Not everyone wears their heart on their sleeve, but tests on a brain-damaged man suggest that we may all sense our heartbeat with our skin. The finding may help to unravel the complex relationship between the heartbeat, our emotions and self-awareness.
Awareness of our internal organs 鈥� the ability to feel your stomach growl and your heart beat 鈥� is known as 鈥渋nteroception鈥�. While the brain subconsciously monitors heartbeat, the mechanism by which the sensation creeps into the consciousness and so influences our emotional state has fascinated researchers studying consciousness, and remains somewhat mysterious.
Neuroimaging studies have previously linked heartbeat interoception to the brain鈥檚 insula and anterior cingulate cortex. But of the University of Iowa in Iowa City wondered whether other parts of the body might be involved too.
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Together with his colleagues, he turned to a brain-damaged patient called Roger who had suffered damage to the insula and ACC on both sides of the brain after suffering a serious bout of hepatic encephalitis.
Pounding heart
Khalsa hooked up Roger, and 11 healthy males of similar age and weight, to an intravenous drip. Each man received seven high and two low doses of isoproterenol 鈥� a drug similar to adrenalin that makes the heart pound 鈥� and the same doses of a saline placebo over the course of a day.
Isoproterenol takes about 30 seconds to work, and its effects last for about 90 seconds before wearing off. All the men were asked to turn a dial to represent how aware of their heartbeats they were throughout the day.
Even without a functioning insula and ACC, Roger was aware of his heart pounding when he was given the isoproterenol 鈥� to the same extent as the other men 鈥� although his response was delayed. This suggests that the two brain regions aren鈥檛 solely responsible for interoception, says Khalsa.
Reducing anxiety
Suspecting that skin might be helping Roger to sense his heartbeat, Khalsa anaesthetised the skin on Roger鈥檚 chest, and then repeated the experiment. This time, Roger couldn鈥檛 tell that his heart rate was increasing when he was given the drug.
鈥淭his suggests that the skin does play a role in interoceptive sensations,鈥� says Khalsa. That at least two independent processes seem to make us aware of our internal organs suggests that the sensations are important ones, he adds. 鈥淲e do know that these sensations are important for everyday functions, such as sensing fear.鈥�
Khalsa hopes that by learning more about interoception, he might be able to develop therapies for people who suffer from anxiety disorders. If an anaesthetic on the chest can reduce the sensations of a quickening heartbeat, it might conceivably reduce palpitations that cause panic attacks.
Music and pain
However, at the Barrow Neurological Institute in Phoenix, Arizona, who has studied the involvement of the insula in interoception and self-awareness, disputes Khalsa鈥檚 assessment of Roger鈥檚 brain damage.
He says that much of Roger鈥檚 insula may still be functioning normally, based on Roger鈥檚 capabilities.
鈥淥ddly, the authors make no comments on Roger鈥檚 ability to feel pain or temperature or itch or sensual touch, feelings that are dependent on insular function,鈥� Craig says. 鈥淚t is highly significant that he still enjoys music鈥�, he notes, something 鈥渆ntirely consistent with an intact left anterior insula鈥�.
Khalsa notes that his own conclusion 鈥� that the skin plays a role in sensing heartbeat 鈥� makes sense. For example, it could help explain why people often link their heart to other parts of the body during moments of extreme stress. People may, for example, describe their heart 鈥渓eaping into their throat鈥�, experience feelings of itchiness, or flushing with embarrassment during moments of anxiety, when their heart rate is also up.
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