Thesis - Instituto de Telecomunicações

3.3. PHYSIOLOGICAL SIGNALS 43Historically, the physiology of the electrodermal activity (EDA) has been studied intwo distinct physiologic paradigms: the vascular [170] and the excretory [46]. The vasculartheory asserted that the skin conductance changes with the increase of blood flow, implyingthe relationship between the electrodermal activity and the circulatory system. The secondparadigm, the excretory function theory, states that the electrodermal activity is related tothe sympathetic system. This last causal model has prevailed and collected increasing evidence.A major support for this view is the verified correlation between sympathetic nerveactivity and electrodermal events [244, 172]. The present physiological knowledge relatesthe electrodermal system directly to the sympathetic activation of the autonomous nervoussystem (ANS). The eccrine glands in the skin produce sweat when the ACh (Acetylcholine)neurotransmitter passes from sudomotor fibers, part of the sympathetic chain, to theseglands [151]. When sweat concentration changes in the eccrine sweat gland, the skin alsochanges it’s electrical characteristics.The brain takes the role of controlling the sympathetic chain via two paths. The sudomotorcontrol is originated in the posterior hypothalamus [42]. The superior functions ofthe brain in the pre-frontal lobe, when some of the regions implied in attention, cognitionactivity or emotion react [235], also produce sympathetic activation.The skin, the largest organ of the human body, presents several functions such as: sensory;substance excretion and absorption; emotional expression and temperature regulation.The skin is pervasively innervated with sudomotor sweat glands that support some of thisregulatory functions of the skin. In figure 3.4 we depict a diagram of an eccrine gland, withthe secretory part in the subdermis and a duct that conduces the sweat to the surface ofthe skin. The eccrine glands are part of the exocrine system, and their main function istemperature regulation.In figure 3.5 we present a 2 minutes acquisition of EDA signal presenting several activationevents.EDA: Concepts and DefinitionsThe electrodermal system has been referred using an extended set of terms. In the 1980’sthe research community adopted the term Electrodermal Activity (EDA) to refer to thisphenomena, expressing the physical nature of the process [41]. Other names (and respective