Brain fingerprinting is a forensic science technique that uses electroencephalography (EEG) to determine whether specific information is stored in a subject's brain. It consists of the measuring and recording a person's electrical brainwaves and their brain response, which is known as P300-MERMER ("Memory and Encoding Related Multifaceted Electroencephalographic Response"), to words, phrases, or pictures on a computer screen.
Brain fingerprinting was invented by Lawrence Farwell. He hypothesized that the brain processes known or relevant information differently than unknown or irrelevant information, and that the brain's processing of information known to the subject is revealed by a specific pattern in the EEG. Farwell's brain fingerprinting technique originally used the P300 brain response to detect the recognition of the known information; however, Farwell later discovered the P300-MERMER response, which extends the basic P300 and is reported to provide greater accuracy and statistical confidence both in the laboratory and in real-life applications, with an error rate of less than 1%. In independent research, William Iacono and others replicated Farwell's technique and results.  In 2001, brain fingerprinting was ruled as admissible for court use in Iowa by the decision in Harrington vs. State of Iowa.  
Brain fingerprinting is premised on the fact that the electrical P300 signal is emitted from an individual's brain approximately 300 milliseconds after he or she is confronted with a stimulus of special significance, for example, a rare stimulus vs. a common stimulus, or a stimulus that the subject is asked to count. In forensics, P300 is used to detect stimuli such as a murder weapon or a victim's face, or any other stimulus related to the crime. Exposure to a stimulus is sufficient to elicit a P300 response; therefore, brain fingerprinting does not require the subject to issue verbal responses to questions or stimuli.
The test subject wears a special headset with electronic sensors that measure the subject's EEG from several locations on the scalp. The person is then exposed to stimuli in the form of words, phrases, or pictures presented on a computer screen. There are three types of stimuli presented to the subject:
- "irrelevant" stimuli are irrelevant to the investigated situation and to the test subject,
- "target" stimuli are relevant to the investigated situation and are known to the subject, and
- "probe" stimuli are relevant to the investigated situation and have never been disclosed to the public or to the subject, and the subject denies knowledge of them.
Probe stimuli contain information that was present at the crime and is thus known only to the perpetrator, investigators, and witnesses. Before the test, the scientist identifies the targets to the subject and verifies their familiarity with these stimuli. The scientist also ensures that the subject is not familiar with irrelevant stimuli, and he confirms that the subject denies familiarity with the probe stimuli. The significance of the probes is revealed to the subject (e.g., "You will see several items, one of which is the murder weapon"), but the subject is not told which items are probes and which are irrelevant.
Since brain fingerprinting uses cognitive brain responses, it is not dependent on the emotions of the subject, nor is it affected by emotional stress that could be caused by the interrogation process. Brain fingerprinting is fundamentally different from the polygraph test, which measures emotion-based physiological signals such as heart rate, perspiration, and blood pressure, because it does not attempt to determine whether or not the subject is lying or telling the truth. Instead, it measures the subject's brain response to specific words, phrases, or pictures to detect whether or not the relevant information is stored in the subject's brain.
By comparing the responses to the different types of stimuli, the brain fingerprinting system mathematically computes a determination of whether the probe stimuli information is "present" (known) or "absent" (unknown), and it provides statistical confidence for this determination. The fact that it is mathematically computed prevents bias on the part of the scientist.
Background and terminology
Brain fingerprinting is a computer-based test that is used to provide evidence that a subject has guilty knowledge regarding crimes, as well as to identify individuals with specific training or expertise, such as the knowledge that a member of a dormant terrorist network would have, or the expertise that a bomb maker might possess. It has also been used to evaluate brain functioning as a means of early detection of Alzheimer's and other cognitively degenerative diseases. It has also been utilized for evaluating the effectiveness of advertising by measuring brain responses.
The technique is described in peer-reviewed publications including Farwell's papers "Brain fingerprinting: a comprehensive tutorial review of detection of concealed information with event-related brain potentials"  and "Brain fingerprinting field studies comparing P300-MERMER and P300 brainwave responses in the detection of concealed information," published in Cognitive Neurodynamics. FBI forensic scientist Drew Richardson was a coauthor on the latter.
In the course of Farwell's research, he developed the test for the P300-MERMER response. The P300, an electrically positive component, is maximal at the midline parietal lobe in the brain, and it has a peak latency between 300 and 800 milliseconds. The P300-MERMER includes both the P300 and an electrically negative component, with a latency between 800 and 1,200 milliseconds. The P300-MERMER includes additional features involving changes in the frequency of the EEG signal, but for the purposes of signal detection and the practical applications the P300-MERMER, the P300 and the following negative response is sufficient without the additional features.
Current uses and research
Brain fingerprinting has two primary applications. First, it is used in detecting whether information about a specific crime, terrorist act, or incident is stored in the brain. Second, it is used to determine whether a subject has a specific type of knowledge, expertise, or training, such as information specific to FBI agents, ISIL-trained terrorists, or bomb-makers.
So far, the brain fingerprinting technique has been successful, with the application of the technique in detecting knowledge of both laboratory mock crimes and real-life events, including major crimes, producing no false positives and no false negatives.   In a study at the FBI Laboratory, Farwell and FBI scientist Drew Richardson, former chief of the FBI's chemical/biological/nuclear counter-terrorism unit, used brain fingerprinting to show that test subjects from specific groups (such as FBI agents) could be identified by detecting specific knowledge which would only be known to members of those groups.
In a study funded by the CIA, Farwell et al. used brain fingerprinting to detect which individuals had US Navy military medical training. All thirty subjects were correctly determined. In another CIA-funded study, Farwell et al. applied brain fingerprinting to detect concealed information regarding real-life crimes. Brain fingerprinting again produced the correct determination in every case.
Use in criminal investigation
Brain fingerprinting was ruled admissible in court in the reversal of the murder conviction of Terry Harrington. Following a hearing on post-conviction relief on November 14, 2000, an Iowa District Court stated that the fundamental science involved in Farwell's brain fingerprinting test was well established in the scientific community, and ruled Farwell's brain fingerprinting results and expert testimony admissible in court. Later, the Iowa Supreme Court also mentioned the test, although it was viewed as incidental to the major points in the appeal that they were handling at the time.
- It must have been tested, peer reviewed, and published.
- It must produce a known, low error rate.
- It must be applied systematically and with set controlling standards.
- It must be well accepted in the relevant scientific community.
In the ruling, the Court stated:
- "In the spring of 2000, Harrington was given a test by Lawrence Farwell. The test is based on a 'P300 effect'."
- "The P-300 effect has been recognized for nearly twenty years."
- "The P-300 effect has been subject to testing and peer review in the scientific community."
- "The consensus in the community of psycho-physiologists is that the P300 effect is valid."
- "The evidence resulting from Harrington's 'brain fingerprinting' test was discovered after the fact. It is newly discovered."
To sum up, the results of the brain fingerprinting test, as well as Farwell's testimony as an expert witness, were admitted. However, the Court noted the distinction between admissibility and weight. In the circumstances of the Harrington case, the Court ruled that the weight of the brain fingerprinting evidence given by Harrington's defense would probably not have been sufficient to change the verdict in the original trial.
In a paper by Farwell and a colleague, attorney Thomas Makeig, he stated the following:
The court determined that Brain Fingerprinting was new evidence not available at the original trial and that it was sufficiently reliable to merit admission of the evidence; however, the court did not regard its weight as sufficiently compelling in light of the record as a whole as meeting its exacting standard, and thus it denied a new trial on this and the other grounds asserted by Harrington.
The Iowa Supreme Court reversed Harrington's conviction, and he was released from prison after serving 24 years of a life sentence. Although the Supreme Court did not give a specific ruling on brain fingerprinting, they allowed the ruling established by the district court to stand, implicitly confirming the district court's finding regarding the admissibility of the test.
Brain fingerprinting produced evidence that, had there been a trial, would have almost surely brought a conviction to serial killer James B. Grinder. In August 1999, Farwell conducted a brain fingerprinting test on Grinder at the request of Sheriff Robert Dawson of Macon County, Missouri. The test proved that the information stored in his brain matched the details of the murder of Julie Helton. Faced with a certain conviction and probable death sentence, Grinder pled guilty to the rape and murder of Julie Helton in exchange for a life sentence without parole. He also confessed to the murders of three other young women.
Both the strengths and limitations of brain fingerprinting are documented in detail in Farwell's expert witness testimony in the Harrington case as well as in his other publications and patents.
Although brain fingerprinting detects brain responses that reveal what information is stored in the subject's brain, it does not detect how that information got there. As a result, if a suspect knows everything that the investigators know about the crime for some legitimate reason, then the test would not be probative. Brain fingerprinting scientific protocols require that a test will not be applied in such circumstances. Examples of cases where a brain fingerprinting test is not applicable and will never be applied include the following:
- If a suspect acknowledges being at the crime scene but claims to be a witness instead of a perpetrator, then the fact that he knows details about the crime would not be incriminating, and the test is not applied.
- If a suspect and an alleged victim—say, of an alleged sexual assault—agree on the details of what events occurred, but disagree on the intent of the parties, then the test will not be applicable and will not be applied, because brain fingerprinting detects only information and not intent.
Another requirement is that a brain fingerprinting test must avoid including public information, as detecting that a suspect is aware information regarding a crime that he obtained by reading a newspaper would not be of use in a criminal investigation. However, this requirement is easily met because standard procedures in the development of the test eliminate all such information from the probes. News accounts containing many of the details of a crime do not typically interfere with the development of a brain fingerprinting test; however, they do limit the material that can be tested. Even so, there are almost always details that are not disclosed to the public, and these are the details that are utilized as the probe stimuli.
Another situation where brain fingerprinting is not applicable is one where the authorities have no information about what crime may have taken place. For example, if an individual simply disappears under suspicious circumstances, but no information is known, then the authorities could not produce any probe stimuli, and it would be impossible to develop a test. Similarly, brain fingerprinting is not applicable for general screening of undesirable actions. If the investigators have no idea of what acts an individual may have committed, then there is no way to structure the appropriate stimuli to detect the telltale knowledge that would result from committing such acts.
Brain fingerprinting, like all forensic techniques, does not determine whether a suspect is guilty or innocent of a crime. This is a legal determination that is made by a judge and jury, not a scientific determination that can be made by a computer or a scientist. To remain within the realm of scientific testimony, a brain fingerprinting expert witness must testify only regarding the scientific test and information stored in the brain as revealed by the test; the expert witness cannot give a verdict.
Brain fingerprinting depends on the memory of the subject; therefore, brain fingerprinting results must be viewed in light of the well-known limitations on human memory and the factors affecting it. Although brain fingerprinting can provide scientific evidence regarding what information is stored in a subject's brain, it does not determine whether that information is an accurate representation of the actual events, as human menory is imperfect. This may create a challenging situation in judicial proceedings, because many judges and juries are not yet familiar with brain fingerprinting. Everyone, however, is familiar with witness testimony. Everyone knows that a truthful witness testifies not to absolute truth, but to the contents of his or her memory. Judges and juries must evaluate all witness testimony, even when the witness is believed to be truthful, in light of the limitations of human memory. This same evaluative process is necessary for brain fingerprinting. Witness testimony provides a subjective (and not always truthful) account of the contents of memory. Brain fingerprinting provides an objective account of the contents of memory. In both cases, judges and juries must evaluate the evidence in light of the well-known limitations on human memory. Because in virtually every trial they apply this same evaluation to the testimony of witnesses, judges and juries are competent and prepared to evaluate and weigh brain fingerprinting evidence even if they have heretofore lacked knowledge of the technology.
Like all forensic science techniques, brain fingerprinting relies on the evidence-gathering process to provide the data to be scientifically tested. Before a brain fingerprinting test can be conducted, an investigator must discover relevant information about the crime. The investigative process depends on the skill and judgment of the investigator, so if the investigative process is carried out incorrectly, it will not produce appropriate probe stimuli. If the information that the investigator provided to the scientist to test was inaccurate, then the results may not be probative with respect to the actual crime. In making their determination about the crime and the suspect's possible role in it, the judge and jury must consider both the scientific determination and also whether the information gathered by the investigator was correct and correctly applied in the test.
Critique of Farwell's conclusions
Although the academic community agrees on the scientific standing of brain fingerprinting and associated P300-based concealed information tests, Farwell’s conclusions regarding the value of applying this science in the real world have been met with skepticism from some commentators. Rosenfeld opined that "the claims on the BF Web site are exaggerated and sometimes misleading." The same journal later published corrections to Rosenfeld's article along with Farwell's reply, stating that "The purpose of this article is as follows: 1) to provide the readers of ...SRMHP with a clear, fundamental understanding of the science and technology of brain fingerprinting; 2) to correct demonstrably false and misleading statements made by Rosenfeld (2005) in a previous article in SRMHP; 3) to present the facts regarding Rosenfeld's misuse of the SRMHP article to support his subsequent false statements and false claims in other forums; and 4) to provide references to sufficient source material to verify all of the above." More comprehensive corrections to the Rosenfeld article were published in a separate monograph. Four authors including Farwell's former PhD supervisor Emanuel Donchin disagreed with one of Farwell's peer-reviewed publications, opining that "Farwell['s 2012 review paper] ... is misleading and misrepresents the scientific status of brain fingerprinting technology." The same journal published a reply by Farwell and FBI forensic scientist Drew Richardson entitled "Brain fingerprinting: Let's focus on the science."  Farwell and Richardson stated that "The authors stated their disagreement with Farwell's hypotheses, but did not cite any data that contradict the three hypotheses, nor did they propose alternative hypotheses or standards. [They] made demonstrable misstatements of fact... We provide supporting evidence for Farwell's three hypotheses, clarify several issues, correct [their] misstatements of fact, and propose that the progress of science is best served by practicing science: designing and conducting research to test and as necessary modify the proposed hypotheses and standards that explain the existing data."
Other scientists agree with Farwell’s conclusions. For example, William Iacono, a scientist unaffiliated with Farwell who testified as an expert witness along with Farwell in the Harrington case, wrote an article entitled “The Forensic Application of ‘Brain Fingerprinting:’ Why Scientists Should Encourage the Use of P300 Memory Detection Methods.” He stated: “To summarize, the GKT [guilty knowledge test or concealed information test] and its ‘brain fingerprinting’ offshoots are based on sound science. They appear to be quite effective at detecting memories stored in the brain.” 
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