Like most of the rest of the country, I followed the recent confirmation hearings for Judge (now Justice) Kavanaugh with great interest.
As the readers of this blog already know, Dr. Christine Blasey Ford testified before the Senate Judiciary Committee that Judge Kavanaugh sexually assaulted her in high school. Much of her testimony recounted her recollection of that event, but some of her testimony was of a different nature. In addition to telling the Committee what she recalled, Dr. Ford also described the biological and chemical processes of memory itself, such as the way that neurotransmitters encode memories into the hippocampus.
In other words, Dr. Ford testified in dual roles: she was both a fact witness and a de facto expert witness.
Most of us will never participate in a Supreme Court confirmation hearing, but a similar type of dual testimony can arise in criminal trials in state court, and it raises some interesting issues.