"I think, therefore I am," pronounced the famed French philosopher René Descartes. What imbues us with this uniquely human sense of self-awareness? Some neuroscientists have pegged an area of the brain known as the insula, which helps us detect what's going on within our bodies. But an unusual case of a man with extensive damage to this region suggests that the insula cannot be the sole source of self-awareness.
Tucked deep inside the brain, the insula responds to pain, a full stomach, changes in body temperature, and other internal sensations. Researchers have proposed that the insula somehow translates these visceral sensations into conscious, subjective experience. But it's a hard hypothesis to test.
Enter "Roger." In 1980, a viral disease known as herpes simplex encephalitis destroyed 25% to 35% of his brain, including nearly all of his insula. Yet Roger functions remarkably well: Although he suffers from amnesia and has lost his sense of smell and taste, he has a normal IQ and very good language skills.
Roger doesn't act like he lacks self-awareness, but researchers led by Sahib Khalsa and David Rudrauf of the University of Iowa in Iowa City were curious about his ability to detect visceral sensations. To investigate, they gave Roger a drug that increases heart rate. Then they asked him to turn a dial to indicate any changes he noticed. Roger detected the increase, as did 11 healthy volunteers.
Khalsa, who is now at the University of California, Los Angeles, suspected that Roger's brain might be getting some help from sensory receptors in the skin capable of detecting a suddenly pounding heart and relaying that information to somatosensory regions of the brain. So Khalsa repeated the experiment, after applying an anesthetic cream to Roger's chest to deaden the sensory receptors. Now, when Roger got the drug, his dial didn't budge. In contrast, the other subjects given the same treatment still detected the change in heart rate.
The researchers conclude that parallel pathways in the brain--one involving the insula, the other involving the sense of touch--mediate the ability to feel the heartbeat. The findings, reported online this week in Nature Neuroscience, undermine the hypothesis that the insula is the sole source of self-awareness in the human brain, Khalsa says.
Not everyone agrees with that interpretation. Bud Craig, a neuroscientist at the Barrow Neurological Institute in Phoenix, Arizona, who has been a major proponent of the idea that the insula has a crucial role in self-awareness, thinks the authors' conclusions are deeply flawed. He points out that Roger does have some residual tissue left in his left insula, which Craig thinks explains his apparent self-awareness. Craig also argues that the pathway that detects a pounding heart via the skin is fundamentally different from the visceral sensations he and others have argued are translated into conscious experience by the insula.
Others offer a more upbeat assessment. "The experiment is very clever," says neuroscientist Antoine Bechara of the University of Southern California in Los Angeles. (Bechara has previously collaborated with the Iowa group but was not associated with the current study.) One possibility that the researchers did not test, he adds, is that the somatosensory system and the insula convey different aspects of the experience of a quickening heart rate: The somatosensory system simply reports the facts (e.g., "My heart is beating faster"), whereas the insula generates a subjective feeling of discomfort or anxiety. And that is similar to the argument Craig has been making all along.