Electrodermal activity

Electrodermal activity (EDA) is the property of the human body that causes continuous variation in the electrical characteristics of the skin. Historically, EDA has also been known as skin conductance, galvanic skin response (GSR), electrodermal response (EDR), psychogalvanic reflex (PGR), skin conductance response (SCR), sympathetic skin response (SSR) and skin conductance level (SCL). The long history of research into the active and passive electrical properties of the skin by a variety of disciplines has resulted in an excess of names, now standardized to electrodermal activity (EDA).^[1][2][3]

The traditional theory of EDA holds that skin resistance varies with the state of sweat glands in the skin. Sweating is controlled by the sympathetic nervous system,^[4] and skin conductance is an indication of psychological or physiological arousal. If the sympathetic branch of the autonomic nervous system is highly aroused, then sweat glands activity also increases, which in turn increases skin conductivity. In this way, skin conductivity can be a measure of emotional and sympathetic responses.^[5] But the theory associating sweat and EDA was already debated decades ago since individuals without sweat glands have an EDA signal : ""The source of the skin potential is presumed to be the sweat glands and the epidermis, although it is present in subjects with congenital absence of sweat glands ... this is not a test of “sweat” function, it is often included in this category as a measure of sudomotor activity".^[6]". This debate is ongoing since more recent technology (see Electrochemical skin conductance) demonstrated a real measure of sweat conductivity with several medical applications.^[7] A good way to differentiate both is to look at measures values and type :

• EDA is a continuous measurement (signal curve with time) given in μS with value mostly <5
• while ESC values ranges from 0 to 100 μS with values under 10 being extremely low and rare. A value of 0 in the case of ESC could be theorically a single point EDA when sweat is totally absent.^[8]

More research is needed and inclusion of additional phenomena (resistance, potential, impedance, Electrochemical skin conductance, and admittance, sometimes responsive and sometimes apparently spontaneous) suggest that EDA is more complex than it seems. There is a knowledge limitation, as wearable brands have included an EDA measure as a feature : "This aspect leads to the conclusion that the reliability of consumer wearables must be further investigated, especially by combining raw data collection with specific preprocessing techniques".^[9]