A new study suggests that the inner sense of our cardiovascular state, our "interoceptive awareness" of the heart pounding, relies on two independent pathways, contrary to what had been asserted by prominent researchers.
The University of Iowa study was published online this week in the journal Nature Neuroscience by researchers in the department of neurology in the Roy J. and Lucille A. Carver College of Medicine and the graduate programs in neuroscience and psychology.
The researchers 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 fibers 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.
The UI team 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," said David Rudrauf, Ph.D., assistant professor of neurology and radiology and director of the laboratory of brain imaging and cognitive neuroscience. Rudrauf is lead author of the study along with Sahib Khalsa, M.D., Ph.D., who received medical and doctoral degrees from the UI and is currently working on his psychiatry residency at UCLA.
Daniel Tranel, Ph.D., a professor of neurology and psychology and director of the postdoctoral residency program in clinical neuropsychology, and Justin Feinstein, a graduate student in clinical neuropsychology, are co-authors on the study, titled "The pathways of interoceptive awareness."
The UI 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.
Roger has been studied in the UI laboratory for 15 years. His brain damage occurred in 1980 following an episode of herpes simplex encephalitis. With Roger, Rudrauf and his colleagues wanted to see if the regions of the brain he's missing are really necessary to feel your own heartbeat.
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," Feinstein said.
This development suggested that the insula and ACC were not necessary, strictly speaking, for interoceptive awareness of heartbeat sensations.