Thanks !

Hmmm...

That would be way too complex for a simple optogenetic circuit, but if you use the optogenetic "switch" as a detector, and combine it with, say, a microchip implanted in the memory centers of the brain capable of taking "snapshots" of all the important parts of memory, reverting to previous snapshots to erase memories, simulating the addition of new details to a snapshot to add them.

Modifying would be... more complex.
It would require, I believe, a full understanding of how the different parts of the memories are stored. Characters, emotions, etc...

You could potentially rewrite it, during sleep, by accessing it constantly and using the brain's capacity to rewrite memories in light of the emotional state during recall to, say, force the memory to be accessed while inducing an emotion of uncontrollable rage at the same time.

Though that kind of manipulation isn't exactly "subtle", and could wake a person up ?

In any case, the brain's good to go if you let it invents its own reasons for why it suddenly associates "rage" with, say, every memory you have of the current president.

Of course, this is all still a "theory"...and one supported mostly by my overactive mind and relative understanding of recent advances in neuroscience (I didn't take a Master in Neuroscience... though I could, after finishing the Bioinformatics one. Bordeaux has an entire Neurocampus and big fancy lab dedicated to it, if I find that interesting... hm....)

As far as I know, we've only recently uncovered the fact that neurons seem to be more than just gateways for processing information, and also store information chemically that alter how fast they transmit their signals, how many connections they can form, and many other characteristics...
All of those things altering how neural patterns form, how strong they are, and thus altering memory storage and a lot of other components of our "mental World".

This is all just supposition, but it's fascinating :)