Amnesia myth challenged as researchers parse regions of the brain

Damage to the brain’s hippocampus may result in more severe loss of personal memory than originally thought, according to a study led by York Professor Shayna Rosenbaum.

The hippocampus is an area buried deep in the forebrain that helps regulate emotion and memory. It is highly vulnerable to a range of neurological conditions, such as Alzheimer’s, encephalitis and loss of oxygen to the brain.

Left: Shayna Rosenbaum

The study – which included an interdisciplinary team of scientists from York, Baycrest’s Rotman Research Institute and Sunnybrook Health Sciences Centre in Toronto, the University of Western Australia, and VA Boston Healthcare System – yielded findings that challenge the long-held notion that damage to the hippocampus results only in the loss of memories formed just before brain damage occurred, while older memories are retained.

Published in the August 2008 issue of the Journal of Cognitive Neuroscience, the study found that damage to the hippocampus may result in more devastating and far-reaching loss of autobiographical episodic memory, the type of memory that allows us to mentally travel back in time and re-experience personal past events.

"The hippocampus is more critical to autobiographical episodic memory than we’d previously thought," said Rosenbaum, the study’s lead author, a professor of psychology in York’s Faculty of Health and a member of the Neuroscience Diploma Program. "In our study, we have found that it is possible to lose all of your past personal memories, even when the hippocampus is the only part of the brain that is damaged."

While previous amnesia studies have included patients with more widespread brain damage, Rosenbaum’s team recruited amnesia patients with damage limited to the hippocampus, thus yielding stronger evidence about the role of the hippocampus in maintaining detailed and personally relevant memories that extend far back in one’s life.

Right: The study found that damage to the hippocampus (highlighted in red) may result in more devastating and far-reaching loss of the type of memory that allows us to mentally travel back in time and re-experience personal past events. 

"We found that one particular patient who had the most severe hippocampal damage also had the most severely impaired memory, even though he had the least brain damage overall. His brain was actually quite intact," Rosenbaum said.

The study used structural magnetic resonance imaging (MRI) to calculate the volume of different brain regions in four amnesic patients with brain injuries. The MRI scans also determined the size and location of brain lesions. Patients were later interviewed to provide fine-grained analysis of their ability to retrieve personal memories established at different times in their lives.

The findings have implications for patients, their families and clinicians, who have previously been unclear about what to expect from damage to the hippocampal region of the brain, Rosenbaum said.

The good news is that patients with damage to the hippocampus still retain semantic memories – general facts about personal identity and events that have been established in the brain through repeated exposure.

Indeed, these semantic memories may help patients with amnesia to compensate for the inability to form or retain new memories.

A separate joint study by York and Baycrest’s Rotman Research Institute, led by Drs. Sandra Moses and Jennifer Ryan and co-authored by Rosenbaum and her former undergraduate student Melanie Ostreicher, showed that a person with severe episodic memory loss was able to rely on intact semantic memories to form new relations in memory.

"Using patients’ semantic knowledge of the game rock, paper, scissors, or playing cards for example, we were able to by-pass the brain-damaged region to allow for intact memory performance," Ostreicher said.

That study, "Successful Transverse Patterning in Amnesia Using Semantic Knowledge", appeared earlier this year in the journal Hippocampus.