Can you feel your heart beating? Most people cannot, unless they are agitated or afraid. The brain masks the sensation of the heart in a delicate balancing act—we need to be able to feel our pulse racing occasionally as an important signal of fear or excitement, but most of the time the constant rhythm would be distracting or maddening. A growing body of research suggests that because of the way the brain compensates for our heartbeat, it may be vulnerable to perceptual illusions—if they are timed just right.
In a study published in May in the Journal of Neuroscience, a team at the Swiss Federal Institute of Technology in Lausanne conducted a series of studies on 143 participants and found that subjects took longer to identify a flashing object when it appeared in sync with the rhythm of their heartbeats. Using functional MRI, they also found that activity in the insula, a brain area associated with self-awareness, was suppressed when people viewed these synchronized images.
The authors suggest that the flashing object was suppressed by the brain because it got lumped in with all the other bodily changes that occur with each heartbeat—the eyes make tiny movements, eye pressure changes slightly, the chest expands and contracts. “The brain knows that the heartbeat is coming from the self, so it doesn't want to be bothered by the sensory consequences of these signals,” says Roy Salomon, one of the study's co-authors.
Other research has previously linked our heart to our sense of self. For example, studies show that people will more readily feel that a virtual-reality body or limb is truly their own when it appears along with a stimulus that flashes in sync with their heartbeats.
On the opposite end of the spectrum are study results showing that cardiac sensations can enhance the processing of threats. Individuals detect scary images that appear in time with heartbeats more easily and find them more intense. Perhaps because a noticeable heartbeat is often linked to fear or anxiety, in this case the brain mistakes the synchronized stimulus as being linked to its internal fight-or-flight reaction. The finding helps to explain why people who are highly sensitive to their internal states, including being aware of their heart beating, tend to be more prone to anxiety and panic disorders. For most of us, however, the heart labors away unnoticed—and the related perceptual quirks may be going unnoticed, too.