Memories are made of this

For the first time scientists have recorded individual brain cells in the act of calling up a memory, revealing where in the brain a specific memory is stored and how the brain is able to recreate it.

Neurons in the hippocampus and entorhinal cortex responded to original stimuli, and when a memory was being recalled

The team - from the US and Israel - recorded the activity of hundreds of individual neurons making memories in the brains of 13 epilepsy patients being treated surgically at UCLA Medical Centre.
 
Surgeons had placed electrodes in the patients’ brains to locate the origin of their seizures before surgical treatment - standard procedure in such cases. They then made use of the same electrodes to record neuron activity as memories were being formed.

The patients watched several video clips while the researchers recorded the activity of many neurons in the hippocampus and a nearby region known the entorhinal cortex that responded strongly to individual clips. Patients were then asked to recall whatever clips came to mind.
 
Dr Itzhak Fried, UCLA professor of neurosurgery said: “They were not prompted to recall any specific clips, but to use 'free recall' - that is, whatever popped into their heads.”

The researchers found that the same neurons that had responded earlier to a specific clip fired strongly a second or two before the subject reported recalling that clip. These neurons did not fire, however, when other clips were recalled. Ultimately, it was possible for the researchers to know which clip a patient was recalling before the patient announced it.
 
The study is significant, say the team, because it confirms for the first time that spontaneous memories arise through the activity of the very same neurons that fired when the memory was first being made. This link between reactivation of neurons in the hippocampus and conscious recall of past experience has been suspected and theorized for sometime, but the study now provides direct evidence for such a link.
 
"In a way, then," Fried said, "reliving past experience in our memory is the resurrection of neuronal activity from the past."

The report appeared in the t online edition of the journal Science.