“In summary, this study is invalid and unreliable,” she wrote in court documents in 2012. “In my opinion, this study questions the ability of Mr. Lewinski to apply relevant and reliable data to answer a question or support an argument.”

Dr. Lewinski said he chose to publish his findings in the magazine because it reached so many officers who would never read a scientific journal. If he were doing it over, he said in an interview, he would have published his early studies in academic journals and summarized them elsewhere for officers. But he said it was unfair for Dr. Fournier to criticize his research based on summaries written for a general audience.While opposing lawyers and experts found his research controversial, they were particularly frustrated by Dr. Lewinski’s tendency to get inside people’s heads. Time and again, his reports to defense lawyers seem to make conclusive statements about what officers saw, what they did not, and what they cannot remember.

Often, these details are hotly disputed. For example, in a 2009 case that revolved around whether a Texas sheriff’s deputy felt threatened by a car coming at him, Dr. Lewinski said that the officer was so focused on firing to stop the threat, he did not immediately recognize that the car had passed him.

Inattentional Blindness

Such gaps in observation and memory, he says, can be explained by a phenomenon called inattentional blindness, in which the brain is so focused on one task that it blocks out everything else. When an officer’s version of events is disproved by video or forensic evidence, Dr. Lewinski says, inattentional blindness may be to blame. It is human nature, he says, to try to fill in the blanks.

“Whenever the cop says something that’s helpful, it’s as good as gold,” said Mr. Burton, the California lawyer. “But when a cop says something that’s inconvenient, it’s a result of this memory loss.”

Experts say Dr. Lewinski is too sure of himself on the subject. “I hate the fact that it’s being used in this way,” said Arien Mack, one of two psychologists who coined the term inattentional blindness. “When we work in a lab, we ask them if they saw something. They have no motivation to lie. A police officer involved in a shooting certainly has a reason to lie.”

Dr. Lewinski acknowledged that there was no clear way to distinguish inattentional blindness from lying. He said he had tried to present it as a possibility, not a conclusion.