Biomedical and physiological foundations of Galvanic Skin Conductance.
Electrodermal activity (EDA), historically termed the Galvanic Skin Response (GSR), provides a sensitive, involuntary peripheral biomarker of sympathetic autonomic nervous system arousal. Innervated exclusively by postganglionic sympathetic sudomotor nerve fibers releasing acetylcholine, human eccrine sweat glands (densely concentrated on the palmar surfaces of the hands and plantar surfaces of the feet) fill with sweat in response to emotional, cognitive, and sensory stimuli. Because sweat is an electrolyte solution rich in NaCl, sweat filling the non-conductive pores of the stratum corneum creates parallel electrical conductance pathways that dramatically lower the electrical resistance of the skin. In modern psychophysiology and lie detection (polygraphy), endosomatic and exosomatic recording techniques are employed. Under the standard Lykken exosomatic method, a constant regulated DC voltage (typically 0.50 V) is applied across two Ag/AgCl finger electrodes, and the resulting current is measured to evaluate skin conductance G (expressed in microsiemens, μS). The signal decomposes into a slowly varying tonic baseline (Skin Conductance Level, SCL) and fast, transient phasic peaks (Skin Conductance Responses, SCR) that occur 1 to 3 seconds following a stimulus. Across standardized sensory stimulus intensities I_stim, the peak phasic conductance amplitude ΔG exhibits direct linear proportionality: ΔG = k_g · I_stim. In this experiment, an instrumented GSR constant-voltage circuit evaluates palmar skin conductance during calibrated arousal tasks.
Viva voce questions & analytical theory
What is the physiological mechanism responsible for changes in skin conductance?
Sympathetic cholinergic nerves stimulate eccrine sweat glands to secrete sweat (a conductive saline solution). As sweat rises into sweat ducts toward the skin surface, it creates low-resistance parallel ionic conduction paths through the insulating stratum corneum, increasing overall skin conductance.
Why is a constant voltage of 0.50 V used rather than a higher voltage?
Voltages above 0.8 to 1.0 V cause non-linear electrical breakdown of cell membranes in the stratum corneum (electroporation) and induce sweat gland damage and polarization overpotentials. A low 0.50 V potential ensures strictly linear ohmic current flow.
What is the difference between tonic and phasic electrodermal activity?
Tonic activity (Skin Conductance Level, SCL) is the slow, continuous baseline conductance that changes over minutes reflecting general arousal or body temperature. Phasic activity (Skin Conductance Response, SCR) refers to rapid, transient spikes occurring 1 to 3 seconds after specific emotional or sensory stimuli.
Why are palmar and plantar skin sites preferred for GSR recording?
The palms and soles have the highest density of eccrine sweat glands in the human body (> 600 glands/cm²) and are primarily responsive to psychological, cognitive, and emotional stimuli rather than thermal regulation.
What is the typical latency of a skin conductance response following a stimulus?
The physiological latency is approximately 1.0 to 3.0 seconds, caused by the time required for sympathetic unmyelinated C-fibers to conduct action potentials to the glands, followed by sweat secretion and duct filling.
How does sweat gland habituation affect repetitive GSR trials?
Repetitive exposure to the same stimulus causes psychological habituation; sympathetic nervous discharge decreases, and subsequent SCR amplitudes diminish unless stimulus intensity is altered or rest intervals are provided.
Experimental precautions and patient safety
- Instruct the subject to keep hand completely relaxed and motionless; finger twitches create mechanical motion artifacts.
- Maintain randomized inter-stimulus intervals (> 30 s) to prevent sensory anticipation and habituation.
- Do not use abrasive skin preps or alcohol swabs; skin abrasion destroys the natural stratum corneum barrier.
- Ensure ambient room temperature is kept at thermoneutral levels (21-23 °C) to avoid thermoregulatory sweating.
Theoretical assumptions & biological limits
- The linear arousal model applies across sub-saturation sensory intensities (1.0 to 6.0 units).
- Assumes normal sympathetic autonomic function; peripheral neuropathies (e.g., in diabetes) abolish SCR.
- Does not differentiate between positive arousal (excitement) and negative arousal (fear/stress); both increase GSR.
- Varies widely between individuals based on basal sweat gland density and skin hydration state.