This post is a revised version of “How does our memory work?” published on 25-5-2017
Our memory is sometimes compared with the hard disc of a computer or a filing cabinet, where data is stored. But that’s not really how our memory works. How does recall actually take place?
Memory: patterns of firing neurones.
In October 2016 I handed out the second Dr. Peter Moleman medal to Mascha Nuijten1. I remember the event very well. I see Mascha reaching out her hand to grasp the award. I hear the applause swelling up, I feel joy, I notice that I am smiling. Huge numbers of neurones in my visual cortex, auditory cortex and frontal cortex are involved in those perceptions and experiences. This explosive activity of billions of firing neurones in my head forms a unique pattern associated with this experience. If Mascha had had longer hair, the light had been brighter, or the applause a little less…..then the pattern of firing neurones would have been just a little different. Of those billions of neurones, perhaps ten or even a thousand would not have fired; or perhaps some of them would have, but a little later. Or different neurones would have fired.
As I write, I remember all these things again. Those firing patterns repeat themselves. In the visual cortex the patterns which represent what I saw are reactivated; in the auditory cortex the patterns representing what I heard, etcetera. What initiates this repetition of the whole event? In this case it starts with the question: what shall I use to illustrate this post? This leads me to think of the medal, to see Mascha’s red hair, feel her slight shoulder, and then the entire scene unfolds. A few details summon a specific memory with all the richness of details.
Central index: the hippocampus
But how does this come about, this activation of all the complicated firing patterns, all perfectly synchronised and in the exact sequence? That is the work of a specific part of the brain: the hippocampus. This is where a sort of index is formed regarding which parts of the brain fired in which way, and the hippocampus lays these down with their connections in space and time. This is how earlier scenes can be replayed coherently and in their entirety in our head2.
I wrote in the previous paragraph: “replayed”. But that is not entirely true, because then I should experience it as if I were standing in that auditorium now. What’s more, I was not feeling nervous thinking about it just then, but at the actual award ceremony I was. That makes my recollection different. So there is a sort of film being played in my brain, and for this film the same firing patterns are used slightly differently than in the original event.
Each time this happens, the index of activated brain areas in the hippocampus is updated. If I recall the event tomorrow, what would be played then? Not the original firing patterns, but the slightly different ones that just raced through my brain!
So you never recall an event exactly the same way twice. An event which is laid down in your memory is always linked to and intertwined with other events, emotions and memories. And if you relive the event, then the recalled experience replaces the original memory in part. If you frequently talk about an important experience in your life, then the narrative changes a little bit each time. Perhaps you will say later: I don’t remember quite how that went, it was all such a long time ago. But the question is if that is because it was such a long time ago or because you have told it so often.
Changing memories: false confessions and trauma therapy
That in itself may not be such a bad thing, but what do you think happens when during a police interrogation they ask for the sixth time: can you tell us one more time exactly what happened. The sixth time around the suspect is bound to tell it somewhat differently than the first time. The policemen think that the interviewee may reveal more, but in fact there is a strong chance that he will “retell” memories that are distorted by his previous five answers to the same question. The memory is especially prone to change if the interrogators have exerted pressure, elicited emotions, and threatened. And that is before you take into account the information about the event that the police themselves have let fall in the course of the interrogation. This information is also assimilated into the memory3. This can even lead to a suspect confessing to something he has not done, and which he believes he has done. This is how strongly your memories can distort or be distorted. And it is also possible that he withdraws his confession because information available after his confession, for instance from other witnesses, makes clear to him that he did not commit the crime after all.
The repeated retrieval of memories in another, safe context, can on the other hand result in very positive effects, as we know from the efficacy of exposure therapy for trauma and the treatment of posttraumatic stress disorder (PTSD); and the use of Eye Movement Desensitisation and Reprocessing (EMDR).
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- For her article “Sustained-release dexamfetamine in the treatment of chronic cocaine-dependent patients on heroin-assisted treatment: a randomised, double-blind, placebo-controlled trial.” Lancet 2016;387(10034): 2226-2234.
- This is called ‘holistic recollection’
- Check out this TED talk by Elizabeth Loftus with some far-reaching examples