For some people, waiting for a particularly unpleasant event, such as a colonoscopy or root canal, can be just as awful as the event itself, according to a new study on the neurobiology of dread, published in the journal Science.

Scientists found that dread activated brain areas similar to those associated with pain, and that people who rated the waiting period as most unbearable had more activity in a part of the brain's pain circuit that mediates attention. They say the findings could help explain how people make some decisions, such as dieting and buying stocks, which involve a long waiting period before seeing results.

The perception of pain is mediated by a complex neural circuit, known as the "pain matrix." Some parts of the matrix control the physical sensations of pain, such as the pain receptors that react when you accidentally bang your knee. Other parts of the matrix control higher-level perceptions of pain, which vary depending on the level of attention and emotional state. One is less likely to feel the pain in a knee, for example, if the joint is banged while running from a bear.

In the current study, researchers used functional magnetic resonance imaging (fMRI), which measures blood flow in the brain, to determine which parts of the brain were most active as subjects waited for an electric shock. They found that the dread associated with waiting for the unpleasant stimulus activated parts of the pain matrix, especially those involved in attention to pain. "These findings underscore the very real nature of dread," says Gregory Berns, the neuroscientist at Emory University in Atlanta who led the research.

Researchers also found that people react to the prospect of the pain in different ways. While almost all participants preferred to get the physical shock over with quickly, about one-third -- whom the researchers dubbed "extreme dreaders" -- actually chose to receive a more severe shock sooner rather than wait for a less painful shock. "Some people found waiting so unpleasant that they were willing to take a worse shock rather than wait," says Berns. And those "extreme dreaders" had more activity in the attention part of the pain matrix.

In a somewhat surprising finding, the researchers discovered that dread was neurologically distinct from both fear and anxiety. An area of the brain known as the amygdala, which plays a key role in regulating emotions such as fear, was not more active in extreme dreaders.

"The prospect that [dread] has more to do with attention than emotion is interesting," says Elizabeth Phelps, a neuroscientist at New York University who studies emotion and decision-making. "Presumably, if you could distract yourself in some way, you could get rid of some of these responses."