How do we form memories? That's a tough one. How do we study them? About equally as difficult.

This 2014 letter in @nature takes an optogenetic approach to beginning to understand the mechanisms behind memory. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4210354/

The study proposes that memories are formed when synapses that encode them are modified by long-term potentiation (LTP) and long-term depression (LTD)

Before we really dive into how the researchers investigated this, we should probably understand what LTP and LTD are (in the context of this paper) and how they can be induced

LTP is the strengthening of a highly activated synapse by NMDA-mediated addition of AMPA receptors to the membrane that are sensitive to glutamate (an excitatory neurotransmitter) making the neuron easier to activate. This was induced in the paper by 100 Hz optogenetic stim.

LTD is the weakening of a highly activated synapse when the neuron becomes significantly depolarized over an extended period of time, causing AMPA receptors to be removed from the membrane. This was induced in the paper by 1 Hz optogenetic stim.

Now that we understand LTP and LTD, we can get into what the researchers did to test their hypothesis

To begin, they had to induce memory formation in their model system, rats, that could be behaviorally measured to test for reinstatement or inactivation following LTP or LTD protocols respectively

They used a fear conditioning method to either teach the rats to associate a specific tone with a foot shock or to elicit a similar conditioned response by optically stimulating a region of the lateral amygdala (the "fear center of the brain") at the same time as a foot shock

How successful the rats were in forming this association was assessed by their performance in a previously trained cued lever test, with less lever presses in response to the tone or optical stim from before correlating to a more fearful rat

Once the rats were fear conditioned, the researchers could finally test the effects of LTP and LTD on the retention and inactivation of this association

After rats underwent training and the LTP protocol, sections of their amygdala showed an increased ratio of AMPA receptors to NMDA receptors, suggesting the LTP worked

Behaviorally, after LTP, rats also exhibited decreased lever pushing activity in response to stimulus, which was reversed and brought to baseline by LTD, and could be brought back by another round of LTP

To ensure that LTP was actually bringing back the memory of the fear association, the researchers tried testing the effect of the LTP protocol without pre-training with the condition stimulus and found that post-LTP there was no change in lever press frequency

Then, to ensure that LTD was actually causing memory inactivation and not extinction, the researchers trained the rats on association and then extinction of the conditioned stimulus...exposure to the LTP protocol after that did not bring back the conditioned response

All of these results support a few conclusions:
1)LTP and LTD are capable of repeatedly activating and inactivating associative memories
2)LTP only works if there is previously trained memory to work on
3)LTD is not extinction

Yet again, why do we care?

We care because, as I said at the beginning of this thread, memory is an extremely complex processes that is difficult to understand and study, but it's so vital to who we are and how we live our lives

If we can get a better understanding of the mechanisms of properly functioning memory, we can hopefully also better understand and treat chronic memory loss diseases like Alzheimer's or dementia