F Grasset
email: grasset@univ-rennes1.fr

N Saito

D Li

D Park

I Sakaguchi

N Ohashi

H Haneda

T Roisnel

S Mornet

E Duguet

Surface Modification of Zinc Oxide NanoParticles by Aminopropyltriethoxysilane

Keywords: Nanostructure, Surface and interfaces, B: Chemical synthesis, C: X-ray diffraction

Commercial zinc oxide nanoparticles (20-30nm) were coated by aminopropyltriethoxysilane (APTES) under varying environments. Three different processes, acidic, basic and toluene were used. The effects of coating conditions (acidic, basic and toluene) on the grafting, structural and optical properties of these nanoparticles were studied.

In the three cases, it was possible to control the coating and according to X-ray diffraction, BET, TEM and SEM results, it is clear that the APTES coating plays a role of growth inhibitor even at 800°C. From diffuse reflectance measurements, one can notice that the grafting process did not modify the transmittance spectra of ZnO.

Introduction

Zinc oxide (ZnO), one of the II-VI compound semiconductor 1 , is an important material for applications such as chemical sensors 2 , varistors 3 , thin film photovoltaic solar cells 4,5 , piezoelectric and luminescent devices 6 , catalysis 7 and cosmetic materials [8][9][10] .

Since it was found that high quality epitaxial ZnO thin film displays excitonic ultraviolet laser action at room temperature (RT) (band gap energy of 3.37 eV) 11 , research on thin film zinc oxide materials has increased enormously during the five past years . At the present time zinc oxide is considered as one of the most promising and novel photonic materials for the blue, violet and UV regions 13 . At the same time, ZnO submicronic particles offer considerable potential as a starting material for such optical applications [36][37][38][39][40][41][42][43][44][45][46] and also for other purposes such as varistors [47][48][49][50][51][52][53] and transparent UV-protection (sunscreen formula) [8][9][10] .

About the last mentioned application, in fact, it is well known that oxides like CeO 2 , TiO 2 and ZnO with band gap energy and refractive index close to 3 eV and 2-3 have ideal characteristics in a broad-spectrum inorganic sunscreen. Nevertheless, the photocatalytic activity of these three oxides can facilitate the generation of reactive species, which can degrade other organic compounds in the formulation. Recently, it was shown that the oxidation catalytic activity of CeO 2 could successfully be decreased by coating with amorphous silica 54 but coating with amorphous silica resulted in decreasing UV-shielding effect. In this study, commercial ZnO nanoparticles were grafted by aminopropyltriethoxysilane (APTES) in various conditions in order to compare the properties of coated and non-coated powders. Three different conditions were used to graft APTES onto the zinc oxide surface. The powders before and after grafting process have been annealed at 800ºC and studied by X-ray diffraction (XRD). Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and specific surface area measurements were performed to control the effect of coating and sintering on the growth of ZnO nanoparticles. The optical transmittance spectrum of the coated and non-coated powders are compared.

Experimental section

Products

Ultrafine ZnO powder (Nanofine P2) was provided by Sakai Chemical Industry.

APTES, anhydrous toluene, nitric acid and tetramethylammonium hydroxide (25% aqueous solution) (TMOH) were provided by Aldrich.

The grafted APTES in acidic condition (or basic condition) was prepared following this process: 1.5 g of commercial zinc oxide was dispersed into distilled water (50 ml, pH = 6.87.2), the starting pH was adjusted to 6.5 with HNO 3 solution (2 M) (or 10.8 with TMOH).

The suspension was stirred for a further 1h and 1 ml of APTES was added. After addition of APTES, because of the basic property of the amino group, the pH was increased to 9.7 and after few minutes was stabilized to 8.99.2 for the acid condition whereas for the basic starting pH (10.8) was decreased and stabilized to 10.410.6. The reaction mixture was stirred for a further 24h then excess APTES was removed by filtration and washing with alcohol and acetone. The powder was dried at 60°C under vacuum.

The grafted APTES in toluene condition was performed with 2 g of commercial zinc oxide. The powder was dried at 110°C under vacuum for 2h and was dispersed into anhydrous toluene (200 ml) under argon flow. The suspension was stirred for a further 1h and 1 ml of APTES was added slowly under argon flow. The reaction mixture was refluxed under an argon flow for further 15h, excess APTES was removed by filtration and washing with fresh toluene and dried under vacuum at 110ºC for 2 h to remove toluene.

In all cases, subsequently one part of the non-grafted and grafted powder was heated at 800ºC in air for 8 hours.

X-ray powder diffraction

X-ray diffraction data were recorded at room temperature on a Philips PW 3020 using

       ² cos ) ) ( ( _ _ _ iso size 2 2 aniso strain iso strain 2 G G tg D 1 U H        cos ) ( _ _ _ _ aniso size iso size aniso strain iso strain L F Y tg D X H
where U, X, , G and Y are refinable parameters, and D and F are analytical functions, depending on a set of additionable refinable parameters, to model the hkl-dependent broadening due to strain and size effects respectively. "Perfect" Y 2 O 3 powder was used as standard to determine the Instrumental Resolution Function (IRF) of our diffractometer. Observed (hkl) dependent line broadening was modeled by different anisotropic sizes models built into Fullprof, which correspond to particular sets of reflections that are affected by broadening. Refinement of tg dependent isotropic strain parameters (U and X parameter in the previous formula) did not improve significantly the profile fitting and only the anisotropic size broadening effects were refined. The best agreement between observed and calculated profiles were obtained considering that the anisotropic size broadening can be written as a linear combination of spherical harmonics (SPH) [START_REF] Rodriguez-Carvajal | Commission on Powder Diffraction[END_REF] . In such a case, anisotropic size is supposed to contribute to the lorentzian component of the total voigt function. The explicit formula for the SPH is given in ref 55 and 56. After refinement of the coefficients, the program calculates the apparent size along each reciprocal lattice vectors. The average apparent crystallite size (  ) can be related to the true size but only if the crystallite shape is known or assumed. For instance, for a spherical crystallite, the true diameter D is simply derived from 4/3(  ). Moreover, this simple relation is strictly valid for a monodisperse system 57 . The true size could be defined, only in the case of single-domain nanoparticles without amorphous shell, as the size estimated from the transmission electron microscopy (TEM) and/or scanning electron microscopic (SEM) images.

Morphological investigation by SEM and TEM

Scanning electron microscopic photographs by Hitachi S-5000 were taken to examine the morphology of non-grafted and grafted ZnO powders.

A transmission electron microscope (JEOL-2000 FX electron microscope operating at 200 kV) was used to study the shape and size of the non grafted and non heated zinc oxide nanoparticles. Samples were prepared by direct deposition of powder dispersed in ethanol on carbon activated Cu-grids.

Specific Surface area

The BET specific surface area of the samples was determined by following the standard N 2 -adsorption method, using a Micromeritics ASAP-200 instrument. Samples were degassed at 200ºC or 350°C for 2 hours and the N 2 adsorption isotherms were determined until 77K.

Chemical analysis

For titration of zinc and silica, Inductive Coupled Plasma (ICP) atomic emission spectroscopy was performed using a spectrophotometer from Seiko Instruments Inc. SPS

1700HVR. The resonance wavelengths were  (Zn) = 213.86 nm and  (Si) = 251.61 nm. The powders were heated in air at 500°C for 1h before analysis to clean the surface.

Optical measurements

Optical transmittance spectra were recorded with a UV-visible spectrophotometer (JASCO V550 UV/Vis.) using diffuse reflectance process.

Results and discussion

Morphological characteristics

The diffraction patterns of non-grafted ZnO nanoparticles before and after heating at 800°C in air for 8 hours are shown in Figure 1A and 1B respectively. As expected, annealing induces an increase of the crystallite size leading to more sharp diffraction peaks. The comparison of the X-ray diffraction pattern of grafted zinc oxide nanoparticles before and after heating at 800°C in air for 8 hours is presented in Figures 2A, 3A, 4A, and 2B, 3B, 4B respectively. Although in the case of basic process the phase Zn 2 SiO 4 appears as impurity, after heating at 800ºC, the particles growth is quasi-inhibited, as we expected, by APTES coating with the three processes. In more detail, the crystallite sizes, determined by the XRD line broadening technique using the recent developments of the program Fullprof, are reported in Table 1. A comparison of the diffraction patterns of ZnO nanoparticles and the well crystalline yttrium oxide used as standard are shown in Figure 5. The observed, calculated and difference X-ray diffraction profiles are shown in Figures 6, 7, 8, 9 and 10. A tentative representation of the shape of the zinc oxide nanoparticles using the program GFourier (J. Gonzalez-Platos and J. Rodriguez-carvajal, ver. 3.10) is given in Figure 11. The shape is represented by GFourier using the apparent size calculated by Fullprof along each of the reciprocal lattice vectors (Table 1). The representation along [110] shows the shape of a "star" whereas the representation along [101] shows a more regular shape. This point will be discussed later. After annealing (Figure 7), the non-coated zinc oxide presents no peak broadening (i.e. the apparent particle size is fundamentally up to 100 nm), whereas, the crystallite size of the three grafted ZnO (Figure 8, 9 and 10) is keeping almost in the same range as the primary zinc oxide particle (Figure 6 and Table 1). The value of the specific surface area confirms these points (Table 2). In fact, the specific surface area of the nongrafted zinc oxide nanoparticles is 30 m²/g. After annealing at 800°C, this material exhibits a reduced specific surface area (2.5 m²/g) with 12-fold decrease as compared to the initial specific surface area (Table 2), which is in accordance with the increase of the crystallite size. For grafted powder by acidic and toluene processes, the value of the specific surface area determinated after annealing is only decreased by 3.5-fold as compared to the initial specific surface area (Table 2), which was unchanged after grafting. In the case of the basic process, the specific surface area is decreased immediately after grafting and remains unchanged after annealing. XRD patterns (Figure 3) show that this result is not due to a reaction with TMOH. The decrease of the specific surface area may be attributed to the mechanism of condensation of aminosilane in basic media (see part 3.2).

The TEM images of the non-grafted and non heated zinc oxide are shown in Figure 12. One can notice on Figure 12 that the zinc oxide nanoparticles are polydisperse in size and that the nanoparticles are spherical or hexagonal in shape. The diameter of the zinc oxide particles is in the range of 20-30 nm and is in a very good agreement with the apparent crystallite size determinated by XRD line broadening technique (Table 1). The calculated shapes shown in Figure 11 cannot completely represent the shape observed by TEM (Figure 12). If we consider that there is not an amorphous shell at the surface of the nanoparticles, this difference could be probably explained by the quality of our data and our model and also by the polydispersity of the size. Nevertheless, if the two calculated shapes could explain the two different shapes (hexagonal or spherical) observed by TEM, high quality diffraction data are necessary to improve our model.

The SEM images of the non-grafted and grafted powder before and after heating at 800°C under isothermal conditions are displayed in Figures 13 (A, B, C, D, E andF). One can notice in Figure 13B that grain growth is significant for the non-grafted ZnO after annealing at 800°C. The zinc oxide crystallite size is considerably increased according to the X-ray data (see Figures 1,7 and Table 1) and it is probable that several crystallites are aggregated into one grain. Indeed, as shown in Figures 13D and13F (acidic process and toluene process respectively heated at 800°C), the zinc oxide nanoparticles consist of an assembly of ultrafine particles. The diameter of the zinc oxide particles is in the range of 25-45 nm. This result is in a very good agreement with the apparent crystallite size determinated by XRD line broadening technique (Table 1). One can distinguish in the Figure 13E (basic process)

"spherical" nanoparticles but accompanied by an strong aggregation process. It seems that SiO 2 interparticle aggregation occurred during sintering, which supports the chemical titration data (Table 1) and the BET measurement (Table 2). The SEM study confirms previous results observed for the zinc oxide particles. The most important feature in Figure 13 is that, for the three grafted powders, the crystallite size is keeping almost unchanged compared to the primary crystallite size. Of course, in the case of basic process, the quantity of APTES seems to be a very important parameter and it should be carefully controlled. This point will be discussed in next section.

Regarding all the results for the three grafted powders after annealing, it is clear that the APTES coating plays a role of growth inhibitor. This point could be related to the studies of Audebrand et al. 57 . In fact, they have shown that the low energy for ZnO crystallite growth must be associated with a surface diffusion process and not a grain boundary or lattice diffusion process, which would require more energy. The silica coating could play a role of diffusion barrier. Generally, aminosilane or derivatives are used to stabilize a cluster solution 58 or to functionalized inorganic materials [59][60][61][62][63][64] . In the broad field of oxide modification, silica is the most widely used. The aminosilane-modified silica may be used as such or after a secondary treatment. Vrancken et al. 65 have already shown that the chemical modification of a silica gel, followed by a controlled thermal treatment, under argon atmosphere, converted the silane layer to a SiC advanced ceramic coating but the phenomenon of growth inhibition by silane was not discussed. Another interesting point is the homogeneity of coating; Although our results, for instance in the acid process, seem to suggest a homogeneous coating, at this time, we cannot completely assume that the coating is homogenous because we can not detect by transmission electron microscopy any homogeneous surface layer of Si-O-Si.

Zinc oxide grafting

Organic silane compounds useful in our method are silane derivatives having at least one organic radical per molecule attached to silicon by a Si-C bond. The organic radical is non-hydrolysable and may possess a functionality that imparts the desired characteristics.

Typical of the useful silane are those with functional groups as 3-aminopropyltriethoxysilane (APTES) and its analogues. Nevertheless, in the case of UV sun block, it is convenient not to use silanes that are too sensitive to UV radiation as for instance silane with phenyl groups 44,66 . One of the problems is that the adhesion between the silane and the oxide surface could be adsorption or covalent bond. The starting point is oxide surface containing hydroxyl groups to which silane molecules could be chemisorbed or physisorbed. It is well known that many metal oxides will hydrolyze in the presence of water (liquid or atmospheric) to form hydroxide layers at the surface. Water molecules may be both physically and chemically adsorbed onto the surface of the oxide particles. An oxide or hydroxide surface can become charged by reacting with H + or OH -ions due to surface amphoteric reactions depending of the zero point of charge (ZPC) of the oxide. The ZPC of zinc oxide found in the literature ranges from 9 to 10 43,67,68 . Below the ZPC, hydroxide surfaces adsorb protons to produce positively charged surfaces. Above the isoelelectric point, they lose protons to produce negatively charged surfaces. The following equilibria must be considered (S represents the surface of oxide):

O H OH S OH OH S 2 2                 O H O S O H OH S 3 2
The number of these sites and the surface charge of the oxide particles are principally dependent of the pH of the solution [START_REF] Jolivet | De la solution à l'oxyde[END_REF] , but the useful pH range may be limited because of the solubility of oxides and the strong pH dependence of hydrolysis. When zinc oxide is in contact with water, the surface of the oxide is hydrolyzed and a layer of zinc hydroxide is built up. It is well established that zinc hydroxide is slightly soluble in water, becoming more soluble as the pH is either lowered or raised. Zinc oxide surface is hydrolyzed again in the reaction of the OH -ions with the surface zinc atoms. Also the generation of surface charge due to amphoteric surface reaction may be described simply, only if H + , OH -and

i 2 i OH M  ) (
are present in the solution [START_REF] Jolivet | De la solution à l'oxyde[END_REF][START_REF] Degen | [END_REF] . When the cation can form hydroxyl complexes, other species are included in the surface charge formation, then a large number of hydrolytic processes and a variety of equilibria determine the species in solution. The establishment of a surface charge on the metal oxide in water may occur by two distinct but essentially equivalent mechanisms: (1) the adsorption of protons or hydroxyls onto amphoteric surface site or (2) the formation of hydroxylated metal species in solution which deposit on the solid surfaces. Regarding the literature about water suspension of zinc oxide [START_REF] Degen | [END_REF] , the surface hydroxide could be in equilibrium with the solution which contains species that can be represented by Zn 2+ , Zn(OH) + , Zn(OH) 2 , Zn(OH) 3 -and Zn(OH) 2- 4 . In fact, in the pH range 6.6-7.2, there is an equilibrium between Zn 2+ and Zn(OH) + species in the solution and the surface hydroxide. At pH > 12, there is an equilibrium between surface hydroxide and zincates ions Zn(OH) 3 -and Zn(OH) 2- 4 . In the region 7.2 < pH < 12 all the different species could be in solution and could also be in equilibrium with the hydrated zinc oxide surface. To resume, when the zinc oxide powder is immersed in the water, the surface of the oxide particles is hydrolyzed because of the physically and chemically adsorbed polar water molecules and a layer of zinc hydroxides is formed. In our range of pH, the surface hydroxides could be transformed into hydrated colloidal particles of Zn 2+ , Zn(OH) + , Zn(OH) 2

and Zn(OH) 3 -depending of the pH value reached.

In the same way, the molecular structure of silane is a function of the pH of the solution and differences in the molecular structure of the silane agent may very well affect the reactivity of functional groups. The structure of usual silane in aqueous solution has been studied in various environments by Fourier transform infrared spectroscopy [69][70][71][72][73] . In the case of APTES and its analogues, the structure obtained from the solution at pH = 2 indicated that the amine group forms an amine salt (NH 3 + ), whereas at pH = 12 the amine group is essentially free of hydrogen bonding. Between these two values of pH, equilibrium is present. Moreover for pH > 3, the silane group after hydrolysis appears to be negatively charged and also regarding the pKa of the NH 3 group (10.3), we must consider the possible intramolecular association between SO -and NH 3 + , for 3 < pH < 10.3, with partial or total proton transfer between these two groups according to the pH value. In all case of pH ranges in water, amino silanes hydrolyze into silanol (SiOH group) rapidly whereas the rate of the condensation reaction of the silanol into siloxane (Si-O-Si) appears very dependent of the pH. Sanchez et al. 74 have shown that at acidic pH, the hydrolysis process of silane is essentially activated whereas for basic pH both hydrolysis and condensation are activated. Regarding our results and principally the chemical titration (Table 1) and specific surface area (Table 1) results, we can propose the following mechanisms:

(i) at pH 6-8.5, the zinc surface should be positively charged whilst the silanol group appears to be negatively charged and the amino group positively charged. Also, we can expect a fast and strong adhesion between the negative silanol and the positive surface of the zinc oxide.

But the final amount of grafted silane is controlled by the fact that small condensation of the silanol into siloxane is expected (1) because condensation is not completely activated by pH < 9 and (2) due to the intramolecular association between SiO -and NH 3 + , most of APTES remain in solution as monomer. After a grafting of a few layers of Si-O-Si, the non-reacted silane is eliminated by washing and centrifuging the solution. In this case, the grafting should be very fast and is controlled by the condensation process.

(ii) at pH 10.5, the hydrolysis of silane into silanol and the condensation into siloxane (Si-O-Si) polymer is activated. Also, we can expect that silanol is condensed into siloxane but not preferentially onto the surface of zinc oxide, because it is not positively charged. However, high condensation is observed by the strong values of weight percentage of silicon ( 5%) obtained by the chemical titration, the decrease of the value of the specific surface area for the basic process before heating and the presence of the Zn 2 SiO 4 impurity after heating. In this case, the starting concentration of silane appears to be a very important parameter in the control of the grafting. Indeed for these concentrations aminosilanetriols form aggregates which later absorb on the ZnO surface.

(iii) In the toluene case condition, the percentage of silicon observed ( 0.3%) by chemical titration could be explained by (i) the low water quantity present during the experiment, (ii) the hard aggregation of ZnO nanoparticles in toluene medium. In the toluene process, although siloxane bonds with the surface of silica may be formed in the absence of water 65 (only in the case of aminosilane), the majority of water is chemisorbed and physisorbed on the surface of zinc oxide, such that the hydrolysis of the aminosilane and the condensation of the silanol are now only possible on the zinc oxide surface. In the condensation reaction this is due to the catalytic effect of the amino group.

Diffuse reflectance properties

The UV-Vis transmittance spectrum of non-grafted zinc oxide and grafted zinc oxide is represented in Figure 14. One can notice that the grafting process did not modify drastically the transmittance spectra of ZnO. Non-grafted and grafted zinc oxide showed excellent UV-Shielding capacity and transparency in the visible light. Moreover, the preliminary results of photocatalytic decomposition of acetaldehyde have shown that the aminosilane coating could increases photostability (results not published). For sunscreen materials, it is known to this art that light radiation of wavelengths from 280 nm to 400 nm promote burning of the human epidermis. Also, we think that aminosilane-grafted zinc oxide ultrafine particles could play an important role as UV sunblock. ZnO silane coating could be used in order to obtain homogeneous dispersion of photostable nanoparticles and functionalized devices for ultrafine inorganic oxide containing sunscreen formulations. The NH 2 group could be used as a reactive function capable of chemisorbing macromolecules or increasing the surface charge of particulate in order to obtain suspensions without the formation of hard agglomerates.

Conclusion

This paper describes the preparation of coated-zinc oxide nanoparticles (APTES-ZnO). The effects of coating conditions (acidic, basic and toluene) on the grafting, the structural and optical properties of these nanoparticles were studied.

1. In the three cases after grafting and annealing at 800°C for 8 hours in air, the crystallite size is remaining almost unchanged compared to the primary crystallite size. It is clear that the APTES coating plays a role of growth inhibitor even at 800°C. This is confirmed both from XRD and SEM studies. This process could be used for the controlled synthesis of advanced ceramics.

2. In the three cases, it was possible to control the coating. In the acid process, the grafting seems to be very fast and is controlled by the condensation process. In basic process, the starting concentration of silane appears to be a very important parameter in the control of the silica contents and the grafting. In the toluene process, the grafting seems to be mainly controlled by the amount of the water on the ZnO nanoparticle surface.

3. From diffuse reflectance measurements, one can notice that the grafting process did not modify the transmittance spectra of ZnO. Moreover, the preliminary results of photocatalytic decomposition of acetaldehyde have shown that the aminosilane coating increases the photostability. According to this results it seems that for a UVA/UVB sun block agent application, it would be desirable to produce silane-coated zinc oxide.

Fig 1

 1 Fig 1 XRD patterns of non-grafted zinc oxide powder before (A) and after (B) annealing at 800°C in air for 8 hours.

Fig 2

 2 Fig 2 XRD patterns of grafted zinc oxide (acidic condition) powder before (A) and after (B) annealing at 800°C in air for 8 hours.

Fig 3

 3 Fig 3 XRD patterns of grafted zinc oxide (basic condition) powder before (A) and after (B) annealing at 800°C in air for 8 hours.

Fig 4

 4 Fig 4 XRD patterns of grafted zinc oxide (Toluene condition) powder before (A) and after (B) annealing at 800°C in air for 8 hours.

Fig 5 Fig 6 Fig 7 Fig 8

 5678 Fig 5 XRD patterns of non-grafted zinc oxide (ZnO) and yttrium oxide (Y2O3) powders.

Fig 9

 9 Fig 9 Observed (dots), calculated (full line) and difference X-ray powder diffraction profiles of zinc oxide after grafting (Basic process) and annealing at 800°C in ait for 8 hours.

Fig 10 Fig 14

 1014 Fig 10 Observed (dots), calculated (full line) and difference X-ray powder diffraction profiles of zinc oxide after grafting (Toluene process) and annealing at 800°C in ait for 8 hours.

Table caption Table 1

 caption1 Apparent size (in Å) non-grafted (before annealing) and grafted (after annealing) zinc oxide powders.

Table 2

 2 Physical and chemical properties of non-grafted and grafted zinc oxide powders.

Acknowledgements

The authors thank N. Pontais, N. Labhsetwar, S. Takenouchi and T. Wada for great technical assistance. This work was funded by the Japan Science and Technology Corporation and under the Science and Technology Agency-Centre National de la Recherche (STA-CNRS) exchange program.


Eve Floriane Fabre

Emilie Soheila Jahanpour

Mickaël Causse
email: mickael.causse@isae.fr

Human Mirror Neuron System Based Alarms in the Cockpit: A Neuroergonomic Evaluation

Keywords: Flight safety, Cockpit warnings, Mirror alarms, Human mirror neuron system, Neuroergonomics, EEG

Controlled Flight Into Terrain (CFIT) events still remain among the deadliest accidents in aviation. When facing the possible occurrence of such an event, pilots have to immediately react to the ground proximity alarm ("Pull Up" alarm) in order to avoid the impending collision. However, the pilots' reaction to this alarm is not always optimal. This may be at least partly due to the low visual saliency of the current alarm and the deleterious effects of stress that alleviate the pilot's reactions. In the present study, two experiments (in a laboratory and in a flight simulator) were conducted to (1) investigate whether hand gesture videos (a hand pulling back the sidestick) can trigger brainwave frequencies related to the mirror neuron system; (2) determine whether enhancing the visual characteristics of the "Pull Up" alarm could improve pilots' response times. Electrophysiological results suggest that hand gesture videos attracted more participants' attention (greater alpha desynchronization in the parieto-occipital area) and possibly triggered greater activity of the mirror neuron system (greater mu and beta desynchronizations at central electrodes). Results obtained in the flight simulator revealed that enhancing the visual characteristics of the original "Pull Up" alarm improved the pilots' reaction times. However, no significant difference in reaction times between an enlarged "Pull Up" inscription and the hand gesture video was found. Further work is needed to determine whether mirror neuron system based alarms could bring benefits for flight safety, in particular, these alarms should be assessed during a high stress context.

Introduction

Controlled Flight into Terrain Accidents

Since the beginnings of commercial aviation, Controlled Flight Into Terrain (CFIT) represents one of the deadliest categories of accidents (International Air Transport Association 2016[START_REF]IATA Controlled Flight Into Terrain Accident Analysis Report-2008-2017 Data[END_REF]. In these events, an airworthy aircraft under the complete control of the crew is inadvertently flown into terrain, water, or an obstacle [START_REF] Moroze | Causes and Remedies of Controlled Flight into Terrain in Military and Civil Aviation[END_REF]. Pilots are generally unaware of the potential danger of the situation until the last moment before the impact. In the early 1970s and after an important number of CFIT accidents [START_REF] Cooper | Controlled Flight into terrain[END_REF], aviation authorities required both aircraft manufacturers and airline companies to equip aircraft with the "Ground Proximity Warning Systems" (GPWS), and later with the Enhanced-GPWS (European Aviation Safety Agency 2007). This alarm system alerts the pilots that the aircraft is in immediate danger of hitting the ground and prompts them to perform an immediate recovery maneuver. In Airbus aircraft, the latter consists of pulling back the sidestick and pushing forward the power levers on the Take-off/Go Around (TO/GA) position to respectively regain altitude and thrust. Ideally, the reaction from the crew should be fast and instinctive [START_REF] Moroze | Causes and Remedies of Controlled Flight into Terrain in Military and Civil Aviation[END_REF]. In most of the modern aircraft, the GPWS alarm is multimodal and combines two channels of sensory information with a spoken "Pull Up" pronounced by a synthetic voice and a red-colored "Pull Up" inscription flashing in the Primary Flight Display (PFD; i.e., display presenting the primary flight parameters, located just in front of the pilots). The "Pull Up" alarm is played repeatedly until the pilots correct the flight path of the aircraft [START_REF] Breen | Controlled Flight Into Terrain and the enhanced Ground Proximity Warning system[END_REF]. While GPWS technology has led to a reduction in the number of CFIT accidents, they still remain the second cause of fatalities in commercial aviation (International Air Transport Association, 2016[START_REF]IATA Controlled Flight Into Terrain Accident Analysis Report-2008-2017 Data[END_REF], with three-quarters of CFIT accidents resulting in the death of all passengers and crew members [START_REF] Moroze | Causes and Remedies of Controlled Flight into Terrain in Military and Civil Aviation[END_REF][START_REF] Scoot | New Research Identifies Causes of CFIT[END_REF].

The Effects of Mental Workload and Stress on Pilots' Reactions to "Pull Up" Alarms

Flying an aircraft requires to handle many tasks (e.g. prioritizing the tasks, switching from a task to another), which is likely to generate a high mental workload in the pilots [START_REF] Chou | Studies of cockpit task management errors[END_REF][START_REF] Funk | Cockpit task management: Preliminary definitions, normative theory, error taxonomy, and design recommendations[END_REF][START_REF] Iani | Factors Affecting Task Management in Aviation[END_REF][START_REF] Woods | The alarm problem and directed attention in dynamic fault management[END_REF]. Various accident investigations concluded that high mental workload can obliterate the pilots' capacity to react properly to warning events (e.g. Korean Air flight 801 in 1997;National Transportation Safety Board 2000). These conclusions are supported by various experimental studies showing that high mental workload can disrupt both the detection and the reaction to the alarms [START_REF] Bliss | Behavioural implications of alarm mistrust as a function of task workload[END_REF][START_REF] Causse | High working memory load impairs language processing during a simulated piloting task: An ERP and pupillometry study[END_REF][START_REF] Dehais | Failure to Detect Critical Auditory Alerts in the Cockpit Evidence for Inattentional Deafness[END_REF][START_REF] Giraudet | P300 event-related potential as an indicator of inattentional deafness?[END_REF]. Not surprisingly, an important proportion of CFIT accidents occurs during the approach and landing phases [START_REF] Corwin | Controlled flight into terrain avoidance-Why the ground proximity warning system is too little, too late[END_REF], where the altitude is low and the pilots' mental workload is high (Causse et al. 2012a;[START_REF] Lee | Inflight workload assessment: Comparison of subjective and physiological measurements[END_REF][START_REF] Wilson | An analysis of mental workload in pilots during flight using multiple psychophysiological measures[END_REF]. Generally, when the GPWS alarm sounds in the cockpit, the crew is unaware of the actual position of the aircraft. Pilots' poor reactions to the GPWS alarm were found to contribute to 38% of the CFIT accidents while the GPWS system was functional (International Air Transport Association, 2018). In these cases, the GPWS alarm left enough time to react, but the crews delayed their response or made an inadequate maneuver [START_REF] Kuchar | Markov Model of Terrain for Evaluation of Ground Proximity Warning System Thresholds[END_REF].

Another explanation for the pilots' poor reaction may be the aggressive and disturbing nature of the GPWS auditory modality, which tends to make the situation more stressful [START_REF] Bliss | Investigation of alarm-related accidents and incidents in aviation[END_REF][START_REF] Doll | Auditory Information Systems in Military Aircraft: Current Configurations Versus the State of the Art[END_REF][START_REF] Edworthy | Improving auditory warning design: Relationship between warning sound parameters and perceived urgency[END_REF][START_REF] Peryer | Auditory alert characteristics: A survey of pilot views[END_REF]. A large body of literature has demonstrated that a high level of stress is likely to disrupt flight performance [START_REF] Rigby | A Predictive Scale of Aircraft Emergencies[END_REF], as it temporarily impairs important cognitive abilities such as reasoning, decision-making, and/or executive functioning [START_REF] Arthur | Flight Simulator Evaluation of Synthetic Vision Display Concepts to Prevent Controlled Flight Into Terrain (CFIT)[END_REF][START_REF] Casner | Vigilance impossible: Diligence, distraction, and daydreaming all lead to failures in a practical monitoring task[END_REF][START_REF] Porcelli | The effects of acute stress on human prefrontal working memory systems[END_REF][START_REF] Scholz | Go no-go performance under psychosocial stress: Beneficial effects of implementation intentions[END_REF][START_REF] Staal | Stress, Cognition, and Human Performance: A Literature Review and Conceptual Framework[END_REF]. A sudden rise in the stress level can also slower reactions [START_REF] Farber | Effects of Anxiety, Stress, and Task Variables on Reaction Time[END_REF], trigger freezing states [START_REF] Schmidt | Exploring human freeze responses to a threat stressor[END_REF] and dangerous reactions such as the startle effect for instance [START_REF] Martin | The Effects of Startle on Pilots During Critical Events: A Case Study Analysis[END_REF][START_REF] Martin | Fear-Potentiated Startle: A Review from an Aviation Perspective[END_REF][START_REF] Martin | A Flight Simulator Study of the Impairment Effects of Startle on Pilots During Unexpected Critical Events[END_REF]. Consequently, "Pull Up" events are likely to impact the crew capacity to evaluate the situation and to react appropriately.

Improving the GPWS Alarm?

Multimodal alarms, like the GPWS, are considered to be more easily detected and consequently more efficient than unimodal alarms [START_REF] Alirezaee | Did You Feel That? Developing Novel Multimodal Alarms for High Consequence Clinical Environments[END_REF][START_REF] Child | The temporal course of the influence of visual stimulation upon the auditory threshold[END_REF][START_REF] Hughes | Visual-auditory interactions in sensorimotor processing: Saccades versus manual responses[END_REF][START_REF] Liu | Comparative study of the effects of auditory, visual and multimodality displays on drivers' performance in advanced traveller information systems[END_REF]). However, the high visual and auditory perceptual load in the cockpit (e.g. numerous displays and communications) is likely to affect the pilots' ability to detect and process the alarms [START_REF] Salzer | Evaluation of an "On-Thigh" Vibrotactile Collision Avoidance Alerting Component in a Simulated Flight Mission[END_REF][START_REF] Van Veen | Tactile information presentation in the cockpit[END_REF]. In this context, the small font size of the "Pull Up" inscription currently used in airline aircraft can make it difficult to detect [START_REF] Corwin | Controlled flight into terrain avoidance-Why the ground proximity warning system is too little, too late[END_REF]. This can be problematic as human beings tend to rely more on visual information than on other forms of sensory information [START_REF] Colavita | Human sensory dominance[END_REF]; and also because visual alarms are overall processed faster than auditory alarms [START_REF] Donohue | Cross-Modal Stimulus Conflict: The Behavioral Effects of Stimulus Input Timing in a Visual-Auditory Stroop Task[END_REF][START_REF] Mrugalska | Human error and response to alarms in process safety[END_REF][START_REF] Posner | Visual dominance: An information-processing account of its origins and significance[END_REF]. Finally, the inattentional deafness phenomenon can alter the ability to perceive the auditory modality of an alarm in the cockpit [START_REF] Dehais | Failure to Detect Critical Auditory Alerts in the Cockpit Evidence for Inattentional Deafness[END_REF][START_REF] Giraudet | P300 event-related potential as an indicator of inattentional deafness?[END_REF]. It is thus desirable to use more visually salient and intuitive alarms.

A first step to enhance the efficiency of the GPWS alarm would be to increase its visual saliency by enlarging the "Pull Up" text font size. However, enlarging the inscription may not be sufficient to cope with the loss of cognitive performance due to the stress generated by the "Pull Up" event. An alternate solution would be to display a video showing the actions to perform in response to the "Pull Up" alarm. This solution could have significant advantages. First, gesture videos were shown to strongly attract attention [START_REF] Abrams | Motion onset captures attention[END_REF]. This recruitment of attentional resources has been associated with a decrease in electroencephalographic (EEG) alpha-band power (8-12 Hz) at parieto-occipital sites [START_REF] Fink | EEG alpha band dissociation with increasing task demands[END_REF][START_REF] Keil | Alpha-band activity reflects reduction of mental effort in a comparison task: A source space analysis[END_REF][START_REF] Klimesch | Alpha-band oscillations, attention, and controlled access to stored information[END_REF][START_REF] Klimesch | Induced alpha band power changes in the human EEG and attention[END_REF][START_REF] Ray | EEG activity during cognitive processing: Influence of attentional factors[END_REF]. Second, a gesture video is also likely to activate the mirror neuron system, which might facilitate the initiation of the motor response to the "Pull Up" alarm [START_REF] Rizzolatti | Premotor cortex and the recognition of motor actions[END_REF]. Mirror neurons have the distinguishing properties of firing both when one makes and/or observes another individual making an action [START_REF] Buccino | The mirror neuron system and action recognition[END_REF][START_REF] Rizzolatti | The mirror-neuron system[END_REF]. They were found to play a key role in the understanding and the imitation of actions [START_REF] Iacoboni | Cortical mechanisms of human imitation[END_REF][START_REF] Jeannerod | The representing brain: Neural correlates of motor intention and imagery[END_REF][START_REF] Meltzoff | The imitative mind: Development, evolution and brain bases[END_REF]. EEG studies investigating the neural signature of the mirror neuron system have shown modulations in the power of the mu (i.e. 8-12 Hz) and the beta rhythms (i.e. 13-25 Hz) in response to both the performance and the observation of actions [START_REF] Debnath | Mu rhythm desynchronization is specific to action execution and observation: Evidence from time-frequency and connectivity analysis[END_REF][START_REF] Hobson | Mu suppression-a good measure of the human mirror neuron system[END_REF][START_REF] Järveläinen | Stronger reactivity of the human primary motor cortex during observation of live rather than video motor acts[END_REF][START_REF] Muthukumaraswamy | Modulation of neuromagnetic oscillatory activity during the observation of oro-facial movements (p. 2)[END_REF][START_REF] Pfurtscheller | Foot and hand area mu rhythms[END_REF]. A desynchronization of the mu and beta rhythms was found at the central electrodes (i.e. C3 and C4; [START_REF] Stancák | The effects of handedness and type of movement on the contralateral preponderance of μ-rhythm desynchronisation[END_REF] when subjects performed and/or observed movements [START_REF] Babiloni | Human movement-related potentials vs desynchronization of EEG alpha rhythm: A highresolution EEG study[END_REF][START_REF] Babiloni | Human cortical electroencephalography (EEG) rhythms during the observation of simple aimless movements: A high-resolution EEG study[END_REF][START_REF] Gastaut | EEG changes during cinematographic presentation (Moving picture activation of the EEG)[END_REF][START_REF] Iacoboni | Cortical mechanisms of human imitation[END_REF][START_REF] Muthukumaraswamy | Modulation of neuromagnetic oscillatory activity during the observation of oro-facial movements (p. 2)[END_REF][START_REF] Nam | Movement imagery-related lateralization of event-related (de) synchronization (ERD/ERS): Motor-imagery duration effects[END_REF][START_REF] Pfurtscheller | Post-movement beta synchronization A correlate of an idling motor area?[END_REF][START_REF] Rizzolatti | The mirror-neuron system[END_REF]. In this sense, mu and beta rhythms are interpreted as being an index of the human mirror neuron system activity. A large body of literature has shown that the mirror neuron system responds to videos of gestures performed by humans, robots, and avatars (e.g. [START_REF] Gangitano | Modulation of premotor mirror neuron activity during observation of unpredictable grasping movements[END_REF][START_REF] Gazzola | The anthropomorphic brain: The mirror neuron system responds to human and robotic actions[END_REF][START_REF] Kilner | Relationship between activity in human primary motor cortex during action observation and the mirror neuron system[END_REF][START_REF] Molnar-Szakacs | Observing complex action sequences: The role of the fronto-parietal mirror neuron system[END_REF][START_REF] Oberman | EEG evidence for mirror neuron activity during the observation of human and robot actions: Toward an analysis of the human qualities of interactive robots[END_REF][START_REF] Press | Robotic movement elicits automatic imitation[END_REF][START_REF] Tai | The human premotor cortex is' mirror'only for biological actions[END_REF]), even though stronger responses are found for real-life actions [START_REF] Järveläinen | Stronger reactivity of the human primary motor cortex during observation of live rather than video motor acts[END_REF]. Some studies suggest that the observation of hand movement associated with transcranial magnetic stimulation of the motor cortex increased motor evoked potentials in hand muscles [START_REF] Fadiga | Motor facilitation during action observation: A magnetic stimulation study[END_REF]. In addition, the simple observation of static snapshots of hands suggesting a pincer grip action was found to induce an increase in corticospinal excitability [START_REF] Urgesi | Mapping implied body actions in the human motor system[END_REF]. Taken together, these studies suggest that displaying a video of the gesture to be performed in case of an imminent collision with the ground may activate the mirror neuron system and then facilitate the initiation of the motor response to the "Pull Up" alarm.

Objectives and Hypotheses

The present study was conducted following a Neuroergonomic approach, whose goal is to better understand the human brain functioning in realistic settings to improve technology and safety [START_REF] Parasuraman | Neuroergonomics: Research and practice[END_REF]. As long as the operator is a key agent in charge of complex systems, the definition of metrics able to understand and anticipate his/ her performance is a great challenge [START_REF] Causse | Monitoring Cognitive and Emotional Processes Through Pupil and Cardiac Response During Dynamic Versus Logical Task[END_REF]. Two separate experiments were conducted aiming at: (1) investigating whether hand gesture videos (i.e. a hand pulling back the sidestick) can increase brainwave frequencies associated with the activity of mirror neuron system; (2) determining whether enhancing the visual characteristics of the "Pull Up" alarm could improve pilots' response times. The first experiment was performed in laboratory conditions to examine the brain signatures to three different alarms: the current "Pull Up" alarm (i.e. currently used in large aircraft); an enlarged "Pull Up" alarm (i.e., current "Pull Up" alarm with an increased font size but displayed at the bottom of the PFD); and the "Pull Up" gesture video also displayed at the bottom of the PFD, presenting the gesture to perform in response to the "Pull Up" alarm (i.e. otherwise called "mirror alarm"). No motor response from the participants was asked during this experiment. The second experiment was performed in a flight simulator and aimed at measuring participants' reaction times to the three different alarms.

Regarding the first experiment with EEG measures, we predicted that (1) the hand gesture videos should attract more attention. This should be indexed in particular by a greatest desynchronization of the alpha rhythm at parietooccipital sites. We also predicted that ( 2) the hand gesture videos should trigger a greater activation of the mirror neuron system, facilitating the motor preparation of the appropriate actions. This increase in mirror neuron system activity should be indexed by greater mu and beta desynchronizations at central sites. Regarding the second experiment, we predicted to observe (3) faster reactions to the two enhanced "Pull Up" alarms compared to the current "Pull Up" alarm, with even faster reactions to the "Pull Up" video. Finally, we also expected (4) a greater detection rate of the two visually enhanced alarms compared to the current alarm.

Experiment 1-EEG Study

Material and Methods

Participants

Twenty-five private pilots (M age = 25.5, SD ± 4.14, age range 21-35 years old; 1 female) participated in this study. Twenty were right-handed, four were left-handed, and one was ambidextrous, as assessed by the Edinburgh handedness inventory [START_REF] Oldfield | The assessment and analysis of handedness: The Edinburgh inventory[END_REF]. All had a normal or corrected-tonormal vision. None of the participants reported a history of prior neurological disorder. Only participants who held a Private Pilot License (PPL) were recruited with an average flight experience of 177.5 h (SD ± 120.9 h) and an average time since licensing of 4.6 years (SD ± 4.5 years). These pilots operated light aircraft where there is generally no GPWS. They had not received any specific training related to GPWS nor had experience with these alarms in real flight conditions. All participants were informed of their rights and gave written informed consent for participation in the study.

Ethics Statement

The present study was carried out in accordance with the recommendations of CERNI no. 2018-107, the Research Ethics Committee of the University of Toulouse (France). All subjects gave written informed consent in accordance with the Declaration of Helsinki.

Material

Stimuli. Three different "Pull Up" alarms (alarms of interest) were presented to the participants. The first alarm was identical to the "Pull Up" alarm currently used in airline aircraft (i.e. current "Pull Up", see Fig. 1a). It consisted of a red 1 Hz-blinking "Pull Up" inscription (Arial font size 18). The second alarm was a "Pull Up" alarm with an enlarged inscription (i.e. enlarged "Pull Up", see Fig. 1b), consisting of a large 1 Hz-blinking red "Pull Up" inscription (Arial font size 85 corresponding to an 18-fold increase in size). The third alarm was a 1024 × 768 video of the gesture that the pilots have to perform in response to the GPWS alarm, namely a pilot's hand pulling the stick back ("Pull Up" video, see Fig. 1c). The alarms were displayed for 2.8 s. The two "Pull Up" inscriptions (current and enlarged) blinked two times and the "Pull Up" video was played two times. In this way, an alarm of interest was composed of one video (1.15 s), one blank (0.5 s), and another video (respectively 1.15 s + 0.5 s + 1.15 s = 2.8 s; see Fig. 2). The auditory component of the GPWS alarm was turned off to prevent potential interaction effects between the auditory modality and the visual modality of the alarm.

Experimental apparatus. The experimental paradigm was presented with a personal computer using E-Prime 2 (Psychology Software Tools, Inc., Pittsburgh, PA). Participants were seated in a chair and placed in front of a 19″ monitor.

Electroencephalographic (EEG) recordings. The EEG signals were recorded and amplified with the Biosemi system using 64 Ag-AgCl active scalp electrodes (Fp1, AF7, AF3, F1, F3, F5, F7, FT7, FC5, FC3, FC1, C2, C3, C5, T7, TP7, CP5, CP3, CP1, P1, P3, P5, P7, P9, PO7, PO3, O1, Iz, Oz, POz, Pz, CPz, Fpz, Fp2, AF8, AF4, AFz, Fz, F2, F4, F6, F8, FT8, FC6, FC4, FC2, FCz, Cz, C2, C4, C6, T8, TP8, CP6, CP4, CP2, P2, P4, P6, P8, P10, PO8, PO4, O2) arranged according to the international 10-20 system. Two electrodes were placed on the mastoids, which served as reference electrodes. Four electrodes (2 vertical electrodes, 2 horizontal electrodes) were placed around the eyes to record the electrooculogram (EOG). The data were sampled at a frequency of 512 Hz. Two additional electrodes placed close to Cz, the Common Mode Sense [CMS] active electrode and the Driven Right Leg [DRL] passive electrode, were used to form the feedback loop that drives the average potential of the participant as close as possible to the AD-box reference potential [START_REF] Metting Van Rijn | High-quality recording of bioelectric events[END_REF]). Skin-electrode contact, obtained using electro-conductive gel, was monitored, keeping voltage offset from the CMS below 20 mV for each measurement site. All the signals were (DC) amplified and digitalized continuously with a sampling rate of 512 Hz with an anti-aliasing filter with 3 dB points at 104 Hz (fifthorder sinc filter); no high pass filtering was applied online. The triggering signals to each video onset were recorded on additional digital channels. EEG data were analyzed using EEGLAB v.13.6.5b open source software [START_REF] Delorme | EEGLAB: an open source tollbox for analysis of single-trial EEG dynamics including independent component analysis[END_REF]) on Matlab 2017a. First, EEG signals were band-pass filtered between 0.1 and 40 Hz. An independent component analysis was performed to isolate eye blinks and movements related to artifacts that have been subsequently subtracted to the signal. A visual inspection of the data was conducted to reject residual artifacts intervals. Data were segmented into epoch from -1000 to 3000 ms.

The alpha rhythm desynchronization was analyzed in order to assess the allocation of attentional resources of participants to the two alarms. It was measured in the 0-3000 ms time window at parieto-occipital sites where the desynchronization was maximal based on visual analysis. These electrodes were then grouped into three clusters (left cluster: PO7/PO3/O1; central cluster: Pz/POz/Oz; right cluster: PO8/PO4/O2) aiming at investigating potential laterality effects. The desynchronization of both mu (i.e., motor alpha, 8-12 Hz) and beta (13-25 Hz; [START_REF] Babiloni | Human cortical electroencephalography (EEG) rhythms during the observation of simple aimless movements: A high-resolution EEG study[END_REF] rhythms were also measured at central sites (i.e. C3 and C4; [START_REF] Stancák | The effects of handedness and type of movement on the contralateral preponderance of μ-rhythm desynchronisation[END_REF] to assess the activity of the mirror neuron system in response to the two alarms. The event-related spectral perturbations (ERSPs) were assessed in the 0-3000 ms time window for the mu rhythm and in the 300-1300 ms time window for the beta rhythm. A baseline correction (-500 to 0 ms) was applied to the alpha, the mu and the beta powers, as presented in the following equation:

Procedure

Participants were installed in a dark and quiet room, at 85 cm from the computer screen. They were equipped with a 64-electrode EEG headset. Participants were instructed to watch the alarms. As we wanted to ensure that the activity of the mirror neuron system reflected the facilitation of the Power = 10. log 10 mean ERSP amplitude window of interest 2 -10. log 10 mean ERSP amplitude baseline 2 motor preparation and not the hand movement performance, participants were asked not to respond to the alarms. The current "Pull up" alarm, the enlarged "Pull Up" alarm, and the "Pull Up" video were each presented 30 times in a row, the order of presentation being counterbalanced between participants. A fixation cross was presented for 1 s, followed by the alarm (2.8 s) and a 2-s black screen (see Fig. 2). In order to facilitate EEG processing and limit ocular artifacts, participants were asked to avoid blinking during the presentation of the alarms.

Data Analysis

A 3 × 3 (Cluster [left cluster: PO7/PO3/O1; central cluster: Pz/POz/Oz; right cluster: PO8/PO4/O2] x Alarm [current "Pull Up"; enlarged "Pull Up"; "Pull Up" video]) two-way ANOVA was performed to examine alpha rhythm power at parieto-occipital sites. Two additional 2 × 3 (Electrode [C3; C4]) x Alarm [current "Pull Up"; enlarged "Pull Up"; "Pull Up" video]) two-way ANOVAs were conducted on both mu and beta rhythm powers at central sites. Bonferroni post-hoc tests were carried out to further examine significant effects (α = 0.05).

Results

Results from the three above described ANOVAs were not significant (F < 1, p > 0.05). However, they were some limitations in comparing the current "Pull Up" to the two new alarms. Indeed, the current alarm inscription is displayed in the middle of the PFD while the new alarms were displayed at the bottom of it, making their brain signatures difficult to compare. We thus conducted a second set of exploratory analyses. We removed the current "Pull Up" from this ANOVA model and focused on the comparison between the enlarged "Pull Up" and the "Pull Up" video. We present below the results of the new three ANOVAs.

Alpha desynchronization. The analysis revealed a significant main effect of the alarm [F (1, 24) = 24.93, p < 0.001, ηp 2 = 0.51; Fig. 3], with a greater alpha desynchronization in response to the "Pull Up" video (M = -15.14 dB ± 1.49) than to the enlarged "Pull Up" (M =-13.52 dB ± 1.78, p < 0.001). The main effect of cluster [F (2, 48) = 1.58, p = 0.20, ηp 2 = 0.07] and the Cluster x Alarm interaction [F (2, 48) = 0.21, p = 0.81, ηp 2 = 0.01] were not significant.

Mu desynchronization. The analysis revealed a significant main effect of the alarm [F (1, 24) = 13.38, p < 0.001, ηp 2 = 0.36; Figs. 4 and5a], with a greater desynchronization in response to the "Pull Up" video (M = -15.39 dB ± 1.34) than to the enlarged "Pull Up" (M = -14.16 dB ± 1.95, p < 0.005). Results revealed no significant main effect of electrode [F (1, 24) = 2.51, p = 0.13, ηp 2 = 0.09] nor Electrode x Alarm interaction [F (1, 24) = 0.01, p = 0.93, ηp 2 = 0.00].

Beta desynchronization. The analysis revealed a significant main effect of the alarm [F (1, 24) = 5.56, p < 0.05, ηp 2 = 0.19; Figs. 4 and5b], with a greater desynchronization in response to the "Pull Up" video (M = -6.76 dB ± 1.52) than to the enlarged "Pull Up" (M = Fig. 3 Time-frequency scalp maps for the alpha rhythm (8-12 Hz) in response to the "Pull Up" video (upper row) and the enlarged "Pull Up" (lower raw). The blue color represents the desynchronization of the alpha rhythm, the darker the blue, the greater the desynchronization. The significant difference in alpha rhythm was significant in the time window of the whole epoch (i.e. 0-3000 ms). Here, we present only the time window where the desynchronization was maximal. Higher alpha rhythm desynchronization for the "Pull Up" video is particularly visible in the figure between 800 and 1200 ms

Experiment 2-Flight Simulator Study

Material and Methods

Participants

The same 25 participants who participated in the first experiment also took part in the second one.

Ethics Statement

The present study was carried out in accordance with the recommendations of CERNI no. 2018-107, the Research Ethics Committee of the University of Toulouse (France).

All subjects gave written informed consent in accordance with the Declaration of Helsinki.

Material

Flight Simulator. The experiment was carried out in the PEGASE A320 flight simulator at ISAE-SUPAERO (French Aeronautical Engineering School, Toulouse, France). During the flight scenario, the participants mainly monitored the PFD that displays flight parameters such as altitude, speed, attitude indicator, and heading. The Flight Control Unit (FCU) was also used for the interactions with the autopilot and the auto-throttle (see Fig. 6).

"Pull Up" alarms. The same alarms as experiment 1 were presented in the PFD. The current "Pull Up" inscription was displayed on the attitude indicator of the PFD (Arial font size 18). The "Pull Up" alarm with an enlarged inscription (i.e., enlarged "Pull Up") was displayed on the bottom part of the PFD (Arial font size 85 corresponding to an 18-fold increase in size). The video of the hand gesture was also displayed on the bottom part of the PFD. The gesture video lasted 1.15 s and was presented six consecutive times with a 0.5 s black screen between each repetition of the video (6 gesture videos of 1.15 s + 6 black screens of 0.5 s = 9.9 s). The three versions of the GPWS alarm were randomly presented three times each for approximately 10 s. The "Pull Up" procedure was simplified (i.e. participants were not required to move the thrust lever), thus the video did not show the application of the maximum thrust. As in experiment 1, the auditory component of the GPWS alarm was turned off to prevent potential interaction effects between the auditory modality and the visual modality of the alarm.

Fillers. In order to prevent participants from responding automatically each time a stimulus was displayed on the PFD, three different fillers for which no response was expected were also presented to the participants. These three fillers were designed to be as similar as possible to the three alarms of interest. The first filler was a 1 Hz-blinking "Jump" inscription on the PFD, similar to the current "Pull Up" alarm (see supplementary Figure A). The second filler was an enlarged 1 Hz-blinking "Jump" inscription displayed on the bottom of the PFD, similar to the enlarged "Pull Up" (see supplementary Figure B). The third filler was a video of a jump with similar characteristics as the "Pull Up" video (i.e., the jump lasted 1.15 s and was presented six times with a black screen of 0.5 s between each jump; see supplementary Figure C). Fillers stimuli were randomly presented three times each for approximately 10 s, as the three alarms of interest.

Procedure

Participants were comfortably installed in an experimental room and asked to fill two questionnaires: an information questionnaire (age, license issue date, total flight hours, etc.) and the Edinburgh Handedness Inventory [START_REF] Oldfield | The assessment and analysis of handedness: The Edinburgh inventory[END_REF] designed to assess the dominance of a person's right or left hand in everyday activities. After that, participants took place in the A320 flight simulator to perform a 30-minute cruise flight from Lyon to Toulouse with the autopilot set (speed: 265 knots, heading: 237°, altitude 15,500 feet). Participants took as much time as necessary to familiarize themselves with the simulator before the beginning of the experiment. The three versions of the alarms were presented to the participants as many times as desired, so they could get acquainted with the stimuli before the experiment started. Participants were asked to apply a simplified Pull-Up maneuver (i.e., pull back the stick frankly a few seconds, then release it, and re-engage the autopilot) as soon as they would detect one of the alarms of interest. There were also instructed to ignore the fillers. Several means were used to prevent the participants from fixing their attention on the PFD only. They were told that some traffic could appear on their flight route, thus they had to monitor the external environment (i.e., the cockpit windows). Moreover, a 300foot thick layer of clouds was set at an altitude of 12,500 feet in order to reduce the visibility and to reinforce the need to actively monitor the external environment. Participants were also distracted by random disengagement of the autopilot and the auto-throttle. They were instructed to reactivate these instruments by pushing the appropriate buttons on the FCU, as soon as they detected the disengagement. All events (i.e., alarms, fillers and autopilot/auto-throttle disengagements) occurred every 20 to 25 s.

Data Analysis

The reaction times of the participants (i.e. the time between the onset of the "Pull Up" alarm and the initiation of the pullback movement) were assessed for the three types of alarms. Statistical analyses were performed using Statistica 10 ©. A one-way (Alarm [Current "Pull Up"; enlarged "Pull Up"; "Pull Up" video]) ANOVA was conducted on the mean reaction times. Bonferroni post-hoc tests were carried out to further examine significant effects (α < 0.05). We also counted two types of errors committed by the participants: omission errors (i.e., the participants did not respond to a "Pull Up" alarm) and commission errors (i.e., responses to the filler alerts that should have been ignored; [START_REF] Falkenstein | ERP components in Go/Nogo tasks and their relation to inhibition[END_REF].

Results

The analysis revealed a significant main effect of the alarm [F (1, 48) = 28.89, p < 0.001, ηp 2 = 0.55]. Bonferroni's post-hoc analysis revealed that participants were longer to react to the current "Pull Up" alarm (M = 2.89 s ± 1.36; see Fig. 7) than to both the "Pull Up" video (M = 1.60 s ± 0.31, p < 0.001) and the enlarged "Pull Up" (M = 1.47 s ± 0.30, p < 0.001). Reaction times to the "Pull Up" video and the enlarged "Pull Up" were not significantly different (F < 1, p = 1.0). Only one participant committed an omission error, missing a current "Pull Up" alarm. No omission errors were found for the two other "Pull Up" alarms. Nine participants made one commission error and one participant made two commission errors in reaction to the small "Jump" filler alarm. One participant made one commission error in reaction to the enlarged "Jump" filler alarm. No commission error was observed in reaction to the "Jump" filler video.

Discussion

The starting point of this study was that mirror neuron based alarms could help improve human reactions and contribute to mitigating the dramatic consequences of human errors in transportation [START_REF] Parker | Driving errors, driving violations and accident involvement[END_REF]World Health Organization 2013). Poor human reactions are not restricted to aviation, for example, the literature shows that 53% of train accidents [START_REF] Reinach | Railroad Yard Safety: Perspectives from Labor and Management[END_REF][START_REF] Reinach | Application of a human error framework to conduct train accident/incident investigations[END_REF] and 76-95% of road transport accidents are at least partly attributable to human error [START_REF] Elgarov | Road crashes and alcohol abusage in Kabardino-Balkaria[END_REF][START_REF] Sabey | Recent developments and research in road safety remedial measures[END_REF].

The mirror neuron system is believed to play a key role in understanding and imitating actions, suggesting that videos of hand gestures could facilitate the initiation of critical motor actions. In this sense, an alarm based on the mirror neuron system functioning could potentially be an efficient solution to display an action to be performed urgently. The aim of the present aviation study was twofold: (1) investigating whether hand gesture videos (i.e., a hand pulling back the sidestick) can activate brainwave frequencies related to the mirror neuron system; (2) determining whether enhancing of the visual characteristics of the "Pull Up" alarm could improve pilots' response times.

A first experiment was conducted to compare the brain signatures of the three alarms: the "Pull Up" alarm currently used in the operational cockpits and two visually-enhanced versions of the "Pull Up" alarm. The latter consisted of an enlarged "Pull Up" inscription and a video of the "Pull Up" gesture ("mirror alarm") to perform in response to the GPWS alarm. Participants passively observed (i.e., no action was expected in response to the alarms) the current "Pull Up" inscription, the enlarged "Pull Up" inscription, and the "Pull Up" video, displayed 30 times each on a computer screen. This experiment was conducted under controlled conditions (i.e. in an experimental room) in order to guarantee a good EEG signal-to-noise ratio. The first round of analysis comparing all three alarms failed to reach significance. At the difference of the enlarged "Pull Up" inscription and the "Pull Up" video that were displayed alone, the current "Pull Up" alarm was embedded in the PFD, where other flight information (i.e., including words) is also displayed. Many studies have shown that written stimuli can trigger a response of the mirror neuron system [START_REF] Baumgaertner | Polymodal conceptual processing of human biological actions in the left inferior frontal lobe[END_REF][START_REF] Hauk | Somatotopic representation of action words in human motor and premotor cortex[END_REF][START_REF] Kemmerer | Neuroanatomical distribution of five semantic components of verbs: Evidence from fMRI[END_REF][START_REF] Kemmerer | The two-level theory of verb meaning: An approach to integrating the semantics of action with the mirror neuron system[END_REF][START_REF] Pulvermüller | Walking or talking?: Behavioral and neurophysiological correlates of action verb processing[END_REF][START_REF] Skipper | Lending a helping hand to hearing: Another motor theory of speech perception. Action to Language via the Mirror Neuron System[END_REF]. This difference in the visual environment may have artificially enhanced the brain response to the current "Pull Up" alarm. A second set of exploratory excluding the current "Pull Up" alarm revealed greater alpha desynchronizations at parieto-occipital sites in response to the "Pull Up" video than to the enlarged "Pull Up" alarm. Previous studies have shown that alpha desynchronizations reflect the allocation of attentional resources to a stimulus [START_REF] Fink | EEG alpha band dissociation with increasing task demands[END_REF][START_REF] Keil | Alpha-band activity reflects reduction of mental effort in a comparison task: A source space analysis[END_REF][START_REF] Klimesch | Alpha-band oscillations, attention, and controlled access to stored information[END_REF][START_REF] Klimesch | Induced alpha band power changes in the human EEG and attention[END_REF][START_REF] Ray | EEG activity during cognitive processing: Influence of attentional factors[END_REF], suggesting that the "Pull Up" video caught more efficiently the attention of the participants than the enlarged "Pull Up" inscription. This result is in line with previous studies showing that motion stimuli attract more attention than static stimuli (e.g., [START_REF] Abrams | Motion onset captures attention[END_REF]. Second, this exploratory analysis revealed greater desynchronizations of mu and beta rhythms at central sites (i.e., C3 and C4) in response to the "Pull Up" video than to the enlarged "Pull Up" inscription. Both mu and beta desynchronizations were found to reflect the activity of the mirror neuron system in response to the observation and/ or the execution of a movement (e.g. [START_REF] Babiloni | Human cortical electroencephalography (EEG) rhythms during the observation of simple aimless movements: A high-resolution EEG study[END_REF][START_REF] Muthukumaraswamy | Modulation of neuromagnetic oscillatory activity during the observation of oro-facial movements (p. 2)[END_REF][START_REF] Nam | Movement imagery-related lateralization of event-related (de) synchronization (ERD/ERS): Motor-imagery duration effects[END_REF]. While some previous studies showed that written action verbs activate the mirror neuron system (e.g. [START_REF] Baumgaertner | Polymodal conceptual processing of human biological actions in the left inferior frontal lobe[END_REF][START_REF] Callan | Neural processes underlying perceptual enhancement by visual speech gestures[END_REF][START_REF] Kemmerer | Neuroanatomical distribution of five semantic components of verbs: Evidence from fMRI[END_REF][START_REF] Kemmerer | The two-level theory of verb meaning: An approach to integrating the semantics of action with the mirror neuron system[END_REF][START_REF] Skipper | Lending a helping hand to hearing: Another motor theory of speech perception. Action to Language via the Mirror Neuron System[END_REF], the present study suggests that the mirror neuron system may be more strongly activated by a gesture video (i.e. "Pull Up" video) than by the corresponding written action verb (i.e. enlarged "Pull Up" inscription). Despite possible contamination of the mu rhythm by the occipital alpha rhythm [START_REF] Hobson | Mu suppression-a good measure of the human mirror neuron system[END_REF], the observed beta rhythm desynchronization on the central areas supports the idea that the mu desynchronization mainly resulted from the mirror neuron system activity. Taken together, the electrophysiological results of the first experiment might support the assumption that the "Pull Up" video attracts more the pilots' attention (i.e. greater alpha desynchronization) and elicits greater motor preparation and pre-activation of the gesture (i.e. greater mu and beta desynchronizations at central sites; [START_REF] Fadiga | Motor facilitation during action observation: A magnetic stimulation study[END_REF] in comparison to the enlarged "Pull Up" inscription.

The second experiment was conducted in a flight simulator and aimed at measuring the participants' reaction times to the same three alarms. While performing a flight scenario, participants were instructed to pull back the stick in response to these three "Pull Up" alarms, each time they were displayed in the PFD. Three filler stimuli that had to be ignored were also presented to the participants in order to prevent automatic responses to any stimulus displayed on the PFD. These fillers consisted of "Jump" stimuli sharing similar characteristics with the three "Pull Up" alarms of interest: one small "Jump" inscription, one enlarged "Jump" inscription and one "Jump" video. The results of this second experiment revealed various errors of commission [START_REF] Falkenstein | ERP components in Go/Nogo tasks and their relation to inhibition[END_REF] in reaction to the "Jump" inscription. This result is a first indication that a small inscription embedded in the PFD is not optimal to indicate the requested action. The results also revealed that enhancing the visual component of the alarm improved the responses to the "Pull Up" alarm (i.e., faster reaction times). While it took the participants around 3 seconds to react to the current "Pull Up" alarm, the reaction times to the two visually enhanced "Pull Up" alarms were below 2 seconds. These shorter reaction times are encouraging as in some cases the time available to react is very limited, for instance when the descent rate of the aircraft is low, the GPWS alerts are delayed [START_REF] Kuchar | Markov Model of Terrain for Evaluation of Ground Proximity Warning System Thresholds[END_REF]. Increasing the size of the area in which the alarm was displayed from 0.5 to 30% of the PFD screen appears to have improved the saliency of the alarm and consequently accelerated the alarm detection process [START_REF] Busch | Size matters: Effects of stimulus size, duration and eccentricity on the visual gamma-band response[END_REF]). This result suggests that enhancing the visual component of the GPWS alarm could improve pilots' reactions in case of an imminent collision with the ground and may help prevent CFIT accidents. Yet, unexpectedly, the results revealed no significant difference in reaction time between the enlarged "Pull Up" inscription and the "Pull Up" video, making it difficult to determine which of the two visual enhancements would be the most effective in a situation of an imminent collision with the ground. The lack of difference in reaction times may be explained by the that participants knew that the alarms would be repeatedly displayed in the PFD. This may have led them to pay more attention to the location where the alarms were displayed, possibly contributing to prevent a potential difference in reaction times between the two alarms. Another explanation may be that the flight scenario generated relatively low levels of stress and workload compared to a real imminent collision with the ground. Beyond the overall negative effect of stress and high mental workload on cognition, these factors can also affect reading skills and the processing of written material [START_REF] Causse | High working memory load impairs language processing during a simulated piloting task: An ERP and pupillometry study[END_REF][START_REF] Daneman | Individual differences in working memory and reading[END_REF][START_REF] Sulpizio | Stress affects articulatory planning in reading aloud[END_REF]. As reacting to mirror alarms does not require decoding complex information nor reading written material, the former might be advantageous when cognitive functions are impacted both by stress and mental workload. Further research is now needed to assess a possible greater efficiency of "mirror alarms" in a context of high stress and/or workload, in which they may specifically trigger faster reactions times than more classical warnings.

The efficiency of this new GPWS alarm should also be evaluated with professional pilots. Moreover, in the present study, we focused on the visual component of the GPWS alarm, and its auditory component was removed. In order to confirm and consolidate the results of the present study, the efficiency of mirror alarms should be investigated when associated with the auditory component of the current GPWS alarm. Previous studies have shown that action sounds can also activate the mirror neuron system [START_REF] Kohler | Hearing Sounds, Understanding Actions: Action Representation in Mirror Neurons[END_REF] and that some specific neurons called audiovisual mirror neurons respond to actions, independently of whether the latter are performed, heard or seen [START_REF] Cook | The ontogenetic origins of mirror neurons: Evidence from 'tool-use' and 'audiovisual' mirror neurons[END_REF][START_REF] Keysers | Audiovisual mirror neurons and action recognition[END_REF]. Multimodal stimuli are also generally better perceived and trigger faster reactions in pilots (e.g. [START_REF] Colcombe | Cockpit display of traffic information automated conflict alerting: Parameters to maximize effectiveness and minimize disruption in multi-task environments[END_REF]. Thus, presenting a gesture alarm with both visual (i.e. gesture video) and auditory components could further improve pilots' reactions. Moreover, using the operational synthetic voice-familiar to airline pilots-in association with the gesture video may facilitate the transition to this new type of mirror alarms, limiting the risk of a change in habits [START_REF] Patterson | Guidelines for Auditory Warning Systems on Civil Aircraft[END_REF]. Future studies could also investigate a combination of text and video as the mirror neuron system was also found to be responsive to the linguistic representation of actions, like written or spoken action verbs [START_REF] Baumgaertner | Polymodal conceptual processing of human biological actions in the left inferior frontal lobe[END_REF][START_REF] Hauk | Somatotopic representation of action words in human motor and premotor cortex[END_REF][START_REF] Kemmerer | Neuroanatomical distribution of five semantic components of verbs: Evidence from fMRI[END_REF][START_REF] Kemmerer | The two-level theory of verb meaning: An approach to integrating the semantics of action with the mirror neuron system[END_REF][START_REF] Skipper | Lending a helping hand to hearing: Another motor theory of speech perception. Action to Language via the Mirror Neuron System[END_REF].

Conclusion and Perspectives

The current study investigated whether mirror neuron based alarms may be useful in situations where accurate and fast reactions are required to handle emergency situations. We found mixed results. The brain signatures of the two visually enhanced alarms' observed during exploratory analysis could suggest that the mirror alarm may: (1) better capture the pilots' attention (i.e., greater alpha desynchronization) and ( 2) improve motor preparation (i.e. greater mu and beta desynchronizations) compared to the enlarged "Pull Up" inscription. However, these preliminary results should be taken with caution and should be replicated using three perfectly comparable alarms. In the flight simulator, both enhanced "Pull Up" alarms improved the response (i.e. faster reaction times) compared to the current "Pull Up" alarm. No reaction time difference was found between the mirror alarm and the enlarged "Pull Up" inscription. Thus we cannot conclude that mirror alarm could be more effective in case of an imminent collision with the ground than written action verbs. However, we can confirm that a more salient "Pull Up" alarm would be desirable to trigger fast reactions in pilots in case of emergencies. In conclusion, very few studies examined the properties of the human mirror neuron system in the aeronautics field [START_REF] Callan | Dynamic Visuomotor Transformation Involved with Remote Flying of a Plane Utilizes the 'Mirror Neuron' System[END_REF]) and this work is probably the first to examine its potential for new alarm systems (with the exception of the preliminary studies of Causse et al. 2012b). Our results are far from being definitive. We encourage additional studies to better understand how and when mirror alarms could be used with significant benefits. If their benefit was confirmed and better understood in future experiments, such alarms might also be used in autonomous vehicles to help drivers take control of the car after a long period of inactivity. Applications of this type of alarms could also be imagined in emergency medicine [START_REF] Brennan | Hospital Characteristics Associated With Adverse Events and Substandard Care[END_REF][START_REF] Wears | Human Error in Emergency Medicine[END_REF] or in the military field, to name a few.

Fig. 1

 1 Fig. 1 Illustration of the three types of alarms. a the Current "Pull Up" alarm; b the Enlarged "Pull Up" alarm; c the "Pull Up" video

Fig. 4

 4 Fig. 4 Time-frequency maps in response to the "Pull Up" video (upper row) and the enlarged "Pull Up" (lower raw) on C3 (left column) and C4 (right column) electrodes. The whole epoch (0-3000 ms) was composed of two consecutive videos for each stim-

Fig. 5 aFig. 6

 56 Fig. 5 a Mu rhythm power and b Beta rhythm power in response to the "Pull Up" Video and the enlarged "Pull Up" alarms. Errors bars represent standard errors

Fig. 7

 7 Fig. 7 Reaction times to the current "Pull Up" alarm, the enlarged "Pull Up", and the "Pull Up" video. Errors bars represent standard errors

Acknowledgements

The authors would like to thank all participants who volunteered in this study. Authors would also thank Vsevolod Peysakhovich for his help with equation for power calculation and Patrice Labedan and Guillaume Garrouste who programmed the flight scenarios.

Funding This work was supported by a chair grant from Dassault Aviation ("CASAC", holder: Prof. Mickaël Causse).

Author Contributions Study conception and design: EJ, EF, MC. Data acquisition: EJ. Data Analysis: EJ, EF. Data interpretation: EJ, EF, MC. Manuscript writing: EJ, EF, MC.

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