It often happens that we can remember something which occurred long back but forget the incident that took place recently. Some memories remain stable while others fade away in minutes.
Researchers from Caltech have found that the memories which remain stable over time are encoded by a group of neurons firing in synchrony thus providing redundancy. The work also tries to understand the effects on memory after tragic events such as brain damage or Alzheimer’s disease.
The experiments were carried out in the laboratory of Carlos Lois, a biology professor and affiliated faculty member of Tianqiao and Chrissy Chen Institute for Neuroscience. It appears in the Science journal.
The team was led by Walter Gonzalez, a postdoc scholar. A test was developed for testing the neural activity of mice when they learn and remember a new place. The mouse was kept in a straight enclosure of length 5 feet. Different symbols were marked in different locations on the walls such as a bold plus sign at the rightmost end and angled slash close to the center. Sugar water was placed at the track ends. As the mouse explored places, the activities of certain neurons in the hippocampus were tracked by the scientists.
When initially placed in the track, the mouse wandered till it found the sugar water. Single neurons were activated when it spotted a symbol on the wall. However, on staying longer, the mouse remembered the sugar locations. As the familiarity increased more neurons got activated in synchrony as it spotted the symbols.
To understand the fading of memories, the mice were kept away from the track for 20 days. On getting back to the track, the mice which had strong memories encoded by high numbers of neurons remembered the task easily. So when large groups of neurons encode an activity, the memories can be easily recalled even if some showed different activity or remained silent.
This can be explained in a way similar to how a long story is remembered. For remembering a long story, it can be told to many different people and when all of them gather each can fill in the gaps which the other member has forgotten. By repeating this every time, the story can be preserved with the strengthening of the memory. In a similar way, neurons help each other in encoding memories which stay over time.
Impairment of memory in any form can affect us a lot since our life is basically driven by memories. Senior citizens get affected by this as a part of the aging process. Alzheimer’s disease also has devastating effects which paralyze even the basic daily functioning of a person. When memory is encoded by fewer neurons it can be forgotten easily. As a result of this, treatments which increase the recruitment of a large number of neurons for encoding a memory help in preventing memory loss.
When an activity is practiced a lot, there are more chances of remembering it as more neurons are encoded for the action. It is usually considered that to make a memory stable, individual connections to a neuron have to be strengthened. However, the study suggests that the memory can be stored for a long period of time with an increase in the neurons which encode it.