Neuro-Tools : GSR October 24, 2016Posted by eyetrackrob in Biometric, Captiv, eye tracking, Glasses, Market Research, neuromarketing, TEA, Tobii, Uncategorized.
As mentioned in my first introduction to this blog, the central nervous system is divided into different branches which monitor and control different body functions. One of the branches, the sympathetic nervous system (SNS), is responsible for quick fight or flight reactions. By constantly accessing the surroundings and scanning for situations that could potentially be dangerous an evaluation takes place which leads to preparations for an adequate fight or flight reaction. These preparations can be measured throughout the body and include changing heart rate, respiration and levels of sweat on hands and feet.
As we start to understand that these non-conscious reactions are strongly and inseparably tied to decision making processes and thus human behaviour, more and more researchers have become interested in using tools to measure these reactions.
In my first post a few weeks ago, I wrote about the general rise of Neuro-Tools and mentioned some such as eyetracking, EEG, facial expression analysis, GSR, heartrate and respiration as well as Implicit Association Tests as examples. The series aims to go through these tools one by one and review what they measure, how they work and of course also where we run into the limitations of those tools. With the general objective to give you a perspective on how these tools can be made a valuable addition for your research, I’d like to continue the series looking at GSR today. Initially I thought of talking about GSR, heartrate and respiration in this post as they could easily be summarized as “biometrics” or “biofeedback measurements”, but it turned out to be a quite long post, so I’ll split them down into individual posts.
Enough of the introductions! Let’s dig into the exciting world of biometrics starting with:
Galvanic Skin Response
GSR isn’t simply around measuring sweat, there is an awful lot more to it than that so before offering some general advice on what to look out for when considering to use GSR, I would like to explain the basics around this tool.
Electrodermal Activity (EDA), Skin Conductance (SC) or Galvanic Skin Response (GSR) refer to the ability of the skin to conduct electricity due to changes in the activity of the sweat glands and thus the secretion of sweat. Those changes are closely related to psychological processes and can be triggered by emotional stimulation. Electricity can be conducted when an external, unnoticeable current of constant voltage is applied, and with more moisture on the skin, electrical resistance decreases and skin conductance increases at a measurable level, although sweat might not necessarily be visible through visual observation.
Skin conductance can be divided into tonic and phasic activity. The level of conductivity of the tonic activity is constantly changing within each individual respondent, depending on their hydration, skin dryness and autonomic regulation in response to environmental factors such as temperature for example. Phasic response in turn are short term peaks in GSR reflecting reactions of the SNS to emotionally arousing events, mostly independent of the tonic level. For most of the time, we will be looking at these reactions which occur in the eccrine sweat glands.
GSR data is measured in microsiemens (μS) and the relevant phasic reactions can be quantified and analysed in different ways. Apart from the number of peaks occurring within a certain period after stimulus onset, peak amplitude, the time to reach peak value and the recovery time can be used for analysis. GSR can be used to determine strength of arousal but can’t be used to determine the valence (like or dislike) of a reaction.
Image 1 is an example of data including tonic and phasic activity.
The density of sweat glands varies across the body being highest on the head, the palms and fingers as well as on the sole of the feet. Most tools that measure the GSR are therefore build to be used on the fingers, where this reaction is strongest. However some instruments on the market allow for measuring the change in sweat levels on the wrist which often results in poorer data quality but might be necessary for some experiments where the hands are needed to interact with objects (i.e. holding mobile devices/products or typing).
Image 2 shows eccrine sweat gland concentration. Red areas indicate a high concentration of eccrine sweat glands (glands.cm−2) allowing to measure sympathetic arousal of low intensity and minimal duration. Green zones indicate a low concentration of relevant sweat glands able to measure only events of high intensity (for example on the wrist). (N. Taylor; C. Machado-Moreira, 2013)
Depending on the manufacturer and kind of system used for the measurements, sensors can be adhesive electrode pads that are already filled with conductive gel in order to reduce preparation time and to avoid electrode movement. Conductive gel is not mandatory but can improve data quality and ensure a good and stable electrical connection. Many GSR device manufacturers that provide systems for the use on fingers and toes, provide Velcro straps to place the electrodes firmly. In any case excessive respiration, movements and talking should be avoided as these can cause noise in the data or variations in the signal that can be misinterpreted.
Image 3 shows a classic sensor (TEA T-Sens GSR) that can be placed on the fingertips adjustable with velcro straps next to an Empatica E4 wristband.
As written in the introduction, reaction times and strength are highly individual and therefore distinct for each participant and they can vary between 400 milliseconds up to 5 seconds after presenting a stimulus. In a controlled lab environment a calibration procedure can help to understand individual differences in reactions but might not always be necessary. It is not advised to use GSR in areas where many low and high impact events can occur uncontrolled at any time and can be mixed with all kinds of artifacts, as it might be complex, if not impossible, to relate an emotional arousal peak to a specific event.
If free movement is a requirement (for example in shopper research) it is highly recommended to calibrate the GSR reaction time and strength for each participant and to complement the GSR measure with a synchronized video and sound feed -ideally even with eyetracking- to understand the source of the arousing events. The synchronization of several feeds can sometimes be a challenge but there are solutions that allow either for a live synchronization or a post-recording-synchronization.
Image 4 shows a synchronized recording of different sensors such as ECG, HR, HRV, Respiration and Cogntitive Workload with eyetracking (top right) and an additional video stream (bottom right). The synchronization can be done for example using the QR code that is visible on the screen (top left) marking a synchronization point in video and sensor feed.
Image 5 shows a TEA T-Log, a small and mobile device that emits a short flash of light that can be picked up by a camera or in the video of the Tobii Glasses marking a visible event in the video and a sync point in the sensor recordings.
How GSR raw data, filtered data and emotion detection works all synchronized with eyetracking, can be seen in the following short video, recorded from TEA Captiv. I also imported data from a wrist-worn GSR device but the data was not usable, which is why I chose to minimize those curves in the software. As you can see in Image 2 the concentration of eccrine sweat glands on the wrist is low which very often means having a very noisy signal or the absence of a signal. To improve the signal quality it is recommended to get a minimum level of tonic sweating, for example through some physical exercises. Although I did this (as you can indirectly and briefly see at the very beginning of the video), it wasn’t enough to make the measurement from the wrist usable. For these types of study (researching and improving the emotional and visual impact of TV commercials), I would usually recommend to use a remote eyetracker such as the Tobii X2-60 as well as sensors worn on the fingers (T-Sens GSR or similar), however I also wanted to show that it can easily be done with a mobile eyetracker if needed as shown below:
In comparison you can also watch a video of a similar test (same commercials) using a remote eyetracker as mentioned above. You’ll notice similarities in the general gaze data but also in the arousal detection, although you might also notice that each participant has a slightly different reaction time and the emotional threshold has an influence on how many emotional moments each person is experiencing:
There is still a bit more to know about GSR and we at Acuity are do offer training on methodologies, technology and best practices for your research. To give you a headstart on some of the things to consider have a think about these 4 questions and then maybe give us a call:
- Where will the data collection happen? Do you need to be completely mobile, or will it be a controlled environment close to a computer? If you go mobile, can you carry a small device to record the data or does the GSR device itself needs to store the data?
- What type of sensor do you need? Is it a viable option to use sensors on the fingers, or will you need to use the hands to hold something or type for example?
- Do you know how to analyse the data? GSR raw data is rarely usable. Do you know how to remove the effects of tonic activity and artifacts and do you need a software that can do it for you and find the relevant events?
- Do you need to synchronize the data with other devices and do you want to accumulate data over several participants?
In the next post I’ll be covering heart rate and respiration to wrap up the more commonly used biofeedback tools before taking on EEG, facial expression analysis, Implicit association tests and others. Stay tuned!