# Camera-Sourced Heart Rate Synchronicity: A Measure of Immersion in Audiovisual Experiences
## 2 Background
### Immersion and Audiovisual Media  
The term _immersion_ is often inconsistently defined and applied in academia and other fields [16]. A filter model, as first proposed in [17], provides a useful reference for exploring these varying interpretations [18; 19]. Three domains are conceptualised in this model: physical, sensory, and affective. Audiovisual experiences start in the physical domain, with physical characteristics such as sound pressure level or the number of loudspeakers being measurable objectively. Next, these physical changes pass through a sensory filter into the perceptual domain, where they can be described as perceptual attributes that we sense, such as loudness or envelopment. Finally, these perceptual attributes pass through a cognitive filter into the affective domain, where they can be described as global measures that we feel, such as fear or enjoyment. Immersion is sometimes considered to be a perceptual attribute or physical characteristic. However, in this paper, a more common interpretation amongst the audio engineering community is applied: immersion is a _global measure_[18; 19; 20]. To describe this global measure in this study, we exercise the following definition: \"placing one's full attention within the film world\" [16]. Some sense of being perceptually located in the film world, rather than the real world, and a sense of focused attention on the narrative are both referenced in this definition. This definition of immersion was derived through an analysis of open-text response data collected from a sample of the public in 2022 [16]; hence, the outcomes of a single-question immersion assessment should reflect this interpretation. Moreover, the definition reflects [PERSON]'s interpretation [21], which is cited and applied in the development of the Film IEQ [22]. Notably, both the single-question approach and Film IEQ are considered to be valid approaches, which have been widely used for assessing immersion in audiovisual experiences, including in combination [9; 23; 24; 25; 26]. Immersion is also used synonymously with many other terms, such as presence and narrative engagement. This semantic issue is an ongoing point of discussion within various research domains, including audio research [16; 20; 27]. This cluster of related attributes has been applied in various publications investigating the influence of audio presentation on non-participatory audiovisual experiences, such as film viewing. [PERSON] et al. [28] found that presence increased with the addition of bass but not with an increase in channel count. [PERSON] et al. [29; 30] demonstrated that downmixing and filtering can significantly decrease basic audio quality. [PERSON] et al. [31] found no significant difference between stereo and a 5.1 presentation mode for music, but there was a tentative increase in _immersiveness_ for film clips. [PERSON] et al. [32] showed that participants did not perceive Dolby Atmos as significantly more immersive, dynamic, or having better localisation or audio quality. [PERSON] et al. [24] found no significant difference in immersion between groups with different channel counts, but there was a significant difference in the sense of envelopment for some stimuli with each incremental increase in channel count. [PERSON] et al. 's [19] results suggest that sounds emanating from the rear surround loudspeakers in a 7.1 soundtrack can be distracting. [PERSON] et al. [33] found that an increase in channel count correlated with higher scores on questions related to realism, emotional involvement, and physical immersion, and some indication of neural correlates were associated with differences in audio presentation. Overall, these empirical studies highlight the complex relationship between audio presentation and non-participatory audiovisual experience as well as the lack of consistent methods for testing.