**1.4 Electrodermal activity (EDA)**

EDA is the preferred term for changes in electrical properties of the skin. It is a set of physiological parameters of sympathetic nervous system activity, and it has been used for physiological measurements due to a strong link with the autonomous activity [8]. However, the EDA phenomenon and its appearance are not sufficiently clarified yet [9, 10]. EDA is measured from the eccrine glands, which cover most of the body. In addition, they are concentrated in the palmar and plantar dermatomes, and, therefore, these are known to be the best sites for measuring EDA [11, 12]. Mainly, there are two categories of electrodermal recordings, namely, endosomatic and exosomatic measurements. In endosomatic measurements, only potential differences originating in the skin itself are recorded without using any external source of current. In exosomatic measurements, externally very small amount of current [either alternating current (AC) or direct current (DC)] is applied to the skin. This is frequently used to measure SC, and in some recently published studies, it is also used to measure skin susceptance. EDA signals are a manifestation of the eccrine sweat gland activity that is innervated by the autonomic nervous system, primary by the sudomotor nerves [13]. When the sudomotor nerves stimulate the sweat production, indeed the SC changes as a result of sweat secretion and alterations in ionic permeability of sweat gland membranes [5, 11, 14].

EDA is composed of two basic components (**Figure 1**): tonic (level) and phasic (response), each with various time scales and relationships with the stimuli. Tonic EDA is represented by SCL which represents the slow-changing baseline level of the SC and skin potential level (SPL) which represents the slow-varying baseline level of the SP. Alterations in the SCL are thought to reflect slow changes in the autonomic nervous system dynamics. Phasic EDA is specified by a fast varying component, known as the SCR and skin potential response (SPR). Both EDA phenomena, tonic (SCL and SPL) and phasic (SCR and SPR), are generated under autonomous nerve control of the active organs of the skin [15], which is reflecting the elicited response of the eccrine sweat glands to external stimuli [11, 14]. Some recent evidences suggest that these two components depend on various neural mechanisms [16] and, consequently, both carry relevant and non-redundant information about the autonomic nervous system dynamic activity [14]. EDA is employed in a broad range of experimental setups since it is a relatively straightforward measurement providing valuable information on the autonomic nervous system response to a wide range of externally applied stimuli. Particularly, SC

**Figure 1.** *Basic components of EDA.*

analysis is commonly used to quantify the levels of arousal related with cognitive and emotional processes [3, 14, 17, 18].

In spite of sweating being basically a means of thermoregulation, sweat glands located on the plantar and palmar surfaces perhaps evolved to increase grip and enhance sensitivity, possibly more responsive to psychologically stimuli than to thermal stimuli [3, 5, 11, 14]. This relationship between EDA, psychological stimuli, and autonomic nervous system makes this physiological signal broadly popular in neuroscience research studies, including quantification of arousal levels during cognitive and emotional processes, information processing, and clinical research examining predictors and correlates of normal and pathological behavior [3, 17–19], such as psychopathology, personality disorders, and neuropsychology [14].
