Brain fingerprinting is a recently developed forensic technique that utilizes brain-wave monitoring to determine if a subject of a crime investigation has memories of specific facts pertinent to the case embedded in his brain. The technology has been used in several high-profile cases in the United States and has been ruled admissible in court. Dr. Lawrence Farwell created the technology in the early 1990s, basing it upon a brain wave known as a P300 that is emitted when a subject recognizes a stimulus placed in front of him or her. This process is limited somewhat in criminal investigations if the subject has already heard about facts in the case, and critics have decried the process as being less than totally accurate.
The brain fingerprinting test is administered via computer and is different from a polygraph, or lie-detector test, in that it seeks no verbal response from its subjects. It measures the brain waves of the subject when visual or audio stimuli is presented to him. Some of the stimuli are called "targets" and are previously known to the subject, thus providing a baseline brain wave for the test. Other stimuli are presented that are irrelevant to the case, but a third group of stimuli, called "probes," are elements of the case or crime scene that have not been previously presented to the subject by investigators.
When the subject sees or hears these probes, his or her brain waves are measured through electronic sensors fitted on a headband worn during the test. It's possible that the probe will produce a brain response that Dr. Farwell categorized as a memory and encoded related multifaceted electroencephalographic response, or MERMER, which is comprised of a traditional P300 response along with other measurable brain patterns that provide further proof that the subject recognizes the stimulus. If that's the case, it is determined that the subject does have an embedded memory of the specific stimulus, also called a "memory present" response. No MERMER response means that he did not recognize the probe, a "memory absent" response.
Courts in the Unites States have used brain fingerprinting in several important cases and have ruled the procedure to be legally admissible as evidence. In 1999, serial killer J.B. Grinder pled guilty to a crime of rape and murder committed 15 years earlier after brain fingerprinting proved he had memory of specific details of the crime. Use of the technology helped to exonerate an Iowa man named Terry Harrington in 2003 of a crime committed 26 years before, as his brain-wave responses backed up his alibi.
Limitations of the process arise when the subject of an investigation becomes aware of evidence and other elements of the case during the investigation or even by learning about it from the media. In addition, brain fingerprinting can usually only prove if a subject was at a crime scene, not whether he or she committed the crime. Critics of brain fingerprinting point to some inaccurate results from P300 studies, although proponents claim that Dr. Farwell's MERMER method is a vast improvement on the original P300 method in terms of accuracy.