Neuroscientists from MIT have identified cell populations that encode different parts of an overall experience. Like the neural code used to recall detailed memories of different locations, these cells are also stored in the hippocampus. Rather than being activated by different location settings however, they are activated whenever similar experiences occur.
It has been known for some time that memories of specific locations are stored in certain cells in the brain’s hippocampus. Research conducted on mice for example has shown that neurons known as place cells activate when the animal is placed in a certain location, and if they dream about it.
For this study, researchers from MIT wanted to understand whether the hippocampus also stores information on more abstract parts of memories. In particular, they wanted to know whether hippocampal cells would also activate when experiencing certain tasks, such as looking through a menu or eating a dessert, regardless of location.
To see whether this would be the case, the research team designed an experiment in which they measured the neuronal activity in the CA1 region of the mouse hippocampus as they ran a four-lap maze. At the end of every fourth lap, the mice were given a reward.
As hypothesized, the researchers noted that place cells fired up at different points along the track. They also found that another set of cells were fired up during one of the four laps, although not others.
To see whether this meant that the mice were also recording chunks of experiences beyond location, the researchers then trained the mice to run a square maze on one day and a circular maze on another, once again receiving a reward every fourth lap. While place cells changed in activity (as expected according to the new environments), they found that the same sets of lap-specific cells fired up during each of the four laps, regardless of the track. These lap-coding cells behaved the same even when laps were made longer or shorter at random.
"Even in the new spatial locations, cells still maintain their coding for the lap number, suggesting that cells that were coding for a square lap 1 have now been transferred to code for a circular lap 1,” says Chen Sun lead author of the study,
From these results, the researchers theorize that the hippocampus consists of ‘two mutually and independently manipulatable codes’. While one encodes factors like location, time and sensory input, the other breaks an overall experience down into smaller categories that fit into consistent experiences like ‘drinking tea’ or ‘reading a book’.
Susumu Tongawa, senior author of the study, said about the two hippocampal codes: “If we want to remember all the details of what happened in a specific experience, moment-to-moment changes that occurred, then the continuous monitoring is effective. But on the other hand, when we have a longer experience, if you put it into chunks, and remember the abstract order of the abstract chunks, that's more effective than monitoring this long process of continuous changes."