A new University of Iowa study has found a second pathway behind the inner sense of our "interoceptive awareness" of the heart pounding.
Researchers have found that, in addition to a pathway involving the insular cortex of the brain-the target of most recent research on interoception-an additional pathway contributing to feeling your own heartbeat exists.
The second pathway goes from fibres in the skin to most likely the somatosensory cortex-a part of the brain involved in mapping the outside of the body and the sense of posture.
They also confirmed the widely held belief by researchers that the insula and anterior cingulate cortex (ACC) regions of the brain are important, but not necessary, for a person to feel his or her own heartbeat.
The insula helps with such higher-order functions as self-awareness, while the ACC is believed to regulate heart rate.
"What's shown in this study is there are probably two pathways that can participate in the conscious representation of these sensations," Nature quoted Dr. David Rudrauf, lead author of the study as saying.
For the study, the researchers studied an extremely rare neurological patient named "Roger" who has virtually complete bilateral insula and ACC damage, but who has the bilateral primary somatosensory cortex intact.
They also studied 11 healthy age-matched male comparison participants.
The researchers injected the participants with a synthetic form of adrenaline to get their hearts to shoot up about 25 beats a minute.
They then had the participants turn a dial to track their moment-to-moment experience of the intensity of their heartbeat sensations.
As it turned out, Roger felt his own heartbeat just like the healthy comparison participants in a dose-response fashion.
"It was a delayed reaction, but he was still feeling it," said Feinstein.
This development suggested that the insula and ACC were not necessary, strictly speaking, for interoceptive awareness of heartbeat sensations.
The researchers suspected that Roger was feeling his heartbeat because his brain was using a different pathway, relying on the impact of the heartbeat on the chest wall and pulsations in blood vessels stretching the skin.
So they applied a topical lidocaine anesthetic to the location on the skin where participants reported feeling the maximal heartbeat sensation.
"There are two pathways. One conveys the heartbeat signal from the surface of the chest wall and blood vessels pulsating under the skin, to the somatosensory cortex, so whenever you feel your heart pounding it's stimulating that pathway," Feinstein said.
"Roger is able to feel his heart beating because that area of his brain-the somatosensory cortex-is still there. When you get rid of that sensation by anaesthetizing the skin, you need areas such as the insular cortex in order to feel the heart pulsing from deep within. That's what is missing in Roger and that's where the healthy person is able to feel it," he added.
The pathways revealed by this study could be involved in everything from the pounding of the heart during a state of panic to the feeling of a "broken heart" during a state of grief, said the researchers.
The study was published online this week in the journal Nature Neuroscience.