It didn’t make the final draft of this post, but we did consider mentioning anesthesia’s ability to turn the brain ‘off’. I like Matta et al 1995, because “make the brain flatline on an EEG” was considered harmless enough by the medical community that it passed an ethics review board to do this to patients who didn’t need it, just because it helped researchers test the exact mechanisms of anesthetic operations:
annafirtree
Karma: 16
The question of how much structure is “enough” is definitely a much-debated topic among preservationists. Nectome team members might not all draw the line at exactly the same point. What we are all agreed on is that our method preserves more than we expect to be necessary, and preserves as much as we possibly can. Our fixatives will lock proteins in place, preserving information at a molecular level, rather than just at the whole-neuron level.
(Minor science point: euthasol wouldn’t be administered in a premortem procedure; it’s a euthanizing drug, so it’s only for the postmortem procedures. Anesthesia/sedation chemicals like ketamine/xylazine would be used for a premortem procedure. Otherwise, you got it all right.)
How much detail are you looking for?
Both premortem and postmortem processes involve a surgery to access the circulatory system, and perfusion (pumping) of fluid chemicals through it.
In the BPF experiments, the premortem process involved cannulating the carotid arteries of a sedated and unconscious, but still living and breathing animal. In a MAiD postmortem process, lethal drugs are taken by the patient, and only after their heart stops and they are declared dead does any surgery begin. That’s why the rat experiment with Critch waited five minutes: to simulate the time it takes to do the surgery on a large mammal.
From a scientific perspective, the important difference is ischemia: in a premortem procedure, the heart and lungs are still active, and the brain is still receiving oxygen up until the time you start pumping chemicals through it. In a postmortem procedure, the heart/lungs have stopped, the brain is deprived of oxygen, and the clock is ticking on the cascade of failures that happen after death.
Evidence points at memories being stored in the physical size of synapses between the neurons in the engram. See Aspirational Neuroscience Journal Club’s video walking through the Goto 2021 paper, where mouse memories are deleted by stopping the synapses’ ability to change size:
https://www.youtube.com/watch?v=saFDeGTYnRU
Also, fixation, because it locks proteins in place, is generally better at retaining chemical information than cryofixation alone. Vitrification itself will lock everything in place, but the process of introducing the cryoprotectants, and the ischemic damage during the delay, causes a lot of disruption at multiple levels. That’s why fixation is standard for neuroscientists looking to label/map specific chemicals in the brain.