X-Message-Number: 5759 Date: Mon, 19 Feb 1996 08:11:49 -0800 (PST) From: Joseph Strout <> Subject: SCI.CRYONICS Re: level of accuracy required Marshall Rice wrote: > >It is possible that all one needs to recover are the approximate > >geometries of the previously existing synaptic connections, and some > >coarse characterizations of their propert ies. > > Unfortunately not. We would need to duplicate the precise properties of each > synapse, i.e. at what level of stimulation it was triggered and for how long. As far as I know, this assertion is not supported by the evidence. Small-scale, local changes in synaptic efficacy (e.g., "long-term" potentiation) have not been shown to be related to long-term memory -- the former "long-term" means hours or days, while the latter means years. LTP and related processes probably depend on the phosphorylation of proteins within the synapse, which is unlikely to last more than a few days; it can take place even when protein synthesis is blocked. Formation of long-term memories, on the other hand, requires protein synthesis, and has been shown to involve the growth of entirely new synapses (or retraction of existing ones). This suggests that it is the presence or absence of a synapse which is vital; fine-tuning happens on a much smaller time scale. Without recovering this molecular information, the patient may lose a few day's worth of recent memories. A small price to pay, I'd think. > The problem lies in re-establishing the neural pathways, not the neurons > and synapses themselves. The fact that there are more potential neural > pathways in a human brain than atoms in the solar system, gives some idea > of the scale of the problem. I think you're right on the money here: the neural pathways are what's important. Fortunately, freezing appears to preserve this information quite well (with an appropriate protocol). The number of potential pathways isn't any more of a problem here, than in worrying about the number of potential books formed by the same number of letters when using a photocopier. (A loose analogy at best, but perhaps you see what I mean.) > I cannot conceive of any way in which neural pathways could be restored > other than by (1) reconstructing each neuron and synapse down to the > molecular level or (2) re-accomodating the synapses. It's possible that the very structure of the pathways would allow one to infer the particular state of the synapses. But as discussed above, this is probably not necessary anyway. > In the case of the latter (and quite probably the former), although you may > be able to duplicate the personality, emotions and memories of an individual, > in no sense would they be the same person. The consciousness of the original > individual would have been lost, just as surely as if they had gone to feed > the worms. This is a very strong -- and, I think, unscientific -- assertion. If the personality, emotions, and memories are the same, then in WHAT sense are they not the same person? What is, to you, the basis of personal identity? To me (and many philosophers), it is exactly those traits (especially memory). And as for consciousness: it seems contradictory to suppose that a person might have a functionally equivalent brain, but a different (or nonexistent) consciousness. ,------------------------------------------------------------------. | Joseph J. Strout Department of Neuroscience, UCSD | | http://www-acs.ucsd.edu/~jstrout/ | `------------------------------------------------------------------' Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=5759