Reweighting and Reconnection
By Ellen Yuan
Curiosity about the brain and memories has been present for centuries – many, including myself, have always been curious about how our brains fail to recall or misremember certain memories. Ideas about how the memory works date all the way back to Plato, who proposed that memory was like wax, having the ability to keep its form, but also flexible enough to change. Neuroscientists have also believed that the connectome, a map of all the neurons and synapses within an organism, functions like wax. Synapses are connections between different neurons; it is the site where most of the brain activity occurs. They can be strengthened or weakened – such changes are known as reweighting – or they can be created or eliminated, also known as reconnection. Neuroscientists have suggested that reweighting and reconnection may have some association with memory and the two changes occur throughout entire lives. Further, through additional findings, like those of Donald Hebb and the Hebbian plasticity rules, scientists have proposed how such synaptic changes occur. By finding solid evidence on these hypotheses, it could help in improving memory and prevent disorders such as Alzheimer’s.
Studies on memory have been occurring for over 2000 years; however, Herman Ebbinghaus was the first to take an experimental approach to studying memory in the late nineteenth century. Ebbinghaus tried to memorize series of nonsense syllables, finding that as the number of syllables increased, so did the number of times he had to review the serious. Further, the more syllables in the series, the longer it took to recite them. His findings relate to those of Hebb, who hypothesized that the memories were stored as the connections between the neurons and were activated through repeated use. He proposed that a nervous system starts out with weak connections in the synapse between neurons, and in order for the synapses to be strengthened there needs to be some sort of demand for those synapses. However, it is important to note that the weak synapses have no effect on the cell assembly. Thus, through repeated sequences, the connections strengthen and will eventually not need additional stimuli to complete the sequence.
Eric Kandel was able to experimentally show Hebbian plasticity rules through sea-slugs, associating the strength in connections with memory. Although Kandel was able to relate connections and memory, his findings are not applicable to the human brain as our memories are not of single synapses. These findings support the hypotheses of reweighting, but there have been thoughts that reweighting and reconnection interact with one another. If the connections between synapses are not strengthened, they will eventually weaken and can potentially be eliminated. If those connections do not exist, then our brain will fail to recall memories. This shows the limited ability of reweighting in storing information. Some have proposed that the brain counteracts this disadvantage through creating new synapses and eliminating those that are not of use. Therefore, reconnection allows for more capacity to store information.
In the 1970s, William Greenough discovered that the thickening in the cortex indicated an increase in a number of synapse. He then went on to propose that memories are stored through an increase in synapse. But this research is not fully supported as there has also been a correlation between cortex thickening and learning. Despite these findings, reconnection has been thought to stabilize memories. It has been associated with short-term and long-term memory. The Profound Hypothermia and Circulatory Arrest is a process is when the heart is slowed to help correct life-threatening conditions. However, even though the body is chilled, the memories still remain intact. This is known as the “dual-trace” theory. The brain has essentially two storages for memories. Constant spiking among the neurons is associated with short-term memory while constant connections are related to long-term memory. When a brain wishes to store long-term memories, information is transferred from the activity to connection. Recalling that memory is simply the process of the brain transferring the information from connection back to activity. This explains some of the synaptic functions in reconnection.
The tradeoff between stability and plasticity can help explain why reconnection is used for further storing of memories. Because neural spiking is continuous, the strength is relatively unstable, thus, it can be further stabilized through reconnection. Recent studies have shown that improving memory is all about strengthening the synapses. As in reweighting, by strengthening the synapses, the connections are further strengthened through the Hebbian plasticity rule. These findings could help prevent disorders such as Alzheimer’s. Alzheimer’s is a brain disease in which the connections in the brain are lost and is usually found in older patients (Figure 2). Many of the changes the brain undergoes as the humans age is associated with loss of synapses. A study in 2007 showed that new information is absorbed through synaptic connections in the hippocampus and cerebral cortex, which are regions of the brain that are associated with memory. In an article featured in the February 2007 issue of Mind, Mood, and Memory, studies show that the neurons are affected by lifestyle and environment. Stress, lack of stimulation, and neurotoxins has been thought to be related to the loss of synaptic connections. Stress can interfere with neurotransmitters and their functions, as high concentrations of glutamate can accumulate in synapses. This can become a toxin, which can affect learning and memory. Constant stress can lead to shrinking in the hippocampus and the cortex and also depression, something which is associated with further memory difficulties. Lack of stimulation can also reduce the number of synaptic connections in the brain. Studies have shown that those with a more enriching environment had more synapses, and a lower risk of getting dementia. Increasing the number of interconnections can prevent a decline in the brain and further improve memory. Lastly, neurotoxins, such as excessive alcohol can cause memory problems, blocking the neurotransmitters.
Other recent studies with memory include a Massachusetts Institute of Technology study conducted this past March, in which researchers discovered that memories are stored in specific neurons. Through experimenting on mice, neuroscientists triggered a couple neurons that forced a specific memory. By removing the neurons, the memory would also be removed. This showed that memories are more physical, as certain memories can be erased. This study could potentially relate to reconnection in creating and eliminating synapses. While many advances have been done on the study of memory, there are still some problems that need to be addressed. Many studies have been done on the idea of reweighting and reconnection, two concepts which have been thought to interact with one another. One is the idea of strengthening and weakening synapses, while the other deals with creating and eliminating synapses. However, it is not fully proven that reweighting and reconnections are associated with memory, yet there have been some significant findings recently. Studies show that strengthening synapses can help improve memory, and that the lifestyle and environment has a huge impact on synapses. However, it is still unknown as to how the synapses functions. Further studies show that memories can be found within certain neurons, an advancement that allowed us to recollect memories. But how these memories are encoded still remains a question. Memories are a pattern of connections, and once the connectome of the human brain has been mapped out, these questions will be answered. Through finding the many connections within the brain, the neurons will be apparent, and thus, the synapses between the neurons can be studied. However, finding the human connectome could take years to complete, but upon completion, the many mysteries of the human brain and memory can be solved.
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