DEVICE, SYSTEM AND METHOD FOR DETERMINING A STRESS LEVEL OF A USER

Abstract

The present invention relates to a device, system and method for determining a stress level, in particular for determining a psychogenic stress level of a user. The device comprises an interface (11) configured to obtain a skin conductance signal trace (22) of the user; a processing unit (12) configured to process the obtained skin conductance signal trace by: identifying a plurality of skin conductance peaks (50) in the skin conductance signal trace; determining, for each of said skin conductance peaks, a normalized parameter (58) of said skin conductance peak, normalized based on a skin conductance value (52, 53) of the respective skin conductance peak (50); and determining a psychogenic stress level (68) of the user based on said normalized parameters of said skin conductance peaks. The invention further relates to a corresponding wearable device (30) comprising such a device (10).

Claims

1. A device for determining a stress level of a user, the device comprising: an interface circuit, wherein the interface circuit is configured to obtain a skin conductance signal trace of the user; and a processing circuit, wherein the processing circuit is configured to process the obtained skin conductance signal trace by: identifying a plurality of skin conductance peaks in the skin conductance signal trace; determining a normalized parameter of each skin conductance peak; normalizing the parameter of each skin conductance peak based on a skin conductance value of the respective skin conductance peak; and determining a psychogenic stress level of the user based on the normalized parameters of the skin conductance peaks.

2. The device as claimed in claim 1, wherein determining the normalized parameter of the each of the skin conductance peaks comprises scaling a first value of the skin conductance signal trace at the respective skin conductance peak based on a second value of the skin conductance signal trace at the respective skin conductance peak.

3. The device as claimed in claim 1, wherein the normalized parameter is a normalized amplitude of the respective skin conductance peak.

4. The device as claimed in claim 1, wherein, the normalized parameter is a normalized steepness of a portion of the respective skin conductance peak.

5. The device as claimed in claim 4, wherein the normalized steepness is determined based on a difference of a logarithm of a first value of the skin conductance signal trace and a logarithm of a second value of the skin conductance signal trace at the respective skin conductance peak.

6. The device as claimed in claim 1, wherein determining the normalized parameter of the skin conductance peak comprises determining at least two points in the skin conductance peak, and taking a relative measurement of the values at these points.

7. The device as claimed in claim 1, wherein the normalized parameter of the skin conductance peak is determined based on a peak value and an onset value of the skin conductance peak, wherein the peak value is indicative of a skin conductance level at a peak of the skin conductance peak, wherein the onset value is indicative of a skin conductance level at an onset of the skin conductance peak.

8. The device as claimed in claim 7, wherein the normalized parameter of the skin conductance peak is determined based on a ratio of a first and a second value, wherein the first value is determined based on a difference between the peak value and the said onset value, wherein the second value is the peak value or the onset value.

9. The device as claimed in claim 1, wherein the processing circuit is arranged to determine a thermogenic sweat level based on the obtained skin conductance signal trace and the determined psychogenic stress level.

10. The device as claimed in claim 1, wherein the processing circuit is arranged to obtain temperature information and/or motion information of the subject, wherein the processing circuit is arranged to determine the psychogenic stress level of the subject based on the skin conductance signal trace and the temperature information and/or motion information of the subject.

11. A System for determining a stress level of a user, the system comprising: a sensor, wherein the sensor is arranged to acquire a skin conductance signal trace from a user; and a device as claimed in any claim 1.

12. The system as claimed in claim 11, wherein the sensor is arranged to acquire the skin conductance signal trace on a dorsal side of a wrist of the user.

13. A wearable device wearable by a user, the wearable device comprising the system as claimed in claim 11.

14. A method for determining a stress level of a user, the method comprising: obtaining a skin conductance signal trace of the user from a database; identifying a plurality of skin conductance peaks in the skin conductance signal trace; determining, for each of the skin conductance peaks, a normalized parameter of the skin conductance peak, wherein the normalized parameter of each skin conductance peak is normalized based on a skin conductance value of the respective skin conductance peak; and determining a psychogenic stress level of the user based on the normalized parameters of at least one of the skin conductance peaks.

15. A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor performs the method as claimed in claim 14.

16. The device as claimed in claim 1, wherein determining the normalized parameter by peak taking a relative measurement of a value of the skin conductance signal trace at two points of a raising edge of a skin conductance peak.

17. The method as claimed in claim 14, wherein determining the normalized parameter of the each of the skin conductance peaks comprises scaling a first value of the skin conductance signal trace at the respective skin conductance peak based on a second value of the skin conductance signal trace at the respective skin conductance peak.

18. The method as claimed in claim 14, wherein the normalized parameter is a normalized amplitude of the respective skin conductance peak.

19. The method as claimed in claim 14, wherein the normalized parameter is a normalized steepness of a portion of the respective skin conductance peak.

20. The method as claimed in claim 19, wherein the normalized steepness is determined based on a difference of a logarithm of a first value of the skin conductance signal trace and a logarithm of a second value of the skin conductance signal trace at the respective skin conductance peak.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

[0055] FIG. 1 shows a schematic diagram of a system and a device for determining a psychogenic stress level of a user;

[0056] FIG. 2 shows an exemplary implementation of a system according to the present invention in form of a wearable device;

[0057] FIG. 3 shows a diagram of a skin conductance trace;

[0058] FIG. 4 shows a diagram of a skin conductance peak or skin conductance response;

[0059] FIG. 5 shows diagrams of a raw skin conductance trace signal, a filtered skin conductance trace signal and of skin conductance peaks identified therein;

[0060] FIG. 6 shows diagrams of a skin conductance trace signal, absolute rising edge heights and normalized rising edge heights extracts from said skin conductance signal trace;

[0061] FIG. 7 shows further diagrams of a skin conductance trace signal, absolute rising edge heights and normalized rising edge heights extracts from said skin conductance signal trace;

[0062] FIG. 8 shows diagrams of a skin conductance trace signal, determined stress level based on absolute rising edge heights and determined stress level based on normalized rising edge heights extracts from said skin conductance signal trace;

[0063] FIG. 9 shows a diagram of a skin conductance trace signal and a positive first order derivative or steepness of the absolute rising edge heights;

[0064] FIG. 10 shows a diagram of a skin conductance trace signal and a positive first order derivative of a logarithmic transformed skin conductance;

[0065] FIG. 11 shows a flow chart of an embodiment of a method for determining a psychogenic stress level of a user.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0066] FIG. 1 shows a schematic diagram of exemplary embodiment of a system 100 for determining a stress level of a user. The system 100 comprises a sensor 20 for measuring a skin conductance signal of the user. The skin conductance signal measured by the sensor 20 over time forms a skin conductance signal trace 22 which is generally indicative of one or more measured stimulus responses corresponding to a neural stress response. The system 100 further comprises a device 10 for determining a stress level of the user.

[0067] The device 10 comprises an interface 11 for obtaining (i.e. receiving or retrieving from the sensor 20 or a (not shown) memory) a skin conductance signal trace 22 of the user and a processing unit 12 for processing the obtained skin conductance signal trace 22. The processing unit 12 can be any type of suitable processing unit or processor, such as for example a microprocessor/microcontroller, or embedded microcontroller but not limited thereto that is adapted accordingly. The interface 11 can be any kind of interface from obtaining data from the sensor 20 or a memory, e.g. a wireless or wired data interface or signal line. It will be understood that the sensor 20 and the device 10 can be part of the same device (e.g. wearable device or wristband) or can be implemented as or in separate devices.

[0068] The processing unit 12 can be adapted to perform the steps of identifying a plurality of skin conductance peaks in the skin conductance signal trace; determining, for each of said skin conductance peaks, a normalized parameter of said skin conductance peak, normalized based on a skin conductance value of the respective skin conductance peak; and determining a psychogenic stress level of the user based on said normalized parameters of said skin conductance peaks. Details of exemplary embodiments of the processing performed by the processing unit 12 will be explained below.

[0069] Optionally, as indicated by the dashed lines in FIG. 1, the system 100 can comprise an output unit 40 for outputting or rendering the determined stress level 24 to a user. The output unit 40 can be a human-machine-interface (HMI). For example, a display, an LED or a buzzer can be provided. Optionally, the output unit may be adapted to discretely indicate a stress level to the user. For example, a buzzer or vibration unit can be provided, which can discreetly signal a stress level increase. In addition or in the alternative, an optical indication can be provided. For example, the stress level may be communicated with a programmable LED, e.g. blue for the lowest level, followed by green, yellow, orange and lastly red for the highest stress level.

[0070] It will be understood that the output unit 40 and the device 10 can be part of the same device (e.g. wearable device or wristband) or can be implemented as or in separate devices. For example, the output unit 40 of the system 100 may be implemented by means of a smartphone or other information processing entity at the same or a remote location. Correspondingly, the processing unit 12 can also be implemented by means of a smartphone that is adapted to perform the afore-mentioned functionality for example by running a corresponding application or another suitable computing device running the corresponding software.

[0071] The system 100 may further comprise a memory 13 for storing program code means for causing the processor 12 to carry out the steps of the method as described herein. Details will be explained below. The memory 13 can be part of the device 10 or can be an external memory. The memory can be any suitable memory such as for example a memory register or RAM (random access memory). Advantageously, a non-volatile memory can be used, for example a microSD flash card. It will be understood that the memory 13 and the processing unit 12 can be part of the same device (e.g. wearable device or wristband) or can be implemented as or in separate devices.

[0072] FIG. 2 shows an embodiment of a wearable device 30 wearable by user. In this embodiment, the wearable device 30 is implemented in the form of a smart watch. The smart watch comprises a wristband 33 and a casing 34. The wristband 33 can loop around the wrist of the user. It will be understood that a wearable device could also be worn around another suitable part of the body such as the ankle foot or hand or may be adapted for attachment to other parts of the body, e.g. in the form of a patch.

[0073] The wearable device 30 can comprise the proposed system 100 for determining a stress level of a user. In this way a corresponding system 100 can be provided in an unobtrusive and wearable format. Alternatively, the wearable device 30 may only comprise the sensor 20, in this embodiment a skin conductance sensor 20. The device 10 of the system 100 may be located at the remote location or implemented in a remote device (e.g. a remote computer, smartphone or patient monitor).

[0074] The skin conductance sensor 20 may comprise a first and a second skin conductance electrode 31, 32 in combination with a skin conductance measuring unit (not shown). In the embodiment of FIG. 2, two skin conductance electrodes 31, 32 are integrated into the casing of the wearable device, however is also possible to integrate them for example into the wristband 33 And thus contact the underside of the wrist. The skin conductance electrodes 31, 32 can be arranged so as to contact the upper side of the wrist when the wearable device 30 is worn by the user. Exemplary implementations of a wearable device comprising a skin conductance sensor are the Philips discreet tension indicator DTI-4 or DTI-5.

[0075] The skin conductance measurement electrodes 31, 32 can be arranged for acquiring the skin conductance signal trace at a dorsal side of a wrist of the user, i.e. at an upper side of the wrist of the user in an as-worn orientation of the wearable device 30. As shown in FIG. 2, the elongated electrodes are arranged such that, in an as worn orientation, they are in line with muscle tendons of an upper (distal) side of the wrist of the user. An advantage of such a combination of shape and orientation can be that dropouts in the skin conductance signal caused by hand or wrist movements can be minimized or at least reduced. Dropouts can be indicative that (part of) the electrode is no longer in contact with the skin.

[0076] Optionally, the device 10, in particular the wearable device 30 can be fitted with a micro climate enhancement electrode surrounding inlay 36 such as described in U.S. Pat. No. 9,706,942 B2, the contents of which are incorporated herein by reference. The micro climate inlay can be shaped such that it is level with the skin conductance electrodes, thus minimizing skin deformation, thus avoiding skin irritation. The micro climate inlay can also be referred to as a micro climate flap or micro climate enhancing rubber.

[0077] The skin conductance sensor 20 is adapted to measure the skin conductance of the user 2 between the skin conductance electrodes 31, 32. For this purpose, the skin conductance measuring sensor may comprise a voltage generator for applying a voltage between the at least two skin conductance electrodes, a sensing unit for sensing a current between the at least two electrodes, and/or a calculating unit for calculating the skin conductance based on the sensed current. The measured skin conductance over time forms the skin conductance signal trace. The skin conductance signal trace (or data) may for example be stored in a memory of the wearable device 30, or may be transmitted (wirelessly or through a wire or signal line) to an external unit.

[0078] The skin conductance measuring sensor 20 and/or the device 10 (as shown in FIG. 1) may be integrated into the casing 34 of the wearable device 30. The wearable device 30 can further comprise a transmitter for transmitting data over a wireless or wired communication link, such as the measurement data and/or the determined stress level 24 of the user. However, it will be understood that the device 10 or processing unit 12 can also be implemented as or in separate parts or devices and that the wearable device 30 then transmits the skin conductance data to the separate part or device via the transmitter.

[0079] Advantageously, the system 100 may also comprise an output unit 40 for outputting the stress level of the user. The output unit 40 may be a separate device or may be integrated into, for example, the wearable device 30 comprising the sensor 20 in form of a smart watch. Furthermore, an external output unit 40, for example a smartphone, tablet or PC running a corresponding application, may be used and coupled to the device 10 or wearable device 30.

[0080] In the following, details of the proposed approach will be explained. FIG. 3 shows an exemplary skin conductance signal trace 22. The horizontal axis denotes the time of the day. The vertical axis denotes the (raw) measured skin conductance in nS (nanoSiemens). In the given example, the skin conductance signal trace 22 comprises three main portions indicated by P1, T1, and P2. The first period P1 (first period of psychogenic sweating) from about 13:50 h to 17:10 h denotes a time of psychogenic sweating, wherein the user was in a temperature-controlled environment and concentrated on a mental task but non engaging in physical activity. Sweating during this period P1 can be attributed to psychogenic sweating. The second period T1 (first period of thermogenic sweating) from about 17:10 h to 17:30 h denotes a period by thermogenic sweating, wherein the user engages in heavy physical activity (cycling for catching the train at 17:27 h). Sweating during this period P2 can be attributed to psychogenic sweating. The third period of time P2 (second period of psychogenic sweating) again denotes a time in a temperature controlled environment without physical activity. Skin conductance responses in this time period may again be attributed to psychogenic sweating.

[0081] FIG. 5 shows an individual skin conductance response or skin conductance peak 50. The curve in FIG. 5 can be seen as an (extremely) magnified portion of the skin conductance signal trace 22 in FIG. 3. The horizontal axis again denotes the time t, whereas the vertical axis denotes the skin conductance in [nS]. The graph in FIG. 4 spans about 15 seconds, and the graph in FIG. 3 spans about 8 hours. As used herein, a skin conductance peak 50 does not only refer to the maximum point but rather refers to a portion of the respective skin conductance response signal 22. A psychogenic stimulus may occur at the moment denoted by 51. In response to said stimulus, with a slight delay, the skin conductance starts to increase at the onset denoted by 52 until the skin conductance peak 50 reaches its maximum value at 53. The delay can for example be about 1 second for the wrist and about 2 seconds for the ankle. This can be attributed to the signal velocity along the sympathetic nerve. The difference 54 between the skin conductance level at the onset 52 and the skin conductance level at the maximum 53 or peak provides the skin conductance peak or skin conductance response amplitudes (SCR.sub.amplitude).

[0082] FIG. 5 illustrates signals during different stages of the signal processing. The horizontal axis in the graphs denotes the time, whereas the vertical axis denotes an amplitude. As shown in the upper graph in FIG. 5, the skin conductance signal trace 22 may be measured by the device at a sampling frequency between 40 and 160 Hz. As shown in the middle graph in FIG. 5, this raw signal can optionally be low-pass filtered to yield a smooth down-sampled signal 22′ having a sampling rate of, for example, 10 Hz. Subsequently, a plurality of skin conductance peaks 50 can be identified in the skin conductance signal trace. For example, rising edges in the filtered signal can be detected by zero-crossings of the first derivative of the (optionally filtered or preprocessed) skin conductance signal 22.

[0083] As an optional further processing step, additional processing and filtering steps can be applied. For example short glitches or slow signal drifts may be eliminated. In the given example, only rising edges that have a duration longer than 0.8 seconds and shorter than 3.0 seconds are attributed to skin conductance responses (SCR). Optionally, it is also possible to count every rising edge, even those lasting longer than 3 seconds or only rising edges lasting longer than a predetermined value, for example longer than 0.8 seconds. It has been found that when there are strong emotions, the skin conductance level may rise so fast that no or only too few zero crossings of the first order derivative are observed (e.g., just ripples in the rising edge). Not taking this into account may lead to missing strong emotions. Optionally, the processing device can thus be configured to determine strong emotional responses based on ripples in the rising edge of (a first derivative of) a skin conductance signal trace. It should be noted that skin conductance responses or skin conductance peaks can be identified using known techniques, as for example described in the aforementioned standard textbooks.

[0084] However, in contrast to the generally accepted procedure, the inventors have recognized that by determining, for each of said skin conductance peaks, a normalized parameter of said skin conductance peak, normalized based on a skin conductance value of the respective skin conductance peak, the impact of thermogenic sweating can be reduced, thereby providing a more meaningful indication of psychogenic sweating. For example, a normalized relative skin conductance peak amplitude, also referred to as normalized skin conductance response amplitude can be determined by:

[00002]$SC{R}_{\mathrm{normalized\_amplitude}}=\frac{\mathrm{peak\_level}\mathrm{\_value}-\mathrm{onset\_level}\mathrm{\_value}}{\mathrm{onset\_level}\mathrm{\_value}},$

wherein the peak (level) value is indicative of a skin conductance level at a peak (see 53 in FIG. 4) of said skin conductance peak and wherein the onset (level) value is indicative of a skin conductance level at the onset (see 52 in FIG. 4) of a skin conductance peak.

[0085] The result of this calculation is shown in the lower graph in FIG. 5, wherein the normalized skin conductance response amplitudes, as exemplary a normalized parameters of said skin conductance peak, normalized based on a skin conductance value of the respective skin conductance peak, are denoted by reference numeral 50.

[0086] The normalized parameter of the skin conductance peak, here SCR.sub.normalized_amplitude can be a dimensionless number that represents the normalized height of the respective skin conductance response. More generally speaking, determining the normalized parameter can comprise scaling a first value, for example the peak level value, of the skin conductance signal trace at the respective skin conductance peak based on a second value, for example the onset level value, of the skin conductance signal trace at the respective skin conductance peak. It should be noted that the proposed approach differs from what is normally used in the practice of extracting meaningful data from a skin conductance trace. In the aforementioned standard textbook Techniques in Psychophysiology the standard method for obtaining the SCRamplitude provides an absolute amplitude, that is determined by

SCR.sub.absolute_amplitude=peak_level_value−onset_level_value

wherein the peak (level) value is indicative of a skin conductance level at a peak (see 53 in FIG. 4) of said skin conductance peak and wherein the onset (level) value is indicative of a skin conductance level at an onset (see 52 in FIG. 4) of the skin conductance peak. In other words, in contrast to the solution proposed herein, the standard method for measuring the amplitude of the skin conductance response only takes the skin conductance level at the top and deducts the skin conductance level at the onset, thus yielding a number with the same dimension as the skin conductance (usually micro Siemens). This (conventional) amplitude can be referred to as absolute skin conductance response amplitude, or SCR.sub.absolute_amplitude.

[0087] In FIG. 6, the top graph shows a skin conductance signal trace 22, acquired with a Philips discreet tension indicator DTI-5 covering a time frame of 3.5 hours. The horizontal axis denotes the time. The vertical axis denotes the skin conductance in nS (nanoSiemens). The level of the skin conductance 22 gradually rises in this time frame. The middle graph in FIG. 6 represents the non-normalized SCR.sub.absolute_amplitude 56 for each of a plurality of skin conductance peaks and the bottom graph represents the normalized SCR.sub.normalized_amplitude 58 for each of a plurality of skin conductance peaks that were extracted from the skin conductance trace 22. The SCR.sub.absolute_amplitude 56 correlates with the average level of the skin conductance, whereas the SCR.sub.normalized_amplitude 58 does not show this correlation. Nonetheless, even though by the proposed peak-to-peak normalization, the information content is reduced, it has been found that the proposed normalization may eliminate or at least reduce thermogenic influences from the extraction of a (psychogenic) stress level from skin conductance data.

[0088] In FIG. 7 this approach is shown for a skin conductance trace 22 that contains thermogenic sweating influences that can be attributed to fast cycling to catch a train in time frame T.sub.1, as shown in the upper graph in FIG. 7. The middle graph in FIG. 7 again represents the non-normalized SCR.sub.absolute_amplitude 56 for each of a plurality of skin conductance peaks and the bottom graph represents the normalized SCR.sub.normalized_amplitude 58 for each of a plurality of skin conductance peaks that were extracted from the skin conductance trace 22. It can be seen clearly that the non-normalized SCR.sub.absolute_amplitude 56 increase caused by the cycling activity has negligible influence on the normalized SCR.sub.normalized_amplitude 58 according to the solution proposed herein. Hence, an influence of the skin conductance level variation of the quantification of the skin conductance response amplitude due to thermogenic sweating can be eliminated or at least reduced.

[0089] From the middle and bottom graphs in FIG. 7, it also becomes clear that in the two time periods before and after bicycling, the height of the SCR.sub.normalized_amplitude peaks is comparable, truthfully reflecting that their levels of stress are comparable as well. Thus the SCR.sub.normalized_amplutide measure may ensure that psychogenic peaks are given equal weight irrespective of the underlying skin conductance level, which might have greatly increased due to thermogenic sweating, especially at the outside of the wrist.

[0090] The conversion of the normalized skin conductance amplitude values to a stress level can be performed using known techniques. For example, a sum of (normalized) rising edge amplitudes per predetermined time interval can be evaluated. Based on a histogram of said sums of (normalized) rising edge amplitudes, different stress levels can be determined and thus user classified or categorized to a corresponding stress level. In the given non-limiting example, five different stress levels are provided, as shown in FIG. 8.

[0091] In FIG. 8, the top graph shows a portion of the skin conductance signal trace 22 of the top graph in FIG. 7. The graph again includes a time frame T.sub.1 of physical activity, here fast cycling to catch a train. The middle graph in FIG. 8 shows a stress level 66 of the user determined based on a sum of conventional non-normalized rising edge amplitudes (cf. FIG. 7, middle graph). On the other hand, the bottom graph in FIG. 8 shows a stress level 68 of the user determined based on as sum of normalized rising edge amplitudes (cf. FIG. 7, bottom graph). As can be seen from a comparison of the middle and bottom graphs in FIG. 8, the impact of thermogenic sweating that is present during the physical activity from 17:20 h onwards. It should be noted that a reduction of mental stress when cycling to the train station also accurately reflects a perceived mental stress level of the user during the measurement.

[0092] Referring again to FIG. 7, the inventors have recognized that, further to evaluating the normalized parameters of said skin conductance peaks, as represented in the bottom graph of FIG. 7, the processing unit can also be configured to evaluate a difference between the non-normalized parameters and the normalized parameters. For example, a difference between the non-normalized SCR.sub.absolute_amplitude 56 for each of a plurality of skin conductance peaks and the normalized SCR.sub.normalized_amplitude 58 for each of a plurality of skin conductance peaks that were extracted from the skin conductance trace 22 can be evaluated. Since the non-normalized SCR.sub.absolute_amplitude 56 comprises contributions due to thermogenic sweating and psychogenic sweating and the normalized SCR.sub.normalized_amplitude 58, as a first order approximation, reflects contributions due to psychogenic sweating (only), the difference between the two may yield the effects of thermogenic sweating (only). This can itself be valuable information.

[0093] Moreover, as indicated in FIG. 1 the device 10 may optionally be provided with a temperature and/or motion sensor 14. Temperature or physical activity data can give information on the cause of the observed thermoregulation. In particular, quantification of thermogenic sweating may offer information on the perception (and impact) of the ambient by (on) the user. Such information may be conveyed to climate control and/or a clothing advice application.

[0094] FIGS. 9 and 10 show graphs regarding a further embodiment of processing skin conductance data for determining a normalized parameter. FIG. 9 shows a diagram of a skin conductance trace signal (SC) 22 (in [pS] or picoSiemens) and a positive first order derivative or steepness curve 91 of the absolute rising edge heights (in [pS/s]). In FIG. 9, the curve 91 denotes the steepness of the absolute rising edge heights (in [ps/s]) that can be calculated by

steepness.sub.absolute=SC.sub.i+1−SC.sub.i

wherein SC.sub.i is the sample value at point i, and SC.sub.i+1 is the sample value at point i+1. On logarithmic scale, this can be rewritten as:

steepness.sub.absolute,log=(10log(SC.sub.i+1−SC.sub.i)).Math.f

[0095] The curve 91 shown in FIG. 9 shows a moving average over the last 30 samples. In the given example, zero and negative steepness values have been discarded. Nonetheless, as can be seen from FIG. 9, the time interval T.sub.1 of physical arousal (here running or cycling to catch a train) shows a strong contribution to the curve 91. Optionally, a maximum rising edge slope of each of a plurality of skin conductance peaks can be identified. Thereby, by only looking at the maximum of the rising edge slope per peak, the amount of data to be stored may be reduced. The terms steepness and slope may be used interchangeably.

[0096] FIG. 10 shows a diagram of a skin conductance trace signal 22 (in [pS] or picoSiemens) and a positive first order derivative of a logarithmic transformed skin conductance 92 (in [log10(pS)/s]), as will be described below. It has been found that the thermogenic contribution to the skin conductance data 22 can also be removed by using a measure for the steepness of the rising edges. The steepness can be calculated by taking the difference of logarithmic values of skin conductance measurement i and measurement i+1. Optionally, the value can be multiplied with the sampling:

steepness.sub.normalized,log=(10log(SC.sub.i)−10log(SC.sub.i+1)).Math.f

wherein SC.sub.i denotes a sample value of the skin conductance signal trace at sample i; SC.sub.i+1 denotes a sample value of the skin conductance signal trace at a subsequent sample i+1; and f denotes the sampling frequency. The sampling frequency is optional in the aforementioned formulae. Hence, a (positive) first order derivative of the skin conductance (SC) signal 22 after conversion to a logarithmic scale, e.g. by log10(SC), can be used as an estimator for arousal. It has been found that this estimation may be less sensitive for one or more of the following effects: thermogenic heating, building up micro-climate after mounting, or intense manual labor or exercise. For such a log-calculation, all negative values can be set to zero, leaving only the rising edges of the skin conductance trace. If the logarithm is not used the thermogenic effects are clearly visible in the steepness curve 91 as is shown in FIG. 9. By calculating steepness according to the formula shown above (i.e. by taking a difference of the logarithms) the thermogenic effects are no longer visible in the steepness curve 92 as is shown in FIG. 10. Instead of steepness, it can also be called the first time derivative. Optionally, a normalized maximum rising edge slope of each of a plurality of respective skin conductance peaks can be identified and used for further processing.

[0097] It should be noted that the proposed calculation may again be considered as obtaining a normalized parameter of said skin conductance peak, normalized based on a skin conductance value of the respective skin conductance peak. The aforementioned equation may also be rewritten as:

[00003]$\mathrm{steepness}=\left(10\log \left(\frac{S{C}_i}{S{C}_{i+1}}\right)\right).Math.f.$

[0098] It will be understood that in the aforementioned equations, the sample SC.sub.i+1 may be replaced by SC.sub.i−1 and vice versa.

[0099] FIG. 11 shows a schematic flow chart of a method for determining a stress level of a user. The method 200 comprises a first step S201 obtaining a skin conductance signal trace (data) of the user. For example, the skin conductance signal trace can be obtained directly as output data from a sensor (or preprocessing unit) or can be retrieved from storage means, for example, a memory or a hospital information system (HIS) or electronic health record (EHR) of the user.

[0100] In the next step S202 a plurality of skin conductance peaks are identified in the skin conductance signal trace. In step S203, for each of said skin conductance peaks, a normalized parameter of said skin conductance peak is determined, wherein the respective parameter is normalized based on a skin conductance value of the respective skin conductance peak. For example, a normalized amplitude or normalized edge steepness of the skin conductance peak may be determined.

[0101] In step S204, a psychogenic stress level of the user can be determined based on said normalized parameters of said skin conductance peaks.

[0102] In conclusion, the proposed solution may assist in quantifying the stress responses that are linked to psychogenic sweating which occur in a skin conductance trace. Advantageously, such a determination may be impervious or less susceptible to skin conductance changes such as those caused by physical activity or climate induced thermogenic sweating.

[0103] Exemplary applications of the propose solution may include, but are not limited to stress response measurement at non-glabrous (hairy) skin locations; climate control for vehicles or rooms based on the measurement of thermoregulation, clothing advice based on the measurement of thermoregulation.

[0104] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0105] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0106] A computer program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0107] Any reference signs in the claims should not be construed as limiting the scope.