Electrodermal Activity Wrist-Based Systems: A Review of Validity, Reliability, and Responsive Measures

Introduction

The electrodermal activity (EDA), also known as galvanic skin response (GSR) or psychogalvanic reflex (PGR), can be defined as the electrical phenomena that occur in the skin, approximately 2 seconds after an internal or external physiological arousal stimulus (Benedek & Kaernbach, 2010; Borrego, Latorre, Alcaniz, & Llorens, 2019; Boucsein et al., 2012; Dawson, Schell, & Filion, 2007; Horvers, Tombeng, Bosse, Lazonder, & Molenaar, 2021; Sharma, Kacker, & Sharma, 2016). EDA is expressed through an increase in the electrical conductance of the skin, typically measured in the palm of the hand and on the sole of the foot (Sharma et al., 2016).

The EDA is mediated by the central nervous system (CNS), specifically the autonomic nervous system through the sudomotor nerve, responsible for activating the sudomotor fibers that activate the sweat glands and consequently the degree of secretion by postganglionic sympathetic fibers with a firing rate of 0.62Hz (Benedek & Kaernbach, 2010; Borrego et al., 2019; Boucsein et al., 2012; Macefield & Wallin, 1996; Schmelz, Schmidt, Bickel, Torebjörk, & Handwerker, 1998; Sharma et al., 2016; Zangróniz, Martínez-Rodrigo, Pastor, López, & Fernández-Caballero, 2017). The sympathetic stimulation, by increasing sweating with a consequent increase in the amount of water and electrolytes in the skin, reduces the skin’s resistance to the electric current (Sharma et al., 2016). In this perspective, EDA has been described as a reachable and reliable reflection of peripheral autonomic change and an index of peripheral sympathetic nervous activity (Sharma et al., 2016). This metric can be used to examine emotional states, such as happiness, sadness, or excitation, and implicit emotional responses such as threat, anticipation, salience, and novelty (Horvers et al., 2021; Sharma et al., 2016).

Different instruments have been indicated for EDA monitoring. Among these, the Ag-AgCl based electrodes attached to the palmar medial phalanges of the index and middle fingers are the most frequent ones (Borrego et al., 2019; Conchell & Calpe, 2022; Horvers et al., 2021; Sharma et al., 2016). However, the technological advances combined with the health and rehabilitation needs regarding continuous and remote monitoring led to the development of EDA monitoring systems associated with other body regions like the wrist (Horvers et al., 2021; Sharma et al., 2016). In fact, despite being associated with high levels of reliability the EDA finger-based measurements are expensive and their usability for long-term monitoring can be questioned (Conchell & Calpe, 2022; Leiner, Fahr, & Früh, 2012; Posada-Quintero & Chon, 2020). Wrist-based measurements through bracelets favor portability, usability, low maintenance, low cost, and continuous measuring (Banks, Bellerose, Douglas, & Jones-Gotman, 2012; Kasos et al., 2018a; Picard, Fedor, & Ayzenberg, 2015). Moreover, wrist bracelets, by measuring other biological signs, such as temperature, heart rate, oxygen saturation, and movement, constitute an attractive solution to conduct real and daily life assessments (Sharma et al., 2016). However, the psychometric properties of this kind of wearable instrument have been questioned (Souza, Alexandre, & Guirardello, 2017). Reliability, validity, and responsiveness are considered the one of the main measurement properties of instruments (Souza et al., 2017). Reliability is the ability to reproduce a result consistently in time and space (Souza et al., 2017; Terwee et al., 2007). Validity refers to the property of an instrument to measure exactly what it proposes and responsiveness is the metric that indicates the capacity of the instrument to identify small but relevant changes in the data measurement in longitudinal context (L. B. Mokkink et al., 2010; Souza et al., 2017).