Everybody agrees that synaptic connections between neurons are needed to access memories, but there is reason to think those connections may not be where memories are ultimately stored, at least not important very long term memories. The theory that memories are stored in the strengthening of synapses based on recent activity (Long Term Potentiation) does a pretty good job at explaining how memory can be retained for hours or days, but when you get beyond a week or so there are problems because the proteins associated with Long Term Potentiation (LTP) are not particularly stable, and experiments with snails have shown that even when LTP has been destroyed with chemicals the loss of memory is not always permanent. So there must be an information storage mechanism other than the pattern of synaptic strengths for at least some memories.
It’s been known for a long time that a mesh of proteins attached to carbohydrates forms something called "the perineuronal net" which sheaths mature brain neurons. And it's been known that synapses form through gaps in the net; but very recently evidence has been found that very long term memories, the sort that endures for an entire lifetime, may be encoded in the holes in the perineuronal net, rather like old fashioned computers once encoded information as holes in punched cards. http://www.sciencemag.org/news/2015/10/mysterious-holes-neuron-net-may-help-store-long-term-memories https://www.braindecoder.com/memory-perineuronal-nets-1418933042.html The proteins that compose the Perineuronal Net are much more stable than those in neurons, but when a synapse is strengthened it produces a small amount of an enzyme that can break down the net, and if the synapse increases a lot over a short amount of time there is enough enzyme to make a small hole in the Perineuronal Net right next to the synapse, and the hole seems to be permanent. Mice that have been genetically engineered to lack this enzyme have normal short and medium term memory but very poor long term memory. Even severe Alzheimer's disease has little effect on Perineuronal Nets, so memory information might still exist in a Alzheimer brain even if the synapses in it are so damaged they can no longer be accessed by them. It seems to me this may also be reason for optimism regarding the success of Cryonics ; the Perineuronal Net is much tougher than neurons are and the space between brain cells where the Perineuronal Net is located, along with blood vessels various glycoproteins and lots of fluid, occupies 20% of brain volume on average but it can get 60% larger during sleep or anesthesia : http://science.sciencemag.org/content/342/6156/373 So if a neuron shrinks or grows in volume during freezing that shouldn't be something it hasn't seen before, and even if a shard of ice does punch a meaningless hole in the net it should be obvious to a intelligent nano-machine that the hole was just caused by ice and not by synaptic behavior because the ice would still be sticking through it. John K Clark -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
