A major topic in research over the past couple of decades has been understanding biological correlates (or as often referred to, biomarkers) of mental health and illness. Millions of dollars have gone into this work, including measuring biometrics such as skin conductance (or how much a person sweats), how much they startle, their heart rate, their brain scans, and genetics, and whether it correlates with their disorder status or personality traits.
I am a biological psychologist by training, and this puzzle is one that has intrigued me, for both scientific and philosophical reasons, since it is inherent in all the work that I have done to date and continue to do in my current role as Research and Innovation Manager at the National Institute of Mental Health’s Research Domain Criteria (NIMH RDoC) Unit. It is also particularly relevant now with the plethora of digital health apps and devices like fitness trackers that have taken over this arena in recent years.
Why is this so fascinating to researchers like me?
Let us take a step back and examine: What is a mental disorder?
One of the curious things involved in diagnosing mental disorders, as compared to other medical disorders such as heart disease, diabetes, or cancer, is that the criteria used for it are mostly based on self-report (or other’s reports) of feelings, cognition, and behavior. In other words, unlike other disorders that implicate other body systems, there are no brains scans, blood markers, or proteins, or related biomarkers that can be used to diagnose, predict, or treat them.
In and of itself, this is not a problem — it does make intuitive sense that mental disorders should after all be based on mental phenomena such as thoughts and feelings, which presumably are related to an individual’s actions.
However, this does lead to some very interesting problems, when researchers try to correlate biology and behavior. One major issue that is not discussed often in the science news about such articles, or even in the more academic articles, is that these two factors often don’t correlate well — especially the larger the sample size of a study grows.
The first time I was exposed to this was in my early graduate school research work, where my lab worked on understanding psychopaths. A common theory about what drives psychopaths is that such individuals are unemotional, unempathetic, and do (or can) not show emotional reactivity the way the rest of the population does.
One of the methods often employed to study emotional reactivity in lab settings is to show images with emotional content to participants and then measure their emotional and biological reactivity to these pictures.
Since a substantial part of mental health and illness deal with emotions, thoughts, and feelings, and what happens when they go awry, the theory behind this idea is that if we can examine differences between groups of people with different mental health issues (including psychopaths), then perhaps we can trace what exactly goes awry in the particular mental disorder they may have.
The images shown in such studies included unpleasant pictures (such as pictures of dead people — some of these can be pretty gory! — or those of insects, or dirty toilets), pleasant pictures (such as soothing natural scenes, babies, or romantic couples, and even erotica), and neutral pictures (such as photos of fire hydrants, everyday furniture, and people with neutral expressions). Participants are also asked to simultaneously rate how pleasant or unpleasant the images while viewing them, and various parameters such as their startle reactivity, sweat rate, heart rate, and brain waves are recorded.
My lab undertook such studies with participants from the general population, incarcerated individuals (some, but not all of them, were psychopaths), and people with mental disorders.
Interestingly, how people across these samples rated these pictures didn’t always match up with the various biomarkers we measured.
For example, one of the biomarkers we measured is called the eyeblink startle response — where we literally quantify how hard the person blinked (eyeblink muscle activity; shown in image below) when they were startled while watching the picture. In the general population, on average, people startle the most when watching unpleasant pictures (think about you how you react when you are startled while watching a horror movie), medium while watching neutral pictures, and the least while watching pleasant pictures.
Generally, the more unpleasant a picture is rated, the more a participant startles while watching it. If you plot this in a graph, the results generally look like this:
Interestingly, when this same effect was studied amongst incarcerated individuals, those high in psychopathic traits appeared to show the linear increase in startle from pleasant to neutral, but not from neutral to unpleasant. On the other hand, prisoners who did not score high on such trait or were in a mixed group reacted more like individuals from the general population.
Perhaps you could conclude that such individuals did not find such pictures as aversive as the general population. However, here’s where the ratings of such pictures came handy. These individuals rated the unpleasant pictures as being similarly aversive to subjects in the general population, but did not startle to them as though they were unpleasant.
How about individuals who are depressed — what is their startle reactivity like?
Yet again, while such participants rated emotional pictures the way you would expect, even more intriguingly didn’t differ statistically in their reactivity between any of the types of pictures — pleasant, neutral, or unpleasant.
There are many other studies that show variations of this discrepancy for other forms of mental disorders such as post-traumatic stress disorder (PTSD), social phobia, and so on.
Why is this the case? Why does biological reactivity not correlate as much with what people say they are feeling? The short answer is: we don’t know.
There are many theories in the scientific literature as to why, but we don’t really know why people’s biology doesn’t correlate that strongly with their behavior or feelings when it comes to mental health.
Now, we could stop our studies here and just shrug our shoulders and move on. But, if you pause and think about it, beyond being just a scientific curiosity, results such as these have very interesting fundamental implications.
Are people really feeling something if they say they are feeling it, but their biology doesn’t show it?
What about the opposite situation — do we rely solely on their biology and disregard what people say? For example, is it alright for me to insist you must be feeling sad because your brain activity showed so, even if you say you aren’t feeling that way?
What is a feeling, anyway — what we say it is, what our brains show, some combination of the above? How much control do we vs our brain have over it? Is there such a thing as a feeling independent of a brain?
Our answers to these sorts questions have many practical implications as well ranging from settings such as mental health treatment facilities to courtroom settings, to more daily circumstances, such as how we feel we should deal with a bad mood. These are topics that individuals from a variety of traditions such as neuroscience, biology, philosophy, and psychology, have tried to answer for hundreds of years without any universal agreement.
My own views of this topic have continued to evolve over the years, from my time as a graduate student to where I am currently in my research career.
At this stage, my personal view is this: mental phenomena such as feelings, thoughts, and disorders wouldn’t exist without the brain. After all, they are not pathogens like a virus or a bacterium that can infect you after floating around in the air or living on surfaces.
However, unlike other disorders such as heart disease, there is no way to diagnose a mental disorder without someone reporting a problem with thoughts or feelings. After all, you don’t go to a doctor saying your frontal lobe isn’t functioning well, but that you are feeling fine.
In other words, my belief is that for us to truly understand mental health and illness, we will need to study both the experience of a feeling and associated biological events. Fortunately for me, we live in a time where both types of research and their integration, especially, are encouraged by various initiatives (such as RDoC).
I look forward in the coming years to see what kinds of light these can shed on this extremely interesting topic.
NOTE FROM THE EDITORS: We are delighted to share with you this fantastic blog from our friend Dr. Uma Vaidyanathan from the RDoC Unit for The National Institute of Mental Health @NIMH_RDoC. You can find out more about the work done by The National Institute of Mental Health’s RDoC Unit here. Thank you so much for writing this piece for InSPIre the Mind, Uma — look forward to reading your next piece soon!