Are Memories Permanent?
The brain creates memory through a near endless conversation. To move ideas from short term, working memory to long term memory the cortex and hippocampus communicate again and again. It may take ten years or more for “permanent” memories to encode in cortical locations – where they first registered as thoughts (see John Medina’s excellent “Brain Rules” for more on this.) Yet are even memories created biochemically over years and years truly permanent?
Probably never. Though memories are what we are left with at the end of life, they are modified whenever retrieved. Our narratives of ourselves, of who we are, change constantly – and most of the time we don’t notice.
Sleep and Learning
Recently Bjorn Rasch and colleagues at Lubeck and Basel Universities did an experiment that did not turn out as expected – so far from expected that its results appeared to international notice in Nature Neuroscience.
The work involved reactivated memories. Basically, young men and women learned a card game matching pictures with places . While they did, a noxious odor (isobutyraldehyde) was wafted into nose masks, on and off, for 30 second cycles. A classical conditioning paradigm, the learning of new information was coupled with this smell.
Next, people learned a similar card game that was meant to destabilize memories. In wakefulness, the process worked as planned. Understanding of the cards went from about 60% to 41% – the brain was confused by the intervening information, making it harder to remember similar, very slightly older material.
Yet when the odor was wafted into the young people experiencing slow wave sleep, memory results improved – from 60-84%. Odor, wafted into the nostrils of sleeping patients, had improved their memory.
Before you go out and waft different scents around while your kids are doing homework, the researchers proceeded to the next step – they looked at what was happening in the waking and sleeping brains.
In the waking brains, different parts of cortex were activated. In the brains reaching deep sleep, within minutes information rapidly passed from cortex to hippocampus to cortex.
The conversation had started, leading to emplacement of longer term memory. Learning was taking place – learning that could not later be removed.
So what does all this mean?
- Sleep is required for learning, especially creation of long term memory.
- The same stimulus in wake and deep sleep had opposite effects – reactivating such memories in wake destabilized them, while reactivating memories in sleep deepened them.
- Providing a stimulus to a sleeping person of a noxious odor, if paired with what they learned during the day, can improve memory and learning. You can modify learning dramatically even when “unconscious.”
- There are very different ways of stabilizing or destabilizing memories – and you can try them in both sleep and wake, with quite different, sometimes opposite effects.
The Chronic Instability of Memory
If this makes you think that memory is something akin to a will o’ the wisp, you’re right.
Memories are remade every time they’re retrieved. They can be destabilized by other memories – particularly similar ones. They can take many years to become properly established.
Of course, most people don’t believe this. They know what they saw – “I was there.”
But the brain, which loves order, will often “fill in the blanks” when it can’t find sufficient information about an event. In other words, it fabricates.
Oliver Sacks is one of the most famous neurologists in the world. In his book “Uncle Tungsten,” he describes perhaps the most salient memory of the childhood – the Blitz of London. There are fires, screams, escapes, a concatenation of annihilation.
Except he wasn’t there. He had been evacuated, living far away in the country.
Sacks came to believe his fabricated memories resulted from talking with family members who had survived the Blitz. He was originally incredulous that he could not have been there.
But that’s what the brain does – information processing. What comes in, conscious or unconscious, mental or physical, must be processed, remembered, or forgotten.
There’s too much information. Yet one hopes what is remembered is important information, information useful to survival and the continuous regeneration the body requires in order to live.
Much of that regeneration goes on in sleep. Rest is active – extremely active. The conversations between cortex and hippocampus matter greatly, as they determine what you recall and what you don’t – a life’s book of memory and forgetting.
And at night, the only awareness you have of the process is the dreams of morning.
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