## 2 Materials and Methods
### PPG Signal  
The original PPG signals used in this work come from 40 students from the UPM (Universidad Politecnica de Madrid), healthy young people between the ages of 18 and 30, who participated in a study of a national investigation that tried to evaluate the degree of incidence of mental stress in different biological signals. The volunteers declared that they were not habitual consumers of psychotropic substances and accredited the absence of a diagnostic history of chronic disease and/or psychopathologies. Through quota sampling, compliance with gender parity was satisfied, 50% men and 50% women [12; 18]. Signals were captured from the middle finger of the left hand and sampled at a frequency of 250 Hz [18], with the psychophysiological telemetric system \"Rehacor-T\" version \"Mini\" from Medicom MTD Ltd., Taganrog, Russia [18]. The signals were compiled in two sessions, of approximately 60 min each, undertaken simultaneously but separated by two weeks [12]. In the first session, called the basal level, the biological signals were captured; in our case, the interest is focused exclusively on the PPG signal under uninterrupted relaxation conditions. During the baseline session, the subjects were relaxed in the supine position and the physiological baseline values of each individual were established in the absence of exposure to stressors. In a second session, two weeks later, the same subjects were subjected to acute emotional stress through a memory test, a stress anticipation test, public exposure to a video and an arithmetic task, following the guidelines defined in the Trier Social Stress Test (TSST) [44]. The Trier Social Stress Test Guidelines have become a standard protocol for inducing and estimating moderate psychosocial stress in controlled settings. Many studies have confirmed its potential to induce significant changes in physiological parameters. At the beginning of the stress level session, each subject is subjected to a relaxation period to avoid the influence of the state with which they arrive at the session. Subsequently, the following stimuli or stressors were applied consecutively: a videotaped memory test, a period in anticipation of the stress in which the patient was the informed subject--we were \"evaluating their results\"--the presentation of the video to an audience and finally an arithmetic task. The timing of the activities contemplated in the basal and stress level sessions is summarized in Figure 1.  
From each PPG signal, 150 segments of 4 s duration (\(\sim\)ten total minutes) randomly chose to have enough temporal traces of the signal that cover the dynamic spectrum captured during the sessions. With all the information collected, it is feasible to record the blood microcirculation of each individual, a faithful reflection of the exclusive peripheral capillary network of each subject. Apart from preliminary preprocessing, to alleviate the  
Figure 1: (Color online) Outline of the protocol for basal and stress sessions, adapted from [12; 18]. The sessions lasted 60 minutes, although the time for acquisition of biological signals, such as the PPG signal, was limited to 40 minutes. **C** (informed consent); **P** (preparation and placement of sensors); **I** (initial interview); **RT** (relaxation time and measurement of normal status); **PR** (pression relaxation); **M** (videotaped memory task); **SA** (stress anticipation test); **V** (video public exposition test); **AT** (arithmetic task); **E** (end of session). impact of the noise that the data-acquisition process entails, the PPG signals undergo a subsequent and crucial transformation that characterizes their diffusive behavior. Figure 2 illustrates the time evolution of the PPG signal both in the basal and stressed states, specifically in this illustrative example, 40 s of PPG signal during a memory task after viewing a video. Figure 2 shows how the harmonic cadence of the time evolution of the PPG signal, characteristic of a relaxed state, is disrupted in the stressed state.