# Biosignals Monitoring of First Responders for Cognitive Load Estimation in Real-Time Operation
## 3 Materials and Methods
### Physiological Variables Selection  
As aforementioned, several biosignals have been identified in the literature with physiological variables that correlate with physical and mental activities. Among them, the following variables are selected for monitoring: electrodermal activity (EDA), heart rate (HR), respiration rate (RR), body temperature, eye gaze, and physiological face changes. These variables have been selected because of the following reasons. For instance, a person's temperature is found to have a correlation with their cognitive workload [47, 48]. Moreover, EDA can be exploited to predict a person's cognitive load in real time [49]. It can also be used to ascertain whether or not a person is under stress [50]. The sympathetic nervous system controls the sweating of the skin. Depending on the sweating level, the skin resistance varies. Therefore, by measuring skin conductance, the emotional and sympathetic responses can also be measured. The human body does not have a constant number of sweat glands per cm\({}^{2}\). The densest areas are in the hands and feet. Consequently, EDA measurements are typically collected from these areas [51]. In addition, HR is quite sensitive in discriminating mental workload [52, 53]. In most modern devices, a heartbeat can be measured by using photoplethysmography (PPG). PPG sensors generally consist of two or more optical emitters which send light waves to the skin. The light is refracted as it enters the skin, and it is captured by the photodetectors. Measuring HR with PPG faces some challenges, with noise in the area where the sensor is placed being the biggest. For example, if the sensor is placed on the wrist, the light from the emitter is also scattered to other parts of the area such as skin, muscles, and tendons. Moreover, the amount of refracted light depends on the person's skin tone (light or dark). Thus, although two people with different skin tones can have the same blood volume, the readings will be different. In addition to the optical devices, an accelerometer is used for extracting the heartbeat. Motion artifacts are introduced in the PPG measurements. With the use of an accelerometer, the motion noise can be removed from the PPG signal. Accelerometers include a mass attached in a spring. When the mass moves, the spring extracts and compresses. By measuring the spring's compression, acceleration is measured. Nonetheless, studies have shown that respiration rate (RR) is directly related to CL [54]. Monitoring respiratory health is possible with several methods including spirometry, full body plethysmography [55], and even arterial blood sampling. These methods work very well in laboratory environments but are difficult to use during other activities. RR can also be measured by attaching optical [56] or strain [57] sensors in the chest area. Therefore,a wearable device that can be attached to the chest is required. The device measures the differential size generated by the expansion and contraction of the thoracic cavity, calculating the RR. The latest studies have also shown that the movement of the eyes is directly related to the CL [46]. Among them, pupillary movements are the most studied physiological aspects related to CL [58]. Unfortunately, it is not possible to measure pupillary movement on real rescue applications, because wearable sensors are not commercially mature or compatible with personal protection equipment (PPE) needed in real scenarios. Nonetheless, eye tracking is also one physiological measuring technique that has received attention over the last decades [59]. According to [60; 61], it should be feasible to differentiate between the mental demands of various interfaces or system designs by depending on eye-tracking-related data. In this context, we aim to evaluate the potential of eye-gaze as a valuable input (among others) to infer CL on FRs. Inference models would then be grounded on such indirect input to estimate FRs' cognitive load and present information in the most suitable form.