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Galvanic Skin Response (GSR)

GSR is arguably the most useful index of changes in sympathetic arousal that are tractable to emotional and cognitive states as it is the only autonomic psychophysiological variable that is not contaminated by parasympathetic activity.

Galvanic Skin Response (GSR), also known as Skin Conductance (SC) and Electrodermal Activity (EDA), is the term used for defining autonomic changes in the electrical properties of the skin.

The GSR signal arise when the skin receives innervating signals from the brain, for instance it caused by the variation of the sweating in the human body and is generally considered one of the strongest features that can be used for detecting emotional arousal.

The most widely studied property of GSR is the skin conductance, which can be quantified by applying an electrical potential between two points of skin contact and measuring the resulting current flow between them.
When a person is under mental load, sweat gland activity is activated and increases skin conductance.

Since the sweat glands are also controlled by the SNS (sympathetic nervous system), skin conductance acts as an indicator for sympathetic activation due to the mental load reaction.

  1. Background tonic component - Skin conductance level (SCL) is a slowly changing part of the GSR signal, and it can be computed as the mean value of skin conductance over a window of data.
  2. Rapid phasic component - a fast changing part of the GSR signal (signal peak) is called skin conductance response (SCR), which result from sympathetic neuronal activity and occurs in relation to a single stimulus.  

There are two major components for GSR analysis.

The GSR signal

There are two types of SCR:

  1. Event-Related SCR (ER-SCR) - SCR caused by a stimulus, typically recorded 1±5 s after the stimulus.
  2. Non-Specific SCR (NS-SCR) - SCR without any detectable cause.

Figure 3 shows two hypothetical skin conductance recording during 20 seconds of rest, then 3 intervals of discrete stimulus. The arrows in the x-axis represent each stimulus.As the Figure shows, the 3 peaks are the SCR of the GSR which indicates the rapid increase in a stimulus.

Most popular phasic and tonic GSR measures such as the SCR magnitude, NS-SCR frequency, and SCL contain considerable information that could be feasible to explain the components of the variance in various emotional states.

Tonic SCL can vary different between person and within the same person in different psychological states, the typical range of SCL values are between 2-20 µS (microsiemens) .

It is common that for SCL to decrease while a person is at rest mode, then rapidly increase in a stimulus.

Regarding figure 1, a widely used parameters for GSR include the amplitude and latency of SCR and average SCL value.

Figure 1 - Graphical representation of ER-SCR components

Figure 2 - Graphical representation of the GSR components flow

Figure 3 - 2 hypothetical skin conductance graphs

References

Galvanic Skin Response (GSR)

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