Device for determining a brain response to a sensory stimulus

Abstract

Device for analyzing a user's brain response to sensory stimulation. The device includes an extended reality system to generate the sensory stimulus, as well as electrodes, a light source, and at least one photodetector intended to be placed against the user's scalp. Figure 1.

Description

Title of the invention: Device for determining a cerebral response to a sensory stimulus Technical field

[0001] The technical field of the invention is a device for collecting, in a non-invasive and synchronized manner, optical and electrical cortical signals, so as to measure a cerebral response to stimuli addressed to a user. PREVIOUS ART

[0002] There are several ways to estimate, in a non-invasive manner, a user's mental state. Currently, the most common mental state estimation systems are based on electrical measurements of brain activity, using "EEG" (ElectroEncephalography) type electrodes distributed on a user's skull. These measurements make it possible to obtain usable information on brain activity. Products intended for the general public, based on EEG electrodes, are now available, particularly in the entertainment field.

[0003] Recently, optical techniques such as functional Near Infrared Spectroscopy (fNIRS) have been developed to provide another method of analyzing brain activity. This type of technique makes it possible to evaluate a local variation in blood oxygenation at the level of the cortex. The principle is based on illumination of the cortex, in a near infrared wavelength, and on detection of an optical signal backscattered by the cortex, a few centimeters from the point of illumination. The detected signal depends on the attenuation induced at the level of the cortex but also at the level of the other tissues crossed.

[0004] The wavelengths used exploit the strong absorption of deoxyhemoglobin or oxyhemoglobin in the near infrared. The fact that brain activity is correlated with the blood oxygenation rate is used to quantify variations in brain activity from optical measurements taken at different times. This type of method is for example described in the following publications: - [1] P Pinti, The present and future use of functional near-infrared spec troscopy (fNIRS) for cognitive neuroscience. Ann NY Acad Sci. 2020 Mar; 1464(1):5-29; - [2] L Lim et al., A Unified Analytical Framework With Multiple fNIRS Features for Mental Workload Assessment in the Prefrontal Cortex, IEEE Trans.on Neural Systems and Rehabilitation Engineering, vol. 28, no. 11, pp. 2367-2376, Nov. 2020.

[0005] The use of fNIRS type techniques currently remains confined to uses in the field of research or in laboratory environments. The inventors propose an integrated device allowing a coupling between electrical and optical modalities, analysis of brain activity, and a virtual reality, augmented reality or mixed reality system. Statement of the invention

[0006] A first object of the invention is a device for analyzing the cerebral response of a user, the device being configured to address a sensory stimulus to the user and to record a cerebral response following the stimulus, the device comprising: - a support, configured to be worn by the user; - an extended reality system, connected to the support, and intended to emit the sensory stimulus; - electrodes, intended to be applied to the skin covering the user's skull, each electrode being configured to record an electrical cortical signal generated by a cortical area of ​​the user located opposite the electrodes;

[0007] the device being characterized in that it comprises - a light source, arranged to illuminate the skin covering the user's skull, forming an illumination zone, the light source being arranged to emit light in a spectral band between 650 nm and 1350 nm; - at least one photodetector, configured to detect light backscattered by the cortex, under the effect of illumination by the light source, the detected light emanating from a detection zone on the skin covering the skull, the distance between the detection zone and the illumination zone preferably being between 1 mm and 7 cm, the detected light forming an optical cortical signal; - a processing unit, programmed to establish: • a variation of an electrical cortical signal detected by at least one electrode following the emission of the sensory stimulus, said variation forming an electrical cerebral response to the sensory stimulus; • a variation of at least one optical cortical signal detected by at least one photodetector following the emission of the sensory stimulus, said variation forming an optical cerebral response to the sensory stimulus.

[0008] According to one embodiment, the processing unit is configured to estimate a mental state of the user from the optical and electrical brain responses. The processing unit may be configured to adjust the sensory stimulus, at a time, based on the mental state of the user estimated at a previous time. The device may comprise a motion sensor, configured to generate an actimetry signal representative of a movement of the user, the processing unit being configured to take into account the actimetry signal when estimating the mental state of the user.

[0009] The light source may be a continuous or frequency modulated light source.

[0010] At least two electrodes, at least one light source and at least one photodetector are preferably arranged less than 5 cm from each other, so as to address the same cortical area.

[0011] According to one possibility, the device comprises an illumination optical fiber, extending from an illumination source, to an illumination end, the illumination end being intended to be placed in contact with the user's skull, the illumination optical fiber forming the light source.

[0012] According to one possibility, the device comprises an optical detection fiber, extending from a light detector, to a detection end, the detection end being intended to be placed in contact with the user's skull, the optical detection fiber forming the photodetector.

[0013] Advantageously, each electrode, the or each light source, the or each photodetector, is connected to the support, thus forming a monolithic device.

[0014] The support may be a frame of a goggle. The goggle may be a virtual reality or augmented reality goggle, forming the extended reality system.

[0015] The extended reality system may comprise: - a projection surface, configured to project visual information, forming a visual stimulus; - and / or a sound generator, configured to generate a sound forming an auditory stimulus.

[0016] The extended reality system may comprise: - a vibration generator, configured to generate a tactile stimulus; - an electrostimulation generator, configured to generate a muscle stimulus.

[0017] A second object of the invention is a method for analyzing a cerebral response of a user following the generation of a sensory stimulus, using a device according to the first object of the invention, the method comprising: a. generation of a sensory stimulus by the extended reality system; b. following step a): - detection of at least one electrical cortical signal by an electrode of the device; - activation of at least one light source and detection of an optical cortical signal by a photodetector of the device; c. using the processing unit, taking into account the signals detected during step b) and determining the user's cerebral response following the sensory stimulus generated in step a).

[0018] Steps a) to c) can be carried out iteratively, the method being such that following a first iteration, the sensory stimulus generated during step a) is adapted according to the cerebral response determined during a previous iteration.

[0019] The sensory stimulus may be intended to cause the user to perform a cognitive task.

[0020] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0021] [Fig.l] represents an example of a device according to the invention.

[0022] [Fig.2] shows the path of a photon through different structures covering the cortex and through the cortex, between an illumination zone and a detection zone located at a distance from the illumination zone. The illumination zone and the detection zone are located on the skin covering the skull.

[0023] [Fig.3] shows schematically the different sensors of the device described in connection with the [Fig.l].

[0024] [Fig.4] shows the main steps of a method implementing the device. PRESENTATION OF SPECIAL EMBODIMENTS

[0025] [Fig. 1] represents an embodiment of the invention. The device 1 comprises a support 10, intended to be held at the level of a user's head. The support carries an extended reality system 11, here taking the form of a pair of glasses. The term extended reality includes virtual reality, augmented reality and mixed reality. The extended reality glasses 11 comprise a projection surface, intended to display a virtual scene for the user.

[0026] The extended reality glasses are intended to address a sensory stimulus or several sensory stimuli to the user. By sensory stimulus, we mean a visual stimulus and / or a sound stimulus. The visual and sound stimuli can be combined. The extended reality glasses can be intended to immerse the user in a virtual environment. The extended reality system 11 can generate other types of stimulus, for example a haptic stimulus or a muscular stimulus, of the electrostimulation type.

[0027] The device 1 is intended to capture and analyze a cerebral response of the user to the displayed virtual scene. It comprises 12 electrodes, of the electroencephalography (EEG) type, intended to be placed on the skin covering the skull (scalp or forehead). In known manner, EEG type electrodes make it possible to capture electrical activity of the cortex. The signal collected by each electrode forms an electrical cortical signal. The number of electrodes can be between two and a few dozen. The diameter of an electrode can be a few millimeters or around 1 cm.

[0028] The device 1 comprises at least one light source 13, intended to illuminate an illumination zone 14 on the skin covering the skull. The light source, or each light source, emits in a spectral band between 650 nm and 1350 nm. This spectral band allows a high penetration depth of the light into the tissues, in this case the skin, the skull and the dura mater. The absorption properties of the photons are mainly governed by the hemoglobin present in the cortex. Each light source may be a light-emitting diode, or one end of an illumination optical fiber connected to an illumination source located on the support 10. In this case, the illumination optical fiber or the end of the illumination optical fiber is assimilated to the light source. The emission spectral band of a light source is preferably less than 100 nm or less than 50 nm. Preferably, the device comprises: - a light source, the emission wavelength of which is less than 800 nm, which corresponds to a spectral band in which the absorption of deoxyhemoglobin is greater than the absorption of oxyhemoglobin; - a light source, the emission wavelength of which is greater than 800 nm, which corresponds to a spectral band in which the absorption of oxyhemoglobin is greater than the absorption of deoxyhemoglobin;

[0029] Preferably, two light sources emitting in two different spectral bands are close to each other, so that the illumination zone of each of the light sources can be considered as merged. Alternatively, each light source can emit successively in several spectral bands.

[0030] The device comprises at least one photodetector 15, configured to detect light backscattered by the user's tissues illuminated by the light source. The photodetector 15 may be a photodiode, directly in contact with the user. It may also be one end of a detection optical fiber, the other end of which is connected to a light detector located on the support 10. In this case, the detection optical fiber or the end of the detection fiber is likened to the photo detector 15. This possibility is illustrated in Figure 2. Each photodetector 15 is configured to collect light emanating from the skin covering the user's skull at a detection area 16. The detection area 16 is located at a distance d from the illumination area 14. The distance d between the illumination area 14 and the detection area 16 is called the backscattering distance. Preferably, the device is configured so that for an illumination area 14, different detection areas 16 are arranged respectively at different backscattering distances. Each backscattering distance is between 1 mm and 30 mm, or even 70 mm.

[0031] In Figure 2, examples of statistical average paths followed by photons penetrating into the tissue at an illumination zone 14 are shown diagrammatically, and emanating, by backscattering, from a detection zone 16 located at a backscattering distance d from the illumination zone 14. The photons propagate through the skin A, the skull B, the dura mater D and the cortex C. It is considered that the greater the backscattering distance, the more the light signal collected at the detection zone is representative of deep layers of the user's tissues. In the present case, taking into account the thickness of the skin and the bone, in the case of an adult, the cortex is located at a depth of between 1 cm and 2 or 3 cm from the skin. It is preferable for the backscattering distance to be approximately 3-3.5 cm for an adult.

[0032] The light source 13 can be continuous or frequency modulated.

[0033] Under the effect of a sensory stimulus, generated by the extended reality system 11, certain parts of the user's cortex can react. One objective of the invention is to detect the cerebral response, whether it is an electrical reaction and / or an optical reaction, the latter reflecting hemodynamic effects induced by cortical activity.

[0034] The electrical response can be captured by the electrodes placed on the skin covering the user's skull. In a manner known per se, the response of the cortex results in a variation in the electrical activity, the latter being able to be recorded by the electrodes 12. The device comprises a processing unit 20, intended to collect and analyze the electrical signals detected by the electrodes or the optical signals from each photodetector. The processing unit 20 comprises different modules.

[0035] A first module 21 is programmed to perform an analysis of the electrical signals collected by the electrodes. Usually, the first module 21 can perform an analysis of the spectral power in different spectral bands, for example one or more of the spectral bands known in the field of frequency analysis of EEG signals: band δ, band 0, band α or band [3. According to one possibility, the frequency analysis can select a spectral band whose power undergoes a temporal variation considered to be significant under the effect of the stimulus sensory and then quantify the temporal variation of spectral power in the spectral band thus selected. This temporal variation corresponds to the electrical response of the cortex to the stimulus addressed to the user. A second way of analyzing EEG signals is to average several cerebral responses by synchronizing with the instant of appearance of the stimuli: this analysis technique called evoked potentials is particularly robust because the measurement noise will decrease during the averaging process.

[0036] A second module 22 is programmed to perform an analysis of the optical signals emanating from each detection zone 16 and detected by each photodetector 15. Known principles of functional near infrared spectroscopy fNIRS, mentioned in the prior art, are then implemented. The fNIRS modality makes it possible to detect a hemodynamic response of the cortex, following the perception of the sensory stimulus. The use of different wavelengths makes it possible to estimate a variation in the concentration of oxyhemoglobin and deoxyhemoglobin under the effect of the stimulus. Certain stimuli can in fact trigger neuronal activity inducing an increase in the demand for oxygen. This local increase results in an increase, at the level of the cortical zone concerned, in the concentration of oxyhemoglobin, and a decrease in the concentration of deoxyhemoglobin.

[0037] The second module 22 is configured to quantify the variations of optical signals detected before and after the stimulus. These variations constitute the optical response of the user to the stimulus.

[0038] The second module 22 can be configured to estimate a heart rate or a respiratory rate. For this, a photodetector is implemented located at a short distance from a light source, typically less than 1 cm. With regard to the estimation of the heart rate, the principles described in French patent application FR2014157 can be implemented. With regard to the estimation of the respiratory rate, the principles described in Charlton PH, Bonnici T, Tarassenko L, Clifton DA, Beale R, Watkinson PJ can be implemented. An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram. Physiol Meas. 2016 Apr;37(4):610-26.

[0039] The first and second modules are configured to establish a cerebral response, respectively electrical and optical, in the cortical area considered, resulting from the perception of the stimulus by the user. By cortical area, we mean the area of ​​the cortex whose activation forms a response detectable by the electrodes and the photodetectors arranged facing the latter. In order to address the same cortical area, at least two electrodes, at least one light source and at least one photodetector, forming two source-detector pairs, are preferably grouped in a perimeter whose diameter is less than 5 cm.

[0040] According to one possibility, the device 1 comprises a motion sensor 17, intended to detect a movement of the user. It may for example be an accelerometer. The motion sensor 17 generates an actimetry signal representative of the movement of the user. The use of such a signal makes it possible to avoid the interpretation of artifacts in the cerebral responses of the user, in particular the electrical response, since the EEG analysis may be sensitive to the movements of the user. The motion sensor 17 is connected to an actimetry module, making it possible to record the actimetry signal and to detect any movement likely to cause a disturbance of the electrical or optical response.

[0041] The extended reality system 11 may be equipped with an inertial unit. The motion sensor 17 may be one of the sensors of the inertial unit.

[0042] According to one possibility, the device comprises an electrodermal activity sensor 18. Such a sensor is configured to estimate a variation in impedance of the skin. A variation in impedance may occur when the user is in a state of stress. The electrodermal activity sensor is formed of two electrodes, a current or voltage source, and an impedancemetry circuit.

[0043] According to one possibility, the device comprises an eye-tracking sensor 19, arranged on the projection surface, so as to track the user's eye movement. Such a sensor may comprise one or more infrared cameras intended to track the movement of the eye.

[0044] The electrical and optical brain responses, resulting from the modules 21 and 22, are used by the processing unit 20 to estimate a mental state of the user and / or a change in the mental state following the successive emission of different stimuli. The processing unit 20 comprises an analysis module 23, configured to estimate the mental state of the user. The mental state of the user may be a level of stress, fatigue, concentration, or emotion, or, more generally, a cognitive load. The movement sensors 17, electrodermal activity sensors 18 or eye tracking sensors 19 may be connected to the analysis module 23, so that their measurements are taken into account in determining the mental state of the user. This involves eliminating artifacts that may be linked to movements of the user or to eye movements. It may also involve obtaining additional measurements, tending to confirm the estimate of mental state.This is the case, for example, when electrodermal activity measurements are used to confirm the occurrence of a stress state. An example of determining the mental state of a user is described in the publication [2] cited in the prior art.

[0045] The processing unit may comprise one or more microprocessors.

[0046] An example of a combination of optical and cortical EEG measurements is described in the publication Mahmoudzadeh M “Usefulness of stimultaneous EEG-NIRS recording in language studies”, Brain and Language, May 2011.

[0047] [Fig. 3] illustrates the connection between the various components previously described. Preferably, the device is a single-piece device, the various components being connected and / or integrated into the support 10.

[0048] An interesting aspect of the device is that, depending on the mental state estimated by the processing unit 20, feedback can be provided so as to adapt each stimulus. For example, a visual stimulus, at a given moment, can be adapted depending on the state of the user estimated at a previous moment. When the user is immersed in a virtual scene, the content of the virtual scene, which forms the stimulus addressed to the user, can thus vary depending on the evolution of the mental state of the user. For example, when the user is requested by the virtual environment to perform tasks, the difficulty of the tasks can vary depending on the mental state of the user, for example his state of stress or fatigue.

[0049] The processing unit 20 can estimate a score, quantifying the mental state of the user, for example a stress level or a relaxation level. The score can be communicated to the user, via the extended reality system, visually or audibly. This allows the user to train himself to change his mental state. Such an embodiment is suitable for the user to practice relaxation activities.

[0050] In [Fig. 3], the solid arrows illustrate signal transmissions by wired or wireless connection. The dotted arrows correspond to the action of the user's cortex, which, under the effect of the scene displayed by the extended reality system 11, generates a cerebral response that can be measured by the electrodes 12 or the photodetectors 15.

[0051] The stimulus can be addressed without the user being focused on it. The stimulus can be a part of the virtual scene, not necessarily directly perceptible by the user, and which makes it possible to cause a change in the mental state of the user, for example generating stress or, on the contrary, relaxation. Thus, it is possible to estimate a mental state of the user without the latter being aware of the stimuli applied, the latter being drowned in the virtual environment in which the user is immersed. For example, the user can be placed in a soothing musical atmosphere, while visual stimuli are addressed to him.

[0052] An advantage of the device is that it allows good synchronization to be obtained between the application of successive stimuli and the measurements of brain responses, and this by using an integrated device.

[0053] The mental state of the user can be transmitted to a remote unit 30, so as to carry out temporal monitoring and correlations between the stimuli applied and the evolution of mental state. It can also involve identifying situations to which the user is sensitive.

[0054] When implementing the device, the user is for example subjected to a continuous musical ambiance, such as background music, and / or to discontinuous noises: in this case, the device comprises a sound emission source, for example headphones. The discontinuous noise may be a signal given to the user to start or stop the execution of a particular instruction, so as to modulate the cognitive load. The discontinuous noise may also be intended to generate a stress effect on the user, for example a high-pitched noise.

[0055] In addition to or as an alternative to a visual and / or auditory stimulus, the device may subject the user to another sensory stimulus. This may in particular be: - a stimulus applied to a part of the user's body: in this case, the device comprises a haptic stimulus generator, for example a vibration generator; - a muscular stimulus, applied to a muscle of the user: in this case, the device may include, for example, an electrostimulation module;

[0056] The user can thus be simultaneously exposed to stimuli of different types: for example visual + auditory, visual + haptic.

[0057] According to one possibility, the visual and sound stimulus aims to generate a cognitive task for the user. This may, for example, involve asking the user, audibly or visually, to perform a task resulting in a cognitive load or a certain state of stress. This may, for example, involve performing an increasing or decreasing count, taking into account a predefined increment (or decrement). For example, the user is asked, visually or audibly, to perform a decreasing count from a value, for example 1000, by successive decrements of 13. The user performs this cognitive task after the emission of an audible or visual instruction signal. As previously mentioned, the complexity of the cognitive task may be adjusted according to the mental state of the user, in this case the cognitive load of the user.

[0058] In one embodiment, the extended reality device is an augmented reality goggle. In this case, the user perceives the real environment in which he is located and visual indications are displayed by augmented reality. These indications can constitute a stimulus. For example, the user can practice a physical activity. The displayed indications are instructions addressed to the user to modulate his physical activity: change of movement, acceleration, etc. Depending on the mental state of the user, the instructions, which form the stimuli, are adapted, by adjusting the level of difficulty.

[0059] [Fig.4] represents the main steps of a method for estimating a mental state of a user using the device previously described.

[0060] Step 100: display of a scene to the user by the extended reality glasses 11. As previously indicated, it may be a virtual scene or a scene comprising augmented reality type information.

[0061] Step 110: acquisition of electrical measurements by the electrodes 12.

[0062] Step 120: acquisition of optical measurements by the photodetectors 15.

[0063] Step 130: acquisition of measurements from other sensors: motion sensor 17 and / or electrodermal activity sensor 18 and / or eye tracking sensor 19.

[0064] Step 140: quantification, by the first module 21, of the electrical response to the displayed scene, from the electrical measurements resulting from step 110 and from previous electrical measurements.

[0065] Step 150: quantification, by the second module 22, of the optical response to the displayed scene, from the optical measurements resulting from step 120 and from previous optical measurements.

[0066] Step 160: estimation of the mental state of the user from the optical response and the electrical response resulting respectively from steps 140 and 150.

[0067] Step 170: Based on the user's mental state, forming a feedback signal to adjust the observed scene. This step is optional.

[0068] Step 180: transmission of the user's mental state to the remote unit 30. This step is also optional.

[0069] Steps 100 to 180 can then be repeated.

[0070] A notable advantage of the invention is to facilitate time synchronization between the generation of the sensory stimulus and the determination of the user's cerebral response following the generation of the stimulus. Synchronization allows for better control of the effect of the sensory stimulus on the user. The device is configured to be worn by the user: it is therefore particularly suitable for nomadic use.

[0071] From the measured cerebral response, the device makes it possible to feedback on the generation of the stimulus, and to adapt the stimulus according to the cerebral response. The stimulus can thus be adapted according to the mental state of the user, in particular the cognitive load of the user.

[0072] The device may be used for analyses carried out for diagnostic purposes, or for functional rehabilitation purposes. For example, it may involve piloting a program for re-educating the user's motor skills, the program indicating motor tasks to be carried out by the user. The device makes it possible to adapt the rehabilitation program, in particular the frequency and / or the difficulty of the motor tasks, to the user's mental load.

[0073] The device can also be used to test the mental abilities of users likely to perform certain operations that may prove stressful. The device can, for example, be used to monitor the evolution of the mental load of a pilot subjected to a virtual environment, simulating difficult flight situations. The principle can be extended to any operator required to perform precise operations in a work environment that may be stressful.

[0074] Although described in connection with glasses, the extended reality device can take other forms. For example, it can be a visor attached to a support connected to the user's head.

Claims

Claims

1. Device (1) for analyzing the cerebral response of a user, the device being configured to address a sensory stimulus to the user and to record a cerebral response following the stimulus, the device comprising: - a support (10), configured to be worn by the user; - an extended reality system (11), connected to the support, and intended to emit the sensory stimulus; - electrodes (12), intended to be applied to the skin covering the user's skull, each electrode being configured to record an electrical cortical signal generated by a cortical area of ​​the user located opposite the electrodes; the device being characterized in that it comprises - a light source (13), arranged to illuminate the skin covering the user's skull, forming an illumination zone (14), the light source being arranged to emit light in a spectral band between 650 nm and 1350 nm; - at least one photodetector (15), configured to detect light backscattered by the cortex, under the effect of illumination by the light source, the detected light emanating from a detection zone (16) on the skin covering the skull, the distance between the detection zone and the illumination zone being between 1 mm and 7 cm, the detected light forming an optical cortical signal; - a processing unit (20), programmed to establish, in synchronization with the instant at which the sensory stimulus is addressed: • a variation of an electrical cortical signal detected by at least one electrode following the emission of the sensory stimulus, said variation forming an electrical cerebral response to the sensory stimulus; • a variation of at least one optical cortical signal detected by at least one photodetector following the emission of the sensory stimulus, said variation forming an optical cerebral response to the stimulus;

2. Device according to claim 1, wherein the processing unit (20) is configured to estimate a mental state of the user from the optical and electrical brain responses.

3. Device according to claim 2, wherein the processing unit is configured to adjust the sensory stimulus, at a time, according to the mental state of the user estimated at a previous time.

4. Device according to any one of claims 2 or 3, comprising a movement sensor (17), configured to generate an actimetry signal representative of a movement of the user, the processing unit (20) being configured to take into account the actimetry signal when estimating the mental state of the user.

5. A device according to any preceding claim, wherein the light source (13) is a continuous or frequency modulated light source.

6. Device according to any one of the preceding claims, in which at least two electrodes, at least one light source and at least one photodetector are arranged less than 5 cm from each other, so as to address the same cortical area.

7. Device according to any one of the preceding claims, comprising an illumination optical fiber, extending from an illumination source, to an illumination end, the illumination end being intended to be arranged in contact with the user's skull, the illumination optical fiber forming the light source (13).

8. Device according to any one of the preceding claims, comprising a detection optical fiber, extending from a light detector, to a detection end, the detection end being intended to be placed in contact with the user's skull, the detection optical fiber forming the photodetector (15).

9. Device according to any one of the preceding claims, in which each electrode, the or each light source, the or each photodetector, is connected to the support (10), forming a monolithic device.

10. A device according to any preceding claim, in wherein the support (10) comprises a frame for a pair of glasses, the glasses being virtual reality or augmented reality glasses, forming the extended reality system (11).

11. Device according to any one of the preceding claims, wherein the extended reality system (11) comprises: - a projection surface, configured to project visual information, forming a visual stimulus; - and / or a sound generator, configured to generate a sound forming an auditory stimulus.

12. Device according to any one of the preceding claims, in which the extended reality system comprises: - a vibration generator, configured to generate a tactile stimulus; - an electrostimulation generator, configured to generate a muscular stimulus.

13. Method for analyzing a cerebral response of a user following the generation of a sensory stimulus, using a device (1) according to any one of the preceding claims, the method comprising: a. generation of a sensory stimulus by the extended reality system (11); b. following step a): - detection of at least one electrical cortical signal by an electrode (12) of the device; - activation of at least one light source and detection of an optical cortical signal by a photodetector (15) of the device; c. using the processing unit, taking into account the signals detected during step b) and determining the cerebral response of the user following the sensory stimulus generated in step a).

14. A method according to claim 13, wherein steps a) to c) are carried out iteratively, the method being such that following a first iteration, the sensory stimulus generated during step a) is adapted in function of the brain response determined during a previous iteration.

15. A method according to any one of claims 12 or 13, wherein the sensory stimulus is intended to cause the user to perform a cognitive task.

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