Neuroscientist David Eagleman, PhD, gets a call about once a week from a defense attorney who wants him to perform a brain scan on a client, usually accused of murder, in hopes of finding something — anything — that might serve as a “biological mitigator.”
After refusing, Dr. Eagleman, director of the Laboratory for Perception and Action at the Baylor College of Medicine, where he also directs the Initiative on Neuroscience and Law, offers the attorney some free advice: “I say that biological mitigators amount to a double-edged sword — if you persuade the jury that your client has little capacity for decision-making — that he's so aggressive he's basically a rabid dog — they're likely to say, ‘Let's fry him for sure.'”
Despite the growing appreciation of how brain dysfunction can contribute to criminal behavior, neuroimaging remains a long way from providing mitigation, much less exculpation. All those detailed and informative images may contribute enormously to diagnosis and treatment, but the data doesn't add up to an explanation of personal responsibility for criminal actions any more than it adds up to an explanation for consciousness itself.
Yet, just as neuroscience now informs philosophical discussions of the nature of consciousness, so it also prods the law to consider the possibility that the human brain does not always provide as much free will as true legal responsibility demands. A symposium at the Society for Neuroscience meeting in Washington, DC, last November considered what neuroscience has to say about criminal responsibility, addiction, memory, and the adolescent brain. The same month members of the American Association for the Advancement of Science conducted a mock trial in which neuroimaging was introduced as evidence demonstrating that a man charged with murdering the mother of an ex-girlfriend was incapable of normal control over his emotions, and therefore could not have planned the attack in advance, making him guilty of second-degree rather than first-degree murder.
Indeed, a report recently released by the Royal Society, a UK-based research consortium — titled “Neuroscience and the Law” — noted there are already some important practical implications of recent neuroscientific discoveries, which should impact on the law — for example, the appropriate age of criminal responsibility, the nature of “shaken baby syndrome,” and “non-accidental head injury.” But, the authors of the report concluded, using neuroimaging as a new type of lie detector, or to identify genes for psychopathy, aggression, and antisocial behavior remain “completely wide of the mark.”
WHAT ABOUT THE FRONT LOBES?
The frontal lobes and the amygdala — the primary focus of such discussions — have long been implicated in criminal acts. In his recent book, Incognito: The Secret Lives of the Brain (Random House, 2011), Dr. Eagleman recounted the case of former Marine Charles Whitman, who, in 1966, when he was 25, climbed to the top of the University of Texas Tower in Austin and shot 45 people, killing 13. He also had murdered his wife and her mother before leaving the house that day. An autopsy revealed a glioblastoma beneath his thalamus, pressing on the hypothalamus and the amygdala, which almost certainly contributed to the “overwhelming violent impulses” and “many unusual and irrational thoughts” that Whitman mentioned in his suicide note.
A case described in the Archives of Neurology in 2003, and often cited as a clear example of how brain damage can induce uncharacteristic and unacceptable behavior, involved a 40-year-old teacher who suddenly developed a voracious appetite for child pornography, and then attempted to molest his prepubescent stepdaughter. He turned out to have a tumor that had displaced a large portion of his right orbitofrontal lobe, a brain region crucial for impulse control and the regulation of social behavior. After removal of the tumor, the inappropriate behavior ceased, but recurred a year later after the tumor had reappeared.
“Is he responsible for his pedophilia?” legal scholar Adrian Raine, DPhil, asked during his presentation at the Society for Neuroscience meeting last November. “He is responsible in the eyes of the law because he knew what he was doing, and he knew it was wrong. That demonstrates rational capacity. But I'm suggesting he is not responsible.”
Dr. Raine, Richard Perry University Professor in the departments of criminology, psychiatry, and psychology at the University of Pennsylvania, has long been trying to produce evidence that shows how dysfunction in the prefrontal cortex and the amygdala can contribute to criminality. For example, he performed PET scans on 41 murderers who pled guilty by reason of insanity and found reduced glucose metabolism in the prefrontal cortex and other brain areas, suggesting a brain signature for a predisposition to violence, as he reported in Biological Psychiatry in 1997. And yet one of the murderers, who had killed 64 people, had high glucose metabolism in the prefrontal cortex, suggesting a normal ability to plan his murders and recognize his responsibility for them.
Dr. Raine also found evidence that a poorly functioning amygdala correlates to some degree with criminality. In a 2009 article in the Archives of General Psychiatry he and his colleagues reported an 18 percent volume reduction in the amygdala of 27 psychopaths scanned with structural MRI, compared with 32 normal controls.
While dysfunctions in the prefrontal cortex and the amygdala are most often linked to criminal behavior, functional connections between those areas, which can be disrupted by abnormal myelination, may also be implicated, according to Christopher M. Filley, MD, professor and chief of neurology at the Denver VA Medical Center. Dr. Filley is also the author of The Behavioral Neurology of White Matter (Oxford University Press), which will be released in its second edition at the AAN annual meeting in April.
“White matter tracts that connect gray matter areas can be involved in the breakdown of the frontal and temporal systems that influence empathy and social cognition,” Dr. Filley said. “A number of papers associate psychopathic and antisocial behavior with problems in the frontal and temporal gray matter structures and the tracts between them. We have DTI and other methods that allow us to see white matter as it connects to various cortical and subcortical areas, so network analysis is becoming more feasible, but we're not ready to use this kind of analysis in the courtroom.”
Even Daniel A. Martell, PhD, who received so many requests to provide expert testimony about the brain's role in criminal acts that he founded Forensic Neuroscience Consultants, has noticed a decline in efforts to enter neuroimaging into evidence in criminal cases.
“I think attorneys and courts are beginning to appreciate that it's not ready for prime time,” said Dr. Martell, assistant clinical professor in the department of psychiatry and behavioral sciences at the David Geffen School of Medicine at the University of California, Los Angeles. “They've coined terms such as the Christmas tree effect to describe efforts to wow the jury with brightly colored pictures, even though science can't explain what they mean. There's a healthy skepticism growing about the limits of neuroscience as it applies to criminal responsibility, but at the same time there's a sense of inevitability that neuroscience will become helpful in some way.”
An MRI image that clearly shows a lesion or a brain bleed may provide useful evidence, according to Dr. Martell, but functional imaging such as a PET scan or voxel-based morphometry studies introduced to suggest that “his brain made him do it” usually can't explain much.
“The defendant's head wasn't in a scanner when he pulled the trigger, and what the jury needs to know is his mental state at moment of the crime,” he said. “There's no way to capture that unless there's a lesion that demonstrably affects his behavior in circumstances that include other contexts than simply the crime itself. You need a clear clinical connection between the brain and the aberrant behavior.”