# Biosignals Monitoring of First Responders for Cognitive Load Estimation in Real-Time Operation
## 2 Theoretical Framework
### Cognitive Load  
In the 1980s, CL was used as a theoretical approach to explain experimental results with very little attempt to directly measure mental load [40]. The theory was used to predict differential learning using particular instructional designs. The first rating scale was introduced in the early 1990s by [PERSON], to provide an overall measure of CL [41]. This measure is a subjective scale that has been extensively used showing good psychometric properties. The main advantages of subjective techniques over physiological techniques are their sensitivity and simplicity. Despite the wide use of psychometrically robust tools, there are limitations regarding the concurrent validity of self-report tools [42].  
Nowadays, the most used self-report instrument is the NASA Task Load Index (NASA-TLX) [43]. The NASA-TLX is a subjective, multidimensional, and widely used assessment tool [44] that rates perceived workload. The total workload is divided into six subjective subscales: mental demand, physical demand, temporal demand, performance, effort, and frustration. Self-perceived cognitive load is rated from 0 to 100 points. The overall workload is calculated by weighting, adding, and averaging each domain rating.  
CL researchers have been using real-time objective and physiological measurements as secondary task techniques. These techniques use performance on a secondary task as an indicator of CL imposed by a primary task, based on the idea of the limited capacity of the working memory. In more recent times, due to advances and availability of new technologies, physiological measurements have become more popular in CL research, specifically [45]:  
* **Cardiovascular and respiratory measurements**: heart rate, heart rate variability, respiratory measurements, blood volume pulse, inter-beat interval, and other cardiovascular features.
* **Eye activity**: pupil dilation, blink rate, fixation, eye-tracking, and other ocular indices. Pupil diameter can be used as an indicator of informational mental strain [46]. Protective eye reflexes at first regulate the incidence of light; increasing mental strain leads to wider pupils; if the parasympathetic system is active, pupil diameter decreases; the action of the sympathetic system leads to an increase in the pupil diameter.  
The blinking rate can be also used as an indicator of mental strain. A decreasing blinking rate is connected to the perception of visual information and a high interest. An increasing blinking rate is an indicator of advanced training, little interest, increasing fatigue, or non-relevant information.  
* **Electrodermal and temperature measurements**: Electrodermal activity (EDA) sensors measure sympathetic nervous system arousal and features related to stress, engagement, and excitement. The infrared thermophile sensor reads body skin temperature.
* **Brain activity**: electroencephalography (EEG) measures electrical potential from the cortex, functional magnetic resonance imaging (fMRI) and functional near-infrared spectroscopy (fNIRS) measure changes in blood flow that occurs with brain activity.  
Therefore, there is considerable evidence that supports that changes in CL can be detected by physiological measures generated by differences in task complexity. Nonetheless, physiological measurements are more sensitive than others to find CL differences across tasks; the most sensitive physiological measures are blink rates, heart rate, pupil dilations, and alpha waves. Regarding subjective measures, it is found that compared with most physiological measures, subjective measurements have a higher level of variability.