An infinitely expandable human mind was once the stuff of science fiction, but new research has suggested that the brain can expand its memory storage capacity in some cases. The research, reported in Proceedings of the National Academy of Sciences, may help our understand of brain diseases in which memory formation is dysfunctional, like Alzheimer’s.
"The brain has the capacity to store an immense amount of information at the synapses between nerve cells. So although we already knew where memories are stored, this work helps clarify how they are stored,” said the co-corresponding author of the study, Professor Terrence Sejnowski, who is head of the Salk's Computational Neurobiology Laboratory. Researchers at Salk conducted the research with scientists at the University of Texas at Austin and the University of Otago.
Neurons in the brain are connected by synapses, and chemical signals move across them, transmitting information through the brain and body (learn more from the video below). When we encounter new information or have a new idea, millions of brain cells communicate with one another, releasing neurotransmitters across synapses. There is still a lot we don’t know about how synapses function in healthy or diseased brains.
Sejnowski has previously used 3D computer modeling to reveal that our brains have a memory capacity that is ten times bigger than had been assumed. To delve deeper into the investigation of memory, researchers from Texas and New Zealand were brought in to the project; they studied the hippocampal region of rodent brains. The hippocampus is critical to memory, and the scientists were studying how new experiences affected it, a part of the brain that mammals all have in common.
With electron microscopy, the investigators expected to see the synapses enlarging, which is known to happen in a type of learning called long-term potentiation. To their surprise, they saw that not only did some synapses get bigger, but some were also getting smaller.
"It's an intuitive idea that as we learn something new, synapses strengthen and get bigger," said Sejnowski. "This shows that there is a balance: some get stronger, some get weaker."
Sejnowski noted that the findings make sense because with synapses that are only getting bigger, eventually a limit will be reached, ending the storage of new information. This is the first time the balance between shrinking and enlarging has been shown, however. The study also indicates that by varying the size of synapses, more synapses overall can be stored, and the capacity of the brain increases.
The scientists also estimated how much information could be stored at synapses in two different parts of the hippocampus, the dentate gyrus, and CA1. It turned out to be very different, which might have to do with the functions of those regions.
"We hope to explore many additional questions such as whether the increase in information storage is accompanied by a compensatory decrease in information storage capacity in adjoining layers, and how long the temporary increase in storage capacity at particular synapses lasts," said the first author of the study Cailey Bromer, a Salk research associate.