When people are asked to identify the cause of an increase in physiological arousal, they often misattribute the source. In an iconic study, Dutton & Aron showed that we’re often not able to discern what exactly is triggering an increase in our physiological arousal. In their study, male participants were asked to walk down either a shaky suspension bridge (that most likely would induce fear) or a sturdy bridge (that would not induce fear) while an attractive female (a confederate of the study) waited at the end. The female then asked questions from a questionnaire, and gave her phone number to the participants, stating that they could contact her if they had any further questions about the experiment.
Why do we bring that up? Let’s explain: The interesting finding was that significantly more males contacted the female when they had just walked down the shaky suspension bridge. Apparently, the increase in physiological arousal as a result of the fear from the bridge was misattributed, and was directed towards how the male participants felt about the female study confederate.
Apart from the fun fact that you might find your dream match after an exhausting session at the gym, self-reported arousal measures apparently are somewhat flawed.
With this in mind, monitoring arousal in an objective way using biosensors might be ideal. Heart activity is closely linked to physiological and psychological arousal. Seeing a dear friend or running from your worst enemy both trigger autonomic arousal processes that result in your heart pumping faster.
How can the beat of your heart be recorded and made available for analysis and interpretation? What is ECG exactly? Let’s figure this out.
Physiology and function of the heart
Before we dig deeper into the fundamentals of ECG, let’s briefly recap on heart physiology and function:
- 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 sending blood through the circulation. 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, bpm).
- The heart operates automatically – it is self-exciting (other muscles in the body 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?
Heart activity can be recorded in two ways:
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 (data unit: microvolt, 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. While not as accurate as ECG recordings, PPG clips use dry sensors and can be attached much quicker compared to ECG setups, making their use easier 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 one’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 beats per minute (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: High-frequency (HF) activity has been found to decrease under conditions of acute time pressure and emotional stress. Also, HRV seems 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, biometric data based on heart rate alone offers valuable insights into subconscious arousal in response to emotionally loaded stimulus material. However, solely based on ECG or PPG data we can‘t be sure 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, that is, the direction of an emotion. The true power of ECG/PPG techniques unfolds as these sensors are combined with other biometric sources such as facial expression analysis, EEG, and eye tracking.
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Curious to dig deeper into ECG? Have a look at our list of must-reads to learn more about cardiac data recordings and analysis.
- Dutton & Aron (1974). Some evidence for heightened sexual attraction under conditions of high anxiety. Journal of Personality and Social Psychology 30: 510–517. (link)
- Nickel & Nachreiner (2003). Sensitivity and Diagnostics of the 0.1-Hz Component of Heart Rate Variability as an Indicator of Mental Workload. Human Factors 45 (4): 575–590. (link)
- Jönsson (2007). Respiratory sinus arrhythmia as a function of state anxiety in healthy individuals. International Journal of Psycho-physiology 63 (1): 48–54. (link)
- Brosschot, Van Dijk, & Thayer (2007). Daily worry is related to low heart rate variability during waking and the subsequent nocturnal sleep period. International Journal of Psychophysiology 63 (1): 39–47. (link)
- Hagit et al. (1998). Analysis of heart rate variability in posttraumatic stress disorder patients in response to a trauma-related reminder. Biological Psychiatry 44.