What are tonic and phasic changes?

Last Updated: Jul 08, 2014 03:25PM EDT

Skin conductance measurement is traditionally characterized into two types – tonic and phasic – which can roughly be thought of as “the smooth underlying slowly-changing levels” vs. “the rapidly changing peaks.”

Tonic – Tonic skin conductance is generally considered to be the level of skin conductance in the absence of any particular discrete environmental event or external stimuli. This slow-changing level is generally referred to as Skin Conductance Level (SCL). Tonic skin conductance level can slowly vary over time in an individual depending upon his or her psychological state, hydration, skin dryness, and autonomic regulation. Tonic changes in the skin conductance level typically occur in a period of from tens of seconds to minutes.

Phasic – Phasic skin conductance measurements are typically associated with short-term events and occur in the presence of discrete environmental stimuli (sight, sound, smell, cognitive processes that precede an event such as anticipation, decision making, etc). Phasic changes usually show up as abrupt increases in the skin conductance, or “peaks” in the skin conductance. These peaks are generally referred to as Skin Conductance Responses (SCRs).
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