Editor (T. Sejnowskyi) comments:
This paper should be rejected on the strength of two negative reviews and my reading, which confirm the conclusion that this paer does not deserve publication, nor is there a likelihood that any revision would improve rating.
Review of "Neurons in the brain cannot implement symbolic systems", by yehouda Harpaz.
The author purports to show that brains cannot implement symbolic processing operations because they cannot copy tokens reliably. But he admits in the first paragraph of section 7 that people can in fact handle symbols. (And contrary to the claim made a few paragraphs later, such symbols are not necessarily tied directly to sensory inputs, since people are perfectly capable of manipulating sentences, formulas, diagrams, and concepts they dream up in their heads.) So the author tries to imply that just because people can handle symbols, this does not mean that BRAINS can handle symbols. Actually, he doesn't say something quite that ridiculous; he argues (p. 14) that "the components of the brain" cannot handle symbols. But this is an entirely different claim than the paper start out with and cocludes with, i.e. that the brain as a whole is not a symbol processor. Since no one ever claimed tha individual neurons or small groups of neurons were symbol processing systems, the author's central argument is revealed as an attack on a straw man.
The paper concludes with a long series of silly statements, such as (p. 18) "handling language does not require any dynamic mechanism" because the meanings of words are static. (What about the neeed to dynamically construct representations of novel sentences?) And (p. 21) "Neuroscientists do not realize the importance of the stochastic connectivity for theoris of cognition".
Obviously, this is not a scholarly paper. And the central argument is nonsense. I recommend that the paper be rejected.
review of "The neurons in the brain cannot implement symbolic systems" by Y. harpaz.
In the manuscript the argumentis made, that the stochastic nature of cortical connectivity precludes the interpretation of brain function as a symbolic processor. Although I sympathize with this view, I do not believe that several of the basic assumptions are true, nor can I follow the line of argumentation. Therefore, I find the manuscript not acceptable for publication in JCN.
1) The connectivity ... does not change as part of the computation. here the author explicitedly excludes to consider changes in synaptic strength.
I do not understand the exclusion of changes in synaptic efficacy. e.g consider the report by markram and tsodyk, that the effect of an presynaptic action potential very much depends on the afferent avtivity. Thus, an existing connection can rendered ineffective by the sheer occurence of an action potential shortly before. What is the difference on ineffectual and a non existing connection?
Furthermore there is no positive of anatomical changes on a timescale of think - what is that timescale anyway? On the other hand, present day techniques can not exclude dynamic changes on a timescale of seconds, i.e. they might be relevant for solving symbolic manipulations.
At least, the asumptions made here do not rest positive text book knowledge, as claimed in the article.
2) The connections that each individual neurons form ar stochastic (i.e. they are not specified accurately by any mechanism).
Along similar lines as above. Right now we do not understand the rules if any dtermining synaptic connective, Thus, local random connections si a sort of zero hypothesis. On theother hand, there is no positive evidence that this hypothesis actually applies, and recent results on the connectivity between retina and lgn (stryker et al.) indicate that a pathway where no apparent order was visible for many years, might reveal such an order, when investigated with the appropriate means. As a second example might serve the specificity of tangenial connections in the cortex. Where a bias of cells with similar physiology properties to form anatomical connections has been found. Thus, given the present rate of results, it is very well possible that we think about precision of cortical connections in different terms in only few years.
3) In too many places phrases like "It is clear ..." It is obvious..." are used. Sorry, most are not clear or obvious to me. take the second paragraph on page 8, incidententally a crucial one for the whole argument.
First, it is NOT clear that the structure of axon trees of individual neurons are not well specified. To conclude from the subjective impression of the complexity of axonal arbors to the randomness of connectivity is just wrong.
Second, what is the relevance of the comparison of different brains? Is a computer not a symbol processor even if it is the only one of its kind? Furthermore, it ignores the influenece of development. The influence of experience onto the formation of cortical connections has been shown in many areas. That this experience is not exactly matched between different individuals may lead to differences in the physical appearance of the respective brains, so what? Is this an argument that, as the brain look different, they cannot implement the same rules?
4) The term stochastic is used inconsistenly. On one hand it refers to the comparison between different brains, on the other hand on the mapping of different activation patterns. Where is the connection? What is the metric to compare different activation patterns? In any instantiation of a brain there is a specific mapping, thus what does it means mean to cal it stochastic?
5) The time scale of storage must be shorter than the time scale ofcomputation.
This relates to theargument above. In the cortex, the expression of LTP(if we take this as related to memory) might take minutes, however, it can be induced on a very short time scale. Furthermore, there might be different mechanisms for storage for different time scales, i.e. reverberating activity - incidentally, this can be developed to an argument against random connectivity, otherwise, how could the sequence X->Y->Z->Z lead to reactivation of X? - which then might lead to long term storage.
6) Why is it necessary to able to store any symbol everywhere? Can you store visual icons in auditory cortex? What is the meaning of anywhere as long the functional unit of brain dynamics has not been elucidated, i.e. locationalist and holistic theories compete?
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