Electrodermal Activity

2.1.2. Peripheral Nervous System

The PNS consists of nerve fibers. It can be divided in an afferent and efferent part. The afferent part transmits signals and information of sensory stimuli of the environment to the CNS. Vice versa, the efferent part receives information, orders and signals from the CNS. [CR02]

The PNS is subdivided into the autonomic nervous system (ANS) and somatic nervous system (SNS). The SNS (shown in green in figure 2.1) covers functionalities that are re-sponses to stimuli of the environment, e.g. regulate motor neurons to control movement of muscles. Most parts of the SNS can be controlled intentional by the human. The au-tonomic nervous system (shown in blue in figure 2.1) covers mainly functions of inner organs, like e.g. breathing and can mostly not be controlled intentional by the human.

It is divided into sympathetic, parasympathetic and enteric nervous system. The enteric nervous system is mainly responsible for the digestive system. Sympathetic and parasym-pathetic nervous system are antagonists respectively to each other. Symparasym-pathetic nervous system goes hand in hand with a high alertness, attention and energy production. The impact on the human body are e.g. higher heart rate and inhibited digestion. On the other hand, parasympathetic system is connected with a relaxation and calm down of the body.

Functionalities like heart rate and energy production slow down. [CR02]

Measures of the PNS are for example cardiovascular activity (autonomous part of the PNS), measures of the eccrine system (e.g. electrodermal activity for the somatic part of the PNS) and respiratory measures (autonomous part of PNS) [RSI98].

2.2. Electrodermal Activity

Electrodermal activity (EDA), often also referred to as Galvanic Skin Response (GSR) is the electrodermal reaction of the skin. The term electrodermal activity covers the electrical characteristics of the skin. EDA is involved in studies of many different research areas.

Research about EDA began in the early 1900s by Vigouroux, who measured tonic skin resistance in 1879 and 1888 and Hermann and Luchsinger who examined innervation of cat sweat glands in 1878 [Bou92].

In the following physiology and the different components of EDA will be described as well as measurement and interpretation of EDA.

2.2.1. Physiology

The skin serves as a barrier between body and environment. It consists of different compo-nents and layers as shown in figure 2.2, for example sweat glands. Two different types of sweat glands exist, eccrine and apocrine sweat glands. Both types have different functions.

Eccrine sweat glands are primarily responsible for the regulation of body temperature. On the palm and plantar position, the eccrine sweat glands respond to psychological stimuli [Ede72], which is based on the high density of sweat glands on the hand [SMFC87]. The apocrine sweat glands are limited to different areas of the body and are less studied than the eccrine sweat glands. Their primary function is as well the regulation of body temper-ature. In contrast to the eccrine sweat gland, the apocrine sweat glands are not directly open on the surface of the skin [SMFC87].

Figure 2.2.: Eccrine sweat gland [CTB07]

Figure 2.2 shows a profile of a eccrine sweat gland in the skin. The secretory part of the sweat gland lays in the subcutis. The duct connects the secretory part with the epidermis.

Sweat, produced in the secretory part rises up the duct. When sweat fills the duct, the skin gets more conductive and the resistance of skin is lowered. EDA reacts within a time frame of 1 to 3 seconds after a stimulus appeared [CTB07]. Several studies showed that the sweat glands are connected to the sympathetic nervous system [CTB07].

2.2.2. Measurement

EDA can be either measured endosomatic or exosomatic. For exosomatic measurement a small direct or alternating current is used to measure conductance of skin. Endosomatic methods measure skin conductance without current. The output of the different measure-ment methods differentiate. Exosomatic measuremeasure-ment with direct current leads to skin resistance or conductance. Measured with alternating current, exosomatic measurement leads to skin impedance and skin admittance. Endosomatic measurement on the other hand leads to skin potential. In many studies, exosomatic measurement with direct cur-rent is the preferred method of measurement. [CTB07]

Several devices for measurement of EDA exist. Most devices have two electrodes which are placed on the palm of the hand. Electrodes are mostly made of silver/silver chloride

2.2. Electrodermal Activity

(Ag/AgCl) to minimize bias potential and polarization [CTB07]. Figure 2.3 shows possible placements for the electrodes. The most recommended electrode position is position 2.

Another possible position for electrode placements are the feet.

Most studies use the non-dominant hand for measurement. This is motivated by the fact, that the skin of the non-dominant hand might show less skin lesion in comparison to the dominant hand. Furthermore, the dominant hand is free for other tasks in this case.

[CTB07]

Figure 2.3.: Possible electrode positions for EDA measurement at the left hand [CTB07]

Measurement can be influenced by different aspects. One influencing factor is the ac-tual condition of the skin. If a subject washes the skin with an abrasive soap, electrical properties of the skin might vary [VC73]. Therefore Venables [VC73] recommends to let subjects wash their hands before electrode placement with a non-abrasive soap. Besides condition of skin, measurement can also be influenced by humidity, ambient temperature and time of day. Several values of EDA can rise, with rising room temperature. [Bou92]

recommends a room temperature of 23 Celsius and keeping humidity constant, if possible.

Due to the issue that time of day can influence the values, these needs to be controlled in studies.

Different aspects of EDA can be measured, which are divided in phasic and tonic mea-sures. The most used measures are Skin Conductance Level (SCL) and Skin Conductance Response (SCR). SCL is a tonic measure and reacts slower over time. SCR, on the other hand, counts to the phasic measures and reacts fast. Both will be described in Detail in the following.

2.2.3. Skin Conductance Level

SCL reacts within a time frame of 10 seconds to minutes [CTB07]. SCL is measured in microsiemens. The range is normally between 2 to 20 µS, when SCL is measured at the

distal phalanges with exosomatic measurement and direct current.

When a new situation or stimulus is happening, SCL rises comparatively fast and de-creases over time when at rest. Figure 2.4 shows the SCL of two different subjects. At the first 20 seconds, both subjects were at rest. After the rest period, three stimuli were pre-sented. The curve shows the variation of SCL values between different subjects. Subject 1 starts at 10 µS, subject 2 at 5 µS. The curves also show the increase in SCL, when a stimulus is presented at 20, 35 and 50 seconds. The first time the stimulus was presented, the rise in SCL was bigger than the second and third time when the stimulus was repeated.

Figure 2.4.: Progress of Skin Conductance Level of two different persons [CTB07]

The measured values of a person cannot be compared with another person’s values. A value of 1 µS might be a high value for one person, for another person it might be the minimum. Due to the individual differences, electrodermal activity has to be normalized.

In this work, SCL is normalized by calculating the percentage of the overall SCL span:

SCL_normalized = SCL(t)−SCLmin

SCL_max−SCL_min ∗100[in%] (2.1) Minimum SCL values can be determined in a baseline measurement during a resting pe-riod. Maximum value can be determined over time or as proposed by [CTB07] initially by blowing up a balloon until it bursts. Interpretation might get more accurate with growing data set.

2.2.4. Skin Conductance Response

SCRs are elevations in form of small waves in the SCL. They are the phasic components of EDA. Figure 2.5 shows the course of a SCR. They can occur after a stimulus or