“I’ll meet you at that spot,” a man says to his friend. To outsiders, it may seem unintelligible, but his friend already knows from past encounters exactly where and when he wants to meet. Thanks to this shared knowledge, or common ground, they are able to make their conversation less detailed and more efficient.

“For example, if I said to you, ‘You know how all politicians are talking about things differently since 9/11?,’ you know what I’m talking about,” said Neal Cohen, psychology and neuroscience professor and faculty member in the Beckman Institute Cognitive Neuroscience Group.

“How is it that by my saying ‘9/11’ you immediately understand that this refers to a date important to America and not just the number you call in an emergency?” Cohen said.

Cohen studies patients with profound memory impairment, or amnesia, in order to identify the many memory systems in the brain and their functions. The concept of common ground has enabled him to observe drastic improvement of his patients’ procedural memory, even though their declarative memory has been damaged.

“Profound memory impairment” refers to the condition of amnesic patients.

“We’ve found really interesting dissociations,” Cohen said. “When you see a distinction or discontinuity (such as loss of autobiographical information due to amnesia), it shows that brain damage can impair one or more systems of memory while leaving others intact.”

In a study Cohen conducted, each amnesic patient was told to bring in someone they interact with frequently, such as a spouse or good friend, to serve as a partner. The patient and partner were each given the same 12 playing cards, but the patient’s cards were in a specific order.

Throughout the experiment, the patients were not allowed to state what exactly appeared on their card.

“Without looking at the partner’s cards, the patient had to teach their partner to put his cards in the same order as his own, saying things like, ‘The card looks like such and such using common ground,'” Cohen said.

“They had to learn to describe these cards as easily and efficiently as possible,” Cohen said.

The experiment showed that amnesic patients’ procedural memory was unaffected by the loss of declarative memory and that it even improved with time, Cohen said.

“Patients didn’t even remember being there,” Cohen said. “One patient got better and better, and told his wife how to do the cards, but he couldn’t even remember if he had brought his coat at the end of the session, and took mine instead.”

By observing areas that patients with specific types of brain damage struggle in, Cohen said he has been able to determine how that area of the brain contributes to every day memory function.

“When you have brain damage, you could lose all memory, but we have found that there are different forms of memory, and different types of brain damage reveal that there are unique mechanisms in the brain controlling different types of memory,” Cohen said.

Amnesic patients have provided Cohen with insight into the workings of the fully functional human brain.

“It is only when something is missing or damaged that we can gauge how it contributes to normal brain function,” Cohen said.