This is an essay I wrote for Fingerprints, JNC’s literary magazine. They’ve asked, for their 2012 issue, for interpretations of the theme “The Colour of Memory”. Nice, no? It’s wonderfully vague. As a nerd, this is my interpretation. [I put up the first full draft I had here. I’ve edited it since, for better continuity and structure.]
“Where were you on the night of the 26th of November, 2008, when a handful of teenagers from Pakistan terrorised the city of Bombay and killed dozens of people?”
For anybody with a personal connection to the events of that night, that question is likely to evoke, or provoke, a lot of thought and produce answers that strike one for their detail. Given the nature of the memories associated with the event in question, you would think these are memories etched deeply in the subject’s mind. Unalterably, permanently. After all, how could one forget something that shakes one up like this?
Memories can be short-term – a phone number you remember just long enough to jot it down – or long-term – your birthday, when India got independence, or what have you. Short-term memories exist as a result of several neurons firing simultaneously and have to be maintained by constant repetition; failing which they cease to exist, or at least exist with any accuracy. Short-term memory is converted to long-term memory in a few hours. Several hormones and brain organs – notably the amygdala and the hippocampus – are involved in this process.
The conversion from short-term to long-term memory involves the creation of neural synapses. Synapses are essentially wires that connect two neurons together. You can imagine that two neurons that are wired together are easier to excite together. Hebbian learning, a theory loosely summarised as ‘neurons that fire together wire together’ tells us which neurons grow synapses between each other. This model has been experimentally verified and is a cornerstone of the modern theory of mind.
Memory, however, is more complicated than this. The memory-as-synapse explanation leaves no room for change. Memories once created are unalterable; the only way to change a memory is to destroy it, as happens with some pathologies. But doctors and psychologists know that people suffering from, say, post-traumatic stress disorder(PTSD) do get better with time and therapy. We are only now beginning to understand this seeming contradiction of the plasticity of memory.
Plasticity of memory – the ability to be modified without getting annihilated – in long-term memory is essential. If a memory once formed could never be altered except by destruction, learning would become impossible. Before I can say anything about how we think plasticity is achieved, however, we need to understand what goes into the formation of long-term memories in the first place.
The amygdala and the hippocampus — parts involved in the consolidation of short-term memory into long-term memory — are also part of the emotional circuit of the brain. They are involved in, for instance, the fear-response of the body – ‘fight or flight’. The body shuts down ‘unnecessary’ functions – growth, for example – and focuses its resources on fighting or running away. There is a surge of adrenaline that opens up the vascular system, increasing blood flow and energy production. The problem with the fear-response is that frequent triggering leads to chronic stress – the body is always on edge, always waiting for a surge of activity even when none is forthcoming. Patients with chronic stress are known to have hippocampi that are 10-15% smaller than usual.
This involvement of the emotional circuit in matters memory-related is a pointer to the complicated nature of memory. Memories with great emotional context are known to be longer-lasting and immutable. However, this only applies to the central details. Peripheral details are either not remembered at all, or are open to suggestion. This means, for example, that while you are likely to remember Ajmal’s face, you aren’t likely to remember whether you had tea or coffee as you watched the events unfold on TV.
The mystery is that whether you remember drinking coffee or tea may be open to suggestion. In fact, this suggestibility is deeper than a simple switch of beverages. Advertisers use this suggestibility to reinforce, or sometimes even change, people’s opinions of products. Called memory morphing, this sort of after-the-fact suggestion makes consumers enthusiastically recommend products that they may not even have liked.
This suggestibility of memory arises because of a process called reconsolidation. Short-term memory has to be consolidated into long-term memory by the formation of new synapses. Karim Nader and colleagues at New York University and elsewhere have now discovered that when an event from the past is recalled, the memory becomes destabilised. The destabilised memory, which is to say the neurons and synapses that make up the memory, has to undergo a process that is similar to the consolidation of short-term memory into long-term memory; it has to be ‘re-consolidated’. It is during this process of reconsolidation of a destabilised memory that the memory can be altered.
Reconsolidation appears to be an updating consolidation intended to modify retrieved memory by a process that integrates updated experience into long-term memory. Previously consolidated memory is partially destabilized and by the infusion of disrupting agents it appears as if the process is intended to consolidate memory again.
The destabilisation of recalled memories also explains the plasticity of memory. New details can be incorporated into existing memories without the existing memory having to be destroyed, by ‘partially rehearsing the older memories’. Which details can be incorporated into existing memories depends on what kind of details they are. The stronger the cue for the memory, the older the memory, the harder it is to destabilise or morph.
These details of memory reconsolidation have been worked out by Akinobu Suzuki from Tokyo University and co-workers. They find that memories recalled for less than a minute don’t get destabilised at all, and don’t have to be reconsolidated. Memories recalled for about a minute undergo destabilisation, and any disruption of the process of reconsolidation tends to destroy the memory. Memories recalled for much longer than a minute are on their way to automatic extinction.
This last fact is well-known; psychiatric therapy works on the premise that talking about traumatic memories can help people deal with them. By recalling memories as they talk about it, patients destabilise their memories. These destabilised memories are rewired with the added environmental cues of a comfortable chair and a doctor’s office. Eventually, the trauma associated with the memories simply fades away. The patient is able to remember what happened without having to re-live it. Pharmaceutical intervention in the reconsolidation process is also possible, now that more details of the reconsolidation process are becoming known.
Suzuki A, Josselyn SA, Frankland PW, Masushige S, Silva AJ, & Kida S (2004). Memory reconsolidation and extinction have distinct temporal and biochemical signatures. The Journal of neuroscience : the official journal of the Society for Neuroscience, 24 (20), 4787-95 PMID: 15152039
Rodriguez-Ortiz CJ, Bermúdez-Rattoni F. Memory Reconsolidation or Updating Consolidation? In: Bermúdez-Rattoni F, editor. Neural Plasticity and Memory: From Genes to Brain Imaging. Boca Raton (FL): CRC Press; 2007. Chapter 11. Available from: http://www.ncbi.nlm.nih.gov/books/NBK3905/