What colour is innocence?

New Scientist, 22 March 1997

MICHAEL ANTHONY JACKSON is on death row in California. One afternoon 14 years ago, high on angel dust, he shot and killed a policeman at close range with the officer's own gun. Jackson was convicted of first-degree murder in 1984. But now his lawyers are trying to overturn this verdict. They argue that persistent drug use damaged Jackson's brain so that he could not have known what he was doing. And they've got brain scans to prove it.

The scans use positron emission tomography (PET), a technique that shows which regions of the brain are active at any one time. Jackson's lawyers say the frontal lobe of his brain is abnormally inactive. Since this part of the brain is believed to control judgment, the lawyers argue that Jackson cannot take responsibility for the shooting, and that his life should be saved. A federal judge is now reviewing the case, and will announce his decision within weeks.

Jackson's is one of a growing number of murder cases in the US where scans have been used to mitigate sentences. Defence lawyers are using the scans to argue that their clients are suffering from behavioural disturbances, such as a lack of judgment or inadequate sexual restraint. The number of trials is unknown, but they have become commonplace in several states including California, Florida and Texas.

Courtroom prop

Scans are particularly popular in the "penalty" phase of American murder trials, when a jury has to decide whether the person they have just convicted should have a life sentence or die for the crime. Some jurors have admitted that brain scans helped them to decide against the death penalty.

Criminal cases like these are highly emotive, but scans are increasingly popular in civil liability cases as well, where they may make an even bigger mark. "The huge sums of money that are at stake are greater than most people think," says Alan Waxman, a radiologist at Cedars Sinai Medical Center in Los Angeles. Several settlements have exceeded a million dollars.

Scans have served as evidence in claims against manufacturers whose products -- such as breast implants -- are alleged to have caused people neurological problems.

Not surprisingly, some neuroscientists are alarmed: science, they say, is being abused by the courts. PET scans simply reveal which parts of the brain are active at the time of the scan, by providing a snapshot of the level of glucose being used in different parts of the person's brain. The more glucose used, the greater the activity in that area.

But whether a scan says anything about a person's behaviour -- even at that moment, let alone at some time in the distant past -- is highly contentious, says Waxman. "We're simply not in a position at this time to use functional imaging to explain behaviour," he says. Nor can a scan prove that a brain has been damaged by toxins.

"It can be very damaging evidence," says Tori Levine, a lawyer in Dallas, who has both won and lost cases for the company Dow Corning over its silicone breast implants. The implants have been blamed for various nonspecific autoimmune illnesses as well as behavioural disturbances.

And the computerised images that translate information about glucose use into "maps" of the brain can be very compelling. "These are pictures with electrifying colours," says Levine. Waxman goes further, arguing that in generating these images, scientists can use whatever colours they like, in whatever combination they like. "That can be misleading," he says.

The debate over what kind of scientific evidence should be admissible in court came to a head in 1993, when two women sued the drugs company Merrell Dow in California, claiming that its product Bendectine had caused birth defects. The women's lawyers wanted to use an unpublished scientific report which linked the drug to birth defects, but in Californian court at that time, only scientific evidence that had been peer-reviewed was allowed. The issue was referred to the Supreme Court.

In a lengthy landmark ruling, the Supreme Court established a new federal standard which allows judges to decide whether scientific evidence is relevant and reliable enough to be put before juries.

Levine says that judges are reluctant to bar scientific evidence because if it is later deemed important, the case can be retried. But if the judge lets irrelevant or unreliable scientific evidence in, it may just be seen as "harmless error", she says. Fearing appeal, most judges open the gates wide.

Helen Mayberg, a behavioural neurologist at the University of Texas Health Science Center in San Antonio, Texas, fears that this approach is turning science into a courtroom prop. Mayberg has been an expert witness in more than a dozen trials. Scientists are still trying to understand the precise links between brain structures and complex behaviours, she says. While those links remain uncertain, she argues, "any scan abnormality can be used to show whatever you want".

"If you take a jury and tell them, this area isn't working right and it's the judgment centre, they just might believe you," says Mayberg. "But if we don't know where judgment is in the brain, then how does the scan explain why the guy ran over to the police car, ripped the gun from its safety [clasps], and cocked it? Does it explain why, when the cop let down his guard, Jackson shot him?" Mayberg says that defence lawyers never claim to be explaining the behaviour in full; they merely suggest that brain abnormalities might have had something to do with it. "But in the criminal court, uncertainty sides with the accused," she says.

She is not alone in her concern. The Society of Nuclear Medicine, an international body whose members produce and interpret scans, was so uneasy about their growing misuse that in 1994 it set up a committee to establish guidelines on their use. Chaired by Mayberg, the committee published its advice in the Journal of Nuclear Medicine last July.

The guidelines exhort scientists not to draw conclusions from brain scans that they cannot support. Scans that have not been published in the literature or repeated by others should never be presented as "objective evidence" of behavioural abnormality, say the guidelines. Nor should scientists make claims about scans when they are not backed up by wider evidence from large-scale studies that a particular scan pattern reliably identifies a particular problem. Brain scans for court use should come with disclaimers about what they can show.

But guidelines are only guidelines, and scientists are free to interpret them as they see fit. Monte Buchsbaum, director of the PET Laboratory at the Mount Sinai Medical Center in New York, believes that scans can provide important objective information in court. "They can't measure the ability to tell right from wrong in centimetres of blood flow per minute," he says, "but they can tell you a lot about human skills like planning and judgment." He has been an expert defence witness some ten times.

It is expert testimony from Buchsbaum's former lab at the University of California, Irvine, that might save Jackson's life. Buchsbaum's successor at the Irvine lab, Joseph Wu, did Jackson's PET scan and interpreted the findings for the defence.

He told the judge that he found evidence of abnormally low activity in Jackson's frontal lobe. This was consistent, he testified, with what his lab had found with other people who abused angel dust. The drug, phencyclidine or PCP, was developed as an anaesthetic, but doctors stopped using it because of its severe reactions. Long-term use can cause memory loss, speech problems and depression. The Irvine team believes that the drug also impairs judgment.

"I limited my testimony to whether the scan was similar or dissimilar to those of other chronic PCP users," says Wu. "And I think we can make the inference that it's compatible." The findings suggest that Jackson's judgment might have been impaired, says Wu.

Personality change

Mayberg, who was called as expert witness for the prosecution, testified that almost nothing is known about the pattern that chronic PCP use might produce on a PET scan. The Irvine group had published only a two-page abstract of their work on PCP, and this was a study of only seven chronic users. Could Jackson be considered a chronic user and have his scan compared with those of chronic users, Mayberg asks, even though he had not used the drug for more than a decade? She also says that a brain scan on a man sentenced to death might be abnormal for other reasons, such as the stress of his situation.

Nevertheless, scans such as those Wu does are having an increasing impact, in civil cases as well as criminal cases. Many of the Irvine lab's clients have sued insurance companies over invisible ailments such as "frontal lobe syndrome". Most have been involved in accidents which they say damaged their concentration or changed their personality. Structural brain scans of these people using computerised tomography -- a technique which, unlike PET, shows only the structures of the brain, not its activity -- show nothing amiss. But Wu's PET scans reveal abnormality. This is objective, says Wu. "You can't fake turning off part of your frontal lobe."

However, PET scans are not as objective as some would claim. As the Society of Nuclear Medicine's committee pointed out, everything from the model of scanner to a twitch of the head can affect the resulting image, and interpretations of it also vary.

While scans hold such sway, cases like Jackson's will remain wide open. Yet the concerns of scientists are growing. "When law and science cross, it's not clean," says Mayberg. "In law, they want everything reduced to black and white. In science we live comfortably in grey."