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1: There seem to be cases where coma patients with a non-active brain (i.e. flat EEG) have regained full consciousness. => Apparently memory and knowledge are stored independent of brain activity.

2: There seem to be animals (e.g. hamsters) that can be frozen to complete organic inactivity and will regain full functionality after being thawed. => Apparently the stored memory does not depend on blood flow and other support.

3: From this I assume that quickly cooling a human brain to a temperature low enough to avoid decomposition would preserve the state of that brain that corresponds to that human's memories, knowledge, cogntivie abilities, and maybe consciousness at the time of cooling. => With the proper technology that "content" is theoretically retrievable.

Q: How long would that state remain after death in a brain left at room temperature?

Or in other words: How long does it take for decomposition to destroy memory?


Addendum:

The fact that the stored memory may not be accessible with current means is not relevant to my question. We cannot access the information stored in Linear A, but this unretrievability does not delete the information.

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I think your premise is off. If someone recovered from a flat EEG, I think the simpler conclusion to draw is that a flat EEG doesn't equal NO brain activity. The brain was still working; but its activity was not detected. –  swbarnes2 May 28 at 18:30
    
looks like this deals with cryopreservation which is still a field in its infancy –  The Last Word May 28 at 19:03
    
@potterbond007 The actual question (in bold) does not pertain to cryopreservation. I used the premises only to deduce that (long term) memory is indepentent of brain activity, so it should still exist after death (unlike consciousness, which probably depends on the brain's activity). The question is: for how long does memory persist without cryopreservation? –  what May 28 at 19:11

2 Answers 2

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There are multiple levels of memory, some of which would die immediately, some of which would take some time. So the answer is: it depends; some immediately, some only very slowly.

At the highest level, the current neuronal firing state of the brain encodes memory on a very short scale - working memory. The memory held on this level does not have a clear anatomical counterpart (but for the potential encoded in the synapses). It equals very short-term memory/STM sequences, such as the words you read just before you read the words you're reading right now. This memory is lost immediately when you lose consciousness, at least to some degree; as this memory is hard to even strictly distinguish from consciousness and attention (though see Jonides et al. 2008).

Other forms of short-term memory/STM are stored in a slightly different form: short-term potentation, the adaption of neuronal responses following brief and intense stimulation. Spike Frequency Adaption/SFA is at an intermediate stage between this and the previous level. Short-term potentation and SFA decay within minutes or even seconds if they are not transferred into some more durable form of memory.

Long-term memory (/LTM) stores have specific anatomical correlates; they are stored in, amongst others, the synaptic weights (i.e. the amount of influence the firing of one neuron has on another). Some forms of LTM are best located in cortical synapses, others in the hippocampus. An even more fundamental, long-term, durable storage form is the wiring itself; not just the weights, but the existence of a synapse between two points, or not. For example, an important part of early learning is synaptic pruning, where synapses which do not play a meaningful role die off, whereas those which connect functionally related brain areas remain. This pruning instantiates one form of learning, and the non-existence of a synapse is a form of memory. Synapses are comparatively stable. Even if the corresponding neurons die, in principle, the synapses still exist - and more importantly, the nonexistence of a synapse is even more durable. This form of memory can be observed in slice preparations of animals long dead.

For a simple example, consider any experiment on the neuronal responses in slice preparations, which can be considered a form of (decontextualised) memory access.

On the most extreme end, epigenetic adaptions and large-scale brain anatomy (which shows developmental traces) can be considered a form of memory that will remain intact until the whole structure rots away.

However, the more direct answer to the question the OP is asking is that once a large amount of neurons have died (brain death), there is currently no power on earth that can access a non-trivial amount of memory. As long as this is averted, nontrivial amounts of memory can be recovered. For the parts that are lost immediately, see the source by Jonides et al. For more durable memory, you could look at for example Purves et al, Neuroscience.

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In some of the founding works of modern neuroscience, Hubel & Wiesel (1977, Phil Trac Royal Soc B) show that plasticity in the domain of ocular dominance induced by partial sensory deprivation results in gross anatomical changes, such as cell shrinkage. This can be seen as the result of a certain kind of information the animal received during its lifetime; and stored information equals memory. Arguably, "one eye was closed off for most of this animal's life" is trivial, but it's recoverable well past death. Contemporary neuroscience of course goes well past that. –  user 49102 Jun 2 at 0:22
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I also fail to see how a tattoo would be relevant to the OP, and consequently do not discuss tattoos in my answer. –  user 49102 Jun 2 at 6:18
    
You have given the OP the answer he wanted. That is what matters. –  daniel Jun 2 at 9:54

Once the thermodynamically irreversible processes we call brain-death have occurred both memories and the machinery to retrieve them are lost.

This is not an answer but a cavil with the premise of the question. Challenges that do not destroy the brain itself are different from those that do. In particular there may be a big difference between hypoxia during periods of low metabolic rate$^1$ and hypoxia at room temperature without lowering of metabolic rates, which by most accounts leads to brain death in about 6-8 minutes.$^2$ The assumption underlying the title of the question is unsupported and opinion-based.

$^1$ See the pretty well-known case of Mitsutaka Uchikoshi, and see News Physiol. Sci. 13: 149-153, 1998, detailing case of a meditation adept who was able to drastically lower metabolic levels.

$^2$ See the Wiki page on apnea. Sources avoid putting a time after which brain death is certain, and the definition is not uniform. The one thing they agree on is that it is irreversible; so if someone (or, say, a hamster) recovers from putative brain-death, they were neither factually nor legally dead.

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That frozen hamster, whose btain was "dead" by the definition of human brain death, showed no sign of having lost the memory of his habitat after reanimation. Quite obviously memory is stored physically and independent of brain activity. –  what May 29 at 18:30
    
The brain is unlike a current computer in many ways (e.g. neurons don' t have binary on/off states), and the relevant difference in the context of this question is that the brain stores info in its neuronal structure, i.e. by changing the hardware. –  what May 29 at 18:48
    
Guyton, Arthur C. (1986). "The Cerebral Cortex and Intellectual Functions of the Brain". Textbook of Medical Physiology (7th ed.). W. B. Saunders Company. p. 658. ISBN 0-7216-1260-1. "We know that secondary memory does not depend on continued activity of the nervous system, [contd.] –  what May 29 at 19:00
    
[contd.] because the brain can be totally inactivated by cooling, by general anesthesia, by hypoxia, by ischemia, or by any method, and yet secondary memories that have been previously stored are still retained when the brain becomes active once again. Therefore, secondary memory must result from some actual alterations of the synapses, either physical or chemical." –  what May 29 at 19:01
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"No activity detectable with this or that instrument" does not equal "no activity", neither is equal to "dead". Your deduction is flawed. –  swbarnes2 May 29 at 19:58

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