Monday, November 30, 2015

In case you missed it; a remark on Gallistel's conjecture.

Patrick Trettenbrein posted a comment on the recent little post on Gallistel's conjecture (here). He included a little paper of his that reviews some interesting new work on aplysia that provides further evidence for the Gallsitel conjecture. Here are the concluding paragraphs.

All in all, it seems that there indeed are two different processes at work in learning and memory, as Chen et al. (2014) also point out. While the exact details about both remain obscure, there appears to be a dissociation between the way in which learning occurs and how memory works. We do not know how the brain implements a read/write memory, but there is good evidence that it does. Similarly, there is ample and convincing evidence, also in Chen et al. (2014), that synaptic conductivity and connectivity play a role in regulating behavior. Consequently, it appears that synaptic plasticity might not so much be a precondition for learning as it is a consequence of it, so that the observed rewiring of synaptic connections might constitute the brain's way of ensuring an “efficient,” or possibly even close to “optimal” (Cherniak et al., 2004; Sporns, 2012), connectivity and therefrom resulting activity pattern that is appropriate to environmental (and presumably also “internal”) conditions. Synaptic plasticity thus might be reinterpreted as a way of regulating behavior (i.e., activity and connectivity patterns) only after learning has already occurred (i.e., after relevant information has been extracted from the environment and stored in memory).

Extrapolating Chen et al.'s (2014) findings stemming from work on Aplysia to claims about much more complex nervous systems is, of course, speculative in nature, to say the least. However, it seems to be no more speculative than the almost universally accepted idea of the synapse being the locus of memory. Similarly to Johansson et al. (2014), the work of Chen et al. (2014) shows that (1) there is plenty of “room” for the implementation of symbols other than synapses, and (2) substantiates the understanding that the network approach of connectionism might indeed best be seen as an implementational theory (Fodor and Pylyshyn, 1988) that still requires representation, computation, and a Turing architecture (i.e., a read/write memory). Gallistel and Balsam (2014) proclaimed that is was about time to rethink the neural mechanisms of learning and memory, Chen et al.'s experimental results add to the urgency of this claim.

 I particularly like the speculation that the wiring is there not to code the relevant information but for efficient use and that it is a consequence of learning rather than a pre-condition for it. I also like the observation, that Gallistel and Matzel also emphasize (see here), that there is, at best, paltry evidence for the standard assumption that the "synapse [is] the locus of memory." The Gallistel conjecture is generally assumed to be some daring edge of thought kind of speculation for which there is little evidence in contrast to the well-established "fact" that memory lives in inter-neural connections. Vast academic enterprises are based on this assumption. This may be more truthy than true, however.

At any rate, take a look at Patrick's note and the Chen paper he links to. It seems that the Gallsitel conjecture is daily becoming less "exciting." About time.

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