Contents
- 1. A little background and physiology of EDA
- 2. Applications
- 3. Measurement methods and electrical equivalent
- 4. Conclusion
What is electrodermal activity (EDA)?
Simply put, anything electrical that is connected to the human skin can be referred to as "electrodermal activity (EDA)"; it is also known as galvanic skin response (GSR). More specifically, it is the physiological measure that reflects changes in the electrical conductivity of the skin's surface and is caused by changes in emotional state.[1].
EDA is a fascinating physical-electrical phenomenon that uses the relative electrical properties of the top layer of skin to provide insights into the intricate relationship between our emotions, stress levels, and the autonomic nervous system. This blog post delves into the world of electrodermal activity, exploring its various properties and measurement methods.
The first part This article deals with the origins, measurement methods, significance in psychology and medicine, and potential applications of EDA. second part This blog series examines factors that influence EDA measurement.
1. Before the electrical part, a little background and physiology of EDA
The history and physiological description of EDA may seem like old news to electrical engineers, but it's essential for developing a reliable and accurate EDA measurement system. Before we delve into the depths of the technical world, let's take a brief overview of EDA's key properties.
1.1. Physiology of electrodermal activity
As mentioned above, the term "EDA" is commonly used to describe changes in skin conductance (impedance) for several practical reasons. Sweat, which is a very strong electrolyte, influences skin conductivity along with skin thickness and other factors. Sweat is secreted by the eccrine sweat glands located beneath the outermost layer of the skin. This layer, which is of interest for EDA measurements, is also called the epidermis or stratum corneum. [2]. Figure 1 shows a cross-sectional image of human skin for better understanding.
Figure 1: Cross-sectional image of the skin [3].
The sweat ducts transport this sweat from the sweat glands to the epidermis. When these sweat ducts are sufficiently wet, the skin's conductance increases significantly. These changes in skin conductance are tracked and processed as EDA for a variety of applications.
The autonomic nervous system (ANS) is responsible for sweating along with other physiological responses such as blood pressure, heart rate and pupil diameter [4]The ANS consists of two branches, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS)[5].
Fun fact: The sweat glands are the only organ whose nerve pathways originate from the SNS branch of the autonomic nervous system. All other physiological responses are controlled by both branches of the ANS. More important than this valuable information, however, is that we can use the SNS to study the effects of emotional stimuli on the sweat glands. Different emotional stimuli can elicit different physiological responses and thus different amounts of sweat.
The degree of hydration of these sweat glands leads to a change in the electrical conductivity of the epidermis. This physiological response is related to the evaluation of emotional valence and the galvanic skin response when exposed to pleasant (positive valence) or unpleasant (negative valence) stimuli. [4], [6].
The physiological aspect of EDA can also be described simply:
The sweat glands produce sweat (wow). The electrolytes it contains increase the skin's electrical conductivity. When a person is emotionally or under a lot of stress, their sweat glands sweat more, increasing the skin's conductivity.
There are two components of skin conductance, which are defined as follows:
- Skin conductance value (SCL):
SCL refers to the baseline electrical conductance of the skin, which reflects the overall activity of the sweat glands and the excitation of the sympathetic nervous system. It provides a general sense of the person's physiological state at a given time. This is also called the tonic response. It is a long-term measurement of skin conductance that is not caused by pseudotumor activity. Therefore, SCL is useful in stress detection and biofeedback systems. - Skin conductance response (SCR):
The SCR, on the other hand, represents the temporary changes in skin conductance that occur in response to certain stimuli. These stimuli can be emotionally charged or elicit a stress response. The SCR is often used to measure emotional responses because it provides insight into a person's emotional experiences. This response is called a phasic response. This response is produced by a pseudotumor burst. This is the component of EDA triggered by emotional stimuli.
1.2. Historical background
Research in the field of EDA began at the end of the 19th century (1879), when Vigoroux observed changes in skin conductance in hysteria patients [2]Although the term EDA was first introduced by Johnson and Lubin in 1966 [7]This observation was later used by the French neurologist Charles Féré to advance his research in this field.
Féré pointed out that skin resistance decreases significantly under the influence of emotional stimuli [8]Around the same time, physiologist Ivan Trachanoff explained the causal relationship between changes in skin conductance. Trachanoff was the one who explored the possibility of endosomatic EDA measurement, which we will discuss later in this article. [9].
Sweat glands are the source of “psychogalvanic phenomena,” according to McClendon and Hemingway in 1930 [10]The epidermis can be electrically remodeled using either passive elements alone or a combination of active and passive elements. The skin can be treated as a galvanic cell, which is why EDA is also referred to as galvanic skin response (GSR). [2].
For the purposes of this article, however, we are only interested in the passive electrical model of the skin. Using the polarization capacitance and psychophysiological components of EDA, Edelberg created an electrical model for the skin in 1971. [11]After more than ten years of EDA study, Edelberg focused on improving and evaluating EDA recordings in 1972. He also introduced additional EDA components, such as the rise time and recovery times of the EDR.
Since then, EDA has been used as one of numerous sensing techniques in psychophysiological research. Despite the widely recognized usefulness of EDA, certain limitations remain, and efforts are continually being made to improve the methods for recording, analyzing, and interpreting EDA data.
2. Applications
Due to its potential to uncover subtle physiological changes related to emotional and cognitive processes, the study of EDA has attracted considerable attention in psychology and medicine. However, there are still some underutilized applications for EDA, while it is prominently used in some others.
2.1. The main applications of EDA
Here are some of the key areas where EDA plays a central role:
- Emotion/stress research and psychophysiological assessments
Electrodermal activity is closely linked to emotional reactions and stress responses. Researchers use EDA to study emotional states such as fear, arousal, and anxiety. By examining changes in skin conductance, scientists can better understand the physiological basis of these emotions and how they relate to psychological experiences. EDA is also commonly used in psychophysiological studies, which monitor physiological responses during tasks or stimuli in real time. These assessments provide valuable information about a person's cognitive processes, emotional responses, and overall well-being. - Biofeedback, stress management and other clinical applications
EDA biofeedback is a technique in which individuals learn to regulate their physiological responses, such as skin conductance, through conscious control. This approach is used in stress management and relaxation training, helping individuals gain greater control over their emotional and physiological state. Electrodermal activity has also been studied in various clinical contexts, such as the diagnosis and monitoring of anxiety disorders, post-traumatic stress disorder (PTSD), and certain types of neuropathies. EDA measurements can provide additional insight into the patient's condition and progress.
2.2. Less explored potential applications
Beyond psychology and medicine, electrodermal activity has potential applications in various fields:
- Human-computer interaction and market research
EDA can be integrated into technological interfaces to adapt to users' emotional and cognitive states. This could lead to more personalized and responsive interactions with computers, robots, and virtual reality systems. Furthermore, EDA is sometimes used in market research to measure consumers' emotional reactions to advertising, products, or experiences. This data helps marketers refine their strategies to elicit the desired emotional responses. - Education and Sports
In education, EDA can help measure student engagement and cognitive load. This information can support the development of more effective teaching methods and materials. The emotional state of athletes can significantly influence their performance. EDA could be used to monitor athletes' stress levels and help them optimize their psychological preparation for competitions.
3. Measurement methods and electrical equivalent
For most practical purposes, however, skin conductance is referred to as electrodermal activity. This electrical activity can be detected by a variety of sensors, electrodes, and sophisticated electronic circuits.
Sounds simple, right? Unfortunately, that's not the case. Numerous elements, ranging from electrical stimulation to complex data processing, affect EDA measurement and are the subject of the second half of this blog post. Nevertheless, some of these aspects will be touched upon in this section. First, the measurement methods for EDA are presented with the basic concepts of skin conductance. A brief introduction to the electrical equivalent of the skin concludes this topic.
3.1. EDA measurement methods
The epidermis of human skin can be modeled as active or as a combination of passive electrical components based on these two approaches. [12]:
- Endosomatic measurement
Recording the electrical potential at the stratum corneum of the skin when no external electrical signals are applied to the skin. Using electrodes, the biopotential of the human skin is recorded during each emotional stimulus applied to the subject. This change in skin voltage is processed to study the effect of various stimuli on the SNS. This measurement is beyond the scope of this article and will therefore not be discussed in detail. - Exosomatic measurement
In these measurements, an external, controlled, and undetectable electrical signal is applied, and the change in resistance (impedance) or conductance (admittance) is encoded based on the nature of the excitation signal. This is a two-step process: First, an electrical signal with "suitable parameters" is applied via electrodes. The same electrodes are then used in a subsequent step to record the electrical response of the epidermis. Skin conductance (or admittance) can be calculated based on the properties of the applied and measured signals.
3.2. Skin conductivity measurements
Skin conductance is divided into two components. Tonic and phasic EDA were introduced and characterized in previous sections. Further information on these components is provided here.
- Skin conductance response (SCR):
This rapidly fluctuating component is also called the phasic response of the EDA; it has a unit of measurement in microsiemens (µS). More specifically, the SCR refers to sudden changes in skin conductance that occur in direct response to environmental stimuli. As shown in Figure 2, these sudden spikes often occur between 1 and 5 seconds after the presentation of a stimulus (e.g., a picture or sound). This time delay is called the latency. These changes in the SCR are closely related to cognitive functions (including decision-making) after a brief event. [13].
Figure 2: Phasic response of EDA (SCR) [13].
The SCR can be characterized by the following five components of the response: (1) latency, (2) rise time, (3) amplitude, (4) peak, and (5) recovery time. While the latency period typically lasts between 1 and 5 seconds, the length of the other four components varies depending on the individual and the stimuli applied. This response can also be elicited without a stimulus. Such an SCR is referred to as a stimulus-independent skin conductance response (NS-SCR or non-SCR).
- Skin conductance value (SCL):
This is a comparatively slowly fluctuating component of the SC that is measured over a long period of time. This component is also referred to as the "tonic response" of the SC (also measured in µS). The SCL undergoes changes over time that are not triggered by specific stimuli or events, but rather reflect a continuous intra-individual course. The SCL varies greatly between subjects and is influenced by autonomic regulation, physical health, and psychological mood. The choice of electronics also has a significant impact on this measurement. Because this component is not event-related (stimulus-related), it has fewer applications than the SCR. The differences between the two components are illustrated in Figure 3 in a prolonged stressful situation such as an exam.[13].
Figure 3: Comparison of the tonic and phasic response during a long stress situation[13].
Because the exam symbolizes a stressful situation that requires a conscious state, this leads to a slow and steady increase in SCL. In contrast, there are numerous abrupt increases in SCL while dealing with specific problems in the exam. These sudden increases are a sign that cognitive processes (including reasoning and decision-making) are heightened for a short period of time. Once the exam is over and the resulting stress has subsided, SCL also gradually decreases to a baseline level.
3.3. Electrical equivalence model of the skin
EDA measurement is a broad field, so before delving into it, it is necessary to select a clear vertical. This study is limited to exosomatic EDA measurement, also to gain a deeper understanding of skin conductance or resistance measurements. All necessary background information and parameters have already been established in this article. To measure the electrical activities of the skin, a credible electrical equivalence model for human skin must be established.
All electrical equivalence models of the skin have in common that the sweat ducts beneath the epidermis form a parallel network of resistances (or impedances). This fact simplifies the analysis when measuring skin admittance rather than skin impedance, because any change in the skin's electrical properties simply has an additive effect. In the case of skin resistances, however, the inverse of the resistances must be added, which complicates the analysis. [14]Therefore, measuring skin conductance is preferred over measuring skin resistance. Although electrodermal levels (SCL) are also measured in addition to the electrodermal response (SCR), skin impedances can easily be converted into skin admittances.
The first credible electrical model of the skin was presented by Montagu and Coles (1966) based on the parallel resistance theory of Thomas and Korr (1957). This model consists of a network of parallel resistances (conductance) and an equivalent parallel capacitance for the entire network. Later, Edelberg (1972) and Fowls (1974) offered the more complex and precise skin impedance model.
In contrast to previous models, this equivalent circuit focuses on the electrical properties of each individual sweat gland. This model takes into account various factors such as the initial state of the sweat glands, the degree of hydration of the epidermis, the activity of the associated membranes, and the influence of the environment of the sweat ducts. [14].
Figure 4: Simplified electrical replacement model of the skin [15].
The parallel resistance model leads to a simpler calculation of the SC instead of the skin resistance, as the effect is additive. A much simpler electrical equivalent model of the skin is shown in Figure 4. This model is only effective enough when measuring skin conductance. When measuring skin resistance, the change in each parallel resistance is influenced by the other members (sweat channels modeled as resistors) of the parallel network.
4. Conclusion
Electrodermal activity reveals the intricate interplay between our emotions, physiology, and the autonomic nervous system. As technology advances, so does our ability to harness the potential of EDA to understand human behavior, enhance well-being, and improve various aspects of our lives.
From psychology to medicine to education and beyond, electrodermal activity continues to be a fascinating avenue of research, shedding light on the hidden currents that shape our inner world.
The next important step in this direction is to discuss the "factors influencing EDA measurements." These elements help make important decisions regarding the design approach. The next article will discuss the key features of signal simulation and the choice of electrodes.
References
[1] Braithwaite, Jason J., et al. “A guide for analyzing electrodermal activity (EDA) & skin conductance responses (SCRs) for psychological experiments.” Psychophysiology 49.1 (2013): 1017-1034.
[2] Boucsein, Wolfram. Electrodermal activity. Springer Science & Business Media, 2012.
[3] WHERE’S THE EVIDENCE?”: IMAGE (WORDPRESS.COM)
[4] Sanchez-Comas, Andres, et al. “Correlation analysis of different measurement sites of the galvanic skin response in test groups with pleasant and unpleasant stimuli.” Sensors 21.12 (2021): 4210.
[5] Electrodermal Activity (EDA) – Where is the evidence? (WORDPRESS.COM)
[6] Borrego, Adrian, et al. “Reliability of the Empatica E4 wristband to measure electrodermal activity to emotional stimulation.” 2019 International Conference on Virtual Rehabilitation (ICVR). IEEE, 2019.
[7] Białowąs, Sylwester, and Adrianna Szyszka. “Measurement of electrodermal activity in marketing research”. Managing Economic Innovations-Methods and Instruments (2019): 73-90.
[8] Dawson ME, Schell AM, Filion DL (2007). The electrodermal system. [In:] Cacioppo, John T., Louis G. Tassinary, and Gary Berntson, eds. Handbook of Psychophysiology. Cambridge University Press, 2007.
[9] Tronstad, Christian, et al. “Current trends and opportunities in the methodology of electrodermal activity measurement”. Physiological Measurement 43.2 (2022): 02TR01.
[10] McClendon, J.F., and Allan Hemingway. “The psychogalvanic reflex as related to the polarization-capacity of the skin”. American Journal of Physiology-Legacy Content 94.1 (1930): 77-83.
[11] Edelberg, Robert. “Electrodermal mechanisms: A critique of the two-effector hypothesis and a proposed replacement.” Advances in Electrodermal Research (1993): 7-29.
[12] Society for Psychophysiological Research Ad Hoc Committee on Electrodermal Measures, et al. “Publication recommendations for electrodermal measurements”. Psychophysiology 49.8 (2012): 1017-1034.
[13] Winter, Michael, et al. “Towards the applicability of measuring the electrodermal activity in the context of process model comprehension: Feasibility study.” Sensors 20.16 (2020): 4561.
[14] Mahon, Mary L. The effect of electrode size on electrodermal measurement. Diss. University of British Columbia, 1986.
[15] Schaefer, Florian, and Wolfram Boucsein. “Comparison of electrodermal constant-voltage and constant-current recording techniques using the phase angle between alternating voltage and current.” Psychophysiology 37.1 (2000): 85–91.
List of abbreviations
| abbreviation | definition |
|---|---|
| FDFA | Electrodermal activity |
| EDR | Electrodermal reaction |
| SC | Skin conductance |
| SR | Skin resistance |
| SCL | Skin conductivity |
| SCR | Skin conductance reaction |
| ANS | Autonomic nervous system |
| SNS | Sympathetic nervous system |
| PNS | Parasympathetic nervous system |