Gel vs. Dry Electrodes, Motion Artifacts & Dermatomes (Skin surface location)
- Gel vs. dry electrodes. The Q Sensor was designed for studies that take place outside of the lab, optimized for 24-hr recordings in which dry electrodes are ideal. Gel electrodes however, are not suitable for long-term recordings. The signal’s amplitude will degrade over time as the gel is absorbed by the skin. In his preeminent work Electrodemal Activity, Boucsein states “during EDA recording the electrolyte-skin system should be disturbed as little as possible”. He suggests using Sodium Chloride (salt) based electrolytes in order to mimic the natural biopotential of the skin surface. With the Q Sensor relying on sweat as the electrolyte solution, the entire system is comfortably mounted on the skin surface. Sweat occurs naturally between the skin surface and electrode. Sometimes this process takes time to initiate so we recommend warm-up and baselining activities be a standard part of any study.
- Movement/Motion artifact. Motion artifacts are a concern in any application involving EDA and there are many ways to address them. Traditional EDA recording requires lead wires to connect the transducer to the electrodes on the palmar surfaces. Movement of the arm has potential to create strain on the electrode connection, so research participants are required to remain still. In these applications, the use of cup electrodes mounted with adhesive pads and conductive gels is one way to help reduce the impact of electrode movement on signal integrity, but these solutions can be uncomfortable and are still prone to failure when adhesives become detached. Most pre-gelled gels are highly conductive, oriented toward EEG monitoring. Pre-gelled electrodes are not suitable for EDA measurement so be careful to select electrodes designated for EDA recordings if you chose this solution. Affectiva has taken a different approach to reducing motion artifacts with the Q Sensor by eliminating external lead wires connecting electrodes to the transducer. While there is no known solution to totally eliminate the occurrence of motion artifacts, proper sensor placement and participant education on the signals recorded are suitable for analysis and summarization. Signal processing is available through Affectiva’s Professional Services divisions and are not only able to extract skin conductance response features, but also distinguish them from most noise caused by typical motion artifacts cause by pressure and/or electrode movement on the skin surface. In the event that a pre-gelled solution is desired, the Q Sensor can be used in conjunction with most snap-in electrodes provided they have sufficiently small conductive surface diameters (the distance between snap connector origins is approximately 1.5 cm.
- Dermatomes (Skin surface location). Affectiva offers two different types of Q Sensors, the Q Pod and Q Curve. Both sensors can be worn on the distal forearm for comfortable long term recordings, but we recommend the Q Pod be worn on the palm for short term recordings in the lab; since the palm has a higher density of sweat glands. During lab studies of activation in awake participants, we usually find the palmar response to have larger amplitude than the forearm response. Note: this is reversed during sleep, and there are exceptions in both cases. After warmup and baselining activities, EDA recordings from the distal forearm recorded with dry electrodes are correlated with recordings from traditional devices. (See the “IEEE Publication” in the “Research” section, p: 27-31 for a discussion of dermatomes and emotional sweating).