Heart rate variability (HRV) is a crucial physiological measure that quantifies the variations in time intervals between successive heartbeats. It provides valuable insights into the dynamic interplay between the sympathetic and parasympathetic branches of the autonomic nervous system (ANS), which collectively regulate various bodily functions, including heart rate. Assessing autonomic dysfunction is critical to medical evaluation, as it can offer valuable diagnostic information. Various tests, such as Respiratory Sinus Arrhythmia (RSA), Valsalva manoeuvre, orthostatic hypotension assessment, and the Isometric Handgrip (IHG) test, can be employed to gauge autonomic function (Ewing et al., 1985). HRV is valuable because it provides instantaneous and non-invasive insights into ANS activity using electrocardiography (ECG).
The neurovisceral integration model (NVM) offers a theoretical framework that further underscores the importance of this investigation. This model posits prefrontal cortex activations associated with increased vagal tone, as reflected in HRV (Nikolin et al., 2017; Thayer & Lane, 2000; Thayer & Sternberg, 2006). This model suggests that better cardiac vagal tone could mean better cognitive functioning. If we can obtain reliable vagal tone measurements through HRV analysis, we can begin to unravel the potential link between autonomic function and cognitive processes. Within HRV, two key metrics, RMSSD (Root Mean Square of Successive Differences) and HF power (High-Frequency Power), emerge as vital indicators of vagal tone. RMSSD quantifies short-term variability in heart rate intervals, offering insights into rapid fluctuations predominantly influenced by parasympathetic activity. On the other hand, HF power represents the high-frequency components of HRV, closely associated with vagal tone and reflecting the body's ability to respond swiftly to physiological changes. By delving into these HRV parameters, we commenced our investigations with a cohort of healthy controls to understand the potential links between HRV, and memory.
Taylor and Lane (2000) introduced the Neurovisceral Integration model, which posits a strong association between parasympathetic activity regulating the heart and cognitive functions. This model suggests that brain areas responsible for autonomic control and cognitive processes overlap, implying that higher heart rate variability (HRV) is linked to better decision-making, attention, and emotional regulation (Thayer & Lane, 2000; Thayer & Sternberg, 2006).
Contemporary studies have explored the impact of vagally mediated resting HRV on memory. These investigations have produced intriguing findings, with several studies suggesting a significant relationship between HRV and memory performance (Gillie et al., 2014). Conversely, Frewen et al. (2013) reported that lower HRV is associated with poorer performance in verbal memory tasks (Frewen et al., 2013). However, not all studies have yielded consistent results, some studies did not establish a significant link between HRV and verbal or visuospatial memory (Britton et al., 2008; Mahinrad et al., 2016) .
Studies on HRV and attention have indicated that individuals with lower HRV may face challenges in tasks requiring sustained attention and concentration (Duschek et al., 2009; Williams et al., 2016). However, there remains a gap in the literature concerning the relationship between HRV and working memory, a cognitive function that demands attention and information updating. We decided to use the n-back-3 task since it requires working memory and attention (Mull & Seyal, 2001).
Additionally, the connection between vagal tone and memory in individuals engaged in regular exercise remains relatively uncharted. It is well-established that athletes and frequent exercisers tend to exhibit enhanced vagal tone. Exercise has also been linked to improved cognitive functions, including memory, and better cardiovascular health, including HRV. For example, studies have demonstrated that three months of pranayama training, a form of yoga, can lead to changes in ventricular performance, characterized by increased parasympathetic activity and decreased sympathetic activity (Udupa et al., 2003). Similarly, meditation and physical exercise (Sime, 1977) have been associated with reduced stress during cognitive tasks, such as exam writing. Considering these factors, our current study adopts the Neurovisceral Integration Model to investigate the interplay between HRV and memory in individuals practising regular exercise, yoga, and meditation.
We hypothesize that individuals who engage in regular physical activity, yoga, or meditation will demonstrate higher vagally mediated heart rate variability (HRV), as indicated by increased RMSSD and HF Power values. This enhanced HRV will correlate positively with improved memory performance, specifically on tasks demanding working memory and attention, such as the n-back-3 task. We also tested these groups for the acute effects of exercise on memory by asking them to exercise or practise yoga/ pranayama for 15 minutes and checking improvements in working memory performance.