Electrodermal Activity (EDA) represents the continuous variation in the electrical characteristics of human skin, providing a non-invasive window into internal physiological states. Often referred to as Galvanic Skin Response (GSR), this measurement quantifies the skin’s ability to conduct electricity. The changes recorded reflect moment-to-moment shifts in an individual’s psychological or cognitive arousal level. This physiological signal is a widely used metric in psychophysiology for objectively assessing changes in emotional and attentional states.
The Biological Basis of Electrodermal Activity
The underlying mechanism of the EDA response is directly linked to the Autonomic Nervous System (ANS). Specifically, the Sympathetic Nervous System (SNS), responsible for the body’s “fight-or-flight” response, is the sole driver of the EDA signal. This makes EDA a pure measure of sympathetic arousal, as it is not influenced by the Parasympathetic Nervous System.
The sympathetic nerves uniquely innervate the eccrine sweat glands, which are densely concentrated on the palms of the hands and the soles of the feet. Emotional or cognitive arousal triggers the SNS to signal these glands, causing them to secrete a small amount of electrolyte-rich sweat.
The presence of this conductive fluid significantly lowers the skin’s electrical resistance, consequently increasing its electrical conductance. This physiological process is highly sensitive, meaning that even subtle psychological stimuli can produce a measurable change in skin conductance. Because these sweat glands are more responsive to psychological stimuli than to temperature regulation, the resulting EDA signal offers a precise index of arousal.
Methods for Measuring Skin Conductance
Measuring the changes in skin conductance requires a non-invasive process known as exosomatic measurement. Two electrodes are typically placed on a hairless area of the skin, most commonly the fingertips or the palm, due to the high density of eccrine sweat glands. These electrodes are often made of silver/silver chloride (\(\text{Ag}/\text{AgCl}\)) because this material provides a stable electrical interface.
The measurement device applies a small, imperceptible direct current (DC) voltage across the two electrodes. The current that flows is measured, which is inversely proportional to the skin’s electrical resistance. Researchers prefer to report the data as conductance (the reciprocal of resistance) because it shows a more linear relationship with the underlying sweat gland activity.
The standard unit of measurement for skin conductance is the microsiemens (\(\mu S\)). Typical baseline levels often fall below \(5\ \mu S\). Since the measurement relies on detecting very small electrical changes, careful preparation of the skin and the use of a constant voltage system are required to accurately capture the signal.
Interpreting EDA Signals (Tonic vs. Phasic)
The raw electrodermal activity signal is composed of two distinct components, each representing a different aspect of sympathetic arousal.
Tonic Activity (Skin Conductance Level – SCL)
Tonic Activity, known as the Skin Conductance Level (SCL), represents the slow-moving, continuous baseline. SCL reflects an individual’s general state of arousal, such as overall mood, attentiveness, or background stress level over a longer period. Changes in SCL can indicate broad shifts in an individual’s baseline physiological state, such as becoming more vigilant or fatigued during a prolonged task. This level is constantly present and provides an overall context against which the more rapid changes are measured.
Phasic Activity (Skin Conductance Response – SCR)
Phasic Activity, known as the Skin Conductance Response (SCR), manifests as rapid, transient spikes above the tonic baseline. An SCR is a short-lived increase in conductance that occurs in direct response to a specific internal or external stimulus. Key metrics used to analyze an SCR include its amplitude (peak height) and its latency (the time delay between the stimulus onset and the start of the response, typically ranging from one to three seconds). Analyzing both the tonic and phasic components provides a comprehensive view of how the sympathetic nervous system is responding.
Real-World Applications
The sensitivity of electrodermal activity to emotional and cognitive states has led to its adoption across a wide range of scientific and commercial fields.
Psychological Research
In psychological research, EDA is frequently used to study emotion regulation, fear conditioning, and attention, providing an objective measure of arousal independent of subjective self-report. Researchers use the magnitude of the SCR to quantify the emotional salience of different stimuli.
Commercial and Marketing Use
In the commercial sector, EDA is a core tool in neuro-marketing and usability testing. By monitoring skin conductance, companies can gauge the level of engagement or cognitive load experienced by consumers as they interact with content or products. This offers data on which elements are most arousing.
Health and Clinical Monitoring
The medical and health monitoring fields utilize EDA for stress and anxiety tracking. Wearable devices, such as smartwatches, incorporate EDA sensors to provide users with real-time feedback on their physiological stress levels. Continuous monitoring is also being explored for clinical applications, including the detection of epileptic seizures and the assessment of chronic pain.
Other Applications
Historically, EDA has been a central component of polygraph testing, where an increase in skin conductance is interpreted as a physiological sign of heightened emotional arousal. Furthermore, it serves as a biofeedback tool, helping individuals learn to consciously regulate their autonomic nervous system responses to manage stress or improve relaxation.