When the brain takes the witness stand

You can’t force a suspect to talk. But what about his brain? Doing a PhD in the emerging field of neurolaw at Tilburg Law School, researcher Sjors Ligthart focuses on an urgent task for a new generation of legal scientists: future-proofing criminal law.

In the new season of the chilling sci-fi series Black Mirror, an investigator uses a ‘Recaller’ device to access people’s memories. A small chip is attached to the temple, while sensory triggers—sounds and smells—help jog the memory. The Recaller then displays your memories, whether you want it or not, like a movie on a television screen.

The technology in Black Mirror is fictional. But in the real world, advancements in our understanding of the brain are slowly but surely making their way into interrogation rooms and criminal courts. With the emergence of sophisticated neurological testing, it may be possible that public prosecutors will soon be calling a new witness to the stand: the human brain. Is our criminal justice system ready for the future? According to Tilburg University researcher Sjors Ligthart, the answer is no. Univers sat down with Ligthart to talk about the legal possibilities—and limitations—of cross-examining the criminal brain.

It’s a scary idea, a device that picks up memories and displays them on a screen to help solve a case. Do you think a device like the Recaller could become reality?

“Actually, something quite similar already exists in the real world: the guilty knowledge test. By measuring unconscious responses, the guilty knowledge test can detect concealed information in a person’s memory. It can determine quite reliably whether a person recognizes a certain object, an event, a piece of information or other details that—in the case of a criminal investigation—would be known to the perpetrator.”

How does that work?

“Roughly, there are two ways to conduct a guilty knowledge test. The old-fashioned way is to hook someone up to a polygraph and to measure the subject’s physiological responses to certain visual stimuli. The other possibility is an EEG test, which doesn’t measure physiological responses, like sweating and respiration rate, but brain electricity. Electrodes are attached to the scalp, which measure changes in brain wave patterns while stimuli are presented to the subject. If a stimulus is familiar and meaningful to the subject in some way, a specific brain wave—‘P300’—is significantly higher than if a stimulus in unfamiliar.”

So if a suspect claims that he doesn’t know anything about a specific object or event, this little brain wave called P300 could tell investigators that he actually does recognize these objects?

“Exactly. We could, for example, show a suspect of a robbery five different guns and five different duffel bags. If the suspect’s brain produces a significantly higher P300 response when he is shown the gun and the bag that were used by the perpetrator to commit the crime, that should raise a red flag. It doesn’t prove that the suspect is guilty, of course, but it does indicate that he knows more than he is letting on.”

“To some extent, neuro-tests could make it possible to take a look inside a suspect’s mind”

Sounds like a dream come true for police and legal authorities. Is the guilty knowledge test ready for use in practice?

“No, it’s not. It has only been tested in lab settings. When carried out under controlled laboratory conditions, the validity and accuracy of the test is very high. The Dutch Ministry of Justice and Security has expressed interest in neurological testing, for instance in the context of risk assessments and forensic diagnostics, and it could indeed be very relevant for the practice of criminal law. Currently, investigations largely depend on the suspect’s willingness to cooperate. Someone may refuse to talk, or tell lies. Police, psychiatrists and other investigators may try to collect information through interrogations and conversations, but they can’t know what’s really going on inside someone’s mind. To some extent, neuro-tests could make that possible. But before you can reliably use such technologies in real-world criminal cases, you must test them outside lab settings. And, in addition, there’s another important task for us legal scientists: before neuro-tests find their way into the courtroom, we must consider the legal implications that these technologies bring.”

Let’s talk about those legal implications. If a suspect refuses to undergo a neuro-test, could he be made to? We all have the right to remain silent. It seems that if you can’t force a suspect to speak, you can’t force his brain to ‘speak’ either. Or can you?

“That’s one of the fundamental questions that my dissertation addresses. Article 29 of the Dutch Criminal Procedure Code states that a suspect has the right not to answer questions. In addition, article 6 of the European Convention on Human Rights implies the right not to incriminate oneself. Those two principles, the right to remain silent and the protection against self-incrimination, lie at the heart of the right to a fair trial. What’s interesting, though, is that article 6 of the Convention distinguishes between material that exists independently of the will of the accused, such as blood, hair and documents, and material that exists dependently of the will of the accused, such as verbal statements or a confession. The second type of evidence cannot be obtained compulsory. In other words, a suspect may to some extent be required to produce documents or a DNA sample, but he cannot be forced to speak. The question, then, is whether brain-based evidence exists independently of the suspect’s will—like blood, hair and documents—or not.”

Does it?

“On the one hand, you could argue that the P300 response in a guilty knowledge test exists independently of the subject’s will. It’s a brain response that simply occurs or not, the subject has no control over it. That would mean that a suspect can be required to undergo a guilty knowledge test, just as he can be required to give a DNA sample. On the other hand, you could argue that the occurrence of a P300 brain does depend on the will of the accused, in the sense that he must actively and attentively look at a certain visual stimulus for his brain to produce a P300 response. So can you really say that such a brain response exists independently of the suspect’s will, then? That’s a very important discussion.”

Do you think the use of neuro-tests—compulsory or not—will revolutionize our legal system?

“Well, the influence of neurological testing should not be overestimated. Neuro-tests could absolutely be helpful in answering legal questions related to guilt, recidivism risk and criminal responsibility. But I think it’s important that we see neuro-tests as an addition to existing tools such as DNA-based technologies and psychiatric evaluations. People tend to be very impressed by pictures of the brain, but an MRI scan isn’t an actual ‘picture’ of the brain at all. We should remain critical of the information that is actually provided by brain imaging techniques.”

“We must prepare for the implementation of neuro-tests in criminal law”

If we shouldn’t expect some sort of neuro revolution in criminal law, why is it so important to anticipate new legal uses of neuro-tests?

“Because the implementation of neuro-tests may have radical effects on the way legal issues are resolved, and as a result, they may have radical effects on legal outcomes. You could compare the emergence of neuro testing technologies to the introduction of DNA technologies, a few decades ago. When DNA first emerged as a way to identify and convict suspects of a crime, legal science was not at all prepared for that. Legal frameworks were established in an along-the-way fashion. If we want to implement neuro testing techniques responsibly, legal scientists must stay ahead of the curve. We must prepare for the implementation of neuro-tests in criminal law, and we must build legal frameworks that allow for those new technologies to be used in a responsible way. That’s what my PhD research focuses on.”

Basically, you’re trying to prevent Black Mirror from becoming reality.

“Ha, I guess you could say that! What’s important is that when compulsory neuro-tests do become reality in our criminal justice system, we’ve carefully thought things through. That means that we haven’t just explored the potential advantages they bring, but that we’ve also explored the potential threats they pose to the legal interests of individuals. Especially to those legal interests that are recognized as human rights.”


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