Hippocampus Is Crucial : Research Supports Theory on Formation of Memory
Scientists have come up with evidence to support a theory raised long ago that a structure deep in the inner labyrinth of the brain is crucial to formation of memories.
Researchers at the San Diego Veterans Administration Medical Center studied an amnesic man known as RB for five years, beginning with the hypothesis that his amnesia was the result of damage to the brain structure called the hippocampus.
The hippocampus extends across the brain’s left and right hemispheres. For nearly a century, scientists suspected that damage caused by illness or injury affects hippocampal function by disrupting the ability to remember.
“Damage to the hippocampus has been implicated in memory impairment since the first case study was published in 1900,” said Dr. Stuart Zola-Morgan, a researcher who also is an assistant professor at the University of California, San Diego, School of Medicine.
“Back then there was no strong evidence supporting that hypothesis. We’re now certain that the hippocampus is involved in the formation of memories, the information that you retain.
Normal Tissue Found
“Prior to our work, it had been impossible in the logic of science to say that definitively,” Zola-Morgan said.
Analysis of RB’s brain after his death showed that the hippocampus was damaged but that tissue surrounding the structure remained normal, a factor that prompts new questions for neuroscientists.
“The hippocampus apparently is involved in the laying down (the formation) of memories, but the manner in which it is involved is still an open question,” Zola-Morgan said.
Zola-Morgan said he believes that RB’s case suggests that the hippocampus plays a role in the process by which new information is transformed and parceled to other areas of the brain for storage. He said the hippocampus also may play a role in retrieving those memories.
The San Diego memory studies bear out earlier investigations conducted by the National Institute of Mental Health near Washington in memory tests of lower primates.
In 1978, Dr. Mortimer Mishkin of the national institute found that surgically induced damage to the hippocampus produced memory impairment in monkeys.
Zola-Morgan said, however, that because surgically induced damage also damages other areas of the brain, it was difficult to say unequivocally that hippocampal damage alone impairs the ability to remember.
‘The Big Mystery’
Researchers at the University of Iowa have found in autopsies of victims of Alzheimer’s disease that lesions were evident throughout the hippocampus.
“We have found that patients with Alzheimer’s disease have damage throughout the brain, but lesions in the hippocampus are quite evident,” said Dr. Anthony Damasio of the University of Iowa.
“The big mystery is why Alzheimer’s patients have lesions (damaged areas) in the hippocampus,” Damasio said.
Zola-Morgan and his team suggest that hippocampal damage probably causes what is known as anterograde memory deficits, an inability to store new information encountered after trauma.
Retrograde memory impairment refers to an inability to recall information stored before trauma.
In RB’s case, the study shows that he was capable of remembering events and information learned before the hippocampal trauma. This is compelling evidence that the hippocampus is not the site of memory storage, Zola-Morgan said.
Many Storage Sites
“We think there is no single storage site in the brain because memory is made up of a number of components. You remember things in terms of color, texture and size, for example, and each component is distributed throughout the brain.”
Tests administered to RB throughout his five years as an amnesia patient at the San Diego Veterans Administration Medical Center show that he had solid recall of information and events that occurred before hippocampal damage.
Zola-Morgan said this suggests that because he was able to recall that information without a functioning hippocampus, “memories aren’t stored there.”
Damasio of the University of Iowa said he believes that “to get retrograde memory deficits, the damage would have had to occur outside of the hippocampus,” in areas where memories have been parceled out for storage.
“In combination with our studies of Alzheimer’s patients, the Zola-Morgan case reinforces the idea of how important the hippocampus is for memory,” Damasio said.
