ARTICLE IN BRIEF
Leading neuroscientists provide an overview of research on the neural underpinnings of human consciousness and empathy.
BALTIMORE—Higher cognitive functions were once considered beyond the reach of traditional neuroscience. But that barrier is falling. That was the implicit message from a symposium held here at the annual meeting of the American Neurological Association, with subjects ranging from the anatomy of consciousness to the neural substrates of empathy.
Antonio Damasio, MD, PhD, professor of neuroscience at the University of Southern California in Los Angeles, has devoted his career to understanding higher brain functions. “Consciousness is a dangerous concept,” because “it means different things to different people,” he began. It means not only the process of mind — the representation of objects and events inside and outside the brain, but also the process of self — the attribution of such representations to an internal protagonist.
“It cannot be reduced to one area of research, one trick,” he said.
But neither is it unavailable to research. Dr. Damasio has explored the hypothesis that, first, “subjectivity is based on establishing a relationship between the organism and any object that becomes conscious, ” and second, that the neural proxy for the organism is the “protoself,” localized in a specific set of brain areas, while the neural proxy of the object is based in cortical and subcortical sensorimotor processes that build up the mind state. Third, and importantly, the relationship of the two is based on cortical and subcortical “integrators,” including the thalamus, deep layers of the superior colliculus, and perhaps most prominently, the posterior medial cortex (PMC).
Several lines of evidence support the centrality of the PMC in consciousness. A variety of anesthetics exert their effect in limited regions of the brain, always including the PMC, and blood flow to the area decreases as consciousness is lost. Normally, the PMC has “probably the highest metabolic rate of the entire brain,” Dr. Damasio said. The local metabolic rate drops in vegetative states, and recovers if the patient regains consciousness. And the PMC “behaves like no other area of the brain,” Dr. Damasio said, in that it is widely connected to multiple other brain areas, including “practically the entire associational cortex.”
“The PMC is a hub of convergence and divergence, ideal for the kind of integration” it presumably serves, he explained. The PMC becomes “dramatically activated” when the brain is engaged in autobiographical memory, moral judgment, imagining action in the future, and contemplating the actions of others in relation to one's own response, with further segregation of response within the PMC based on the character of the mental action.
“We are obviously not going to find a trick to understanding consciousness, because there is not just one problem,” Dr. Damasio concluded, but this new region “could contribute to the process of integration” that lies at the heart of consciousness.
MIRROR NEURONS, EMPATHY
In another plenary lecture, Vittorio Gallese, MD, a co-discoverer of mirror neurons, got right to the point: “The punch line of my talk is, we must rethink the motor system.” The system is built not just for “mere displacement of different body parts, but in terms of goal-directed motor acts,” he continued.
Mirror neurons are a distinct class of motor neurons that fire both when an act is performed, and when it is observed. The working model culled from research to date is that the activity of mirror neurons “reflects an internal motor description of the perceived motor act's goal, rather than a mere visual description of its features,” he explained. Thus, they are the basis of “embodied simulation,” “the basis of shared experience,” and are “the building blocks” of social cognition — that is, how we understand the intentions of others, said Dr. Gallese, a professor in the department of neuroscience at the University of Parma, Italy.
Mirror neurons have been well studied at the single-cell level in primates. Evidence for their existence in humans is still indirect, but growing. “Single-neuron evidence is not easy to obtain, but not impossible,” Dr. Gallese said, and his group is currently endeavoring to do such recordings in patients preparing for epilepsy surgery.
Perhaps the boldest claim about the mirror system is that deficits in it underlie autism. Evidence includes a reduction in mu wave suppression in high-functioning autistic individuals. Mu wave activity involves the motor cortex and is suppressed with movement or intent to move. The magnitude of the observed suppression deficit is proportional to the deficit in these individuals' ability to imitate the actions of others, “most likely due to malfunctioning of the mirror system,” Dr. Gallese noted.
Further work in stroke victims, who lose abilities to both experience and recognize specific emotions such as disgust, suggest that “embodied simulation may not be confined to the domain of action, but encompass other areas of activity, such as emotion and sensation,” Dr. Gallese said. “We do not just see an action, or an emotion or a sensation, and then understand it through an inference by analogy.” Instead, the observer generates an internal simulation of the observed state, as if experiencing it himself. “I think this to be an important component of what being empathic is about,” he explained.
Commenting on the lectures, Daniel Lowenstein, MD, professor of neurology at the University of California-San Francisco, and moderator of the symposium, told Neurology Today:“I consider discovery of mirror neurons one of the great discoveries of the last decade. The findings are absolutely extraordinary and robust. There is no question they exist in humans, and I think it is very likely that they explain the notion of empathy.”
Added Mahlon Delong, MD, professor of neurology at Emory University: I don't think we'll ever reach “the top of Everest” in the sense of really understanding these things (consciousness), because we are dealing with the problem of emergent properties, properties that arise as systems become more complex. “We can understand the circuits,” he said, “but there is nothing in our brains that can understand how they are put together to do what they do. I can't put together any model of the mind that lets me understand consciousness.”
AAN ANNUAL MEETINGS
• toronto, Ontario • April 10–17, 2010
• Honolulu, Hawaii • April 9–16, 2011
• New Orleans, LA • April 21–28, 2012