# Estimation of Mental Distress from Photoplethysmography
## 1 Introduction  
Mental distress (or psychological distress) is a general term used to describe unpleasant feelings or emotions that impact your level of functioning. In other words, it is psychological discomfort that interferes with your activities of daily living. Mental distress can result in negative views of the environment, others, and the self. This is why it is important to investigate on devices and environments capable of recognizing and/or regulating negative emotions [1; 2; 3; 4; 5]. Sadness, anxiety, distraction, and symptoms of mental illness are manifestations of psychological distress. Mental stress is accompanied by dynamic changes in activity of the autonomic nervous system (ANS). Although mental stress cannot be measured directly, the physiological response can be interpreted to assess the level of mental stress. Several physiological parameters (like electroencephalograph, heart rate variability, blood pressure, event-related potentials, and electromyography, among others) have been found sensitive toward any changes occurring in mental stress level [6]. Moreover, heart rate variability (HRV), the quantification of beat-to-beat variability in cardiac cycle over time, is one of the most determinant measures of ANS status [7; 8]. The heart rate (HR) represents successive heart polarization and depolarization caused by the electrical impulses generated on the sinoatrial node and transmitted to the ventricles [9]. The sympathetic nervous system increases HR in response to stress, exercise or heart disease byacting on the accelerans nerve [10]. On the contrary, the parasympathetic nervous system decreases HR through acting on the vagus nerve [11]. During ventricular polarization, blood is pumped into the cells throughout the circulatory system. This process is reflected in an electrocardiogram as the QRS complex, where R-peaks are the most significant points within this wave. Accordingly, R-peaks are used as reference for computing HR. Indeed, HR is defined as a time series sequence of non-uniform RR intervals [12]. One of the most extended methodologies to measure HR consists in quantifying blood volume changes caused by the circulatory system functioning in veins or capillaries [13; 14; 15]. This technique is based in photoplethysmography (PPG), which consists of a low-cost optical technique capable of measuring small variations in reflected/transmitted light intensity, associated with changes in blood pumping function [16]. Technology has enabled to measure HR using inexpensive PPG sensors. By using robust software running a good algorithm it is possible to measure HR and HR related parameters like HRV, and in turn stress [17; 18; 19]. In this respect, blood movement in vessels goes from heart to fingertips in a wave-like motion, generating a lag by the time required for transmission of the pulse wave between heart pumping, depicted in EEG as QRS complex, and maximum blood volume in the vessels, depicted in PPG signals as PPG-peak (systolic peak). Although there is no time synchronization between R-peak and PPG-peak due to time lag, some authors have used peak-to-peak PPG signal interval instead of R-R interval in ECG signal. The use of pulse cycle interval is often called pulse rate variability (PRV) for this purpose. While there are many studies that analyze the use of PRV as an estimation of HRV, it is difficult to obtain quantitative conclusions due to the differences among experiments and methodologies. However, a high correlation between both metrics has been reported for subjects at rest [20]. Our research team has decided to design, build, and assess its own wearable photoplethysmography sensor aimed at exploiting HRV for the sake of estimating mental distress. In this case, estimating mental distress it is not conceived at this point of the design to be used in clinical applications.