When we exercise, our brains and bodies process a non-stop stream of information, but scientists have now shown how one data point can be manipulated to play tricks on the mind.

In a paper published Monday in the journal Proceedings of the National Academy of Sciences, researchers from France and Italy show it doesn’t take much to fool our brains into overestimating the intensity of a workout. One only needs the sound of a fake heartbeat.

Giovanni Pezzulo, Ph.D., a researcher with Italy’s National Research Council and the study’s corresponding author, notes that while we’re susceptible to trickery, our brains remain quite discerning: Humans in his test were easily fooled into thinking that their workouts were more difficult by hearing a faster-that-actual heartbeat, but they couldn’t be tricked into thinking they were easier while hearing a slower-than-actual heartbeat.

“Our study suggests that this is possible, but also that some illusions are harder to believe, possibly because they are maladaptive,” Pezzulo tells Inverse.

Pezzulo's study showed that the brain can be easily fooled into overestimating how hard a workout is, but it's hard to fool someone into thinking that a workout is easier than it actually is. Unsplash/ i yunmai

How the Experiment Worked

In his experiment, Pezzulo had a small sample of 18 participants perform an indoor cycling exercise, pedaling at seven intensity levels while listening to one of two heartbeat sounds. One heartbeat sound was laboriously pounding, while the other slower, evenly paced.

The exercise workouts were identical, but the false heartbeat information changed how people perceived their effort levels during those workouts. If they heard a pounding heartbeat, they thought they were working out more intensely than they actually were.

The heartbeat sound was intended to investigate how the body maintains an “interoceptive schema,” Pezzulo says. The interoceptive schema is defined as the process “by which the nervous system senses, interprets, and integrates signals originating from within the body.” One of those signals is a heartbeat.

“Most of us believe that the body maintains some internal model or internal representation of interoceptive, physiological, and homeostatic variables, for example how hungry, thirsty, or fatigued one is,” explains Pezzulo.

In the experiment, the sound of faster heartbeat filled that “internal model” with false information. Over the course of the tests, Pezzulo and his co-authors found that people believed that information, but only in one scenario. When participants heard the sound of a rapidly beating heart, they overestimated their efforts during the cycling workout. When they heard chill heartbeats, by comparison, they did not tend to underestimate their effort levels.

On one hand, these results indicate that it’s possible to create an “interoceptive illusion,” or a fake feeling of exertion. But the key, explains Pezzulo, is that we seem to be fooled in an “asymmetrical” way. We’re convinced by an audio recording of a pounding heart when it tell us we’re grinding on a bike, not when it tells us we’re chilling.

Overall, he explains that this is likely the vestige of an ancient survival technique. Not recognizing how hard the body is working would have been disastrous for ancient humans, he speculates, so the body listens very carefully to information suggesting we’re overdoing it.

“We also expected that it would have been much more difficult to convince somebody that he/she was not making effort, or is not fatigued,” he says. “This is adaptively very dangerous, as our evolutionary ancestors made life-or-death decisions based on effort and fatigue information, for example how long to run, whether to hide or attack.”

Pezzulo’s results aren’t evidence that it’s impossible to make a workout seem easier — working out at different times of day, for example, has demonstrated effects on performance. But they do suggest that our brains are more likely to embrace information telling us we’re working hard — even if the evidence is as flimsy as an audio recording of a fake heartbeat.

Abstract: Interoception, or the sense of the internal state of the body, is key to the adaptive regulation of our physiological needs. Recent theories contextualize interception within a predictive coding framework, according to which the brain both estimates and controls homeostatic and physiological variables, such as hunger, thirst, and effort levels, by orchestrating sensory, proprioceptive, and interoceptive signals from inside the body. This framework suggests that providing false interoceptive feedback may induce misperceptions of physiological variables, or “interoceptive illusions.” Here we ask whether it is possible to produce an illusory perception of effort by giving participants false acoustic feedback about their heart-rate frequency during an effortful cycling task. We found that participants reported higher levels of perceived effort when their heart-rate feedback was faster compared with when they cycled at the same level of intensity with a veridical feedback. However, participants did not report lower effort when their heart-rate feedback was slower, which is reassuring, given that failing to notice one’s own effort is dangerous in ecologically valid conditions. Our results demonstrate that false cardiac feedback can produce interoceptive illusions. Furthermore, our results pave the way for novel experimental manipulations that use illusions to study interoceptive processing.