Signals from the inside
The sense that lets us notice hunger, breathlessness, nausea or a heart that speeds up with excitement is called interoception. The five senses perceive the outside world; interoception perceives the inside of the body. Psychology has long linked it with the bodily basis of emotions: noticing that your heart is racing may be part of noticing that you are afraid.
Measuring interoception is not easy, though. The heart became the most popular target because its rhythm is regular and countable. In 1981 Rainer Schandry used a simple method: for set periods, participants counted the heartbeats they felt without touching their pulse, and these counts were compared with the actual number of beats. Today, labs record the actual beats with an electrocardiogram.
How is the score calculated?
For each count, accuracy is 1 − |actual − counted| / actual. Someone who counts 30 beats when there were actually 40 gets 0.75; someone who counts 40 gets 1. The average of the three counts is the person’s score. In one common version of the task the periods last 25, 35 and 45 seconds, and participants are usually not told how long they are.
In Zamariola and colleagues’ sample of 572 people (2018), average accuracy was 0.65, 0.65 and 0.61 for the 25, 35 and 45-second counts. In many studies, 0.85 and above is referred to as “heartbeat perceivers”. In this experiment we do not know the actual number of beats; instead we ask you to count your pulse with your fingers or type the value shown on your device.
Accuracy, sensibility, awareness
Garfinkel and colleagues (2015) showed that interoception is not a single thing. On one side there is accuracy on the task, how precisely you count your beats. On another there is how people rate themselves, the tendency to say “I can feel my heart well”. And then there is how well the two match: whether people with high scores are actually more confident.
In a normative sample of 80 people these three dimensions were distinct; meaningful correspondence between them emerged only in the subgroup with the highest accuracy. That is why we ask at the end how well you could feel your heartbeats: it lets us compare the scores of confident and unconfident people in the crowd.
Problems with a famous task
Over the past decade the heartbeat counting task has drawn serious criticism. In a sample of 572 people, Zamariola and colleagues (2018) found that more than 95% of participants under-counted their beats and that the correlation between counted and actual beats was only r = 0.16. They also showed that people with slower hearts scored higher (r = −0.36) and that the score varies with the length of the counting period.
Desmedt and colleagues (2018) reported that scores dropped by about half when participants were urged to count only the beats they actually felt; in the original task participants described three different routes: feeling beats in a specific place, feeling them diffusely, and estimating their heart rate. Earlier, Ring and Brener (1996) had shown that counted rates are closer to people’s beliefs about their heart rate than to their actual pulse, and in 2018 that counting scores are unrelated to another task that measures sensing the timing of heartbeats.
Belief or feeling?
Ring and colleagues (2015) ran an elegant experiment: they gave participants heartbeat feedback either in sync with each beat or with a delay. Synchronous feedback could teach people to feel their heart; delayed feedback only conveyed information about heart rate. Both kinds of feedback improved counting to the same extent.
The researchers concluded that the improvement came not from feeling the heart better but from updated knowledge about heart rate. This suggests that part of the counting task is really good estimation. Still, the task remains widely used, and its results need to be read with these caveats in mind.
What does this experiment do?
You do three counts: 25, 35 and 45 seconds, in shuffled order and without knowing their length. Start and stop are signalled by a soft tone, and the screen turns red if your sound is off; you can close your eyes. Following Desmedt and colleagues’ warning, the instruction asks you to count only the beats you actually feel.
Then you count your pulse with your fingers for 30 seconds. We calculate the expected number of beats from that count, so both the number we compare against and your own count rest on reports. That is why the results screen explains these limitations and the task’s known problems next to your score.