Skin Conductance Response (SCR)

What are Skin Conductance Responses (SCR)?

Skin Conductance Responses (SCR) are brief, phasic fluctuations in skin conductance above the tonic baseline. The eSense Skin Response counts detected SCR per minute. Use SCR/min to observe your responses during relaxation training. Also use the following existing Mindfield experience-based ranges as rough orientation within newer sessions.

Skin conductance contains two components: the tonic, static “level” and the phasic, fluctuating “reactions”. These phasic reactions are called Skin Conductance Responses (SCR).

SCR/min describes the frequency of detected responses; µS describes measured skin conductance. Background on electrodermal activity is provided in [1]. The existing Mindfield experience-based ranges are provisionally retained as rough practical guidance: 0–5 SCR/min relatively relaxed, 6–9 SCR/min more activated, from 10 SCR/min markedly more activated, and over 16 SCR/min very strongly activated. They originated with the earlier counting method up to and including app version 8.1.3 and are not scientific reference ranges; recalibration for the detection documented from 8.1.6 is still pending. Also use the ranges for rough orientation within newer sessions. Assess progress against your own baseline with the same detection method. Positive excitement can also increase the value. If your value rises during relaxation training, rest your hand, relax deliberately and compare the subsequent signal with your own resting phase.

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[1] Boucsein, W. (2012). Electrodermal Activity (2nd ed.). Springer, New York. DOI: 10.1007/978-1-4614-1126-0.

[2] Boucsein, W., Fowles, D. C., Grimnes, S., Ben-Shakhar, G., Roth, W. T., Dawson, M. E., & Filion, D. L. (2012). Publication recommendations for electrodermal measurements. Psychophysiology, 49(8), 1017–1034.

Start with a 10-minute resting baseline recording without watching the readings. Afterwards, note SCR/min, mean conductance in µS and how tense you felt. In the app archive, compare this recording with further resting and training recordings of the same length: use the same app version and detection method, electrode type and position, and preferably the same time of day and similar room temperature. Note the app version yourself if it is not identifiable in the recording. During relaxation training, practise reducing SCR/min relative to your own baseline over several sessions; you do not need to reach 0 SCR/min. After a change of detection method, record a new resting baseline rather than interpreting the difference from the old counter as training progress.

The mean value in µS includes both the baseline and SCR occurring during the recording. Many peaks can raise the mean, so it is not the same as an isolated tonic level. To assess your baseline, select a quiet phase with few, small SCR and note its conductance. In relaxation training, practise lowering this personal level under the same recording conditions. Compare quiet phases of equal length. If readings keep rising without an apparent reason, the electrodes may be too tight and the skin underneath may be sweating; loosen them slightly and repeat the recording.

We have colored the SCR/min in the eSense app. Low SCR are green or yellow and high SCR/min are orange or red. While making a measurement, the recognized SCR are colored within the curve. Two examples from our app:

The diagrams on the right show the proportions of time with rising, falling and stable readings. Compare resting and exercise phases of equal length: try calm, comfortable breathing or deliberately relaxing your muscles, and observe whether conductance falls or remains stable more often and fewer SCR occur. Note which exercise helps you. After a phase with pronounced rises, return to rest and check recovery. Rising readings can also accompany interest or positive excitement; interpret the signal alongside the situation and your own experience.

How the app detects a spontaneous fluctuation (from version 8.1.6)

The following detection method is documented from app version 8.1.6: the app examines the rise from a local minimum to the following maximum. It counts as an SCR if it reaches at least 0.02 µS and its rise time is between 1 and 20 seconds. Check and note your app version before starting a comparison series. The four-step description further below applies to the earlier detection method up to and including 8.1.3. The app also includes a plausibility check against contact and saturation artifacts. If counts seem implausible, check electrode contact first and keep your hand still.

Boucsein et al. (2012, section 3.1)[2] describe amplitude criteria from 0.01 to 0.05 µS, with the choice depending on recording conditions and equipment noise. The manufacturer documentation describes steps of 0.01 µS for the eSense Skin Response and explains the 0.02 µS threshold as intended to prevent a single resolution step from counting as a response. Boucsein et al. give a typical rise time of 0.5 to 5 seconds from response onset to the peak. The window of 1 to 20 seconds documented for the app from version 8.1.6 differs from this. The publication validates neither the device resolution nor the app’s detection criteria.

In practice this means the SCR rate now depends far less on how calm or restless the signal is overall. With low readings, part of the measurement noise used to be counted as a fluctuation. Keep your recording hand still and check electrode contact first if readings change abruptly and implausibly. Do not interpret a lower SCR rate immediately after changing app versions as better relaxation on its own.

Can older and newer recordings be compared?

SCR values from recordings made before version 8.1.6 cannot be compared one to one with newer ones. This concerns the number of spontaneous fluctuations, the SCR per minute figure and the colour classification derived from it. The shape of the curve, the baseline level in microsiemens and the pronounced rises and falls remain comparable. Start a new baseline with the version you use to assess further progress.

A second change works in the same direction. Since versions 8.1.x the app smooths the measurement signal with an adaptive filter, which can also lower the number of counted fluctuations. What that filter does and what it does not do is explained in Why does my curve look smoother than before?.

When comparing SCR/min and the colour classification in the archive, select recordings with the same app version and detection method and, as far as possible, matching recording conditions; being close in time is not enough. You can also compare the shape of the curve, the baseline level in µS and pronounced rises and falls with older recordings. If the earlier version is unknown, record a new baseline for comparing SCR/min.

The existing Mindfield experience-based ranges of 0–5, 6–9, from 10 and over 16 SCR/min are provisionally retained as rough practical guidance for newer sessions too. They originated with the earlier counting method; recalibration for the detection method documented from app version 8.1.6 is still pending. They are not scientific reference ranges. Use them for a broad interpretation within a session and assess training progress against your own baseline under matching conditions and with the same detection method. Do not combine the two counting methods when assessing progress.

How detection worked up to version 8.1.3

We are keeping the earlier description because many users hold series of recordings that reach across this version boundary. Up to and including version 8.1.3, detecting and calculating the SCR was a cycle of four steps:

1. Listening state
2. Determining if the signal raise is really a SCR
3. Gathering and calculating SCR and listening asynchronously for SCR recovery phase
4. Initiate recovery phase and end of fluctuation

Listening state		Determining if the signal raise is really a SCR
Initiate recovery phase and end of fluctuation		Gathering and calculating SCR and listening asynchronously for SCR recovery phase

The 4 Steps in detail

1. Listening state

The cycle starts in the first step, the listening state. This is the standard default state.

2. Determining signal raise

There are two ways for detecting the start of an SCR start. The values could either raise slowly with a small step or very fast with a big amplitude. This creates the following to conditions:

1. The signal is constantly rising for 2 seconds 
2. The difference between the current signal and the pretended base value (the value that is considered to be the first value in the rising signal. But on this stage, it is not confirmed yet as a base value) is greater than 0.5 micro siemens.
   In simple words the app will react to a sudden signal raise and confirm it as the fluctuation start and will not wait for 2 seconds (as in the first condition above)

If the signal will fall just even 0.1 micro siemens during a raise, the app will drop out of this second step and return to the first step (the listening state) again. Otherwise it will continue with the next two steps 3 and 4.

3. Gathering and calculating SCR (fluctuation)

If one of the two conditions from the second step above are met, the app enters this third step, the fluctuation. The first signal of the fluctuation period is the base value of a fluctuation signal. During the whole fluctuation period, the app is detecting the amplitude which is constantly changing. Then the app starts to calculate the SCR events globally and within a one-minute time window. This results in the SCR/min value.

4. Initiate recovery phase

The fluctuation is considered to enter the recovery phase (the end of fluctuation) when the signal falls for more than 50% of the greatest amplitude of the fluctuation period. 

This is also the step where the color in the graph is changed (based on the quantity of SCR events that were occurring). The colors for the SCR background are also calculated (by dividing time intervals into equal segments and getting the sum of the SCR events from each segment).