_REF][START_REF] Olson | The Enigmatic temporal pole: a review of findings on social and emotional processing[END_REF][START_REF] Ross | Social cognition and the anterior temporal lobes[END_REF] . Although its role is still under discussion, it has been proposed that this brain area is involved in encoding and retrieving social knowledge [START_REF] Olson | Social cognition and the anterior temporal lobes: a review and theoretical framework[END_REF] . As was the case in this study, assigning social status mobilizes stereotypical social knowledge (e.g., the chief must have the most ornaments) and entails encoding: The participants associated a type of ornamentation with a social role and created an arbitrary social code that they reused throughout the task. Thus, we propose that the parahippocampus and the temporal pole, which are strongly functionally connected, work in synergy to facilitate the association of a type of ornamentation with a specific social status and then to encode and restore this association.

Inferior and orbitofrontal cortex

Assigning a social status involved many frontal regions not solicited during the ornament type attribution condition. However, the specific areas for explicit processes, i.e., activated in the social attribution task compared to the 1-back task, were mainly in the lateral part of the inferior frontal gyrus and the orbitofrontal cortex as defined by Rudebeck and Rich [START_REF] Rudebeck | Orbitofrontal cortex[END_REF] . The resting-state connectivity analysis showed that these regions were highly functionally linked.

Previous studies have emphasized the role of the orbitofrontal cortex in social cognition in non-human primates and humans. It has been argued that this cortical area contains neurons sensitive to representing social categories [START_REF] Barat | Face cells in orbitofrontal cortex represent social categories[END_REF] and evaluating social information [START_REF] Azzi | Modulation of value representation by social context in the primate orbitofrontal cortex[END_REF] in nonhuman primates. In humans, a deficit in social perception after orbitofrontal cortex lesions [START_REF] Mah | Impairment of Social Perception Associated With Lesions of the Prefrontal Cortex[END_REF] , an inability to judge social traits in a decision-making task [START_REF] Xia | Lateral Orbitofrontal Cortex Links Social Impressions to Political Choices[END_REF] , or acquired sociopathy have been reported [START_REF] Blair | Impaired social response reversal: A case of `acquired sociopathy[END_REF] .

In healthy participants, fMRI studies have emphasized the role of the orbitofrontal cortex in social cognition and social behavior ( [START_REF] Marsh | Dominance and Submission: The Ventrolateral Prefrontal Cortex and Responses to Status Cues[END_REF][START_REF] Koski | The neural representation of social status in the extended face-processing network[END_REF] ; See 60 for a review) and, more specifically, in explicit processing [START_REF] Cunningham | Attitudes and evaluations: a social cognitive neuroscience perspective[END_REF] . The orbitofrontal cortex is sensitive to non-verbal social signals [START_REF] Marsh | Dominance and Submission: The Ventrolateral Prefrontal Cortex and Responses to Status Cues[END_REF] . Recent results indicate that this area is critical in representing social status [START_REF] Koski | The neural representation of social status in the extended face-processing network[END_REF] . A recent fMRI study showed that the OFC represented the stereotypic social traits of others and that its pattern of activity was predictive of individual choices, highlighting its critical role in social decision-making [START_REF] Kobayashi | Neural representations of others' traits predict social decisions[END_REF] . In these studies, participants had to behave according to the facial expression, attitude, or social category of the individuals presented in the experiment. Our results extend these findings. Unlike previous studies, participants based their decision on symbolic features (the type and arrangement of ornamentations), to which they arbitrarily attributed social meaning. This implies that the role of the orbitofrontal cortex in social decision-making is not restricted to processing stereotypical attitudes or social groups, a capacity shared with non-human primates. Social evaluation based on symbolic external attributes also involves this region in humans.

The social status attribution task heavily relies on high-order executive functions such as attentional control, selection, and flexibility. The activation of the pars triangularis of the inferior frontal gyrus extending to the inferior frontal sulcus reflects these aspects [START_REF] Koechlin | An information theoretical approach to prefrontal executive function[END_REF] .

Although the activation was bilateral, the right and left inferior frontal gyrus probably played a different role in the task. The right inferior frontal gyrus is explicitly associated with highlevel social cognition [START_REF] Hartwigsen | Functional Segregation of the Right Inferior Frontal Gyrus: Evidence From Coactivation-Based Parcellation[END_REF] . The left inferior frontal gyrus is involved in selecting some aspects or subsets of available information among competing alternatives [START_REF] Thompson-Schill | Role of left inferior prefrontal cortex in retrieval of semantic knowledge: A reevaluation[END_REF][START_REF] Zhang | Is left inferior frontal gyrus a general mechanism for selection?[END_REF] . This region also plays a role in processing non-linguistic symbolic information [START_REF] Muayqil | Representation of visual symbols in the visual word processing network[END_REF]100 , consistent with the symbolic value attributed by the participants to face adornments.

Overall, the prefrontal cortex's involvement in the present study underlines its role in social decision-making. Our results extend their contribution to symbolic social communication, here materialized by face ornamentations.

Salience network

Social status attribution elicited activation in the anterior insula, the dorsal anterior cingulate cortex (dACC)/pre-SMA, and subcortical structures, such as the thalamus and the caudate nuclei. These regions constitute the so-called salience network, whose key components are the insula anterior and the dACC/pre-SMA 101-103 . This network is involved in selecting relevant elements of the environment for perceptual decision-making 104-106 . In our case, participants had to extract salient information from the ornamented faces to associate the proposed social status with one of the three faces presented to them. The salience network was also activated in the 1-back task by the need to detect the repetition of ornamental patterns. However, the greater uncertainty in decision-making during the attribution task can explain why activation of the salience network was more extensive during status attribution than the 1-back task 107 .

To attribute a social status, participants had to make a forced choice among three possibilities, with several plausible answers and had to compare the different options and arbitrate to choose only one. These aspects of the task have probably triggered the activation of the dACC/pre-SMA, belonging to the salience network. The dACC/pre-SMA has been reported as involved in conflict and performance monitoring [108][109][110] , and more recently in social categorization domain 111 .

Resting-state functional connectivity

Resting-state functional connectivity provides insight into the potential interactions between neural assemblies activated by the social status assignment task. The G_Fusiform-4-R and the G_Temporal_Pole_Sup-1-R were characterized by many connections with other activated regions ( Figure 6). These two regions were connected with 22 and 27 hROIs, respectively. The G_Fusiform-4-R included the FFA (see results) and is likely involved in the initial processing phase. The functional relationships between the medial temporal lobe, the fusiform gyrus, and the temporal pole reflected the association of the perceptive, social, mnemonic, and associative aspects of the task. In addition, the G_Temporal_Pole_Sup-1-R was connected with frontal regions and could act as a hub, allowing communication between visual areas and executive frontal regions. The connection between the temporal pole and the salience network enables the exchange of information necessary for evaluating and selecting inputs relevant to social decision-making. The temporal pole and the salience network were related to the inferior frontal gyrus, contributing to the evaluation of subjective confidence about a perceptual decision 112 .

The orbitofrontal cortex was functionally connected to the G_Temporal_Pole_Sup-1-R and the G_Fusiform-4-R. These regions, whose essential role in social status evaluation has been discussed above, could constitute the core network in the social attribution task ( Figure 6). They must have allowed the integration of information leading to the assignment of social status based on the perception of symbolic cues. Notably, none of the regions involved in assigning social status exclusively dealt with social information. Most of these regions are involved in many cognitive functions. The functional connection of structures whose processing properties are beneficial for the execution of the task allows the social judgment function to emerge. Human connections exceed those of animals, including primates, at both the structural and functional levels [113][114][115] .

Thanks to creating functional connections (linking social cognition, memory, and executive functions), humans could use symbolic items and markings to signify social status.

Conclusion

This study delved into the neural mechanisms involved in the social interpretation of facial adornments and found that various brain regions, including the FFA, temporal poles, salience network, and orbitofrontal cortex, were involved in this process. Furthermore, assigning a social status from symbolic cues also activated the medial temporal regions and the inferior frontal gyrus, reflecting the role of episodic memory, contextual association, and executive functions. The complexity of this neural network raises questions about when it became fully functional in our ancestors and whether it resulted from a gradual process of integration and complexification or was already fully functional when the first archaeological evidence of culturalization of the human face was recorded.

The gradual complexification and patchy emergence of face adornment technologies over the last 500,000 years suggest a scenario of increasing but asynchronous integration of brain areas involved in social status recognition based on facial culturalization. This growing integration allowed the decoding of increasingly complex symbolic codes, supported by more demanding technologies for face adornment.

The interplay between cultural and biological mechanisms likely drove this process, with individuals gifted in acquiring, decoding, and creating these symbolic messages having selective advantages that favored the permanent inscription of a more integrated connectivity in the brain [116][117][118] . A progressive co-option of brain regions has also been suggested for the evolution of tool-making [119][120][121] . It would have enabled the development of increasingly complex tools.

The period between 140,000 and 70,000 years ago may have represented a key moment in this integration process, as this was when red pigments use became almost ubiquitous at African Middle Stone Age sites and marine shell beads were used for the first time in North Africa, the Near East, and Southern Africa. This diversification of colors, shapes, and technologies indicates a complexification of practices allowing wearers to use their faces to communicate information about their social role using more complex shared symbolic codes.

It is reasonable to think that the human brain had largely equipped itself with the necessary connections to process and interpret these stimuli 70,000 years ago.

Figure 1 .

 1 Figure 1. Face ornamentations used in the tasks. Top row: paint only. The middle row: beadworks only. Bottom row: a combination of paint and beadworks.

Figure 2 .

 2 Figure 2. Drawn version of some triplets of stimuli used in the selection task. Each triplet was composed of individuals of the same sex (male or female). The three ranks correspond to three levels of richness. Note that in the study, photographs of real people with adorned face were presented but could not be displayed here because of their identifying nature.

Figure 3 .

 3 Figure 3. Organization of one event of the selection task.

Figure 4 .

 4 Figure 4. Activated regions in [Social status minus Ornament check] contrast superimposed on an MRI template.

Figure 5 .

 5 Figure 5. Resting-state connectivity matrix of 37 hROIs specific to the social status assignment (i.e., positive in the contrast [Social status minus Ornament check] and positive in contrast [Ornament (1-back) minus Faces].

Figure 6 .

 6 Figure 6. Schematic resting-state functional connectivity network between regions activated during a social status attribution task based on symbolic culturalized faces. Black arrows indicate the reciprocal resting-state functional connectivity between brain regions (univariate t-test, p < 0.05, FDR corrected).

Table 1 .

 1 Questions in the social status and ornament check conditions. The wording of the questions was gendered according to the stimulus.

	Questions	Social status attribution	Ornament (control)
	1	Chief?	Painted eyes?
	2	Healer?	Double necklace?
	3	Warrior?	Diadem?
	4	Hunter / Huntress?	Painted cheeks?
	5	Shaman?	Painted circle?
	6	Musician?	No beads?
	7	Storyteller?	No necklace?
	8	Married?	No earrings?
	9	Mother / Father?	No paint?
	10	Scout?	No lines?

Table 2 .

 2

Mean, standard deviation, and p-value of the activated regions in the [Social status minus Ornament check] contrast.

hROI Social status -Ornament check (* Specific to Social status - Ornament check, i.e., more or not activated in Ornament block -faces) MNI coordinates of the center of mass Mean (BOLD)

	G_Frontal_Mid-1-R G_ParaHippocampal-1-R*	41 44 13 14 -4 -18	0.09 0.17	0.16 0.20	0.0075 0.0001
	G_Frontal_Mid-5-L G_ParaHippocampal-2-L*	-43 20 37 -28 -27 -19	0.21 0.07	0.35 0.17	0.0038 0.0253
	G_Frontal_Mid-5-R G_ParaHippocampal-2-R	42 17 41 29 -25 -19	0.09 0.07	0.17 0.14	0.0132 0.0169
	S_Inf_Frontal-1-L* G_ParaHippocampal-4-L*	-44 38 12 -17 -27 -13	0.37 0.12	0.31 0.21	<.0001 0.0071
	S_Inf_Frontal-1-R* G_ParaHippocampal-4-R*	46 40 10 17 -27 -10	0.24 0.15	0.21 0.18	<.0001 0.0002
	S_Inf_Frontal-2-L N_Caudate-4-R	-43 15 29 14 20 8	0.38 0.06	0.34 0.13	<.0001 0.0237
	S_Inf_Frontal-2-R* N_Caudate-5-L	44 19 28 -13 10 8	0.28 0.17	0.21 0.21	<.0001 0.0003
	G_Frontal_Inf_Tri-1-L* N_Caudate-5-R	-49 26 5 12 10 9	0.15 0.16	0.22 0.22	0.0011 0.0006
	G_Frontal_Inf_Tri-1-R* N_Thalamus-1-L	50 29 5 -4 0 1	0.07 0.18	0.17 0.29	0.0434 0.0032
	S_Precentral-1-R N_Thalamus-1-R	50 10 24 4 0 1	0.11 0.16	0.24 0.24	0.0178 0.0012
	G_Frontal_Sup_Medial-3-L* N_Thalamus-2-R	-5 35 43 9 -7 13	0.19 0.09	0.23 0.18	0.0002 0.0120
	G_Frontal_Sup_Medial-3-R N_Thalamus-3-L	6 33 44 -3 -7 -1	0.14 0.15	0.19 0.22	0.0006 0.0017
	G_Supp_Motor_Area-1-L* N_Thalamus-4-L*	-6 22 46 -3 -14 8	0.39 0.20	0.23 0.27	<.0001 0.0005
	G_Supp_Motor_Area-1-R* N_Thalamus-4-R	6 21 48 3 -14 9	0.37 0.17	0.26 0.25	<.0001 0.0017
	G_Supp_Motor_Area-2-L N_Thalamus-5-L	-11 18 61 -12 -19 7	0.08 0.09	0.16 0.14	0.0134 0.0026
	G_Insula-anterior-1-L* N_Thalamus-5-R	-20 5 -19 13 -17 6	0.12 0.07	0.18 0.14	0.0013 0.0148
	G_Insula-anterior-1-R* N_Thalamus-6-R	19 7 -19 15 -27 13	0.07 0.05	0.17 0.12	0.0329 0.0455
	G_Insula-anterior-2-L* N_Thalamus-7-L	-34 17 -13 -9 -28 11	0.17 0.09	0.24 0.15	0.0009 0.0054
	G_Insula-anterior-2-R N_Thalamus-9-L	35 18 -13 -5 -11 -7	0.10 0.12	0.18 0.13	0.0054 <.0001
	G_Insula-anterior-3-L* N_Thalamus-9-R	-34 24 1 5 -10 -6	0.31 0.11	0.22 0.16	<.0001 0.0007
	G_Insula-anterior-3-R	37 24 0	0.20	0.15	<.0001
	G_Insula-anterior-4-L	-41 15 3	0.08	0.21	0.0457
	G_Occipital_Inf-1-R	50 -60 -9	0.22	0.19	<.0001
	G_Occipital_Inf-2-L	-45 -71 -7	0.13	0.17	0.0003
	G_Occipital_Inf-2-R	47 -65 -7	0.19	0.18	<.0001
	G_Occipital_Lat-2-L	-26 -94 -1	0.06	0.15	0.0253
	G_Occipital_Lat-3-L	-40 -84 -12	0.10	0.16	0.0041
	G_Occipital_Lat-3-R	43 -81 -10	0.11	0.20	0.0078
	G_Occipital_Lat-4-L	-31 -89 8	0.08	0.15	0.0078
	G_Occipital_Lat-4-R	34 -85 9	0.09	0.15	0.0054
	G_Occipital_Lat-5-L	-35 -79 -1	0.07	0.15	0.0132
	G_Occipital_Lat-5-R	36 -76 2	0.07	0.14	0.0116
	G_Fusiform-2-L*	-35 -26 -23	0.09	0.10	<.0001
	G_Fusiform-4-L	-43 -50 -17	0.19	0.18	<,0001
	G_Fusiform-4-R* G_Fusiform-5-L G_Fusiform-5-R G_Fusiform-6-R	44 -46 -18 -31 -50 -12 32 -47 -41 29 -62 -9	0.22 0.09 0.09 0.05	Standard deviation 0.15 0.12 0.12 (BOLD) 0.12	<.0001 p (FDR 0.0006 corrected) 0.0006 0.0489
	G_Cingulum_Ant-2-L* S_Intraoccipital-1-L G_Cingulum_Ant-2-R S_Intraoccipital-1-R G_Cingulum_Mid-2-L G_Precuneus-2-R* G_Cingulum_Mid-2-R G_Precuneus-7-L* S_Cingulate-1-L* G_Precuneus-7-R* S_Cingulate-1-R S_Intraparietal-2-L S_Cingulate-2-L S_Intraparietal-2-R S_Cingulate-2-R S_Intraparietal-3-L G_Cingulum_Post-1-L S_Intraparietal-3-R G_Cingulum_Post-2-L G_Temporal_Inf-4-R G_Cingulum_Post-3-L G_Temporal_Pole_Sup-1-L* G_Cingulum_Post-3-R G_Temporal_Pole_Sup-1-R* G_Frontal_Inf_Orb-1-L* G_Temporal_Pole_Mid-2-L* G_Frontal_Inf_Orb-1-R* G_Temporal_Pole_Mid-2-R* G_Frontal_Mid_Orb-2-L* G_Hippocampus-1-L*	-7 34 22 -24 -72 32 7 33 23 28 -69 33 -4 3 30 5 -56 20 4 4 30 -6 -65 35 -7 27 30 7 -63 36 7 27 31 -34 -58 45 -7 16 41 37 -52 48 8 14 46 -27 -60 43 -4 -26 29 27 -61 46 -4 -39 27 54 -58 -11 -5 -43 10 -35 11 -24 6 -42 10 36 16 -24 -42 31 -17 -35 9 -33 44 33 -14 35 12 -34 -41 49 -5 -30 -7 -19	0.12 0.09 0.08 0.15 0.15 0.27 0.11 0.16 0.23 0.21 0.20 0.22 0.22 0.14 0.14 0.16 0.12 0.13 0.11 0.10 0.13 0.09 0.10 0.10 0,21 0.05 0.13 0.04 0.23 0.06	0.16 0.23 0.16 0.21 0.16 0.29 0.18 0.31 0.20 0.29 0.19 0.29 0.22 0.20 0.20 0.27 0.22 0.19 0.23 0.19 0.24 0.13 0.22 0.16 0.22 0.10 0.16 0.09 0.30 0.11	0.0455 0.0005 0.0007 0.0173 <.0001 <.0001 0.0097 0.0045 0.0006 <.0001 0.0005 <.0001 0.0010 <.0001 0.0051 0.0011 0.0012 0.0072 0.0086 0.0121 0.0019 0.0071 0.0044 0.0253 0.0097 <.0001 0.0121 0.0001 0.0005 0.0115
	S_Orbital-1-R G_Hippocampus-1-R*	25 41 -15 30 -5 -18	0.07 0.08	0.11 0.11	0.0023 0.0005
	S_Orbital-2-L* G_Hippocampus-2-L S_Orbital-2-R* G_Hippocampus-2-R G_ParaHippocampal-1-L*	-31 34 -13 -25 -32 -3 29 34 -13 25 -31 -2 -16 -4 -18	0.26 0.04 0.18 0.06 0.24	0.21 0.09 0.17 0.09 0.23	0.0430 <.0001 0.0041 <.0001 <.0001

Acknowledgments

We thank the Ginesis Lab (GIN, Fealinks, Labcom Programme 2016, ANR 16LCV2-0006-01) for their help with data management and processing. We are also indebted to Violaine Verrecchia and Marc Joliot for their help in data analysis. Our thanks also go to Marie Guerlain, Annie Bardon-Lay, and her team members for their help with the face painting.

Warm thanks also go to all those who agreed to participate in our experiments. 7. Authors' contributions MS, FE, SR, EM designed the study MS, EM acquired the data MS, EM analyzed the data MS, FE, SR, EM wrote the article 8. Fundings This work was supported by the CNRS project 80 Prime Neurobeads and a grant from the IdEx Bordeaux/CNRS (PEPS 2015). Francesco d'Errico's work is supported by the European Research Council through a Synergy Grant for the project Evolution of Cognitive Tools for Quantification (QUANTA), No. 951388; the Research Council of Norway through its Centres of Excellence funding scheme, SFF Centre for Early Sapiens Behaviour (SapienCE), project number 262618, the Talents Program of the Bordeaux University [grant number: 191022_001] and the Grand Programme de Recherche 'Human Past' of the Initiative d'Excellence (IdEx) of the Bordeaux University.

Data Availability

Raw BOLD values for all the hROIs and contrasts are available as supplementary material


Giovanna Franklin

El-Mahdi Halim

Celine Merlet

Pierre-Louis Taberna
email: pierre-louis.taberna@univ-tlse3.fr

Patrice Simon

-Insights into the effect of oxygen functional groups on the heterogeneous electron transfer at activated carbons from scanning electrochemical microscopy

Keywords: Scanning electrochemical microscopy, Activated carbon, heterogeneous electron transfer rate constant, surface oxygen groups influence

The influence of surface chemistry on heterogeneous electron transfer at activated carbon (AC) electrodes was studied using scanning electrochemical microscopy (SECM) and conventional electrochemical measurements, which allow for measuring respectively the local and the overall electrode electron transfer rate constants. Using a thermal treatment in air and under argon, commercial activated carbons were either oxidized or defunctionalized. Both treatments lead to a slight decrease of the specific surface area of the material, without altering its pore size distribution and morphology. The nanogap voltammetry and approach curves obtained by SECM were used to characterize the rate constants at the interface between an aqueous solution of ferrocenemethanol (FcMeOH) and the AC electrodes. The heterogeneous electron transfer (HET) rate constant for the untreated AC was 3.4 10 -2 ± 0.2 cm s -1 and was increased by about 2.5 and 1.5 times, after oxidizing and defunctionalizing the surface, respectively. SECM results showed that the heat treatment increases the conductivity of AC, and the oxidation of the surface increases the wettability and accessibility of the pores, which are assumed to promote the electron transfer. The local heterogeneous electron transfer rate constant values obtained by SECM are higher, by an order of magnitude, than the global ones obtained from conventional electrochemical macroelectrode measurements. The local information obtained by SECM is valuable for understanding the nanoscale interfacial mechanisms in porous carbon electrodes as a better understanding of the charge storage mechanism is key to improve their electrochemical performances.

Introduction

The massive growth of population around the world accompanied by a tremendous industrial development result in an enormous demand for global energy consumption. In addition, the evolution of the world towards smart and connected technologies and the daily need for electronic devices such as laptops, smartphones and tablets require highly efficient energy storage systems. [START_REF] Simon | Perspectives for Electrochemical Capacitors and Related Devices[END_REF] Electrochemical double layer capacitors (EDLCs), also called supercapacitors (SCs), are energy storage devices that store energy via adsorption-desorption of ions at the electrode/electrolyte interface. Porous carbons such as activated carbons (ACs) with high specific surface area (more than 1500 m 2 g -1 ) and good electrical and electrochemical properties have been extensively used as electrode material for EDLCs. [START_REF] Simon | Perspectives for Electrochemical Capacitors and Related Devices[END_REF][START_REF] Shao | Nanoporous Carbon for Electrochemical Capacitive Energy Storage[END_REF] At the start of the 21 st century, the use of nanoporous carbons showed a drastic increase of the capacitive performance. [START_REF] Chmiola | Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer[END_REF] Indeed, a surprising increase in capacitance was achieved by reducing the pore size to the sub-nanometer level (< 1 nm). This discovery has impacted the performance of commercial devices and provided fundamental new insights into the electrical double layer in nanopores. Accordingly, exploring the attractive properties on the nanometric scale to understand the charge storage mechanism of nanoporous carbon-based electrodes is a very effective way to boost the performance of electrochemical capacitors. Many studies have been devoted to investigate the relationship between pore size and ion size, more precisely ion confinement inside nanopores, to describe the ion exchange mechanisms in nanoporous carbon and their influence on the capacitance. [START_REF] Chmiola | Desolvation of Ions in Subnanometer Pores and Its Effect on Capacitance and Double-Layer Theory[END_REF][START_REF] Merlet | On the Molecular Origin of Supercapacitance in Nanoporous Carbon Electrodes[END_REF][START_REF] Merlet | Highly Confined Ions Store Charge More Efficiently in Supercapacitors[END_REF] Reducing the pore size to a size smaller than that of the solvated ions resulted in a partial desolvation and consequently a significant increase in the capacitance. [START_REF] He | Capacitive Mechanism of Oxygen Functional Groups on Carbon Surface in Supercapacitors[END_REF] As capacitive electrodes store charges at the carbon surface, it is of essential to study the influence of the surface chemistry on the specific capacitance, in addition to the porosity. [START_REF] Qu | Studies of Activated Carbons Used in Double-Layer Capacitors[END_REF] Thermal treatments (TT) under oxygen or under argon are expected to affect the material's surface chemistry, and specific capacitance. [START_REF] Yumak | Effect of Synthesis Methods on the Surface and Electrochemical Characteristics of Metal Oxide/Activated Carbon Composites for Supercapacitor Applications[END_REF] For instance, subjecting a carbon surface to a TT under oxygen lead to the introduction of Oxygen Functional Groups (OFGs) such as carboxyl, carbonyl and hydroxyl as a consequence of the gas/active material interaction. The accessibility of the pores may be impeded and/or a pseudocapacitive behavior may be enabled due this effect. [START_REF] Dastgheib | Adsorption of Oxygen by Heat-Treated Granular and FIbrous Activated Carbons[END_REF] Indeed, it has been proven that OFGs on the material's surface can lead to pseudo-capacitance coming from the reaction from the functional groups and the electrolyte, increasing the global capacitance. [START_REF] Chen | Effects of Oxygen-Containing Functional Groups on the Supercapacitor Performance of Incompletely Reduced Graphene Oxides[END_REF] In this work, two different thermal treatments are conducted on commercial ACs aiming at their oxidation or reduction. As the presence of OFGs can alter the surface electronic conductivity, the surface accessibility is also expected to be modified. Such changes can be probed by assessing the heterogeneous electron transfer (HET) rate constants measuring the dynamic effects in the electrode/electrolyte interface. [START_REF] Fawcet | The Kinetics of Heterogeneous Electron Transfer Reaction in Polar Solvents[END_REF][START_REF] Park | Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for Redox-Active-Electrolyte Supercapacitors[END_REF] As a matter of fact, a higher surface conductivity and a better access to the carbon surface should lead to higher HET. Measurements of the overall (global) and local HET rate constants were conducted. Regarding the global measurement, Nicholson's method was applied for each carbon. [START_REF] Nicholson | Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics[END_REF] However, it is known that the global measurement is affected by the electrode resistance. For instance, the grain-grain contact resistance will make the effective HET lower [START_REF] Park | Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for Redox-Active-Electrolyte Supercapacitors[END_REF] . It is where local measurement comes into play as it makes the grain-grain resistance less prominent and in turns should lead to a higher effective HET. [START_REF] Gao | Situ and Operando Force-Based Atomic Force Microscopy for Probing Local Functionality in Energy Storage Materials[END_REF] Scanning electrochemical microscopy (SECM) is a perfect tool for measuring such local rate constants. Global and local measurements of the effective HET were done and compared depending on the TT conditions applied in this work.

Experimental

Materials

In this work, the amount of surface oxygen functional groups on commercially available YP-50F activated carbons (AC), designated was modified through thermal treatments. Air or argon atmosphere was used whether the purpose was to increase or decrease the amount of surface oxygen functional groups, respectively. The appropriate thermal conditions were determined using thermogravimetric analyses (TGA), performed in air and in argon. Accordingly, YP-50F oxidation was done in air at 300 °C for 12h. Then, the powder was kept in the oven until it cooled down to room temperature. YP-50F reduction was carried out under argon atmosphere at 250 °C for 3h. The sample was then cooled to room temperature in argon. In the following, the air oxidized AC was named oAC and the argon reduced AC, rAC.

Materials characterizations

TGA measurements were carried out using a SETARAM TAG 16 by heating the AC up to 800°C with a 5°C min -1 heating rate in air for AC oxidation, and in argon atmosphere for AC reduction.

Porosity properties of AC materials were investigated using a Micromeritics ASAP2020 instrument, with argon gas as adsorbate. AC materials were outgassed at 200°C under vacuum for 24h. Argon isotherm was measured at 77 K. Specific surface area (SSA) values were determined using Brunauer-Emmett-Taller equation (BET), [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF] while pore size distributions (PSD) were obtained from 2D non-local density functional theory (2D-NLDFT) [START_REF] Seaton | A New Analysis Method for the Determination of the Pore Size Distribution of Porous Carbons from Nitrogen Adsorption Measurements[END_REF] implemented in SAEUIS, from Micrometrics. The surface morphology of all electrodes was characterized by scanning electron microscopy (SEM) VEGA3-Tescan and the chemical composition was analyzed by energy dispersive X-ray spectrometer (EDX) using a JED 2300 detector coupled to SEM.

Electrochemical global measurements

The carbon powders (AC, oAC, and rAC -90 wt%) were mixed with polyvinylidene fluoride (PVDF) binder (10 wt%) and dispersed in N-methyl-2-pyrrolidone (NMP) solvent using an ultrasonic bath. The obtained dispersion was coated by spray gun on stainless steel foil (12 mm in diameter) and dried at 80°C under vacuum for 12 hours. The mass loading of the coating was about 0.4 mg cm -2 . The electrochemical tests were carried out in a three-electrode configuration, at room temperature, using a Biologic SP-300 potentiostat. The carbon electrodes were used as working electrode, a Pt foil as counter electrode, and a Ag/AgCl as reference electrode (Figure 1A). Cyclic voltammetries (CV) were done from 0 to 0.5 V vs. Ag/AgCl operational potential window (OPW), in a 0.1 M potassium chloride (KCl) electrolyte containing 1 mM ferrocenemethanol (FcMeOH) as an electrochemical probe. Various scan rates (5, 10, 20, 50 and 100 mV s -1 ) were applied in order to realize kinetic analyses. The CV curves were normalized with respect to the mass of active material. Nicholson's method [START_REF] Nicholson | Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics[END_REF] was used to determine the effective electron transfer rate constant (keff) for the three carbon electrodes using the CV results.

SECM measurements

SECM experiments were performed under feedback mode with a Biologic SECM instrument (model M470) associated with a SP-300 bipotentiostat. A commercial platinum ultramicroelectrode (Pt-UME with a diameter of ~10 µm) was used together with Pt and Ag/AgCl as working, counter and reference electrodes, respectively (Figure 1B). The platinum ultramicroelectrode (Pt-UME) as SECM tip was fixed on an x,y,z-positioning system to control its position over the carbon substrate at the bottom of the SECM cell. All electrodes were emerged in an aqueous electrolyte containing 0.1 M KCl and 1 mM FcMeOH. After each test, the electrolyte was renewed. It is worth noting that for all SECM tests, the carbon substrates were not polarized. The initial approach of the Pt-UME over the substrates was monitored using a video microscope. Initially, it is essential to establish the intermittent contact position between the tip and substrate. This can be achieved by utilizing the piezoelectric mode offered by the Biologic software. In this mode, specific vibration frequencies are associated with different current values, reflecting the distance between the probe and the carbon. The approach of the Pt-UME tip is halted when the physical distance matches the frequency corresponding to the highest calculated current peak observed during intermittent contact. Then, the remaining distance between the substrate and the tip before contact was controlled via the variation of the feedback current during the approach curve while the tip was biased at 0.5 V vs. Ag/AgCl. The definitive contact, defined by the origin of z-axis, is defined as being the point where the tip current value doubles compared with the bulk. The accuracy of the origin determination was about 5 µm. SECM-CV curves were measured between the Pt-UME at ~ 200 ± 5 µm (in the bulk solution) and at ~ 1±5 µm from the carbon substrates, in the potential range from 0 to 0.6 V vs. Ag/AgCl at a scan rate of 20 mV s -1 . The distance standard variation is acquired according to the delimited displacement measured at each experiment, CV and approach current curves were normalized by the tip current measured in the bulk solution (Ibulk). [START_REF] Kai | Scanning Electrochemical Microscopy at the Nanometer Level[END_REF]

Results and discussion

In this work, the goal was to study the influence of the oxygen group content on the effective heterogeneous electron charge transfer between a microporous activated carbon and an aqueous electrolyte. To do so, electrochemical characterizations of commercial and thermally treated ACs were performed using conventional CV measurements on the overall AC electrode (global measurement) and using SECM-local approach curve measurements. To determine the proper thermal conditions for oxidation and reduction of the initial activated carbon (YP-50F), TGA analyses were conducted. The results of TGA together with Derivative Thermogravimetry (DTG) of AC, under air and argon atmosphere are shown in Figure 2. In the initial heating range, below 150°C, in both cases, AC exhibits a small weight loss of about 4%, due to the elimination of physiosorbed water molecules. Under air, AC does not experience any weight loss until it gets fully decomposed beyond 550°C (Figure 2A); while only a weight loss of about 11%, around 235°C, is observed for AC heated under argon atmosphere. This weight loss is ascribed to the thermal desorption of surface oxygen groups. [START_REF] Mcallister | Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite[END_REF][START_REF] Kariim | Development and Characterization of MWCNTs from Activated Carbon as Adsorbent for Metronidazole and Levofloxacin Sorption from Pharmaceutical Wastewater: Kinetics, Isotherms and Thermodynamic Studies[END_REF] Following the TGA analyses, an AC oxidized at 300°C under air for 12 hours, designated as oAC, and an AC reduced at 250°C under argon for 3 hours, designated as rAC, were prepared. Gas sorption analyses were performed on these two carbons and the isotherms are presented in Figure 3A. The isotherm of the as-received AC is also shown for comparison. PSDs determined using nonlocal density functional theory (NL-DFT) [START_REF] Tarazona | Free-Energy Density Functional for Hard Spheres[END_REF] are plotted in Figure 3B. All the textural properties are summarized in Table 1. ) oAC

AC

The isotherm diagrams (Figure 3A) exhibit a typical Type I isotherm [START_REF] Lykiema | Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity[END_REF][START_REF] Donohue | Classification of Gibbs Adsorption Isotherms[END_REF] , as expected for a microporous carbon. A slight hysteresis is observed and is ascribed to the presence of small amount of mesopores. The untreated AC material presents specific surface area (SSA) values of about 1836 ± 27 m 2 g -1 with a total pore volume of 0.99 cm 3 g -1 and an average pore size of about 1.2 nm. After oxidation or reduction treatment, the SSA and the total pore volume of both materials decreased by ~25%. However, the overall pore size distribution (PSD) stays the same leading to the same average pore size. Indeed, the three materials showed the same two adsorptions modes, one centered at about 0.9 nm, the other at 1.5 nm. It confirms that the micropore structure was not modified by the oxidation or reduction AC treatment. The SSA decrease for both thermal treated materials could be explained by the blocking of some micropores over the carbon surface thermal pretreatment. For the oxidized activated carbon, there is a small weight increase due the addition of oxygen atoms over its surface, while, in the case of defunctionalized AC, local structural rearrangement in the carbon could explain the decrease of micropore volume. [START_REF] Bleda-Martınez | Chemical and Electrochemical Characterization of Porous Carbon Materials[END_REF] The surface morphology of AC, oAC, and rAC was characterized by SEM analysis. The SEM images and EDX analysis of the films surface are shown in Figure 4. According to SEM images, no severe morphology change is observed after the different treatments.

EDX analysis shows a slight decrease of the C/O ratio after the thermal oxidation, due the oxygen amount increase, while the opposite is seen after thermal reduction. Those characterization results

indicate some surface modifications have occurred during both treatments.

Following Nicholson's paper, [START_REF] Nicholson | Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics[END_REF] the current peak potential separation is dependent on the heterogeneous electron transfer rate constant of the electrode: the larger the current peak potential separation, the lower the HET. In this case, CV curves of AC, oAC, and rAC electrodes, performed in 0.1M KCl containing FcMeOH as mediator at various scan rates, are presented in Figure 5 (A-C). No major changes with oxidation are noticed, as the overall shape and current magnitude are very similar. The results of CVs of the three carbon electrodes are as expected for quasi-reversible redox reactions with a single electron transfer. Going further, the variations of the peak current with the square root of the scan rate are plotted in Figure 5 (D-F) for both the oxidation and the reduction.

For all carbons, the peak currents vary linearly, as expected for diffusion-controlled kinetics. [START_REF] Neudeck | The Determination of Diffusion Coefficients and Rate Constants from the Dependence of the Peak Separation and Peak Current on the Scan Rate of Cyclic Voltammograms at Micro-Cylindrical Electrodes[END_REF] 0 The oxidized AC sample (oAC) exhibits a 20% higher current than the as-received activated carbon (AC) which could be linked to the presence of higher amount of oxygen groups adding a pseudo capacitive contribution in the material. For the carbon having a lower oxygen content (rAC), the current density is around 10% lower, especially at slower scan rates. The influence of oxygen groups content is more marked when comparing the peak-to-peak separation (∆Ep = |𝐸 𝑝𝑎 -𝐸 𝑝𝑐 |) values (Figure 6A).

Starting with the same mass loadings for all the electrodes (0.15 mg/cm 2 ), the cycling voltammetry in presence of no redox mediator at the electrolyte, presented a no redox peaks profile. The observed peaks can be attributed to the interaction between the electrode and the redox mediator in the )

 1/2 (V s -1 ) 1/2 D C E A B F electrolyte.
In the presence of FcMeOH dissolved in the electrolyte, the oxidation treatment of AC leads to a decrease of ∆Ep at different scan rates, while the reduction of AC shows a distinctly different electrochemical behavior. For the lowest scan rates, rAC electrode exhibits higher ∆Ep values, and at faster scan rates, lower ∆Ep values compared to AC electrode. The ∆Ep value variation for both different treatments are due to the different interaction between the carbon surface and the electrolyte's redox mediator, since different functional groups are reacting for both treatments. [START_REF] Neudeck | The Determination of Diffusion Coefficients and Rate Constants from the Dependence of the Peak Separation and Peak Current on the Scan Rate of Cyclic Voltammograms at Micro-Cylindrical Electrodes[END_REF] The ∆Ep values were used to calculate the charge transfer rate constant (keff) using the following equation [START_REF] Nicholson | Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics[END_REF] :

𝑘 𝑒𝑓𝑓 = (𝐷𝑛𝐹𝜈 𝑅𝑇 ⁄ ) 1 2 ⁄ 𝛹 ( 1
)
where Ψ is the kinetic parameter related to the ∆Ep value (Table S1) [START_REF] Nioradze | Quasi-Steady-State Voltammetry of Rapid Electron Transfer Reactions at the Macroscopic Substrate of the Scanning Electrochemical Microscope[END_REF] , D is the diffusion coefficient of the redox mediator (cm 2 s -1 ), calculated from Randles-Sevcick equation [START_REF] Park | Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for Redox-Active-Electrolyte Supercapacitors[END_REF] , n is the number of the electrons involved in the redox reaction, F is the Faraday constant (96 485 C mol -1 ), ν is the scan rate (V s -1 ), R is the gas constant (8.3145 J mol -1 K -1 ),and T is the temperature (K). The calculated keff values of AC, oAC, and rAC electrodes at different scan rates, are reported in Figure 6B. In figure 6B are presented the effective HET rate constants for the three carbons at different scan rates. As observed, the effective HET is mostly constant for the non-treated AC while it increases with scan rate for the other two carbons. Between scan rates of 50 mV/s and 100 mV/s, the absolute values of the HET for both treated carbons exhibit an approximate twofold increase and a similar behavior. For the lower scan rates, on the other hand, the charge transfer constant is approximately twice as important for oAC than for rAC. This characteristic, first observed by Reinmuth [START_REF] Reinmuth | Inversible Systems in Stationary Electrode Polarography[END_REF] and further explained by Klingler and Kochi [START_REF] Klingler | Electron-Transfer Kinetics from Cyclic Voltammetry. Quantitative Description of Electrochemical Reversibility[END_REF] , support a 𝑘 𝑒𝑓𝑓 behavior change with different scan rates is due a lower reversibility. Then, as the sweep rate increases, the peak separation potential slightly increases, while the 𝑖 𝑝 𝑎 𝑖 𝑝 𝑐 ⁄ remains constant for every ν.

Based on the eq 2 provided by Nicholson and Shain [START_REF] Nicholson | Experimental Verification of an ECE Mechanism for the Reduction of P-Nitrosophenol, Using Stationary Stationary Electrode Polarography[END_REF] as an alternative method for the evaluation of 𝑘 𝑒𝑓𝑓 , expressed in eq 1, the calculated value of the effective constant will increase regularly with the sweep rate, that influences Ψ for reversible reactions However, it will eventually reach a point where further increases in the scan rate will offset the complementary changes in the peak separation and in this case, keff will become independent of the peak separation and as consequence,

 (V s -1 )

AC oAC rAC

the scan rate. Kingler and Kochi also confirmed that the simulated 𝑘 𝑒𝑓𝑓 values evaluated by

Nicholson's method in eq 1 and eq 2 are the same under the same experimental conditions.

𝑘(𝐸) = 2.18 [ 𝐷𝛽𝑛𝐹𝜐 𝑅𝑇 ] 1 2 exp[ 𝛽𝑛𝐹 𝑅𝑇 (𝐸 -𝐸 °)] (2)
Where D is the diffusion coefficient, β is the transfer coefficient for the electrode process, describing the degree of reversibility of a redox reaction between 0 and 1, calculated by independent methods [START_REF] Klingler | Electron-Transfer Kinetics from Cyclic Voltammetry. Quantitative Description of Electrochemical Reversibility[END_REF] , n is the number of electrons transferred in the rate-limiting step, F is the Faraday constant, E is the electrode's potential between the electrode and electrolyte solution and E ° denotes the standard potential, measured under standard conditions, used as reference point.

Local measurements

SECM measurements were performed to investigate the local influence of oxygen groups on the heterogenous electron transfer rate at activated carbon. Cyclic voltammograms measured using a Pt-UME over the AC, oAC, and rAC substrates, are shown in Figure 7. The CV current was normalized with respect to the tip current measured in the bulk of the electrolyte, 𝑖 𝑇,∞ . As observed, the voltammograms measured at about 200 µm from the carbon substrates are sigmoidal, without hysteresis: a steady-state response independent of 𝑣 is reached, as under these experimental conditions spherical diffusion prevails. [START_REF] Fu | Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions[END_REF] Furthermore, these curves have the same current variation at the bulk, as the Pt-UME is far enough from the substrate to avoid any feedback. [START_REF] Lee | Scanning Electrochemical Microscopy. 41. Theory and Characterization of Ring Electrodes[END_REF] When the tip is in the vicinity of the substrate (~1 µm), the voltammograms exhibits a higher current compared to that measured in the bulk, showing a positive feedback, for all the carbons studied. Indeed, as the three carbons tested are conductive, they allow for the regeneration of the mediator, causing an increase in the ultramicroelectrode (UME) current. As it is assumed that the diffusion coefficient is the same for the oxidized and the reduced forms of the electrochemical mediator, the current increase is likely related to the adsorption and regeneration of the mediator at the carbon surface. [START_REF] Park | Investigation of Charge Transfer Kinetics at Carbon/Hydroquinone Interfaces for Redox-Active-Electrolyte Supercapacitors[END_REF] Even though the current increases for all the carbons, different current amplification is found depending on the carbon tested. For the as-received AC substrate the current increased by about 2 times at the tip/substrate contact level, meanwhile, for oAC and rAC substrates, the current increased by about 3.5 and 2.75 times respectively, with respect to the CV current measured in the bulk electrolyte. These results confirmed the significant influence of the carbon surface state on the effective HET rate. In the experimental conditions chosen in this work, it turns out that both thermal treatments lead to higher current so higher HET rate.

In order to calculate the HET rate constant (𝑘 𝑒𝑓𝑓 ), the SECM approach curves were performed by approaching the Pt-UME toward the carbon substrates in feedback mode under the same electrolyte as the global measurements. The tip was biased at a potential of 0.5 V vs. Ag/AgCl to oxidize FcMeOH while approaching the surface of the substrates and measuring the current. All along the measurement, the carbon being tested are kept at open circuit potential. The approach curves of Pt-UME over AC, oAC, and rAC substrates are reported in Figure 8. As shown in figure 8, for normalized distances L higher than 2, the current remains stable and does not depend on the carbon surface state. It is considered that there is no tip-substrate interaction.

Differently, for tip distances from the surface lower than 2, the current increases in accordance with a positive feedback, as expected for conductive samples, it is related to the oxidation of Fc(MeOH) at the Pt-UME. [START_REF] Lindsey | Scanning Electrochemical Microscopy: Approach Curves for Sphere-Cap Scanning Electrochemical Microscopy Tips[END_REF] The experimental approach curves were fitted accordingly with the method described by Bard and Mirking [START_REF] Klingler | Electron-Transfer Kinetics from Cyclic Voltammetry. Quantitative Description of Electrochemical Reversibility[END_REF][START_REF] Lee | Scanning Electrochemical Microscopy. 41. Theory and Characterization of Ring Electrodes[END_REF] to determine the HET rate constant, 𝑘 𝑒𝑓𝑓 , values. According to the fitting process, the normalized rate constant, K, was determined, then, 𝑘 𝑒𝑓𝑓 was calculated using the following equation [START_REF] Wei | Scanning Electrochemical Microscopy: Application of SECM to the Study of Charge Transfer Processes at the Liquid/Liquid Interface[END_REF] :

𝐾 = 𝑘 𝑒𝑓𝑓 𝑟 𝑇 𝐷 (3)
where 𝑟 𝑇 is the radius of Pt-UME (~5 µm) and D is the diffusion coefficient of FcMeOH. The obtained results are reported in Table 3 as it is compared with the local values for AC, oAC and rAC. Among the three carbons, the oAC has demonstrated the higher 𝑘 𝑒𝑓𝑓 value, 8.3 10 -2 cm s -1 , compared to that of AC and rAC, which present 𝑘 𝑒𝑓𝑓 respectively a 𝑘 𝑒𝑓𝑓 of about 3.4 10 -2 cm s -1 , and 6.3 10 -2 cm s -1 , agreeing with the global HET order calculated by Nicholson's method (Figure 6B). Such findings show that carbon surface state plays a key role in the electrochemical charge transfer kinetic. Although the SSA of oAC and rAC are lower than that of AC, these two carbons exhibit a higher HET. This result may be linked to local carbon's electronic structure that is highly likely to change with the amount of surface oxygen functional groups. [START_REF] Chen | Effects of Oxygen-Containing Functional Groups on the Supercapacitor Performance of Incompletely Reduced Graphene Oxides[END_REF] Table 2 shows the 𝑘 𝑒𝑓𝑓 value in function of their respective SSA, understanding the influence of the porosity at the HET in a local way. The resolution of an in situ and local measurement versus a global measurement can be noticed trough the different effective constants for the same material versus the "averaged" response in the global [START_REF] Lindsey | Scanning Electrochemical Microscopy: Approach Curves for Sphere-Cap Scanning Electrochemical Microscopy Tips[END_REF] way, being seen as a limitation, as the value may be lower because of a lower electron transport throughout the AC film. Comparing the HET values obtained by the micro and macro electrode analysis, we can see that the order of each material's reactivity is kept, but the absolute value is increased by one order of magnitude with SECM measurements. Comparing local and global effective HET may bring information about the electron transport within the AC film-a large keff local/keff global may indicate the electron transport through the AC film is impeded by a higher grain-grain electrical contact resistance. Table 3 gives the calculated keff through the local and global technique for comparison, their ratios have been also calculated. The oAC presents the highest HET in both analysis and for all scan rates, consistent with the higher specific current during the electrochemical tests and confirming the increase of reactivity and capacity through the addition of oxygen functional groups in the carbon's surface. For the rAC, the HET increases in relation to AC in higher scan rates (50 mV s -1 and 100 mV s -1 ), as an evidence that the thermal treatment for the AC improve its performance and kinetics. Besides, the HET ratios show the TT lead to higher ratio that may suggest that the contact resistance has been affected by the TT and the change of surface chemistry.

Conclusions

Local and global measurement were applied on an as-received activated carbon and two different thermal treated ACs (oxidized oAC and reduced rAC) in order to investigate the effective HET rate constants showing the reactivity of the electrode surface for a given electrolyte. For all three cases, the sigmoidal CV shape obtained during macroelectrode's cycling shows a reversible reaction in which the oAC presented the highest current and highest charge transfer constant (8.6 10 -2 cm s -1 ) followed by rAC (6.3 10 -2 cm s -1 ) and AC (3.4 10 -2 cm s -1 ), demonstrating that a thermal treatment under air and argon might improve carbon's capacitance. The effective HET rate constants measured from SECM at the local scale were always higher than the one measured for the overall AC electrode, for the AC tested. A higher difference was observed for both TT AC that may be due to the modification of the surface chemistry after TT, which in turns leads to a higher grain-grain contact resistance.

Figure 1 :

 1 Figure 1: Schematic representation of the experimental setup of (A) global measurements and (B) local measurements. For the global measurements, The AC electrode was used as working electrode, and for the local measurements, a Pt UME was used as working electrode

Figure 2 :

 2 Figure 2: TGA and differential thermal analysis thermograms of the commercial AC powder in (A) air and (B) argon.

Figure 3 :

 3 Figure 3: (A) Ar adsorption/desorption isotherms and (B) pore size distributions calculated using NL-DFT for AC, oAC, and rAC.

Figure 4 :

 4 Figure 4: SEM images of (A) AC, (B) oAC, and (C) rAC films. Inset: percentages of oxygen and carbon, and C/O ratio values measured by EDX detector.

Figure 5 :

 5 Figure 5: CV curves of (A) AC, (B) oAC, and (C) rAC electrodes performed in 0.1 M KCl aqueous solution containing 1 mM FcMeOH from 0 to 0.5 V vs. Ag/AgCl at various scan rates (5 -100 mV s-1). Randles-Sevcik plots for (D) AC, (E) oAC, and (F) rAC.

Figure 6 :

 6 Figure 6: (A) Peak-to-peak separation (∆ Ep ) and (B) charge transfer constant (keff) values for AC, oAC, and rAC at different scan rates (5 -100 mV s -1 ) calculated by eq 1.

Figure 7 :

 7 Figure 7: CV curves measured by Pt-UME in the bulk (~200 µm) and near (~1 µm) (A) AC, (B) oAC, and (C) rAC electrodes, in an 0.1 M KCl aqueous solution containing 1 mM FcMeOH from 0 to 0.6 V vs. Ag/AgCl at a scan rate of 20 mV s -1 .

Figure 8 :

 8 Figure 8: Approach curves performed with Pt-UME (10 um in diameter) on AC, oAC, and rAC, in 0.1 M KCl solution containing 1 mM FcMeOH. The Pt-UME was biased at 0.5 V vs. Ag/AgCl and positioned at an initial distance of about 200 µm from the carbon electrodes.

Table 1 :

 1 Textural properties of AC, oAC, and rAC powders obtained by Ar

	adsorption/desorption
	Material	SBET	SNLDFT	Vt	Vmicro	Average pore size
		(m 2 g -1 )	(m 2 g -1 )	(cm 3 g -1 )	(cm 3 g -1 )	(nm)
	AC	1836 ± 27	1656	0.99	0.87	1.2
	oAC	1332 ± 19	1210	0.71	0.63	1.2
	rAC	1348 ± 20	1211	0.72	0.64	1.2

Table 2 :

 2 Normalized local HET by their Specific Surface Area

	Electrode	keff/SSA (g m -1 s -1 )
	AC	1.85 10 -7
	oAC	6.23 10 -7
	rAC	4.67 10 -7

Table 3 :

 3 keff values calculated using CV curves of Nicholson's method for 20 mV s -1 and SECM approach curves.

		𝒌 𝒆𝒇𝒇 (cm s -1 )
	Electrode	Global measurements (CV)	Local measurements curve) (SECM-approach	𝒌 𝒆𝒇𝒇𝒍𝒐𝒄𝒂𝒍 𝒌 𝒆𝒇𝒇𝒈𝒍𝒐𝒃𝒂𝒍
	AC	2.1 10 -3	3.4 10 -2	16
	oAC	4.1 10 -3	8.6 10 -2	20
	rAC	3.1 10 -3	6.3 10 -2	20

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 714581) and ANR SPICS program (ANR-19-CE05-0035).