Washington: Scientists, including one of Indian-origin, have found that our brain stores memories by their "temporal context" - what happened before, and what came after - and not by content.
Very often, our memories must distinguish not just what happened and where, but when an event occurred - and what came before and after, researchers said.
"We need to remember not just what happened, but when," said graduate student Liang-Tien (Frank) Hsieh, first author on the study published in the journal Neuron.
The hippocampus is thought to be involved in forming memories. But it's not clear whether the hippocampus stores representations of specific objects, or if it represents them in context.
Hsieh and Charan Ranganath, professor in the Department of Psychology and the Center for Neuroscience, looked for hippocampus activity linked to particular memories.
First, they showed volunteers a series of pictures of animals and objects. Then they scanned the volunteers' brains as they showed them the same series again, with questions such as, "is this alive?" or "does this generate heat?"
The questions prompted the volunteers to search their memories for information. When the images were shown in the same sequence as before, the volunteers could anticipate the next image, making for a faster response.
From brain scans of the hippocampus as the volunteers were answering questions, Hsieh and Ranganath could identify patterns of activity specific to each image.
But when they showed the volunteers the same images in a different sequence, they got different patterns of activity.
In other words, the coding of the memory in the hippocampus was dependent on its context, not just on content.
"It turns out that when you take the image out of sequence, the pattern disappears. For the hippocampus, context is critical, not content, and it's fairly unique in how it pulls things together," Ranganath said.
Other parts of the brain store memories of objects that are independent of their context, Ranganath said.