Electrodermal activity (EDA) refers to changes in the electrical properties of the skin, primarily influenced by sweat gland activity. It is monitored through two methods:
- Galvanic Skin Resistance (GSR): An active method measuring electrical resistance when a current is applied.
- Galvanic Skin Potential (GSP): A passive method measuring voltage without applied current.
Ohm's Law:
Describes the relationship between current (I), voltage (E), and resistance (R):
I = \frac{E}{R}
or, using conductance (G = \frac{1}{R}):
I = E \times G .
Units:
- Current (I): Amperes (A)
- Voltage (E): Volts (V)
- Resistance (R): Ohms (Ω)
- Conductance (G): Siemens (S)
Normal Conductance Values:
For human skin, normal conductance ranges from 1 to 20 microSiemens (μS).
Measurement Equipment:
The BIOPAC system uses a constant 0.5 V DC to measure the current required to maintain this voltage, with conductance inferred from current readings.
Anatomical Aspects
- Measurement Sites: Inner surfaces of palms and soles are commonly used due to rich eccrine sweat gland density in these areas.
- Neurotransmission: Eccrine glands are stimulated by acetylcholine, contrasting with other sympathetic nervous system components that are controlled by noradrenergic fibers.
- Sympathetic Activation: Increases in skin conductance occur with heightened sympathetic nervous system activation, characterized by decreased resistance.
- Function of Eccrine Glands: Primarily linked to thermoregulation through cooling via evaporation; however, in palms and soles, responses are often due to psychological stimuli.
Response Dynamics
- Measurement Protocol: Electrodes placed on fingers (index and middle or ring fingers). Skin must be cleaned with soap before placement, avoiding abrasion.
- Response Latency: The EDA response exhibits a 1-3 second delay before onset and a further 1-3 seconds to peak in response to stimuli due to physiological processing.
- Typical Changes: Short-term (phasic) conductance changes measured before and after stimuli typically range between 0.2 and 1 μS.
- Challenges in Measurement: Common artifacts from loose electrodes, movement, and deep breaths can affect readings.
Spontaneous Fluctuations
- Definition: Fluctuations in EDA can occur in the absence of external stimuli, termed spontaneous (non-specific) responses, often larger than 0.01 μS.
- Electrodermally Labile vs. Stable:
- Labile: Individuals with high spontaneous fluctuations, greater emotional responsiveness.
- Stable: Individuals with lower fluctuations, exhibiting regulated emotional responses.
Historical Context and Applications
- Historical Use: EDA has roots in clinical studies dating back to Jean Charcot and emotional research by Carl Gustav Jung.
- Research Applications: Widely studied in emotions, arousal, attention, and cognitive load since the early 20th century.
- Psychophysiological Traits: Differences in lability/stability are partly genetically determined (up to 50%).
- Behavioural Associations: Electrodermal lability corresponds with effective attention span and emotional regulation, while psychopathy often shows diminished EDA responses.
Utility in Investigative Contexts
- Polygraph Testing: EDA is a crucial component in polygraphic lie detection, a method based on monitoring psychophysiological responses, though its scientific validity is debated.
Class Goal
- Objective: Demonstrate basic EDA phenomena and the polygraphic method using the Biopac Student