Neuroscience lessons in body awareness from the man with two hearts

Researchers have been taking a keen interest lately in how the brain represents the internal state of the body – a process called interoceptive awareness (IA). There's evidence that poor IA is associated with eating disorders and other mental illnesses, and also that IA is important for social cognition, including empathising with other people. The most popular measure of IA is a person's sensitivity to their own heart beat – a topic we've covered on the Digest before.

Neuroscientists think we detect our own heart-beats via two routes – one is "somatosensory"; that is, we feel the movement of the heart's beat in our skin and this information is processed in the part of the brain that deals with touch. The other route is "vagal", based on fibres that feed directly from receptors in the heart to the brainstem and then onto two brain areas known to be vital for IA: the insular cortex and the anterior cingulate cortex.

Now a study led by Blas Couto and Agustin Ibanez and their colleagues, at the University of Cambridge and INECO, has explored this theory by testing a male patient "with two hearts". The 32-year-old man has a heart condition and is awaiting a transplant. In the meantime he has been fitted with a "left ventricular assist device" (see pic). This is an external pump that aids his failing left ventricle. Approximately the size of a large watch, it sits on his abdomen. Crucially, the frequency of this external pump is out of sync with the man's own heart-beat.

Couto's team tested the patient known as C.S. on a series of heart-beat detection tasks and compared his performance to a small group of healthy controls. Asked to tap a keyboard key in time with his own heart-beat, C.S. tapped in time with his assist device rather than his own heart. The researchers also recorded C.S.'s brain activity with EEG while he attended to his own heart and this showed that activity associated with interoceptive awareness was reduced in his brain compared with controls.

These findings support the idea that there is a dual route to interoceptive awareness and they show how the process of IA is subject to change. The researchers said their results "suggest that somatosensory input produced by the artificial pump … dominates the input from his endogenous heartbeat." Perhaps most intriguing of all are the results from social and emotional tests. Compared with controls, C.S. showed impairments in empathy, theory of mind (representing other people's mental states), and a decision making task. Against a background of otherwise normal performance on tests of intelligence, language and memory, these selective impairments are consistent with past research showing how IA is important for social and emotional cognition.

The researchers advised the usual caution when interpreting a single case study. They also acknowledged that C.S. was anxious and that this may have affected the results. These concerns aside, Couto and his team said their results show an artificial heart pump may cause an imbalance in cardiac interoception and in turn this could have subtle but important consequences for social and emotional processing.

Further reading

Couto B, Salles A, Sedeño L, Peradejordi M, Barttfeld P, Canales-Johnson A, Dos Santos YV, Huepe D, Bekinschtein T, Sigman M, Favaloro R, Manes F, and Ibanez A (2013). The man who feels two hearts: The different pathways of interoception. Social cognitive and affective neuroscience PMID: 23887813