**1. Introduction**

Numerous studies had been done by researchers to analyze the basic of EDA responses as indicators via employing various methods and techniques of measurement since long time ago. However, results and hypotheses of several such studies were rather conflicting. These investigations were apparently lost of sight by later investigators, and many of the same errors and conflicting results have appeared in later studies. The purpose of this review was to advance the understanding of simultaneous recordings of EDA parameters at the same skin site and show its advantages over sequential or alternate recordings.

#### **1.1 Skin**

To reach our goal in this chapter, a brief introduction about the skin, which is one of the most complex organs of the human body, should be depicted.

The human skin is a complex and a large organ (in terms of both weight and surface area) that covers the body and forms a remarkable protective barrier against the external environment [1, 2]. It is facilitating to regulate the core body temperature and water balance via bloodstream to the exterior of the body [3].

EDA responses are frequently used as the peripheral indicators of sympathetic activation. The EDA measurement by psychophysiologists is basically concerned with sweat gland activity that is psychologically induced. Different internal and external stimuli cause mental stress; as a result, sweat glands produce various amounts of sweat that are propelled up to the sweat ducts and hence result in different EDA responses. Numerous models have been proposed to explain how these peripheral mechanisms are associated to the electrical activity of the skin and to the transient increases in EDA parameters evoked by external stimuli. According to Edelberg [4] one can account for the several EDA phenomena, including alteration in tonic skin conductance level (SCL) and phasic skin conductance response (SCR) amplitude, with a model based completely on the sweat glands [4]. As noted by Edelberg [5], one should not be surprise that an organ with such vital and dynamic functions continuously receives signals from control centers in the brain, and the author suggested that "we can listen in on such signals by taking advantage of the fact that their arrival at the skin is heralded by measurable electrical changes that we call electrodermal activity" (p. 368).

In order to clearly understand how EDA is linked to the sweat glands, it is useful to imagine the sweat ducts as a set of variable resistors with parallel connection. Sweat columns will rise in the ducts with different amounts and different sweat gland numbers, depending on the level of the sympathetic nervous system activation. As the ducts are filled through sweating, there is a more conductive path through the relatively resistant stratum corneum layer. As the sweat level further rises, the resistance in that variable resistor is further lowered. Changes in the sweat level in the ducts alter the values of the variable resistors and thus yield observable changes in EDA [3].

#### **1.2 Electrical bioimpedance**

Electrical bioimpedance is a measure of how well the biological tissues such as the skin impede alternating current flow at different frequencies. Electrical impedance has two components: the resistive and the reactive parts. Mathematically, the electrical impedance (Z) is expressed as a complex number by the sum of the resistance (R) and the reactance (X):

$$\mathbf{Z} = \mathbf{R} + \mathbf{jX} \tag{1}$$

Electrical impedance is the ratio between the voltage and current. When a known current is applied to a material, the impedance is found by measuring the voltage between the electrodes and dividing it by the current. However, in many cases, applying a known voltage to the material and measuring the resulting current between the electrodes are more practical. The measured current then becomes inversely proportional to the impedance. This quantity is called electrical admittance (Y), which allow current to flow. It is also expressed as a complex number with two components: conductance (G) and susceptance (B):

$$\mathbf{Y} = \mathbf{1}/\mathbf{Z} = \mathbf{G} + \mathbf{jB} \tag{2}$$

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*Electrodermal Activity: Simultaneous Recordings DOI: http://dx.doi.org/10.5772/intechopen.89025*

material can be calculated from Eq. (3).

**1.3 Electrical impedance spectroscopy**

**1.4 Electrodermal activity (EDA)**

current. B is proportional to this frequency, and the electrical capacitance (C) of the

Electrical impedance spectroscopy (EIS) is applying a sinusoidal voltage or current to the sample under test to calculate impedance parameters within a wide range of frequencies, where frequency-dependent electrical properties of biological tissues can be detected. Therefore, EIS has been proven as an effective technique for noninvasive tissue characterization in medical, biomedical, and biological applications [6]. The real part of the impedance is associated with resistive pathways across the tissues, which is typically large at low frequencies but decreases with increasing frequency, whereas the imaginary part of the impedance is associated with capaci-

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

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

tive pathways, which decreased (not noticeable) at high frequencies [7].

ionic permeability of sweat gland membranes [5, 11, 14].

B = 2П FC (3)

Both the X and B are dependent on the frequency (F) of the applied current through the material and only can be measured by alternating the direction of the *Electrochemical Impedance Spectroscopy*

we call electrodermal activity" (p. 368).

changes in EDA [3].

**1.2 Electrical bioimpedance**

resistance (R) and the reactance (X):

The human skin is a complex and a large organ (in terms of both weight and surface area) that covers the body and forms a remarkable protective barrier against the external environment [1, 2]. It is facilitating to regulate the core body tempera-

EDA responses are frequently used as the peripheral indicators of sympathetic activation. The EDA measurement by psychophysiologists is basically concerned with sweat gland activity that is psychologically induced. Different internal and external stimuli cause mental stress; as a result, sweat glands produce various amounts of sweat that are propelled up to the sweat ducts and hence result in different EDA responses. Numerous models have been proposed to explain how these peripheral mechanisms are associated to the electrical activity of the skin and to the transient increases in EDA parameters evoked by external stimuli. According to Edelberg [4] one can account for the several EDA phenomena, including alteration in tonic skin conductance level (SCL) and phasic skin conductance response (SCR) amplitude, with a model based completely on the sweat glands [4]. As noted by Edelberg [5], one should not be surprise that an organ with such vital and dynamic functions continuously receives signals from control centers in the brain, and the author suggested that "we can listen in on such signals by taking advantage of the fact that their arrival at the skin is heralded by measurable electrical changes that

In order to clearly understand how EDA is linked to the sweat glands, it is useful to imagine the sweat ducts as a set of variable resistors with parallel connection. Sweat columns will rise in the ducts with different amounts and different sweat gland numbers, depending on the level of the sympathetic nervous system activation. As the ducts are filled through sweating, there is a more conductive path through the relatively resistant stratum corneum layer. As the sweat level further rises, the resistance in that variable resistor is further lowered. Changes in the sweat level in the ducts alter the values of the variable resistors and thus yield observable

Electrical bioimpedance is a measure of how well the biological tissues such as the skin impede alternating current flow at different frequencies. Electrical impedance has two components: the resistive and the reactive parts. Mathematically, the electrical impedance (Z) is expressed as a complex number by the sum of the

Electrical impedance is the ratio between the voltage and current. When a known current is applied to a material, the impedance is found by measuring the voltage between the electrodes and dividing it by the current. However, in many cases, applying a known voltage to the material and measuring the resulting current between the electrodes are more practical. The measured current then becomes inversely proportional to the impedance. This quantity is called electrical admittance (Y), which allow current to flow. It is also expressed as a complex number

Both the X and B are dependent on the frequency (F) of the applied current through the material and only can be measured by alternating the direction of the

with two components: conductance (G) and susceptance (B):

Z = R + jX (1)

Y = 1/Z = G + jB (2)

ture and water balance via bloodstream to the exterior of the body [3].

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current. B is proportional to this frequency, and the electrical capacitance (C) of the material can be calculated from Eq. (3).

$$\mathbf{B} = \mathbf{2} \Pi \,\mathbf{F} \mathbf{C} \tag{3}$$

#### **1.3 Electrical impedance spectroscopy**

Electrical impedance spectroscopy (EIS) is applying a sinusoidal voltage or current to the sample under test to calculate impedance parameters within a wide range of frequencies, where frequency-dependent electrical properties of biological tissues can be detected. Therefore, EIS has been proven as an effective technique for noninvasive tissue characterization in medical, biomedical, and biological applications [6]. The real part of the impedance is associated with resistive pathways across the tissues, which is typically large at low frequencies but decreases with increasing frequency, whereas the imaginary part of the impedance is associated with capacitive pathways, which decreased (not noticeable) at high frequencies [7].
