What are tonic and phasic changes?

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.


Fig.1The two gray shaded areas contain no intentional events while the person was resting. The EDA is smooth and slowly changing, and can be used to estimate tonic SCL. The tonic SCL in each smooth region can be computed as the average of that region. Here the tonic level is higher after this person exercised.

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).