# Camera-Sourced Heart Rate Synchronicity: A Measure of Immersion in Audiovisual Experiences
## 2 Background
### Psychophysiological Methods and Audiovisual Media  
Psychophysiological methods explore the interrelation between psychological phenomena and physiological processes. A variety of these methods, which often include both self-report questionnaires and physiological monitoring, have been applied for the assessment of audiovisual experiences. Common physiological features applied in this domain include heart rate variability (HRV), which is considered to be an indicator of stress and emotional response [39]. In this research, we concentrate on ISC-HR analyses. The principle underlying ISC analyses is to assess the degree of synchrony in physiological responses among participants. Hence, these measures can only be used when subjects' experiences are time-locked, such as non-participatory film and TV viewing. In some cases, ISC features are calculated by comparing the physiological response of one participant with the average response of all participants. However, more commonly, ISC features are calculated by taking the average pairwise correlation of one participant with all other participants [40]. This second approach is considered advantageous as it allows for commonality amongst clusters of participants to be represented in the ISC feature rather than the ISC feature only representing the similarity of a participant's response with the overall trend [41]. The ISC is commonly calculated for each participant \(i\) within a set of length \(N\) as the mean of all pairwise Pearson correlation coefficients (PCCs) [42]. Hence, in the equation below, variable \(j\) is used to index all other participants in the set, excluding participant \(i\). \[\text{ISC}_{i}\ =\ \frac{1}{N-1}\sum_{j\
eq i}^{N}\text{PCC}(HR_{i},HR_{j}) \tag{1}\]  
Research by [PERSON] et al. showed that a viewer's level of attentional engagement, defined as the combination of attracting the attention of the eye and the cognitive processing of the target of this attention, correlates with ISC features associated with electroencephalography (ECG), gaze position, pupil size, and heart rate (ISC-HR) [11]. Notably, in this study, each participant viewed the media individually within this study, showing that these physiological modalities synchronise even when participants are not co-present. It has been found that heart rate synchronises during media experiences in accordance with an individual's level of immersion. This statement is supported by experimental data showing how ISC-HR extracted from ECG data correlates with existing immersion measures, including the Film IEQ and a single-item approach [9]. Assessing both audiovisual and audio-only stimulus across a set of experiments, [PERSON] et al. showed that ISC-HR is reduced when subjects are distracted from the narrative, and a higher ISC-HR predicts a better recall of the narrative [12]. [PERSON] also revealed that slow changes in heart rate were most indicative of whether viewers were exposed to a distraction or not. Specifically, frequency analyses suggest that this difference is greatest when ISC is calculated using an ECG-sourced HR signal bandpass filtered between 0.08 Hz and 0.153 Hz [12]. This finding is also supported by [PERSON] et al. [11], although their data seem to suggest that even lower frequencies may also be effective indicators of distraction. [PERSON] et al. [13] applied this consideration to the processing of their heart rate data when assessing the relationship between ISC-HR and flow during musical performance, low-pass filtering their interbeat interval signal with a cutoff of 0.05 Hz. The frequency of synchronisation found to be indicative of the immersion-related measures documented in these studies may have been affected by the pacing and duration of the stimulus used [41]. [PERSON] et al. used 5 to 10 min educational videos [11], [PERSON] used 1 to 11 min audio and audiovisual stories [12], and [PERSON] used performances of length 5 to 13 min [13]. Commonly used statistical tests, such as the \(t\)-test [43] and Pearson correlation [42], assume that observations are independent.