What happened when? How the brain stores memories by time

Before I left the house this morning, I let the cat out and started the dishwasher. Or was that yesterday? Very often, our memories must distinguish not just what happened and where, but when an event occurred — and what came before and after. New research from the University of California, Davis, Center for Neuroscience shows that a part of the brain called the hippocampus stores memories by their “temporal context” — what happened before, and what came after.

"We need to remember not just what happened, but when," said graduate student Liang-Tien (Frank) Hsieh, first author on the paper published March 5 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," Ranganath said. "For the hippocampus, context is critical, not content, and it’s fairly unique in how it pulls things together."

Other parts of the brain store memories of objects that are independent of their context, Ranganath noted.

"For patients with memory problems this is a big deal," Ranganath said. "It’s not just something that’s useful in understanding healthy memory, but allows us to understand and intervene in memory problems."

Human memory study adds to global debate

An international study involving researchers from the University of Adelaide has made a major contribution to the ongoing scientific debate about how processes in the human brain support memory and recognition.

The study used a rare technique in which data was obtained from within the brain itself, using electrodes placed inside the brains of surgery patients.

Obtained in Germany, the data was sent to the University of Adelaide’s School of Psychology for further analysis using new techniques developed there. The results are published today in the Proceedings of the National Academy of Sciences (PNAS).

"Being able to understand how human memory works is important because there is a range of conditions that affect memory, such as Alzheimer’s disease, head injury, and ageing," says Professor John Dunn, Head of the School of Psychology at the University of Adelaide and a co-author of the study, which was led by researchers at the universities of Cambridge, UK, and Bonn, Germany.

"Scientists know a lot about memory from years of study, but there is an ongoing debate about how certain mechanisms in the brain process memory, and how those mechanisms work together.

"What we’re looking at is how the human brain processes ‘recognition memory’, which is our ability to recognise people, objects or events that we’ve encountered in the past."

The debate has centered on two key regions in the brain:

• the hippocampus, which is very important to memory and is one of the first regions of the brain to suffer damage from Alzheimer’s disease; and
• the perirhinal cortex, which receives sensory information from all of the body’s sensory regions.

"The debate is whether or not these two regions work in the same or different ways to support memory and recognition Studies over the years have led to both conclusions," Professor Dunn says.

He says this new study, which uses data from inside the brain instead of from electrodes on the scalp, far from the critical regions, revealed that different processes are at work in the hippocampus and the perirhinal cortex.

"Our analysis shows that these regions are responding to and processing memory in two very different ways. The activity levels in those regions changed in different ways according to the amount of information that could be remembered," Professor Dunn says.

"This study won’t settle the debate once and for all, but it does add weight to those scientists who believe that these two distinct parts of the brain respond to memory in different ways," he says.