The new study is the latest chapter in an extraordinary, decades-long collaboration among cognitive scientists, brain surgeons and people with severe epilepsy being evaluated for an operation. The preoperative “evaluation” is a fishing expedition of sorts, in which doctors sink an array of electrodes through the top of the skull and wait for a seizure to occur, to see whether it’s operable. Many of the electrodes sit in or near memory areas, and the wait can take weeks in the hospital. Cognitive scientists use this opportunity, with patients’ consent, to present memory tests and take recordings.

This approach — called direct neural recording, and piggybacking entirely on the clinical placement of the electrodes — has become the leading edge of research into the biology of human memory. This study used data from 150 patients, and had 20 collaborators from institutions around the country, including Emory University, the University of Washington, the Mayo Clinic and the University of California, San Francisco.

In a series of experiments, the researchers had patients memorize lists of words and later, after a distraction, asked them to freely recall as many of the words as they could. All the while, the scientists monitored a handful of “hot spots” in the brain which, previous work had shown, were strongly related to memory encoding. Before the stimulation tests, the team determined the precise settings for each patient’s high- and low-functioning states.

Each participant carried out the word-memorization tests repeatedly, with different words every time; some lists were memorized with brain stimulation, and other lists with no stimulation, which served as a control. The authors then examined memory performance based on whether stimulation arrived during low- compared with high-functioning brain states.

The team then statistically analyzed the results and found that people scored slightly higher than usual on words when stimulation arrived during a low or foggy state — and worse, when the pulse arrived in a high state. “The average enhancement effect was about 12 to 13 percent,” Dr. Kahana said. “And when stimulation arrived in a good state, the average was about 15 to 20 percent worse than usual.”

Dr. Doris Greenblatt, a psychiatrist who participated in the trial at Emory, said she sought the surgery because her epilepsy had long caused memory problems. “Each seizure I had tore at the fabric of memory, and it was as if my memories weren’t attached to anything,” Dr. Greenblatt said.