After decades of research, scientist who study the brain -- including a Johns Hopkins University professor -- are at last poised to explain how the brain is organized to form long-term, conscious memory and later to recall a particular memory in its entirety.
In a paper published in the current Science, two California scientists describe the anatomical components of the brain's conscious memory system, and suggest how it operates in connecting the many separate storage sites throughout the cortex that together represent a whole memory.
The organ, known as the hippocampus, links the separate parts of a memory as it is formed, so enabling all to be evoked when the memory is recalled, they suggest.
A major insight into the working of conscious memory has been gained from a patient known as H.M. who had his hippocampus removed to relieve epilepsy.
The California team has re-created H.M.'s situation in monkeys, and managed to plot in much greater detail the precise anatomical structures involved in laying down long-term memories.
They find that the hippocampus, and nearby regions with extensive two-way connections with the neocortex, are essential for establishing long-term memory for facts and events.
Dr. David Olton, a professor of psychology at Johns Hopkins who has studied hippocampal memory systems in depth, said that the work on H.M. "gave us access to the human brain and memory for the first time. That in turn gave us ways to make animal models of the same memory system, and that gave us the power to compare memory across species -- comparative cognition. And finally, that in turn has given us the possibility of having real practical advances."
"We have found for example that if we look at old rats and old monkeys, their memory systems get old just as humans' do," the Hopkins professor said.
The new findings may suggest ways to protect the hippocampal cells, which are the quickest to die when oxygen to the brain is cut off.
Other damage during head injury or aging also may be prevented if the brain's key chemicals or repair mechanisms can be mimicked.
The article in Science by Drs. Larry R. Squire and Stuart Zola-Morgan describes a concept of memory that is new -- worked out over the past three decades and confirmed in elaborate human and animal studies by several scientists.
The work has shown that in both monkeys and humans, when the hippocampus and nearby tissue is damaged or removed, the ability to form new memory is lost. But it appears that only one specific type of memory is lost, the conscious memory or recall of facts and events.
Unconscious memory, including the ability to learn both mental and physical skills, remains intact. Nor is there damage to the memory used in immediate recall -- that is, matters to which a person is paying current attention.
Through experiments with monkeys trained to perform a simple task requiring recall, the California scientists have traced the path of events that occur between the perception of an event and the formation of a long-term memory.
A particular region of the neocortex determines that a perceived object is in motion. Once this area has begun tracking an object in space, it passes this information along to the perirhinal cortex, and thence to the parahippocampal formation or the entorhinal cortex.
These three structures are close to the hippocampus, and they channel the information through the dentate gyrus at the doorway to the hippocampus, and then through the several parts of the hippocampus in sequence.
One significant result of the work is that the amygdala, often thought to be part of the hippocampal memory system, is in fact not directly involved.
For a memory to be laid down, the nervous messages reaching the hippocampus from a particular perception must then be passed back through the medial temporal lobe and out to the relevant regions of the neocortex.
The specific memory can then be recalled from its various storage sites through the links made by the hippocampus.
The mapping of this complex pathway in the monkey brain was spurred by studies of the original patient, H.M., who was completely unable to form new memories after his hippocampus had been removed.
Students of memory believed at the time that memories were not located in any one place.
They were influenced by the well-known experiments of Dr. Karl Lashley which seemed to show that successive destructions of every part of a rat's brain failed to destroy it's memory.
The inference was that memory must be distributed across the brain in such a way that, if one part is damaged, its functions are taken over by another.
It was not realized at the time that Dr. Lashley's surgery over the previous decades was too imprecise to find or eliminate the central location of memory.
H.M. is now 65 and has been in a nursing home for many years. After the accident, he could not work as he had as an electrician's assistant, nor could he take even the simplest of jobs.