"I had to do it. What other choice did you give me?"

These words, spoken by Cho Seung-Hui on a video in between the two sets of killings at Virginia Tech last week, raise more questions than answers. What made him believe that such a tragic act was necessary? Was he a psychopath, a man who killed in cold blood or in anger set off by the slightest provocation? Did he embody what most religions would simply classify as "evil"?

Psychiatrists and neuroscientists are making extraordinary advances in understanding the psychopathic or sociopathic mind, a mind that lacks empathy, compassion, fear, or remorse. In some of the most exciting research, advanced brain-imaging techniques are revealing that certain sections of psychopaths' brains seem to be misfiring.

Using functional magnetic resonance imaging (fMRI), researchers in the United States, Germany, and elsewhere have started taking scans of the brains of psychopaths while the patients view horrific images, such as photographs of bloody stabbings, shootings, or evisceration. When normal people view these images, fMRI scans light up to indicate heavy brain activity in sections of the emotion-generating limbic system, primarily the amygdala, which is believed to generate feelings of empathy. But in psychopathic patients, these sections of the amygdala remain dark, showing greatly reduced activity or none at all. This phenomenon, known as limbic underactivation, may indicate that some of these people lack the ability to generate the basic emotions that keep primitive killer instincts in check.

Other researchers see similar deficits from fMRI scans of the frontal cortex, part of the reasoning center of the brain, which helps regulate impulsive and irrational actions. These researchers say that frontal-deficit syndrome creates a psychopathic inability to rein in overly emotional, impulsive, and violent reactions to the slightest provocation.

James Blair, head of the National Institute of Mental Health's Unit on Affective Cognitive Neuroscience, believes that a dysfunctional amygdala affects the frontal cortex. In just-completed studies of psychopathic brains, to be published late this year or early next, Blair's fMRI scans show that a lack of normal activity in the amygdala is mirrored in the frontal cortex. He believes that the amygdala forwards the wrong signals to the frontal cortex.