1. Electrocardiography (ECG, EKG)
ECG records the electrical activity generated by heart muscle depolarizations, which propagate in pulsating electrical waves towards the skin. Although the electricity amount is in fact very small, it can be picked up reliably with ECG electrodes attached to the skin (in microvolts, or uV).
The full ECG setup comprises at least four electrodes which are placed on the chest or at the four extremities according to standard nomenclature (RA = right arm; LA = left arm; RL = right leg; LL = left leg). Of course, variations of this setup exist in order to allow more flexible and less intrusive recordings, for example, by attaching the electrodes to the forearms and legs. ECG electrodes are typically wet sensors, requiring the use of a conductive gel to increase conductivity between skin and electrodes.
2. Photo-Plethysmography (PPG).
Throughout the cardiac cycle, blood pressure throughout the body increases and decreases – even in the outer layers and small vessels of the skin. Peripheral blood flow can be measured using optical sensors attached to the fingertip, the ear lobe or other capillary tissue.
The device has an LED that sends light into the tissue and records how much light is either absorbed or reflected to the photodiode (a light-sensitive sensor). PPG clips use dry sensors and can be attached much quicker compared to ECG setups, making the device relatively easy to use, and less bothersome for participants.
Cardiac parameters of interest
Recording heart rate data gives you access to the following parameters that can be interpreted with respect to a participant’s arousal:
Heart Rate (HR). HR reflects the frequency of a complete heartbeat from its generation to the beginning of the next beat within a specific time window. It is typically expressed as bpm. HR can be extracted using ECG and PPG sensors.
Inter-Beat Interval (IBI). The IBI is the time interval between individual beats of the heart, generally measured in units of milliseconds (ms). Typically, the RR-interval is used for the analysis.
Heart Rate Variability (HRV). HRV expresses the natural variation of IBI values from beat to beat. HRV is closely related to emotional arousal: HRV has been found to decrease under conditions of acute time pressure and emotional stress (meaning that the heartbeat is more consistent).
HRV has also been found to be significantly reduced in individuals reporting a greater frequency and duration of daily worry , as well as in patients suffering from post-traumatic stress disorder (PTSD) . For IBI and HRV analysis, ECG sensors are recommended as they are more sensitive to certain signal characteristics which PPG sensors cannot pick up.
Why combine ECG with other sensors?
Of course, data based on heart rate alone offers valuable insights into nonconscious arousal in response to emotionally loaded stimulus material. However, data solely based on ECG or PPG data can‘t tell us whether the arousal was due to positive or negative stimulus content.
Why? The change in heart rate is in fact identical. Both positive and negative stimuli can result in an increase in arousal triggering changes in heart rate.
In other words: While ECG/PPG are ideal measures to track emotional arousal, they are not able to reveal emotional valence, the direction of an emotion. The true power of ECG/PPG techniques unfolds as these sensors are combined with other data sources such as facial expression analysis, EEG, and eye tracking.