# Estimation of Mental Distress from Photoplethysmography
## 2 Materials and Methods
### Experimental Methodology  
In order to classify distress versus calm condition we have used the well-known International Affective Picture System (IAPS) [23]. IAPS consists of a standard and categorized database of color photographs created to provide a wide range of affective stimuli. Moreover, the two primary dimensions recorded in the database are valence (ranging from pleasant to unpleasant) and arousal (from calm to excited). So, for each IAPS picture the mean and standard deviation of arousal and valence is provided in four different tables constructed from responses of men, women and children who responded to the emotion felt when exposed to pictures by means of the Self-Assessment Manikin (SAM), an affective rating system [24]. Thus, the idea is to use IAPS database to show a series of images to some volunteer participants. Each image used in the experiment should belong to one of two classes \"high arousal-low valence\" and \"low arousal-high valence\", corresponding to distress and calm, respectively. Fifty healthy participants (twenty-eight men and twenty-two women; 20 to 28 years old) not suffering from evident mental pathologies were recruited to participate in the experiment. Moreover, the participants who agreed did not present cardiovascular or anatomic nervous system diseases that could alter their PRV. All participants were informed on the high emotional content of some pictures that they will be shown. All participants were students from Technical School at Cuenca, Spain. The students had to pass the PHQ-9 Depression Test Questionnaire to be accepted in the experiment. The exclusion criteria used to discard subjects from the experimental section were mainly based on the results of PHQ-9. In this regard, the exclusion threshold was fixed at scores greater than eight. Considering this prerequisite, four students were not welcomed, and one experiment was not valid due to technical problems. Thus, the number of valid experiments was forty-five (twenty-five males and twenty females). This study was approved by Universidad de Castilla-La Mancha institutional committee on human experimentation. All participants gave written informed consent in accordance with the Declaration of Helsinki. The procedure for performing the experiment is described next (see Figure 2). The participant sits in front of the experimentation monitor to keep his/her movement minimal. The developed wearable sensor, described in Section 2.1, is put on his/her right wrist. In this regard, the experimentation monitor consists of a high resolution 28 inches screen. When the technician verifies the proper functioning of the wearable and its communication with the software, the experiment starts. Firstly, the participant has to carefully read the general instructions of the experiment. Next, ten pictures randomly chosen from a set of pictures that fulfill the condition to belong to negative stress (or distress) are shown consecutively to the participant during 6 s each. Silences consisting of blank images with a fixed duration of 1 s are inserted between two consecutive images. Afterwards, a distracting task is presented to the participant so that his/her emotional state comes to neutral. Next, the experiment continues by showing randomly another set of ten IAPS images that fulfill the condition to belong to calmness. Again, silences are used between each pair of images. Lastly, the distracting task is offered again. Pictures were classified into calm and stress subsets depending on their score of arousal and valence reported in IAPS database. Thus, the inclusion criterion for a picture to be added to the stress subset was that the picture rated an arousal level higher than 5 and a valence level lower than 3. Similarly, the inclusion criterion for a picture to be inserted into the calm subset was that it rated an arousal level lower than 4 and a valence level higher than 4 and lower than 6. Several studies have reported these ranges as those corresponding to negative stress and calmness, respectively [25].