The ancient Greeks (Aristotle) considered the heart the center of our thinking. While over the last 2,000 years we have altered our view, and now know the brain is our control center (for most of us), we can still get a window into our brain from listening to our heart.
I previously covered some simple measurements we can use to track our overall health, and then went into details about heart rate measurements to get insights into your health.
Review of the general background:
Heart rate is controlled by the autonomic nervous system (ANS). There are two main branches of the ANS: the sympathetic system which is involved in energy mobilization (think increased HR), and parasympathetic system acting as the opposite and hence restoration (decreased HR). When the system is nice and healthy there is an appropriate balance between these two controllers and your HR changes according to the appropriate response to the current situation. A perfect example of this is when you exercise your HR increases to meet the demand of the higher energy requirements. In contrast, in an ‘unhealthy’ system one of the two branches of the ANS can come to dominate which leads to various diseases, or accelerate certain diseases, with the most prominent being cardiovascular disease (CVD). Typically the unbalance occurs with an overactive sympathetic and/or underactive parasympathetic.
Normally, the HR is tonically inhibited by the parasympathetic component of the ANS and is mainly mediated by the vagal nerve innervation of the heart. So resting HR can be considered a measurement of the parasympathetic system.
But the topic today is what can the heart tell us about the brain.
So the first question would be how does the brain actually influence the heart? Well, one anatomical pathway starts in your frontal lobe, which in general inhibits the amygdala (part of the limbic system). One of the amygdala functions is to automatically mediate defensive behavior (fight or flight). Hence, the frontal cortex via direct and indirect pathways (including the amygdala) of the parasympathetic system has an inhibitory effect on heart rate (and other measurements of the balance between parasympathetic and sympathetic systems). This could be how various people can control their heart rate by mediation and other techniques.
Now what would you guess occurs when people face conditions of uncertainty or threat? The activity level of the frontal cortex is reduced - thereby releasing its normal inhibitory activity on the amygdala/limbic system and other parasympathetic pathways and your hear rate increases. Along with heart rate increase comes changes in other measurements of the balance between parasympathetic and sympathetic systems - hence your HR variability (HRV) decreases.
Interestingly, various altered brain states such as depression, anxiety, schizophrenia, and post-traumatic stress disorder (PTSD) are associated with reduced frontal cortex activity (hypoactivity) (Thayer and Lane 2007). Now these same people have poor executive function and memory, which is consistent with reduced frontal cortex activity, but also do not adapt well to novel neutral stimuli, and are more prone to think something is a threat (when it is not). The question then does these same mental health conditions also result in altered HR measurements? Lowered heart rate variability (HRV) has been linked to anxiety, panic disorder, depression, and schizophrenia (Mujica-Parodi et al., 2007). For a quick primer of HRV see this piece.
Heart rate variability can be broken into time and frequency. Regarding time, though there are still many measurements of this, you can think of one as the standard deviation between interbeat intervals (IBI). As for frequency: both low frequency and high frequency spectral power have been used an indices of vagal activity.
Another way to look at these two measurement of variability is that frequency domains are an attempt to try to dissect out the excitatory (sympathetic) and inhibitory (parasympathetic) components/contributions, while the time series measurements are seeking to measure the overall chaos or complexity in the system.
As for the general take home message, people with a higher rate of variability are considered healthier and less likely to die. So at first this might sound somewhat counterintuitive for those not familiar to the heart field or chaos theory. But think of it as the more dynamic the system the better it can react to external changes.
Anxiety and HRV.
But is there really a link between the limbic activity of the amygdala and heart rate measurements? Mujica-Parodi et. al., 2007 examined the link between anxiety, heart rate variability, and limbic dysregulation. The researchers showed happy, neutral, and fearful faces while measuring their heart and scanning (fMRI) their brains. They used time ratio heart rate variability (HRV) - with high HRV reflecting a greater degree of chaos and a system with a greater capacity for adaptation to changing environmental condition (in this case a good thing - see my previous article). Overall, they found the subjects with the highest anxiety displayed the most limbic dysregulations - e.g. heightened and longer activation of the limbic system to neutral faces. Additionally, the higher the limbic dysregulation (e.g. left amygdala) there was an increase in the sympathetic activation and the lower the chaos in the heart-rate (lower HRV). The authors go on to argue/explain that a healthy mind is able to quickly react to changing excitatory (sympathetic) environmental conditions with equally strong and quick inhibitory activity via the parasympathetic to head back toward homeostasis. If the inhibitory parasympatheic system is slow and/or non responsive to the outside threats then the sympathetic drive dominates, which results in lower HRV (because once elevated the heart rate tends to stay there). Now from this paper I did not think we could know where the problems originates. However, the author argues that the limbic dysregulation is their best educated guess, based on their results. But I would think if the frontal lobe is not functioning correctly then you would also not get the appropriate response of the limbic system, feel high anxiety, and have altered heart rate readings. And remember many of the mental diseases mention earlier including anxiety is related to hypoactive frontal lobe. Take home: higher anxiety is associated with reduced HRV and greater sympathetic drive.
Pain and HRV.
Could pain threshold and HRV be related? Applehans and Luecken 2008 found that people that had greater low-frequencey (LF) HRV had a lower rating of pain to 4 degree water (cold) - a higher threshold of pain. However, high-frequency (HF) HRV was not associated with pain ratings. Why did LF but not HF variability correlated to pain threshold? According to the authors argument LF plays a role in blood pressure by affecting arterial baroreflex and interestingly the baroreflex mediates pain inhibitory pathways during conditions of high negative affective arousal (e.g conditions that cause large drop in blood pressure). Whatever the mechanism, we can see how your current natural setting of your autonomic nervous system, which in this case most appropriate measurement is LF heart rate variability is correlated with your pain threshold.
Heart rate and ability to handle major life stresses.
If heart rate measurements can be related to handling specific pain what about general resilience to life situations? Oldehinkle et al., 2008 examined this question and used simple heart-rate (reflects a balance between sympathetic and parasympathetic) and respiratory sinus arrhythmia (RSA: measures magnitude of rhythmic fluctuations, which is said to be mainly influenced by parasympathetic) . What they found was those subjects with a low resting HR were better able to handle major life stressors than the subjects with normal or high resting HR. The normal/high HR subjects suffered more mental health problems per same unit of major life stress. The researchers found no relationships with the RSA measurement (parasympathetic driven measurement). They conclude that the parasympathetic system was not playing a major role in the relationship they observed since RSA did not correlated with the ability to handle stress, but low HR did. Hence, they suggest that a low sympathetic system is the important variable.
To take this to a more general psychology theory - the stimulation-seeking theory suggests that people with low arousal state feel ‘unpleasant’ and therefore seek out arousing activity to put them into a more ‘balanced state’ for them. Research cited in the above paper point out that people with low HR prefer high intensity activities along with greater novelty. Interesting to me would be the chicken or egg problem of does a person who does seek out novel and high intensity activity (e.g. looks of physical activity) result in low HR, or is it vice-versa ? Therefore, when life becomes hectic and uncertain (high stress) this might be the preferred situation by the people with a naturally low HR, since it can be considered their desired state for appropriate level of arousal and/or it does not put them over the red line of excessive stress. While those with normal or high HR that encounter the same high stress conditions it might put them above the red line, which eventually leads to mental health issues.
One very interesting line of research would be does exercise which lowers your resting HR make you more resilient to life stresses?
Interestingly, training has been shown to result in a large increase in total power (LF + HF) and even great increase in LF, but only a moderate increase in HF (Gamelin et al., 2007). Along with the well known affect of exercise on lowering resting HR it could be suggested that exercise would increase your pain threshold and make you more resilient to life stresses.
You the reader might say well you already knew all that - of course exercise makes you have a higher pain theshold, we have all experienced that with exercise - you can push harder up the hill after a few months of running, and that exercise is well known to reduce stress. And if you the reader are thinking this I would agree with you, in many cases we already know manyy of the answers, and what we should be doing for our health. However, maybe now we have more objective readings of these improvements by examining various HR measurements.
More so, looking at research like this give us additional reasons/motives to do what we already know we should be doing. This is a very interesting topic that I find constantly popping up - we know what we should be doing for our health (backed by 100s of papers and all the logic and mechanisms) but we still struggle with doing what we know is good for us.
Our heart rate measurements can give us a glimpse into at least parts of our brain function and a way to track changes as we work to improve the health of our brain.
The above is just a working draft of my thoughts - please feel free to send me your thoughts and knowledge on this subject.
To better mental and physical health.