Patent Publication Number: US-2023157635-A1

Title: Multimodal biometric human machine interface headset

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to U.S. Provisional Pat. Application Serial No. 63/283,192, filed on Nov. 24, 2021, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a headset and noninvasive sensors for real-time measuring of electroencephalography (EEG), photoplethysmography (PPG), and inertial measurement unit (IMU) signals from locations on the forehead and head in the detection of brain activity. 
     BACKGROUND OF THE INVENTION 
     Brain waves can be detected via EEG, which involves monitoring and recording electrical impulse activity of the brain, typically noninvasively. EEG data can be generated by placing a number of electrodes (often part of a brain-computer interface (BCI) headset) on or near the subject’s scalp. The electrodes detect the electrical impulses generated by the brain and send signals to a computer that records the results. The data from each electrode may be deemed a channel representing data from a portion of the brain where the electrode is located. Each channel may have an active and reference electrode in a montage used in the differential amplification of the source signal. 
     Brain waves may be detected as a time-varying signal and comprise components having different spectral characteristics such as frequency and power. As an example, brain waves may include the following brain wave components: delta waves, theta waves, alpha waves, beta waves, and gamma waves. The spectral characteristics of the brain waves may indicate different mental states based on factors such as source location, duration, coherence and dominance or amplitude. 
     SUMMARY OF THE INVENTION 
     Traditional systems in the field of non-invasive plethysmography (e.g., using an instrument to measure changes in volume within an organ of a body) have been implementing improvised calculations of PPG signals in order to measure changes in volume of human organs. For example, one method of signal quality analysis uses the perfusion index (PI) to measure a peripheral perfusion or ratio of pulsatile blood flow to non-pulsatile static blood flow in a patient’s peripheral tissue. Another method of signal quality analysis uses skewness to measure the probability distribution of a real valued random variable about its mean value. Another method of signal quality analysis uses regression techniques to map the values of these parameters to PPG signal classifications. 
     However each of these traditional methods have limitations. For example, PI works best for signals that are received through transmissive type PPG sensors and not for reflective type PPG. In some examples, PI is not comparable with reflective type signals and traditional SQI may be limited due to the reflective type sensor. The signal may be classified based on SNR and standard deviation values combined, for example, the SNR calculation equal to 20 * log10 (Mean / standard deviation of signal) dB. 
     Various embodiments of the present disclosure may include capacitive non-dermal EEG electrodes mounted within the headset device for brain wave signal detection and data acquisition. The headset can enable real-time measuring of EEG as well as PPG signals from locations of the user’s head in the detection of brain activity. 
     The headset may be embodied in a durable polymer. The headset may also include one or more adjustable slider arms on either side of the headset (e.g., for a better fit of the headset to the user’s head via, for example, extending or retracting across the user’s forehead), LED lights on either side (e.g., to identify power, charging, or programmable function indicators), one or more replaceable EEG sensors (e.g., with surfaces coated in silver, silver chloride, or conductive polymer), one or more PPG sensors (e.g., on the forehead), and/or a charging port. A single frontal piece may be employed in addition to or instead of the slider arms. Other components of the headset may be included as well, including a microphone, adhesive, and/or connectors between components. 
     The slider arms can be placed on either side of the headset and may be adjustable. The slider arms may extend or retract for positioning on the forehead of the user. The single frontal piece extending across the user’s forehead may optionally be employed connecting the slider arms or in place of the slider arms. 
     The LED lights can be placed on either side of the headset or at other locations of the headset to indicate power, charging status, any connections to a host device, or other programmable functions. 
     The EEG sensors can detect electricity generated by a user’s brain as brain waves and generate data. The data corresponding with the brain waves can be filtered, amplified, analyzed, and/or recorded (e.g., in a wave pattern). 
     The PPG sensors can determine the PPG data. The PPG sensors may be placed along the headset to correspond with forehead locations when the user is wearing the headset. The sensor placement may identify one or more optimal locations for determining PPG signals. 
     In some examples, the headset may apply automatic gain control (AGC) and signal to noise ratio (SNR) calculations of the PPG signals. The AGC may compensate for different skin tones using positive or negative feedback loops from standard values. In some examples, the signal strength of PPG recorded is highly dependent on skin tone. Since the skin tissues lie on the optical path of PPG, darker skin tones absorb large amounts of 660 nm wavelength — red light, whereas the lighter / pale skin tones reflect back most of the light causing amplifier saturation. To ensure optimum signal quality through different demographics, AGC may be applied to keep the PPG signal amplitude under or above desired threshold value. 
     The SNR is also determined. In some examples, the PPG signals may be highly sensitive to sensor and skin movement. When a reflective type sensor is implemented with the headset, light may be traveling toward the artery or blood vessel and may also be traveling from the artery or blood vessel as reflected light. This slight change in optical path between the light traveling towards and reflected from the user’s skin (e.g., based on sensor movement or skin movement) can cause a disturbance in recorded PPG signal. When the sensor is not placed properly or there is a motion artifact, the SNR value may decrease and standard deviation of that data recording may increase. This change indication may be provided to the user (e.g., via a software application or graphical user interface) to adjust the PPG sensor location or fix the PPG sensor to properly record the values. 
     Other noise reduction methods may be implemented other than SNR. For example, due to the nature of implementing an indirect measurement process, wearable devices may inevitably face challenges caused by baseline drift and Motion Artifacts (MAs), especially during exercise and under free living conditions. 
     Although the classical Adaptive Threshold Peak Detection (ATPD) algorithm is capable of resolving baseline drift in PPG signal analysis by detection of peak positions in the time domain, ATPD may be vulnerable to MAs. Adaptive Noise Cancellation (ANC) has the ability to reduced unwanted MAs by introducing multi-sensor accelerometer and gyroscope signals and it is being widely used for cancelling MAs and noise in PPG signals. However, the ANC algorithm fails if the MAs have a close enough main frequency component to the heartbeat rate in the PPG signal. Adaptive noise cancellation algorithm utilizes Discrete cosine transform and Hilbert transform calculation over complete frequency range (i.e. 0 to 50 Hz) for every second of data of PPG signal. While implementing this, there may be a lag in data acquisition due to high computational complexity of the algorithm. To overcome these problems, a Discrete cosine transform and Hilbert transform calculation may be computed over complete frequency range (i.e. 0 to 8.53 Hz or 512 data points) for every second of data of PPG signal and instead of processing 0 to 50 Hz which corresponds to 3000 data points of DCT values. Real-time data acquisition (with no lag) of PPG may be obtained. 
     Once the optimal value of PPG is captured, AGC may set the LED current. Once the LED current is set, the AGC can calculate SNR and standard deviation to estimate signal quality of the signal from the site of the recording (e.g., fixed to the headset). 
     As illustrative examples, the LED current range can be correlated for various skin tones. For example, for light brown skin tones, the LED current range (Red) may be 8 to 9 mA and the LED current range (IR) may be 6. In another example, for moderate brown skin tones, the LED current range (Red) may be 12 to 16 mA and the LED current range (IR) may be 6. In another example, for dark to deep dark skin tones, the LED current range (Red) may be 20 to 30 mA and the LED current range (IR) may be 6. 
     One or more capacitive non-dermal EEG electrodes may be incorporated with the headset. Each of the electrodes may have an elongated shape (e.g., rectangle or ellipse) or a non-elongated shape (e.g., square or circle), and/or a curved shape (e.g., a curvature to match a natural curvature of a surface of a head). The electrodes’ surfaces may be coated in gold, silver, silver chloride, or other conductive polymer. The texture of the electrodes may be smooth or any other texture that can increase a surface area of the electrode. 
     In some examples, the electrodes collect data using noninvasive, electrical brain signal measurements absent the use of an interface material between the electrode and the skin (e.g., an electrolyte, in a EEG gel or paste form, etc.). The user may not need to add gel or saline solution to improve signal quality of the capacitive non-dermal contact electrodes. 
     Each electrode may be coupled with foam, spring, gel-containing material, or other support that can provide some pressure from the headset to the electrode, in order to help apply pressure to the electrode to remain in contact with the hair or head of the user. The pressure can help ensure a better fit and help conform the electrode (and headset overall) to the curvature of portions of the head and/or irregular (or regular) bumps on a surface of the head. 
     The electrodes may be placed at locations of the headset based on general head anthropometry and experimental trials. The electrodes may be curved. The curvature of the electrodes may vary based on the placement of the electrode location. Different head parts may correspond with different curvatures. In some examples, eight electrodes may be placed around the headset in various designs and utilitarian functions for capacitive non-dermal EEG electrodes. 
     The electrodes may be replaceable. For example, the headset may have one or more ports to plug in an electrode into headset. When an electrode becomes inoperable, the electrode may be unplugged from the port and replaced with a new electrode for easy replacement. 
     The charging port can provide power to the components of the headset and optional wired connectivity for transmitting data. In a rechargeable scenario, a cable may be removably coupled with the charging port in the headset and a power outlet to provide electrical connectivity while charging the headset. In some examples, the cable may remain attached to the charging port in the headset and a power outlet during operation of the headset (e.g., in research and gaming environments where even millisecond delays have significance). 
     The headset may be used to collect data using an IMU (also referred to herein as an IMU sensor) within the headset as a control system. The IMU may include an accelerometer, gyroscope, feature extraction, sensor fusion, and other components or engines for generating data. The data can be stored, inferred, or retrieved from a memory incorporated with the headset as well. In some examples, the data may be transmitted to a computer system for processing and analysis. 
     The computer system may receive the data from the headset and implement controls based on the data. As an illustrative example, the headset may be moved along an X and Y axis and, on a corresponding graphical user interface provided at a display coupled with the computer system, an object can be moved in accordance with the headset movement. In other words, the headset may be used to control the object using head motion as an alternative human-computer interface device. 
     In some examples, the computer system may improve the data using a spectrum noise cancellation process to remove motion artifacts (e.g., generated by the EEG or PPG sensors). The computer system may execute spectrum denoising to help reduce the noise in the data. As an illustrative example, the denoised data may be used to estimate heart rate and respiration rate of the user in a non-invasive form. 
     These and other objects, features, and characteristics of the system and/or method disclosed herein, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings are provided for purposes of illustration only and merely depict typical or example implementations. These drawings are provided to facilitate the reader’s understanding and shall not be considered limiting of the breadth, scope, or applicability of the disclosure. For clarity and ease of illustration, these drawings are not necessarily drawn to scale. 
         FIG.  1    provides a left-perspective view of an embodiment of the headset, in accordance with one or more implementations of the invention. 
         FIG.  2    provides a right-perspective view of an embodiment of the headset, in accordance with one or more implementations of the invention. 
         FIGS.  3 - 4    provide illustrative views of the headset as worn by a user, in accordance with one or more implementations of the invention. 
         FIG.  5    provides an illustrative view of a slider arm of the headset, in accordance with one or more implementations of the invention. 
         FIG.  6    provides an illustrative view of PPG and EEG sensors of the headset, in accordance with one or more implementations of the invention. 
         FIG.  7    provides a projection view of an EEG electrode of the headset, in accordance with one or more implementations of the invention. 
         FIG.  8    provides an isometric view of an EEG electrode of the headset, in accordance with one or more implementations of the invention. 
         FIG.  9    provides an exploded view of an EEG electrode of the headset, in accordance with one or more implementations of the invention. 
         FIG.  10    provides an illustrative placement of a set of EEG electrodes in a first layout of the headset, in accordance with one or more implementations of the invention. 
         FIGS.  11 - 14    provide illustrative embodiments electrodes of the headset, in accordance with one or more implementations of the invention. 
         FIG.  15    provides an illustrative placement of a set of EEG electrodes in a second layout of the headset, in accordance with one or more implementations of the invention. 
         FIGS.  16 - 19    provide illustrative embodiments electrodes of the headset, in accordance with one or more implementations of the invention. 
         FIG.  20    provides an illustrative placement of a set of EEG electrodes in a third layout of the headset, in accordance with one or more implementations of the invention. 
         FIGS.  21 - 24    provide illustrative embodiments electrodes of the headset, in accordance with one or more implementations of the invention. 
         FIG.  25    provides an illustrative process for replacing electrodes of the headset, in accordance with one or more implementations of the invention. 
         FIG.  26    provides an illustrative example of generating inertial data, in accordance with one or more implementations of the invention. 
         FIG.  27    provides an illustrative example of generating accelerometer data, in accordance with one or more implementations of the invention. 
         FIG.  28    provides an illustrative example of generating gyroscope data, in accordance with one or more implementations of the invention. 
         FIG.  29    illustrates 3D angular mapping using various components of the headset, in accordance with one or more implementations of the invention. 
         FIG.  30    illustrates the user’s head movement for generating data, in accordance with one or more implementations of the invention. 
         FIG.  31    illustrates a process of converting data to interactions with a desktop display, in accordance with one or more implementations of the invention. 
         FIG.  32    illustrates an example of a process of calculating a heart rate, in accordance with one or more implementations of the invention. 
         FIG.  33    illustrates an example of a process of calculating a heart rate and/or respiration rate, in accordance with one or more implementations of the invention. 
         FIG.  34    illustrates an example of a process of determining LED current, in accordance with one or more implementations of the invention. 
         FIG.  35    illustrates an example of a process of determining a good or improper signal, in accordance with one or more implementations of the invention. 
         FIG.  36    provides a left-perspective view of an embodiment of the headset, in accordance with one or more implementations of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     It is to be understood that the figures and descriptions of the present invention may have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements found in a typical headset or typical method of using a headset. Those of ordinary skill in the art will recognize that other elements may be desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. It is also to be understood that the drawings included herewith only provide diagrammatic representations of the presently preferred structures of the present invention and that structures falling within the scope of the present invention may include structures different than those shown in the drawings. Reference will now be made to the drawings wherein like structures are provided with like reference designations. 
     Before explaining at least one embodiment in detail, it should be understood that the inventive concepts set forth herein are not limited in their application to the construction details or component arrangements set forth in the following description or illustrated in the drawings. It should also be understood that the phraseology and terminology employed herein are merely for descriptive purposes and should not be considered limiting. 
     It should further be understood that any one of the described features may be used separately or in combination with other features. Other invented devices, structures, apparatuses, systems, methods, features, and advantages will be or become apparent to one with skill in the art upon examining the drawings and the detailed description herein. It is intended that all such additional devices, structures, apparatuses, systems, methods, features, and advantages be protected by the accompanying claims. 
     For purposes of this disclosure, the term “headset” is interchangeable with the terms helmet, cap, hat, lid, wrap, band, and/or any combination thereof, or other head covering. 
     The invention described herein relates to a headset comprising a curved frame; a plurality of capacitive non-dermal EEG sensors connected with the curved frame; one or more PPG sensors connected with the forehead portion of the curved frame, wherein locations and curves of the plurality of EEG sensors are formatted in accordance with a head shape of a user; and one or more adjustable slider arms on either side of the headset. 
     The invention described herein also relates to systems and methods for measuring of PPG signals from a forehead location and capacitive non-dermal EEG sensors distributed over the surface of the head in the detection of brain activity having one or more physical processors programmed with computer program instructions that, when executed by the one or more physical processors, cause the computer system to perform the method, the method comprising: receiving, by one or more sensors, raw EEG data; applying spectral analysis to one or more channels of the raw EEG data to isolate spectral components in the channels; and using this data to create data outputs based on the spectral analysis that may be either displayed in raw form or utilized in algorithms to determine mental states of the user. 
     It will be appreciated by those having skill in the art that the implementations described herein may be practiced without these specific details or with an equivalent arrangement. In various instances, well-known structures and devices are shown in block diagram form to avoid unnecessarily obscuring the implementations. 
     Example Headset Architecture 
       FIGS.  1 - 2    provide a left-perspective view and a right-perspective view of an embodiment of the headset, respectively, in accordance with one or more implementations of the invention. In these figures, an embodiment of headset  100  is provided. Headset  100  may be a non-dermal contact headset device for brain wave signal detection and data acquisition. Headset  100  can enable real-time measuring of PPG signals from forehead location of the user’s head in the detection of brain activity, and can be employed absent the use of traditional methods like the perfusion index, skewness, and regression techniques. 
     Headset  100  may be embodied in a durable polymer or other flexible or non-flexible material. Headset  100  may also include a frontal piece including one or more adjustable slider arms  110 , including left slider arm  110 A and right slider arm  110 B, on either side of headset  100  (e.g., for a better fit of the headset to the user’s head), LED lights  120  on either side (e.g., to identify power, charging, or programmable function indicators), one or more replaceable EEG sensors  130  (e.g., with surfaces coated in silver, silver chloride, or conductive polymer), reference electrode  133  (for the EEG sensors  130 ), one or more PPG sensors  140  (e.g., on the forehead), and a charging port  160 . Other components of headset  100  may be included as well, including a mic, adhesive, and/or connectors between components. 
     Additional views of headset  100  are illustrated in  FIGS.  3 - 4   . While headset  100  is worn, EEG signals are determined from the capacitive non-dermal EEG electrodes, illustrated as first user  300 A and second user  300 B. The EEG signals correspond with brain activity of each user. 
     Adjustable slider arm  110  and LED light  120  are illustrated in  FIG.  5   . For example, adjustable slider arm  110  can be placed on either side of headset  100  and may be adjustable/movable (see, for example, slider arms  110   a ,  110   b  in  FIGS.  1 - 2   ). Adjustable slider arm  110  may extend or retract for positioning on the forehead of the user. Additionally, LED lights  120  can be placed on either side of headset  100  or at other locations of the headset  100 , to indicate changes with the devices, including increasing or decreasing power, active or deactive charging status, initiating or dropping connections to a host device, or other programmable functions. 
     One of the EEG sensors  130  and one of the PPG sensors  140  are illustrated in  FIG.  6   . The EEG sensors  130  can detect electricity generated by a user’s brain as brain waves and generate data. More or fewer EEG sensors may be provided with headset  100 . The data corresponding with the brain waves can be processed (e.g., filtered, amplified, analyzed, and/or recorded (e.g., in a wave pattern)). 
     PPG sensors  140  can determine the PPG data. PPG sensors  140  may be placed along headset  100  to correspond with forehead location when the user is wearing the headset. 
     The PPG data may be altered. In some examples, headset  100  may apply automatic gain control (AGC) and signal to noise ratio (SNR) calculations of the PPG signals. The AGC may compensate for different skin tones using positive or negative feedback loops from standard values. In some examples, the signal strength of PPG recorded is highly dependent on skin tone. Since the skin tissues lie on the optical path of PPG, darker skin tones absorb large amounts of 660 nm wavelength — red light, whereas the lighter / pale skin tones reflect back most of the light causing amplifier saturation. To ensure optimum signal quality through different demographics, AGC may be applied to keep the PPG signal amplitude under or above desired threshold value. 
     The SNR may also be determined. In some examples, the PPG signals generated by PPG sensors  140  may be highly sensitive to sensor and skin movement. When a reflective type sensor is implemented with headset  100 , light may be traveling toward the artery or blood vessel and may also be traveling from the artery or blood vessel as reflected light. This slight change in optical path between the light traveling towards and reflected from the user’s skin (e.g., based on sensor movement or skin movement) can cause a disturbance in recorded PPG signal. When the sensor is not placed properly or there is a motion artifact, the SNR value may decrease and standard deviation of that data recording may increase. This change indication may be provided to the user (e.g., via a software application or graphical user interface) to adjust the PPG sensor location or fix the PPG sensor to properly record the values. 
     Other noise reduction methods may be implemented other than SNR. For example, due to the nature of implementing an indirect measurement process, wearable devices may inevitably face challenges caused by baseline drift and Motion Artifacts (MAs), especially during exercise and under free living conditions. 
     Although the classical Adaptive Threshold Peak Detection (ATPD) algorithm is capable of resolving baseline drift in PPG signal analysis by detection of peak positions in the time domain, ATPD may be vulnerable to Mas. Adaptive Noise Cancellation (ANC) has the ability to reduced unwanted Mas by introducing multi-sensor accelerometer and gyroscope signals and it is being widely used for cancelling Mas and noise in PPG signals. However, the ANC algorithm fails if the Mas have a close enough main frequency component to the heartbeat rate in the PPG signal. Adaptive noise cancellation algorithm utilizes Discrete cosine transform and Hilbert transform calculation over complete frequency range (i.e. 0 to 50 Hz) for every second of data of PPG signal. While implementing this, there may be a lag in data acquisition due to high computational complexity of the algorithm. To overcome these problems, a Discrete cosine transform and Hilbert transform calculation may be computed over complete frequency range (i.e. 0 to 8.53 Hz or 512 data points) for every second of data of PPG signal and instead of processing 0 to 50 Hz which corresponds to 3000 data points of DCT values. Real-time data acquisition (with no lag) of PPG may be obtained. 
     Once the optimal value of PPG is captured, AGC may set the LED current. Once the LED current is set, the AGC can calculate SNR and standard deviation to estimate signal quality of the signal from the site of the recording (e.g., fixed to headset  100 ). 
     As illustrative examples, the LED current range can be correlated for various skin tones. For example, for light brown skin tones, the LED current range (Red) may be 8 to 9 mA and the LED current range (IR) may be 6. In another example, for moderate brown skin tones, the LED current range (Red) may be 12 to 16 mA and the LED current range (IR) may be 6. In another example, for dark to deep dark skin tones, the LED current range (Red) may be 20 to 30 mA and the LED current range (IR) may be 6. 
     Charging port  160  can provide power to the components of the headset and optional wired connectivity. A cable may be removably coupled with the charging port in the headset and a power outlet to provide electrical connectivity to the headset. In some examples, the cable may remain attached to headset  100  during operation of the headset (e.g., in research and gaming environments where even millisecond delays have significance). 
     Embodiments are directed to a headset  100  including a curved frame  102  (e.g.,  FIG.  1   ) including: a frontal curved frame portion  104  configured to be worn on a forehead portion of a head of a user; and an upper curved frame portion  106  configured to be worn on an upper head portion of the head of the user. The headset  100  also includes: one or more PPG sensors  140  coupled to the frontal curved frame portion  104 ; One or more (capacitive non-dermal) EEG sensors (and a bias sensor/electrode) are coupled to the frontal curved frame portion  104  and/or upper curved frame portion  106 . One or more additional capacitive non-dermal EEG sensors  150  (also referred to throughout the disclosure as electrodes  150 ) may be optionally coupled to the posterior curved frame portion  108 . 
     In an embodiment, the one or more additional EEG sensors are coupled to only the upper curved frame portion. 
     In an embodiment, each of the PPG sensors  140  includes a curved outer surface, wherein the coupling location of each PPG sensor to the front curved frame portion  104  and a shape of the curved outer surface of each PPG sensor are configured to correspond with a corresponding location and curvature of the forehead or temporal portion of the head of the user. 
     In an embodiment, each of the EEG sensors  130  includes a curved outer surface, wherein the coupling location of each EEG sensor  130  to the frontal curved frame portion  104  and/or upper curved frame portion  106  and a shape of the curved outer surface of each EEG sensor  130  are configured to correspond with a corresponding location and curvature of the upper head portion of the head of the user. 
     In an embodiment, each of the EEG sensors  130  is a capacitive non-dermal contact type EEG sensor that is configured to be positioned either in direct contact with skin or be positioned over hair (i.e., not in direct contact with skin) on the head of the user, when the headset  100  is worn by the user. 
     In an embodiment, the frontal curved frame portion  104  includes two length-adjustable slider arms  110 A,  110 B that are retractable and extendable from opposite side portions of the curved frame. 
     In an embodiment, the frontal curved frame portion  104  includes an adjustable single frontal piece  3610  (see  FIG.  36    which is described more fully below) coupled between opposite side portions of the curved frame  102 , wherein a PPG sensor  3640  and EEG sensors  3630  are positioned along the single frontal piece  3610 . The single frontal piece  3610  may be adjustable with respect to the opposite side portions of the curved frame via rigid or flexible retractable/extendable slider arms  3612  or, alternatively, via elastic arms (not shown). 
     In an embodiment, the curved frame  102  further includes a posterior curved frame portion  108 . One or more additional EEG sensors  150  are coupled to the posterior curved frame portion  108 . 
     In an embodiment, the curved frame  102  further includes a posterior curved frame portion  108  including two posterior parts  108   a ,  108   b . The headset may further include one or more electrodes  150  coupled to at least one of the two posterior parts  108   a ,  108   b . As shown in  FIG.  36   , the corresponding two posterior parts  3608   a ,  3608   b  may be coupled together via an elastic member  3690 . 
     In an embodiment, each of the EEG sensors  130  (again, for example,  FIG.  1   ) may be removable or replaceable. 
     In an embodiment, the headset  100  further includes a charging port  160 . 
     In an embodiment, the headset  100  further includes one or more LED lights  120  indicating power, charging status, and/or connection to a host device. 
     In an embodiment, the headset  100  further includes an IMU sensor. 
     Electrode Embodiments 
     Various embodiments of electrode construction, placement, and design are described throughout the disclosure. For example,  FIGS.  7 - 9    illustrates various views of an electrode design, in accordance with one or more implementations of the invention. One or more electrodes  150  may be removably attached to headset  100  (and may be replaceable) and configured to receive brain waves. 
     In the projection view of an electrode design shown in  FIG.  7   , electrode top  710 , electrode middle  720 , electrode bottom  730 , and electrode side  740  are provided for illustrative purposes and should not be limiting to the disclosure. 
     In the isometric view of an electrode design shown in  FIG.  8   , electrode top  810  and electrode bottom  820  are provided for illustrative purposes and should not be limiting to the disclosure. 
     In the exploded view of an electrode design shown in  FIG.  9   , electrode top  910 , first adhesive  920 , first side of fastener  930 , first side of connector  932 , second side of fastener  940 , second adhesive  950 , foam  960 , third adhesive  970 , electrode  980 , and second side of connector  982 . These and other components of the exploded view of an electrode design are provided for illustrative purposes and should not be limiting to the disclosure. For example, foam  960  can provide some pressure from the headset to the electrode  910 , in order to help apply pressure to electrode  980  to remain in contact with the hair or head of the user. The pressure can help ensure a better fit and help conform electrode  980  (and headset  100  overall) to the curvature of portions of the head and/or irregular (or regular) bumps on a surface of the head. 
     Each of the electrodes  150  may have an elongated shape (e.g., rectangle or ellipse) or a non-elongated shape (e.g., square or circle), and/or a curved shape (e.g., a curvature to match a natural curvature of a surface of a head). The surfaces of electrodes  150  may be coated in, for example, gold, silver, silver chloride, or other conductive polymer. The texture of electrodes  150  may be smooth or any other texture that can increase a surface area of electrode  150 . 
     The configuration of each electrode  150  may form a low density (e.g., 2-channel system) to a high density (e.g., 256-channel system) array. For example, headset  100  may comprise a 8-channel EEG system with an active and reference electrode. In some implementations, each electrode  150  may correspond to a specific channel input of the scanner. For example, first electrode  150 A may correspond to a first channel, second electrode  150 B may correspond to a second channel, and so on. In some embodiments, each channel may have an active and reference electrode in a montage used in the differential amplification of the source signal. 
     The channels of each electrode may be configured to receive different components, for example such as delta, theta, alpha, beta, and/or gamma signals, each of which may correspond to a given frequency range. In a non-limiting example implementation, delta waves may correspond to signals between 0 and 3.5 Hz, theta waves may correspond to signals between 3.5 and 8 Hz, alpha waves may correspond to signals between 8 and 12 Hz, beta waves may correspond to signals between 12 and 30 Hz, and gamma waves may correspond to signals above 30 Hz. These example frequency ranges are not intended to be limiting and are to be considered exemplary only. 
     In some implementations, electrodes  150  may be attached at locations spread out across headset  100 . Electrodes  150  may be configured to detect electric potentials generated by the brain from the low ionic current given off by the firing of synapses and neural impulses traveling within neurons in the brain. These electric potentials may repeat or be synchronized at different spectral characteristics such as frequency and power according to the previously listed brain wave types (e.g. alpha and beta). These spectral characteristics of the brain waves may be separated from the single superimposed frequency signal detected at each electrode  150 . In various implementations, this isolation, separation, decomposition, or deconstruction of the signal is performed via spectral analysis. 
     In various implementations, headset  100  may be configured to receive raw EEG data generated by one or more electrodes  150 . In some implementations, headset  100  may be configured to perform initial signal processing on the detected brain waves. For example, headset  100  may be configured to run the raw EEG data through a high and low bandpass filter prior to the filtered data being run through a fast Fourier transform (FFT) to isolate the spectral frequencies of each channel. Each channel may be run through a high and low bandpass filter. In some implementations, headset  100  may be configured to perform error detection, correction, signal decomposition, signal recombination, and other signal analysis. Accordingly, headset  100  may be configured to filter, analyze, and/or otherwise process the signals captured by one or more electrodes  150 . 
     In some examples of the “10-20 international system” of electrode placement, Channel 1 may correspond to the F p   1  location and Channel 2 may correspond to Fp2. The active electrode may be placed along the frontal curved portion and the reference electrode may be placed on the temporal region. As described herein, filtered data for each channel may be run through spectral analysis to isolate the spectral frequencies of each channel. The power of the theta (e.g., 4-7 Hz), alpha (e.g., 8-12 Hz), beta (e.g., 13-20 Hz), and gamma (e.g., 21-50 Hz) components of each channel for a given sampled timeframe (e.g., 3 seconds) may be determined. The power of each of the isolated components may be used to generate a numerical output of the data that may be used for graphical visualization or incorporated into algorithms for brain-related measurements. 
     In some examples, electrodes  150  may collect data using noninvasive, electrical brain signal measurements absent the use of an interface material between electrode  150  and the skin (e.g., an electrolyte, in a EEG gel or paste form, etc.). The user may not need to add gel or saline solution to improve signal quality of the capacitive non-dermal contact electrodes  150 . 
     Each electrode  150  may be coupled with foam, spring, gel-containing material, or other support that can provide some pressure from the headset to the electrode  150 , in order to help apply pressure to electrode  150  to remain in contact with the hair or head of the user. The pressure can help ensure a better fit and help conform electrode  150  (and headset  100  overall) to the curvature of portions of the head and/or irregular (or regular) bumps on a surface of the head. 
     Electrodes  150  may be placed at various locations of headset  100  based on general head anthropometry and experimental trials. Electrodes  150  may be curved. The curvature of electrodes  150  may vary based on the placement of the electrode location. Different head parts may correspond with different curvatures. In some examples, eight electrodes may be placed around the headset in various designs and utilitarian functions for non-dermal EEG systems. 
       FIG.  10    illustrates a first layout of a plurality of electrodes and a curvature of the headset, in accordance with one or more implementations of the invention. In this illustration, a user’s head  1010  is provided relative to placement of one or more electrodes  1020 , illustrated as first electrode  1020 A, second electrode  1020 B, and third electrode  1020 C, and F p   1 /F p   2  location electrodes  1030 . 
       FIG.  11    illustrates a top view of an electrode in the first layout of  FIG.  10   . For example, the height  1110  of the electrode may be around 49 mm with a curvature of around 20° and a focal length of around 71 mm. 
       FIG.  12    illustrates a side view of an electrode in the first layout of  FIG.  10   . For example, the initial depth  1210  of the electrode may be around 0.5 mm and the secondary depth may be around 0.8 mm. 
       FIG.  13    illustrates a back view of an electrode in the first layout of  FIG.  10   . For example, the width of the innermost portion  1310  of the electrode may be around 8 mm, the width of the secondary portion  1320  of the electrode may be around 12 mm, and the width of the outermost portion  1330  of the electrode may be around 15 mm. 
       FIG.  14    illustrates a projected view of an electrode in the first layout of  FIG.  10   . In some examples, the height of the electrode is 50 mm and the width of the electrode is 15 mm. 
       FIG.  15    illustrates a second layout of a plurality of electrodes and a curvature of the headset, in accordance with one or more implementations of the invention. In this illustration, a user’s head  1510  is provided relative to placement of one or more electrodes  1520 , and F p   1 /F p   2  location electrodes  1530 . 
       FIG.  16    illustrates a top view of an electrode in the second layout of  FIG.  15   . For example, the height  1610  of the electrode may be around 50 mm with a curvature of around 12°. 
       FIG.  17    illustrates a side view of an electrode in the second layout of  FIG.  15   . For example, the depth  1710  of the electrode may be around 0.5 mm. 
       FIG.  18    illustrates a back view of an electrode in the second layout of  FIG.  15   . For example, the width of the innermost portion  1810  of the electrode may be around 8 mm, the width of the secondary portion  1820  of the electrode may be around 12 mm, and the width of the outermost portion  1830  of the electrode may be around 15 mm. The height of the innermost portion  1840  may be around 14 mm and the height of the secondary portion  1850  may be around 20 mm. 
       FIG.  19    illustrates a projected view of an electrode in the second layout of  FIG.  15   . In some examples, the height of the electrode is 50 mm and the width of the electrode is 15 mm. 
       FIG.  20    illustrates a third layout of a plurality of electrodes and a curvature of the headset, in accordance with one or more implementations of the invention. In this illustration, a user’s head  2010  is provided relative to placement of one or more electrodes  2020 , illustrated as first electrode  2020 A and second electrode  2020 B, and F p   1 /F p   2  location electrodes  2030 . 
       FIG.  21    illustrates a top view of an electrode in the third layout of  FIG.  20   . For example, the height  2110  of the electrode may be around 40 mm with a curvature of around 12 °. 
       FIG.  22    illustrates a side view of an electrode in the third layout of  FIG.  20   . For example, the depth  2210  of the electrode may be around 0.5 mm. The width of the innermost portion  2220  of the electrode may be around 10 mm and the width of the outermost portion  2230  of the electrode may be around 12 mm. 
       FIG.  23    illustrates a back view of an electrode in the third layout of  FIG.  20   . For example, the width of the innermost portion  2310  of the electrode may be around 6 mm, the width of the secondary portion  2320  of the electrode may be around 10 mm, and the width of the outermost portion  2330  of the electrode may be around 12 mm. The height of the innermost portion  2340  may be around 14 mm and the height of the secondary portion  2350  may be around 20 mm. 
       FIG.  24    illustrates a projected view of an electrode in the third layout of  FIG.  20   . In some examples, the height of the electrode is 40 mm and the width of the electrode is 12 mm. 
     In any of the embodiments described throughout the disclosure, including the embodiments illustrated in  FIGS.  10 - 24   , electrodes  150  may be replaceable. For example, headset  100  may have one or more ports to plug in an electrode into headset. When an electrode becomes inoperable, the electrode may be unplugged from the port and replaced with a new electrode for easy replacement. 
     Electrodes  150  may have an elongated shape (e.g., rectangle or ellipse) and/or a curved shape (e.g., a curvature to match natural curvature of surface of a head) as illustrated. Electrode  150  surfaces may be coated in gold, silver, silver chloride, or other conductive polymer. The texture of electrodes  150  may be smooth or any other texture that can increase a surface area of the electrode. 
     An illustrative process for replacing electrodes is illustrated in  FIG.  25   . In this illustration, headset  2500  is provided with a replaceable EEG electrode  2510 . Headset  2500  and electrode  2510  may be similar to headset  100  and electrode  150  illustrated in  FIG.  1   . For example, three connection points may be connected to electrode  2510 . When electrode  2510  is communicatively coupled with headset  2500 , the data signals, power signals, and other communications may be transmitted between electrode  2510  and headset  2500 . 
     In some examples, the connection points may be ports to plug in electrode  2510  into headset  2500 . When electrode  2510  becomes inoperable, electrode  2510  may be unplugged from the port of headset  2500  (e.g., each of the three connection points) and replaced with a new electrode for easy replacement. 
     3D Angular Mapping 
     Headset  100  may be used to collect data using an IMU within the headset as a control system, which may also perform the analysis and processing. The IMU may include an accelerometer, gyroscope, feature extraction, sensor fusion, and other components or engines for generating data. The data can be stored, inferred, or retrieved from a memory incorporated with headset  100  as well. In some examples, the data may be transmitted to a computer system for processing and analysis. 
     The IMU may be an electronic device incorporated with headset  100  that measures and reports the specific force, angular rate, and orientation of a user’s body (e.g., head). For example, this data may be generated using one or more of an accelerometer, gyroscope, or magnetometer as components of the IMU. In some examples, six degrees of freedom (DOF) may be measured by the IMU, including acceleration, force, and angular velocity acting upon the X, Y, and Z axis. 
     In some examples, the IMU may be mounted on the motherboard (e.g., PCB board) which will be located on the expanded area extending behind the ear and wrapping around the back of the head. In some examples, the IMU may be mounted on the right side of the headset or the left side of the headset. In some examples, the exact location of the IMU may not matter. 
     An illustrative example of generating inertial data is provided with  FIG.  26    in relation to the X, Y, and Z axis. The data may comprise a roll or pitch along the X-axis, the linear acceleration along the X or Y axis, and the yaw, heading, or gravity direction along the Z-axis. 
     An illustrative example of generating accelerometer data is provided with  FIG.  27   . In this illustration, one or more electrodes  150  are installed with headset  100  and the headset incorporates an accelerometer. The accelerometer may comprise anchor  2710 , fixed electrodes  2720 , movable seismic mass  2730 , and tether or spring  2740 . A differential capacitor pair  2750  is also illustrated to show the relation of the movable seismic mass  2730  to be fixed electrodes  2720  during acceleration. 
     For example, the accelerometer may measure acceleration (e.g., the rate of change of the velocity of an object). The accelerometer of headset  100  may measure the acceleration in meters per second squared (m/s 2 ) or in G-forces (g) by sensing either static forces (e.g., gravity) or dynamic forces (e.g., vibrations and movement) of acceleration. Additionally, accelerometers are useful for sensing vibrations in systems or for orientation applications. 
     An illustrative example of generating gyroscope data is provided with  FIG.  28   . The gyroscope of headset  100  may measure rotational motion microelectromechanical system (MEMS) or angular velocity (e.g., in degrees per second) or the rate of change of the angular position over time (angular velocity) with a unit of (deg./s). Gyroscopes are useful for sensing an angle of rotation in systems or for orientation applications. 
     In some examples, feature extraction may be implemented, including instantaneous force exerted on object (e.g., using accelerometer data) or instantaneous angle of object (e.g., using gyroscope data). For example, an angle of an object can be calculated by integrating gyroscope data over time t. To obtain the angular position, the angular velocity may be integrated with t=0 theta=0. The angular position can be determined at any moment t with the following equation: 
     
       
         
           
             θ 
             
               t 
             
             = 
             
               
                 
                   ∫ 
                   0 
                   t 
                 
                 
                   
                     θ 
                     ˙ 
                   
                   
                     t 
                   
                   d 
                   t 
                 
               
             
             ≈ 
             
               
                 ∑ 
                 0 
                 t 
               
               
                 
                   θ 
                   ˙ 
                 
                 
                   t 
                 
                 
                   T 
                   s 
                 
               
             
           
         
       
     
     In some examples, the angle of an object can be calculated from the accelerometer data using the gravity vector (e.g., gravitational force acting upon the sensor at all times). Along with the gravitational force, other systems may act upon object as well and accelerometer data may be filtered to remove high frequency noise (e.g., caused due to vibration or shocks). 
     In some examples, sensor fusion may be implemented (e.g., to bring together inputs from accelerometer and gyroscope to form a single model). The data may include drift over time due to continuous integration over time and accelerometer data observed at instant time t and/or stable over a long time interval (e.g., using a complementary filter or Kalman filter). The output of the analysis may include a 3D angular position of an object that can be mapped in 3D space using pitch, roll, and yaw values. The analysis may also consider whether the user is stationary or in motion. 
       FIG.  29    illustrates 3D angular mapping using various components of the headset, in accordance with one or more implementations of the invention. Headset  100  illustrated in  FIG.  1    may implement the process described herein. 
     At block  2910 , the IMU may collect data from an accelerometer, gyroscope, feature extraction, sensor fusion, and other components or engines of the headset. 
     At block  2920 , the microcontroller unit (MCU) may receive sensor information via the inter-integrated circuit (I 2 C) or serial peripheral interface (SPI). 
     At block  2930 , the data may be calibrated. 
     At block  2940 , the data may be pre-processed and/or filtered. 
     At block  2950 , the data may be provided for sensor fusion to combine the sensory data or data derived from disparate sources. The resulting information may have less uncertainty than would be possible when these sources were used individually. 
     At block  2960 , the roll, pitch, and yaw calculation may be implemented. 
     At block  2970 , the 3D angular motion mapping may be implemented. 
       FIG.  30    illustrates the user’s head movement for generating data, in accordance with one or more implementations of the invention. The headset may move along the X, Y, and Z axis. While the user wears the headset  100 , the headset may record data along each axis. 
     Headset as a Human Computer Interface 
     Headset  100  may receive the data from the sensors and implement controls based on the data. As an illustrative example, the headset may be moved along an X and Y axis and, on a corresponding graphical user interface provided at a display coupled with the computer system, an object can be moved in accordance with the headset movement. In other words, the headset may be used to control the object using head motion as an alternative human-computer interface device. 
       FIG.  31    illustrates a process of converting data to interactions with a desktop display, in accordance with one or more implementations of the invention. 
     At block  3110 , the IMU may collect data from an accelerometer, gyroscope, feature extraction, sensor fusion, and other components or engines of the headset. 
     At block  3120 , the microcontroller unit (MCU) may receive sensor information via the inter-integrated circuit (I 2 C) or serial peripheral interface (SPI). 
     At block  3130 , the data may be calibrated. 
     At block  3140 , the data may be pre-processed and/or filtered. 
     At block  3150 , the movement of object (e.g., displacement) over 2 axis (e.g., X and Y, or X and Z) is calculated over time interval dt. For example, the derivatives of x, y, or z (e.g., Dx, Dy, and Dz) may hold information of movement of headset  100  in the x, y, or z direction over the time interval dt. 
     At block  3160 , various methods of output may be implemented, including wired or wireless communication protocols. When wireless communication is implemented, the derivative data may be transmitted using the Bluetooth/BLE protocol with HID profile, which is transmitted wirelessly to a client device or desktop computer. When wired communication is implemented, the derivative data may be transmitted using a native USB protocol with HID support, which is transmitted through the wired USB connection to a client device or desktop computer. 
     System Visualizations and Architecture 
     One or more other visualizations may be generated using the techniques described herein. For example, in some implementations, a computer system may utilize the various components and techniques described in U.S. Pat. Application No. 17/411,676, the entirety of which is incorporated by reference. 
     Example Processes Performed With the Headset 
     Headset  100  may perform various processes with the sensor data. for example, headset  100  may implement an optimized adaptive spectrum noise cancellation to remove motion artifacts from PPG data, non-invasive heart rate and respiration rate estimation, adaptive spectrum noise cancellation (ASNC), frequency domain artifact removal, or real-time motion artifact removal. 
     Headset  100  may perform real-time motion artifact removal as the user is in motion while using headset  100 . The motion artifact affects the frequency spectrum especially in heart rate and respiration rate frequency range. To prevent false identification of heart rate and respiration rate, an adaptive spectrum noise cancellation algorithm may be used to remove the motion noise from the spectrum. In order to enable online processing, the process may implement spectrum denoising, heart rate calculation, and respiration rate calculation before the next data is received to a data buffer. Faster calculations can be achieved by reducing the number of iterations required in mathematical calculations. 
     In PPG, useful information may be identified in very low to low frequency regions (e.g., 0.01 Hz to 10 Hz). Any frequency data after 10 Hz may be removed to achieve a faster execution time and lower use of memory. This may also optimize DCT and Hilbert transform calculations to the required frequency range. By removing the unnecessary data, the process may perform faster than other algorithms to remove the motion activity of the head of the user with noise cancellation. 
     The first stage may implement digital filters to filter signals out of frequency of interest. Each data may pass through different filters for heart rate, respiration rate, and SpO2 calculations. This filtered data may be transformed to frequency spectrum using Discrete Fourier Transform (DFT). The process may also implement discrete cosine transform (DCT) from a frequency range 0 Hz to 5 Hz for accelerometer data, red PPG data, and IR PPG data. The process may also implement a Hilbert transform function to envelope a noisy PPG signal from 0 Hz to 5 Hz for accelerometer data, red PPG data, and IR PPG data. The process may also implement a Moore-Penrose inverse of accelerometer data. The adaptive gain may be calculated based on the envelope of accelerometer data and IR signal. Using the calculated adaptive gain value, the accelerometer data may be scaled up to match the magnitude of the PPG spectrum. The final cleaning of the motion artifact spectrum from IR PPG spectrum may be implemented to remove false motion induced peaks from PPG spectrum. 
       FIG.  32    illustrates an example of a process of calculating a heart rate, in accordance with one or more implementations of the invention. 
     At block  3210 , the process begins. 
     At block  3212 , the process receives the PPG and MEMS raw data. 
     At block  3214 , the process identifies the PPG raw data. 
     At block  3216 , the process provides the PPG raw data to the IIR bandpass filter. 
     At block  3218 , the process provides the data to a discrete cosine transform (DCT). 
     At block  3220 , the process provides the data to the envelop detection. 
     At block  3230 , the process identifies the accelerometer raw data. 
     At block  3232 , the process provides the accelerometer raw data to the IIR bandpass filter. 
     At block  3234 , the process identifies the gyroscope raw data. 
     At block  3236 , the process provides the gyroscope raw data to the IIR bandpass filter. 
     At block  3240 , the process provides the accelerometer data and the gyroscope data to the motion artifacts estimation. 
     At block  3242 , the process provides the data to a discrete cosine transform (DCT). 
     At block  3244 , the process provides the data to the envelop detection. 
     At block  3250 , the process provides the data is provided to the adaptive spectrum noise cancellation. 
     At block  3252 , the process provides the data to a spectrum PPG signal without Mas. 
     At block  3254 , the process calculates a heart rate by spectrum peak. 
     At block  3260 , the process ends. 
       FIG.  33    illustrates an example of a process of calculating a heart rate and/or respiration rate, in accordance with one or more implementations of the invention. 
     At block  3310 , the process begins by receiving red data, IR data, or accelerometer data. 
     At block  3312 , the process identifies the red data. 
     At block  3314 , the process identifies the IR data. 
     At block  3316 , the process stores a plurality of samples (e.g.,  100 ) of IR data and red data into an array. 
     At block  3318 , the process calculates DC_Val = DCT at 0 Hz. 
     At block  3320 , the process implements the DC blocker. 
     At block  3322 , the process determines a second order Butterworth Bandpass filter (e.g., 0.5 Hz - 5 Hz). 
     At block  3324 , the process determines a DCT and Hilbert Transform (e.g., 0 Hz - 5 Hz). 
     At block  3326 , the process calculates AC_Val = Amplitude of heart rate peak (e.g., 0.7 Hz - 4 Hz). 
     At block  3328 , the process calculates R = (AC_Red/DC_Red)/(AC_IR/DC_IR) and SPO2 = 94.485 + (30.354*R)-(45.06*R*R) 
     At block  3330 , the process stores a plurality of samples (e.g., 3,000) of IR data into an array (e.g., IR[3000]). 
     At block  3332 , the process implements the DC blocker. 
     At block  3334 , the process determines a second order Butterworth Bandpass low pass filter (e.g., 5 Hz). 
     At block  3336 , the process determines a DCT (e.g., 0 Hz - 5 Hz). 
     At block  3338 , the process determines a Hilbert Transform of DCT (e.g., 0 Hz - 5 Hz). 
     At block  3350 , the process identifies the accelerometer data. 
     At block  3352 , the process stores a plurality of samples (e.g., 3,000) of accelerometer data into an array (e.g., ax[3000], ay[3000], and az[3000]). 
     At block  3354 , the process implements the DC blocker. 
     At block  3356 , the process calculates a = (ax^2 + ay^2 + az^2)^(0.5). The result may be stored into array a[3000]. 
     At block  3358 , the process determines a second order Butterworth Bandpass filter (e.g., 0.07 Hz - 5 Hz). 
     At block  3360 , the process determines a DCT (e.g., 0 Hz - 5 Hz). 
     At block  3362 , the process determines a Hilbert Transform of DCT (e.g., 0 Hz - 5 Hz). 
     At block  3364 , the process implements Moore Penrose-pseudo inverse of M(M+^). 
     At block  3366 , the process implements Adaptive gain(H) = (M+^)*(S). 
     At block  3370 , the process implements Scaled Motion artifacts (W-) = H*M. 
     At block  3372 , the process implements Clean PPG (Y) = S - (W-). 
     At block  3374 , the process calculates the frequency of max peak in freq(0.7 - 4 Hz) = HR_freq. 
     At block  3376 , the process calculates the heart rate as 60 * HR_freq. 
     At block  3378 , the process calculates the frequency of max peak in freq(0.083 - 5 Hz) = RR_freq. 
     At block  3380 , the process calculates the respiration rate as 60 * RR_freq. 
       FIG.  34    illustrates an example of a process of determining LED current, in accordance with one or more implementations of the invention. 
     At block  3410 , the process begins. 
     At block  3420 , the process sets a default LED current 
     At block  3430 , the process determines a subset of data (e.g., one second of data). 
     At block  3440 , if the DC value is greater than a desired value, the process proceeds to block  3450 . If not, the process proceeds to block  3460 . 
     At block  3450 , the process reduces i LED by 2 mA. 
     At block  3460 , if the DC value is less than a desired value, the process proceeds to block  3470 . If not, the process proceeds to block  3480 . 
     At block  3470 , the process increases i LED by 2 mA. 
     At block  3480 , the process ends. 
       FIG.  35    illustrates an example of a process of determining a good or improper signal, in accordance with one or more implementations of the invention. 
     At block  3510 , the process begins 
     At block  3520 , the process receives a plurality of sample data (e.g., 100 data samples). 
     At block  3530 , the process calculates the mean and standard deviations. 
     At block  3540 , the process calculates the SNR as 20*log(Mean / standard deviation) dB. 
     At block  3550 , if the SNR value is greater than  120  and SD is less than  350 , the process proceeds to block  3580 . If not, the process proceeds to block  3570 . 
     At block  3570 , the process determines an improper signal. 
     At block  3580 , the process determines a good signal. 
     At block  3590 , the process receives a next sample and proceeds back to block  3510 . 
     For purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the description. It will be appreciated by those having skill in the art that the implementations described herein may be practiced without these specific details or with an equivalent arrangement. Accordingly, it is to be understood that the technology is not limited to the disclosed implementations, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present technology contemplates that, to the extent possible, one or more features of any implementation can be combined with one or more features of any other implementation. 
     Implementations of the disclosure may be made in hardware, firmware, software, or any suitable combination thereof. Aspects of the disclosure may be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a tangible computer readable storage medium may include read only memory, random access memory, magnetic disk storage media, optical storage media, flash memory devices, and others, and a machine-readable transmission media may include forms of propagated signals, such as carrier waves, infrared signals, digital signals, and others. Firmware, software, routines, or instructions may be described herein in terms of specific exemplary aspects and implementations of the disclosure, and performing certain actions. 
     The various instructions described herein are exemplary only. Other configurations and numbers of instructions may be used, so long as the processor(s) are programmed to perform the functions described herein. The description of the functionality provided by the different instructions described herein is for illustrative purposes, and is not intended to be limiting, as any of instructions may provide more or less functionality than is described. For example, one or more of the instructions may be eliminated, and some or all of its functionality may be provided by other ones of the instructions. 
     In some implementations, the headset may comprise one or more processing units. These processing units may be physically located within the same device. In some implementations, one or more processors may be implemented by a cloud of computing platforms operating together as one or more processors. Processor(s) be configured to execute one or more components by software; hardware; firmware; some combination of software, hardware, and/or firmware; and/or other mechanisms for configuring processing capabilities on processor(s). As used herein, the term “component” may refer to any component or set of components that perform the functionality attributed to the component. This may include one or more physical processors during execution of processor readable instructions, the processor readable instructions, circuitry, hardware, storage media, or any other components. Furthermore, it should be appreciated that various instructions may be executed locally or remotely from the other instructions. 
     The various instructions described herein may be stored in a storage device, which may comprise random access memory (RAM), read only memory (ROM), and/or other memory. For example, one or more storage devices may comprise any tangible computer readable storage medium, including random access memory, read only memory, magnetic disk storage media, optical storage media, flash memory devices, and/or other memory configured to computer-program instructions. In various implementations, one or more storage device may be configured to store the computer program instructions (e.g., the aforementioned instructions) to be executed by the processors as well as data that may be manipulated by the processors. The storage device may comprise floppy disks, hard disks, optical disks, tapes, or other storage media for storing computer-executable instructions and/or data. 
     One or more databases may be stored in one or more storage devices. The databases described herein may be, include, or interface to, for example, an Oracle™ relational database sold commercially by Oracle Corporation. Other databases, such as Informix™, DB2 (Database 2) or other data storage, including file-based, or query formats, platforms, or resources such as OLAP (On Line Analytical Processing), SQL (Structured Query Language), a SAN (storage area network), Microsoft Access™ or others may also be used, incorporated, or accessed. The database may comprise one or more such databases that reside in one or more physical devices and in one or more physical locations. The database may store a plurality of types of data and/or files and associated data or file descriptions, administrative information, or any other data. 
     The various components illustrated throughout the disclosure may be coupled to at least one other component via a network, which may include any one or more of, for instance, the Internet, an intranet, a PAN (Personal Area Network), a LAN (Local Area Network), a WAN (Wide Area Network), a SAN (Storage Area Network), a MAN (Metropolitan Area Network), a wireless network, a cellular communications network, a Public Switched Telephone Network, and/or other network. Furthermore, according to various implementations, the components described herein may be implemented in hardware and/or software that configure hardware. 
     In some instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the description. In other instances, functional block diagrams and flow diagrams are shown to represent data and logic flows. The components of block diagrams and flow diagrams (e.g., modules, blocks, structures, devices, features, etc.) may be variously combined, separated, removed, reordered, and replaced in a manner other than as expressly described and depicted herein. 
     Although embodiments are described above with reference to replaceable EEG sensor(s), the EEG sensor(s) described in any of the above embodiments may alternatively be another type of EEG sensor such as a fixed EEG sensor, removable EEG sensor, disposable EEG sensor, etc. Similarly, the PPG sensors and the electrodes described in any of the above embodiments may be replaceable, fixed, removable, and/or disposable. Such alternatives are considered to be within the spirit and scope of the present invention, and may therefore utilize the advantages of the configurations and embodiments described above. 
     In addition, although embodiments are described above with reference to a frontal piece including two separated adjustable slider arms  110 A,  110 B, the frontal piece described in any of the above embodiments may alternatively be an adjustable single frontal piece  3610  coupled between opposite side portions of the curved frame, wherein the PPG sensor  3640  and two EEG sensors (3630) along with a bias sensor  3631  (which acts as ground, for example) are positioned along the single frontal piece  3610 , as illustrated in  FIG.  36   .  FIG.  36    also illustrates an optional elastic member  3690  that couples the two posterior parts  3608   a ,  3608   b  of the posterior curved frame portion together. The elastic member  3690  aids in maintaining adequate tension of the headset across the head so there is sufficient pressure on the electrodes against the head to improve signal quality. The two posterior parts may further optionally be separated with a gap (i.e., without a connection or coupling to each other) when worn by the user, as illustrated in  FIG.  4   . As a yet further option, the two posterior parts may be a unitary structure. Such alternatives are considered to be within the spirit and scope of the present invention, and may therefore utilize the advantages of the configurations and embodiments described above. 
     Further, although embodiments are described above with reference to sensors  140 ,  3640  being PPG sensors, any or all the PPG sensors described in any of the above embodiments may alternatively be EEG sensors (and can utilize the locations of those PPG sensors). Such alternatives are considered to be within the spirit and scope of the present invention, and may therefore utilize the advantages of the configurations and embodiments described above. 
     The method steps in any of the embodiments described herein are not restricted to being performed in any particular order. Also, structures or systems mentioned in any of the method embodiments may utilize structures or systems mentioned in any of the device/system embodiments. Such structures or systems may be described in detail with respect to the device/system embodiments only but are applicable to any of the method embodiments. 
     Features in any of the embodiments described in this disclosure may be employed in combination with features in other embodiments described herein, such combinations are considered to be within the spirit and scope of the present invention. 
     The contemplated modifications and variations specifically mentioned in this disclosure are considered to be within the spirit and scope of the present invention. 
     Reference in this specification to “one implementation”, “an implementation”, “some implementations”, “various implementations”, “certain implementations”, “other implementations”, “one series of implementations”, or the like means that a particular feature, design, structure, or characteristic described in connection with the implementation is included in at least one implementation of the disclosure. The appearances of, for example, the phrase “in one implementation” or “in an implementation” in various places in the specification are not necessarily all referring to the same implementation, nor are separate or alternative implementations mutually exclusive of other implementations. Moreover, whether or not there is express reference to an “implementation” or the like, various features are described, which may be variously combined and included in some implementations, but also variously omitted in other implementations. Similarly, various features are described that may be preferences or requirements for some implementations, but not other implementations. 
     The language used herein has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. Other implementations, uses, and advantages of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The specification should be considered exemplary only, and the scope of the invention is accordingly intended to be limited only by the following claims.