What colour is innocence?
New Scientist, 22 March 1997
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
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
Courtroom propScans 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.
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
Scans have served as evidence in claims against
manufacturers whose products -- such as breast implants -- are alleged
to have caused people neurological problems.
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.
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.
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,"
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.
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".
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.
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.
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.
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
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,
Personality changeMayberg, 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
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.
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."