Galvanic Skin Response

Galvanic skin response (GSR), also referred to as skin conductance response (SCR) or electrodermal activity (EDA), is the property that momentarily makes our skin a better conductor of electricity when we are physiologically aroused.

When we are aroused, a number of body processes get activated – for instance, our heart beats faster and pulse rise. We also sweat more, and as we sweat, our skin conductance increases.

So, GSR is essentially triggered by sweating, and is measured in micro-Siemens (µS) or micro-Mho (µM). (It is actually a more complex phenomenon, particularly in light of recent research findings. Practitioner’s, however, need not get into the finer details).

Arousal is one of the two main dimensions of emotion, the other being valence. GSR can capture emotional arousal, or the level of stimulation, and this makes it a strong predictor of attention and memory. It does not reveal emotional valence, i.e. it is unable to detect whether we are feeling good (positive) or whether we are feeling bad (negative). In this respect, GSR and facial coding are complementary. Facial coding captures the valence of emotion, but cannot gauge the associated arousal.

Since sweat secretion cannot be consciously controlled, GSR is able to track autonomic arousals. This is what makes it a non-deceptive indicator of our emotions. It also explains its use along with other technologies, in lie-detectors.

The other prime advantage is that GSR is relatively inexpensive and easy-to-measure, requiring fairly simple devices.

Devices


Exhibit 25.19   Shimmer GSR sensor. (Source: iMotions).

A GSR sensor is a non-invasive device that requires minimum preparation/calibration.

The operation is fairly straightforward. Two electrodes are placed where there is high density of the eccrine sweat glands, such as fingers, palms or the soles of the feet. A low voltage is applied. The skin conductance is computed by measuring the current flowing between the electrodes.

The fingers, palms and foot soles are the parts of the body that respond to emotional and other psychological stimuli. As shown in Exhibit 25.19, the middle and index finger are ideal locations to place the electrodes, typically on the non-dominant hand so that respondent is free to use the dominant hand.

If respondents need to use their fingers, such as for typing, then the palm is an option, or else the inner side of the sole of the foot.

As can be seen from the exhibit, the electrodes are mounted on Velcro straps. The device is easily worn by fastening the straps.

n addition to the electrodes, the device comprises of an amplifier to boost the signal, and a digitizer to convert from the analog signal to digital.

Analysing GSR Signals

The process of translating the raw GSR signals into meaningful information that minds can cope with is a little complicated. But, since this is automated, it not something a marketer needs to study in much detail.

As mentioned, skin conductance is computed by measuring the current flowing between the electrodes. The formulae, which you might recall from your physics lessons in school:

$$ Ω=\frac{V}{A};\, S=\frac{1}{Ω} $$

where Ω is ohm (resistance),V is volt,A is ampere and S is siemens.


Exhibit 25.20   GSR original and phasic response patterns.

Filtering out the tonic level: The GSR signal has two main components – skin conductance level (SCL), also known as the tonic level, and skin conductance response (SCR), aka phasic response.

The tonic level varies slowly over time, and from individual to individual. Moreover, since it is dependent on factors other than emotional arousal, it is not of relevance for analysis purposes.

Because it moves much more gradually than the phasic response, the tonic component is approximately the same as the smoothened GSR signal. It can be filtered out simply by applying a smoothing function and subtracting it from the data.

Take for example the data depicted in Exhibit 25.20, where the tonic level is removed using a fairly basic smoothing algorithm.

Baselining: Individuals have different baseline patterns of electrodermal activity. Some tend to have skin conductance signals that do not vary much. These individuals are called stabiles. Others have many more skin conductance responses, even when they are resting. These individuals are called labiles.

At the start of a recording, we need to establish the respondent’s baseline. A neutral baseline is recorded when there is no stimuli and the respondent is relaxed. The respondent is then shown a video with content that should elicit a range of positive and negative emotions. His responses to these stimuli help establish his baseline pattern.

Incidentally, stabiles and labiles tend to have different personalities. According to some studies a labile tends to be more inhibited in emotional expression, whereas the stabile is more expressive and antagonistic. A stabile also has difficulty maintaining attention over time and is more likely to fall asleep in boring situations.

Response time and test stimulus: There exists a lag, varying from 1 to 5 seconds, between the appearance of the stimulus and the phasic response. In order to capture the complete pattern and clearly distinguish the responses, the stimulus is presented for at least 5 seconds. In between the test stimuli, a calming down stimulus is shown so that respondents return to baseline.

Metrics


Exhibit 25.21   GSR response pattern and metrics.

The GSR follows the pattern shown in Exhibit 25.21. As mentioned above, it takes a few seconds after the stimulus is shown, for the amplitude to start rising. This is called the latency period.

The onset is auto-detected by setting a cut-off amplitude, for instance, 0.01 µ.

The amplitude rises to a peak, before returning to the baseline. The offset is point of return when the GSR phasic dips to zero/below zero. And the peak amplitude is the increase in amplitude, from onset to peak.

The time duration between onset and peak is called the rise time. And the time duration from peak to offset, is referred to as the recovery time.

Given the cut-offs defined above, algorithms are able to detect the GSR pattern and record the metrics.

Applications

Marketing applications of GSR include product development, packaging development, copy testing, and testing of websites.

Typically respondents, at a controlled location, strapped with GSR sensors, are exposed to the stimulus that is to be tested. Key metrics such as the number of peaks, and the peak amplitude are recorded for each respondent, and averaged across the total sample as well as relevant demographics.

The GSR metrics quantify the emotional arousal of the individuals, sub-groups, as well as the total sample. Marketers can compare the arousal level for different prototypes, or they can compare the metrics against benchmarks to gauge the prototype’s performance.

Common analysis include comparison of peaks/minute and highs and lows across respondents.

For instance, consider advertising. If the objective of an advertisement video is to penetrate consumers’ minds associating the brand with positive or negative emotional states, then GSR can help determine if this objective is likely to be met. More peaks/minute and peaks of higher amplitude would indicate that respondents are experiencing frequent emotional arousals of greater intensity.

Importantly, while GSR is indicative of emotional arousal, it is not an indicator of emotional valence (positive/negative). So one should consider using it in combination with other devices such as facial coders that are able to capture the nature of respondent’s emotions.

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