Is fMRI the New Polygraph?

In an age of rapidly advancing technology, what are the moral and ethical implications of employing neuroimaging techniques in the pursuit of truth? Following the demise of the polygraph, fMRI has been proposed as an effective alternative- capable of detecting both familiarity and deception in the courtroom. Yet, despite its promise, such evidence remains inadmissible under current U.S. standards.  

   Functional magnetic resonance imaging (fMRI) combines a strong magnetic field to map biologic tissues and record differences in MR signals from oxyhemoglobin and deoxyhemoglobin to measure brain activity by detecting changes in blood flow. A brain area that is active will have a higher oxyhemoglobin:deoxyhemoglobin ratio because naturally, active tissue consumes more oxygen. For the brain field to become active, fMRI must be used congruent with cognitive tasks. Once neuronal tissues are stimulated, local microvasculature is employed, increasing arterial blood flow and consequent deliverance of oxygen to the area via oxyhemoglobin. The rapid change in blood oxygen levels (after ~1-2s) following neuronal activity provokes a change in the MR signal, labelled the “blood-oxygenation level-dependent” (BOLD) signal. Fluctuations in the BOLD signal allow scientists to confine brain activity on a second by second basis within millimeters of the source, showing remarkable spatial precision. 

     Since the brain state displayed in fMRI is the dependent variable, it is necessary to correlate fMRI data with complementary interrogation techniques. Two widely used models of investigative interviewing are the PEACE model and more commonly in the U.S., the Reid model. The PEACE model of investigative interrogation is expansive, and can be used in any interview situation with any type of respondent. Additionally, it has been in use for over two decades- representative of its general positive results. The PEACE model emphasizes rapport-building, transparency, and structured dialogue through five stages: Planning and Preparation, Engage and Explain, Account/Clarification/Challenge, Closure, and Evaluation. In contrast, the Reid model employs behavioral cues and a confrontational sequence of nine stages aimed at eliciting confessions. 

      In principle, fMRI could supplement these models by providing neural correlates of two critical interrogation goals: assessing familiarity and detecting deception. For instance, during an fMRI session, investigators might display an image of a crime scene. If a suspect’s BOLD signal map reveals activation patterns consistent with prior recognition, it could suggest familiarity with that location. 

     Another potential application of fMRI lies in detecting deception by identifying neural indicators of conflict between truth and falsehood. This conflict arises when an individual lies, as truth represents the automatic or dominant response- requiring that person to inhibit it in favor of fabrication. Lie detection studies using fMRI are grounded in the analysis of BOLD signals and their contrast across different conditions. Researchers compare brain activation patterns during lying versus truth-telling, allowing them to identify the neural correlates associated with deception. 

     Although fMRI studies have revealed intriguing patterns, no method of “truth detection” has been scientifically validated for courtroom use. U.S. judges have repeatedly excluded such evidence, citing Daubert standards that demand established reliability, general acceptance, and reproducibility. 

     Moreover, fMRI faces significant technical and ethical hurdles. The technique relies on strong magnetic fields, which are contraindicated in certain individuals or medical conditions. Additionally, fMRI does not directly measure neuronal activity and instead detects changes in blood oxygenation, specifically the oxyhemoglobin:deoxyhemoglobin ratio, which can be influenced by various vascular processes. 

     In some investigative contexts, neuroimaging may be conducted under conditions of coercion to increase the validity of results. Physical coercion might involve restraining a suspect to minimize motion during scanning, while pharmacological intervention may be employed to reduce involuntary movement. Legal coercion, on the other hand, occurs when individuals are pressured to participate under threat of legal consequences. This raises obvious ethical concerns, especially since techniques like fMRI often require the subject’s active engagement for accurate results. 

     Another complication of fMRI validity is seen in individuals exhibiting psychopathic traits. A considerable proportion of criminal offenders meet diagnostic criteria for psychopathy, where deception is a defining feature. Studies examining offenders with antisocial personality disorder have shown atypical or absent BOLD responses in the prefrontal cortex during tasks involving instructed deception, challenging assumptions about fMRI’s reliability across populations. 

     Moreover, individual differences and situational stress can affect scan accuracy. Even minor movements may produce artifacts that distort results. Certain subjects—such as those with claustrophobia, ferromagnetic implants, or other medical contraindications—cannot safely undergo MRI procedures. Additional barriers include the unsuitability of some suspects (e.g., juveniles, individuals with substance use disorders, or mental instability) and the high cost, limited availability, and need for specialized expertise in both medical imaging and forensic interpretation.

     While technology remains scientifically captivating, its legal and moral complexities are profound. As neuroscience continues to blur the line between mind and behavior, society must decide: Should the brain ever be called to testify?