FOR DECADES, SCIENTISTS HAVE used electrodes to study how the brain reacts to sensory inputs. When a patient sees a face or hears a voice, scientists can watch how certain cells become active and interact with other cells in different parts of the brain. The question that has been hardest to answer is how the cells store information and remember what has happened – how is memory made and how is it recalled?
Itzhak Fried, M.D., Ph.D., a UCLA professor of neurosurgery, and colleagues at the Weitzmann Institute of Science in Israel, have developed a new research model to look at the memory process. Dr. Fried’s research involves patients with epilepsy who have undergone surgical treatment. As a standard process before surgery, surgeons place electrodes inside the patients’ brains to locate the origin of their seizures. Dr Fried made use of those electrodes to record activity as memories are being formed.
Thirteen patients who had electrodes implanted in single neuron cells before their surgery were shown short clips of “episodes” featuring various characters and people such as Homer Simpson, Jerry Seinfeld and Tom Cruise. As the patients watched, the researchers recorded the activity of various single neurons in the hippocampus and in a nearby part of the brain called the entorhinal cortex.
A few minutes later, after performing an intervening task, the patients were asked to recall whatever clips came to mind. “They were not prompted to recall any specific clips,” Dr. Fried says, “but to use ‘free recall’ – that is, whatever popped into their heads.”
The researchers found that the same neurons that had responded to a specific clip when it was first shown fired strongly a second or two before the patient recalled – or “remembered” – that clip.
The study is significant because it confirms that spontaneous memories arise through the activity of the same neurons that became active when the memory was first made. This link between reactivation of neurons in the hippocampus and conscious recall of past experience has long been suspected. The study provides clear evidence that memory can be tracked to single cells in the brain, and as Dr. Fried says, that “reliving past experience in our memory is the resurrection of neuronal activity from the past.”