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Assuming you could "read" the memories of a once living person, how long after death could you read the memories stored in the human's head? How long does it take for the pathways (dendrite, axon, etc.) to break down?
Is there a difference in the literal decay times for areas & physical methods used by the brain to store short, long, and sensory memory?
Few minutes (4 to 5) of anoxia (lack of oxygen supply) are sufficient to cause permanent damage to neurons, compromising a brain's functionality.
https://www.spinalcord.com/anoxic-brain-injury
On average, the brain can survive a mere four minutes without oxygen, so if you suspect a stroke or other injury that is cutting off oxygen to the brain, you need to act immediately.
As easily as 30 seconds in, brain damage can begin, and at two minutes brain damage becomes almost inevitable.
With the neuron gone, gone is also the capability of releasing neurotransmitters and electric signals, which is how the collective behaviour of the brain neurons emerges.
Since with death also the respiratory function ceases together with the blood stream and neurons are the first to be damaged, it's likely that your technique will read increasingly noisy signals the more minutes are passed from the moment of death, until the read out just becomes a white noise.
Although L. Dutch's answer probably contains all the information you need, I think it would also be important to take into account the manner of the person's death.
If an elderly person passes away in their sleep, I would guess that their memories would be readable for longer than someone who fell down a cliff and died of concussion, especially if they damaged their memory centre. Some diseases and poisons could also harm the victim's brain before killing them.
An entity such as the army of the United States of Nowhere In Particular might want to use such a technology to harvest sensitive information from prisoners. They would therefore need to choose an appropriate manner of "putting down" their source. Of course the electric chair is out of the question, but even modern day lethal injection isn't as "peaceful" as one may think...
i don't think you can, since memory isn't stored in the way you think it is (at least, base on my understanding of this subject).. to recall something, your neuron network need to be "connected" in a certain way - the fact that memory can be faulty or changed indicates that this is an active, ongoing process: and if this fail, then the memory is lost..
for simplicity, i'll compare our head with a computer.. if you download things from the internet and print them out, then you can always access these.. however, our "brain" computer only save things as a bunch of 1 and 0, that need to be arranged a certain way to access.. and as you may know, once the computer s down, everything will be unretrievable..
