ECG (electrocardiography) is a method of collecting electrical signals generated by the heart. This allows us to understand the level of physiological arousal that someone is experiencing, but it can also be used to better understand someone’s psychological state.
Below we’ll go through the importance of physiological arousal in emotions, the physiology of the heart, how the activity can be measured, and what parameters are of interest.

Emotional Arousal

When people are asked to identify the cause of an increase in physiological arousal, they often misattribute the source. We might think we understand everything that’s going on with our body, but studies show that this is often not the case.
An influential study by Schacter and Singer in 1962 showed how this occurs. Participants were told that their vision would be tested after they were given vitamins. They were instead either given epinephrine (also known as adrenaline), or a placebo drug.

They were then told that the the effects of the drug were the same as epinephrine, or that they might feel some discomfort, or they were told nothing at all.
Epinephrine is known to activate the central nervous system, bringing about such changes as increased heart rate and pupil dilation. The participants were then placed in a waiting room with a confederate (an actor employed by the researchers) of the study who either pretended to be angry, or happy.
The angry or happy conditions were a series of standardised steps (step fifteen of the happy condition: “Stooge replaces the hula hoop and sits down with his feet on the table. Shortly thereafter the experimenter returns to the room.”) that were intended to influence the emotional state of the participant.
The participant then had to complete a questionnaire about how they were feeling. They found that not only was the emotional state of the participant influenced by the apparent emotion of the confederate (and more so in the epinephrine condition), but that the participants also attributed that emotion to events entirely unrelated to the situation they were currently in.

Physiology and function of the heart

  • The heart has four chambers. The upper two chambers (left/right atria) are entry-points into the heart, while the lower two chambers (left/right ventricles) are contraction chambers that send blood out to the body. The circulation is split into a “loop” through the lungs (pulmonary) and another “loop” through the body (systemic).
  • The cardiac cycle refers to a complete heartbeat from its generation to the beginning of the next beat, comprising several stages of filling and emptying of the chambers. The frequency of the cardiac cycle is reflected as heart rate (beats per minute, or bpm).
  • The heart operates automatically – it is self-exciting (this is a unique feature compared to other muscles in the body that require nervous stimuli for excitation). The rhythmic contractions of the heart occur spontaneously, but are sensitive to nervous or hormonal influences, particularly to sympathetic (arousing) and parasympathetic (decelerating) activity.
 

How to measure heart activity?

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 [2], as well as in patients suffering from post-traumatic stress disorder (PTSD) [3]. 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.

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