Memories are being formed when, in principle, different groups of neurons fire together - they become active together, and that leads them to making new connections.
To give you an example; let's say you're cooking dinner while listening to your favourite band, and there will be some neurons that will fire in response to the music you're listening to. There will be other neurons representing your kitchen, or the fish cakes in the pan. So, these neurons start forming connections amongst each other.
And let's say a week later, you are somewhere completely different - you listen to the same song again on the radio. Because the neurons have formed these connections, listening to the song will also re-elicit the neurons that responded to the fish cake and your kitchen. And that reactivation of those neurons will make you remember the evening when you were cooking dinner in the kitchen.
It may be disappointing to some, I guess, but from a scientific point of view, a photographic, perfect memory really doesn't exist. So, there are no cases that have ever been reported of a perfect memory that works like a video camera. This process of neurons forming connections, it happens all across the brain.
But we know that the most important structure for it is called the hippocampus. It works less like a video camera and really more like a librarian. So, it doesn't hold all of the details of what happens - it doesn't store a record of those. But it stores what we call an ‘index’ or a ‘pointer’. So, it knows where to find the different bits and pieces in the rest of the brain. And the hippocampus kind of glues all these different bits and pieces together. So, we might recall some of the basic elements of a memory, but the details get lost. And so the brain has to fill in these gaps from its general knowledge about how the world works or from other past memories.
So, we introduce a few biases and have to, you know, memory is more like a puzzle than a snapshot. There is something that is called a ‘flashbulb memory’. And that happens when something very emotionally salient happens to us. This will activate a little part of the brain called the amygdala that sits very close to the hippocampus. The amygdala will kind of tag the memory as very salient and gear the whole hippocampal visual system towards storing a very detailed memory of the event.
There is a condition that is called ‘eidetic memory’. It’s less of a condition but more of a superior ability that some people - especially kids - have. Roughly between the age of six and 12 is common. An eidetic memory means that these kids are able to hold on to an image in mind with very high precision, right? So, even when you show them an image, even when the image is gone, they can mentally scan through it and report a lot of the details as if the image was still in front of them. And obviously, it's an ability that's hardly ever been reported in adults. It's a developmental stage.
I would personally say these are not memories in the strict sense because they will never gone from your mind, and then you have to reload them back from memory. They've been there all along and you just, you know, it's a capacity to hold on in your visual imagery. But it's not the capacity to load a past event from your long-term memory. I think that's what what makes the nature of memory being so reconstructive, and not photographic, is that the hippocampus just doesn't have the capacity to store any details. And so memory can never be like a photo album.
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