Electrodermal Activity (EDA) in Polygraph Testing: A Deep Dive into the Science of Deception Detection

Electrodermal Activity (EDA) is one of the most crucial components in polygraph testing, playing a significant role in detecting deception by measuring physiological responses linked to emotional stress. This article explores EDA’s role in polygraph testing, its scientific basis, methods of measurement, and the factors influencing its accuracy.

What Is Electrodermal Activity (EDA)?

Electrodermal Activity refers to changes in the skin’s electrical properties due to sweat gland activity, especially in response to emotional or psychological stimuli. EDA is a broad term encompassing various phenomena, including changes in skin conductance (SC) and skin resistance (SR). It is a non-invasive, real-time indicator of sympathetic nervous system activity, providing insight into a person’s psychological and physiological states.

Historical Context

The history of EDA dates back to the mid-1800s when German physiologist DuBois-Reymond first discovered the skin’s electrical activity. Later, in 1878, Hermann and Luchsinger demonstrated a correlation between sweat gland activity and electrical currents in the skin. By the 1920s, polygraph pioneer Leonard Keeler introduced the use of EDA in polygraphy, solidifying its place in deception detection systems. Today, EDA remains one of the primary physiological measures in modern polygraph tests.

Structure of the Skin and Sweat Gland Activity

The skin, as part of the integumentary system, serves as a protective barrier and plays a role in thermoregulation and waste elimination through sweating. The skin consists of two layers: the epidermis and the dermis. The sweat glands, primarily the eccrine glands, are distributed throughout the body but are most concentrated on the palms and soles of the feet. These glands are responsible for secreting sweat, which directly influences the skin’s electrical properties and forms the basis of EDA measurements.

The sweat produced by eccrine glands contains ions that affect the skin’s electrical conductance, making the skin more or less conductive depending on the amount of moisture present. This change in conductance is what polygraph tests measure when analyzing EDA responses.

Measurement of EDA in Polygraph Testing

In polygraph testing, EDA is measured using electrodes placed on the skin—usually on the fingers or palms. These electrodes detect variations in electrical conductance caused by the activation of sweat glands. There are two main methods to measure EDA:

1. Endosomatic Method: Measures the internal electric potential of the body without applying an external current.
2. Exosomatic Method: Applies either direct current (DC) or alternating current (AC) through the electrodes and measures the skin’s resistance or conductance.

In polygraphy, the exosomatic method is most commonly used. The system records changes in skin resistance (measured in ohms) or skin conductance (measured in microsiemens), allowing polygraph examiners to analyze emotional and physiological reactions to specific stimuli.

Phasic vs. Tonic Responses

EDA responses can be divided into two categories:

• Phasic Responses: These are rapid, transient changes in skin conductance triggered by specific stimuli, such as polygraph questions. Phasic responses are often referred to as Skin Conductance Responses (SCRs) and last only a few seconds.
• Tonic Responses: These represent the baseline level of skin conductance, known as Skin Conductance Level (SCL), which can fluctuate over longer periods. Tonic responses are used to assess overall physiological arousal.

Polygraph tests rely on both phasic and tonic responses to evaluate a subject’s physiological state during questioning.

Types of Questions in Polygraph Testing

Polygraph tests commonly use three types of questions to assess deception:

1. Irrelevant Questions: General questions designed to establish a physiological baseline. These questions elicit minimal emotional response, allowing examiners to observe the subject’s natural physiological state.
2. Relevant Questions: Directly related to the crime or situation under investigation. These questions often trigger a strong physiological response if the subject is being deceptive.
3. Comparison Questions: Questions that are unrelated to the crime but designed to provoke a deceptive response. The examiner compares the subject’s reactions to these questions with their responses to the relevant questions.

Analysis of EDA Data in Polygraph Testing

When analyzing EDA data, several parameters are essential:

• Amplitude: The magnitude of the EDA response.
• Latency: The time interval between the stimulus and the onset of the response, typically ranging from 0.5 to 5 seconds.
• Duration: The length of time the response lasts.
• Curve Complexity: The shape and pattern of the EDA response, which can indicate the subject’s level of arousal.

Two scoring systems are commonly used to interpret EDA data:

1. 3-Position Scoring: Assigns values of -1, 0, or +1 based on the differences in responses between comparison and relevant questions.
2. 7-Position Scoring: A more detailed system that assigns values between -3 and +3, with higher values indicating greater differences between the responses.

Factors Influencing EDA Responses

EDA is sensitive to various factors, which can impact its accuracy during polygraph testing:

• Medications: Some medications can alter sweat gland activity, affecting EDA responses.
• Demographics: Age, gender, and even skin color can influence EDA measurements. Older individuals and men tend to have lower sweat gland activity, which may reduce EDA responses.
• Emotional State: Emotions such as stress, fear, and anxiety can increase EDA responses, while relaxation can decrease them.
• Environmental Conditions: Temperature and humidity can affect the skin’s electrical properties, influencing the accuracy of EDA measurements.

Conclusion: EDA’s Critical Role in Deception Detection

Electrodermal Activity remains one of the most valuable physiological markers in polygraph testing. Its sensitivity to emotional and psychological states makes it a reliable indicator of stress and arousal. While EDA alone cannot conclusively determine deception, when used in conjunction with other physiological measures—such as heart rate and respiration—it provides a powerful tool for detecting lies.

Polygraph systems continue to evolve, with software advancements allowing for more accurate and detailed analysis of EDA data. By filtering out noise and emphasizing critical responses, modern polygraph systems offer greater clarity and reliability, ensuring that EDA remains a cornerstone of deception detection technology.