In one of our previous articles you already learned a bit about two well known concepts in neuroscience, synaptic plasticity and long-term potentiation (LTP), both important for learning and memory. According to the synaptic plasticity theory, LTP is involved in memory creation and retrieval. However, the brain is an incredibly complex organ and we know very little about it. In this article, we will see what the issues are with this theory and what other explanations exist on how our brains store memories. 

Neurons can change in many ways: their shapes, connections, functions and more. This property is called  synaptic plasticity. LTP is when the connections of neurons become stronger , the stronger the connections between neurons become, the easier it is to access the network forming the memory. If the connection becomes weaker, the opposite happens and it will be harder to remember. This is called Long term depression (LTD). We can compare this with lifting weights: if the weight is heavy (a memory rich in information) or we lift it many times (repetition and revision), the muscles become stronger, if we don’t lift/ use the muscle (don’t use the information), it becomes weaker.

The synaptic plasticity theory seems very plausible and there is research to support its role in learning and memory. In one study, scientists manipulated rats' brains to be able to activate a specific synapse (where 2 neurons communicate) of the brain. They taught the rat to link the activation of the part to an electric shock. Once linked, they used LTP and LTD on the same synapse. LTD caused the rat to forget about the link while with LTP, the rat remembered that there was a shock to come. They were even able to do this cycle of LTD and LTP multiple times and it still worked.  

Although synaptic plasticity theory holds promise in explaining the mechanisms behind memory, there are still some slight problems. One of these problems is that we aren’t able to create a memory by manipulating the brain. If we were to try this, we would need to have a detailed record of all the connections of neurons before and after learning while also inducing LTP in the right network for the memory we want to create. For this, we would first need a large amount of data to record all connections and their strengths. Secondly, we would need to know precisely which neurons create a specific memory and the strength of its connections.  This is currently impossible because of the complexity of the brain and the 80+ billion neurons inside it! Other issues surrounding the synaptic plasticity theory concern the properties of LTP. Research is inconsistent regarding the exact role of LTP in memory formation. Problems that arise are the following: studies found that how easy it was to induce LTP in an animal didn’t correlate to their ability to form new memories, others found that LTP wasn’t involved at all in the formation of certain types of memories and some found the duration of LTP an issue, thinking that it doesn’t last long enough to maintain a long term memory. Other researchers even believe that the connections and the neurons aren’t stable enough to store information. Overall, some scientists believe LTP cannot be the basis of memory as theoretically, the properties such as duration, stability and time to induce LTP do not match those of learning. 

In response to these issues found with the traditional synaptic plasticity theory, several other theories for memory storage have been developed. The first is a modification of the synaptic plasticity theory. In the original version, an experience causes a change in the network of synapses and these enhanced connections are what store the information. The altered version states that this change doesn’t have to be permanent and that a memory can exist even if the enhanced network is later modified. The only condition for maintaining the memory is that the overall strength of the connections remains the same. So, as long as the strength of the whole network isn’t affected, individual connections can be modified and newer connections for new information can be included. This theory could be supported by the current evidence for the original theory while removing the issue of the stability of neurons.

Secondly, another version of the synaptic plasticity theory exists in which LTP isn’t the key factor for forming memories. Instead, neurons create connections and the connections themselves are what store memories. The strength isn’t relevant for the storage. However, later, the strength of the connections, enhanced by LTP or lowered by LTD, are what determine how easy the memory is going to retrieve. This means LTP doesn’t have to last for the lifetime of the memory and neurons, as long as they stay connected, can be as unstable as they want.

Up to now, all the theories we discussed were based on synaptic plasticity. However, there are theories of memory storage that do not involve LTP and LTD at all. The cell intrinsic memory storage theory is one of them, where the memories are stored inside the cells’ DNA. The suggested mechanism for this is a process in which parts of DNA are turned on or off, coding information as if programming a computer using 0s and 1s.

Another theory is non-synaptic plasticity. This theory keeps the idea that memories are created by changes in the brain, but this time, it isn’t about the connections. Non-synaptic plasticity is a phenomenon in which a neuron becomes easier or harder to activate (more/ less excitable) and was found in mammals such as cats. A neuron requires a signal from other neurons to be activated, this signal must be stronger than a certain point, called a threshold. When the threshold gets lowered for example, the neuron can be activated from weaker signals, when the threshold becomes higher, the neuron will require stronger signals to be activated. This is what happens in nonsynaptic plasticity and this is a way the brain may store information. However, it is usually found to happen together with synaptic plasticity and it is still unknown whether nonsynaptic plasticity alone can store a memory.

Apart from synaptic plasticity theory, there are many theories on how the brain can store information. Although this theory has the most evidence and is the more accepted theory, other theories also have some support. One being right doesn’t have to mean the others are all wrong. Memory is a broad term ranging from the definition of a word to riding bikes to our daily habits. Maybe the theories that don’t work for one type of memory, work for another. This again shows how complex the brain can be and how much we still have to learn about it. But, it also makes us wonder, how on earth does the brain create all these pictures, sounds and memories in our heads and how do we keep everything we know? 

Author: Kenzo Le Vaillant

References

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