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In the ''[[Drosophila]]'' [[olfactory system]], sparse odor coding by the [[Kenyon cell]]s of the [[Mushroom bodies|mushroom body]] is thought to generate a large number of precisely addressable locations for the storage of odor-specific memories.<ref>A sparse memory is a precise memory. Oxford Science blog. 28 Feb 2014. http://www.ox.ac.uk/news/science-blog/sparse-memory-precise-memory</ref> Sparseness is controlled by a negative feedback circuit between Kenyon cells and [[GABAergic]] anterior paired lateral (APL) neurons. Systematic activation and blockade of each leg of this feedback circuit shows that Kenyon cells activate APL neurons and APL neurons inhibit Kenyon cells. Disrupting the Kenyon cell–APL feedback loop decreases the sparseness of Kenyon cell odor responses, increases inter-odor correlations, and prevents flies from learning to discriminate similar, but not dissimilar, odors. These results suggest that feedback inhibition suppresses Kenyon cell activity to maintain sparse, decorrelated odor coding and thus the odor-specificity of memories.<ref>Lin, Andrew C., et al. "[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4000970/ Sparse, decorrelated odor coding in the mushroom body enhances learned odor discrimination]." Nature Neuroscience 17.4 (2014): 559-568.</ref>
===Non-spike ultramicro-coding (for advanced intelligence)===
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Whatever the merits and ubiquity of ''Action-Potential/Synaptic'' (“Spike”) signalling and its coding, it seems unable to offer any plausible account of ''higher-intelligence'' such as human abstract thought; e.g. see <ref>Rose, S.P.R.(2015 Feb 14) 50 years of neuroscience. ''Lancet'', '''385'''(9968), 598-9. https://doi.org/10.1016/S0140-6736(15)60224-0</ref><ref>
Trettenbrein PC (2016) The demise of the synapse as the locus of memory: a looming paradigm shift? ''Frontiers in Systems Neuroscience'', '''10''': 88. https://doi.org/10.3389/fnsys.2016.00088</ref> Hence a search for an alternative capable of ''reliable digital'' performance, but the only plausible candidate seemed to be the use of ‘spare’ RNA ('''n'''ot involved in protein-'''c'''oding, hence “[[ncRNA]]”). That ncRNA would offer the “written-down” '''static coding'''. Such ultramicro sites could not routinely intercommunicate using action-potentials, but they would almost certainly have to use [[infra-red]] or nearby optical wavelengths. Such wavelengths would conveniently fit in with the diameters of myelinated nerve fibres — here seen as [[coaxial cables]] — thus offering a second fast signalling system (with significantly different properties) operating simultaneously with the traditional system, on the same axons whenever appropriate.
<ref>Traill, R.R. (2008/2005b). Thinking by Molecule, Synapse, or both? — From Piaget’s schema, to the Selecting/Editing of ncRNA. ''Gen.Sci.J.'', https://www.gsjournal.net/Science-Journals/Research%20Papers/View/891</ref><ref name="doi.org">Traill R.R. (2022) Coding for the Brain: RNA, its Photons, and Piagetian Higher-Intelligence through Action. ''Journal of Psychiatry and Psychiatric Disorders'', '''6''', 276-297. https://doi.org/10.26502/jppd.2572-519X0175</ref><ref>Traill, R.R. (1988). The case that mammalian intelligence is based on sub-molecular memory coding and fibre-optic capabilities of myelinated nerve axons. ''Speculations in Science and Technology'', '''11'''(3), 173-181. https://www.ondwelle.com/OSM10en.pdf</ref>
Even if we accept it as true, such activity is mostly [[unobservable]] — not ''directly'' observable for practical reasons — so the extent one should accept this model depends on one’s [[philosophy of science]]. The model is based on a considerable quantity of mutually-supporting interdisciplinary evidence, so [[scientific realism]] should presumably accept it (just as it does for unseen [[Black Holes]] or [[neutrinos]]), at least until some ''observed'' disproof arises — while [[instrumentalism]] could be expected to mix disbelief with a willingness to simply use the model as practically useful, given that it does answer several mysteries.
But then additionally there are two minor items of ''direct'' evidence in the form of fulfilled predictions: (i) (more a hope than a prediction) that there would be enough spare RNA available — a doubt which was dispelled when Mattick<ref>Mattick, J.S. (2001). Noncoding RNAs: the architects of eukaryotic complexity. ''EMBO Reports'', '''2'''(11), 986-991. http://emboreports.npgjournals.com/cgi/content/full/2/11/986 {{Webarchive|url=https://web.archive.org/web/20051227234650/http://emboreports.npgjournals.com/cgi/content/full/2/11/986 |date=2005-12-27 }}</ref>
disclosed that (in humans) only about 3% of RNA was used for protein-making, ''so 97% was available for other tasks''. (ii) The feasibility of the coaxial-cable sub-hypothesis was justified by experiments showing that infra-red and other light-frequencies ''can'' be transmitted via axons.<ref>Sun Y[an], Chao Wang, & Jiapei Dai (2010, Jan). "Bio-photons as neural communication signals demonstrated by in situ biophoton autography". ''Photochem. Photobiol. Sci.'', '''9''', 315-322. https://doi.org/10.1039/b9pp00125e</ref><ref>Zangari A., D.Micheli, R.Galeazzi & A.Tozzi, V.Balzano, G.Bellavia & M.E.Caristo (2021) "Photons detected in the active nerve by photographic technique" ''Scientific Reports'', '''11''', 3022. https://doi.org/10.1038/s41598-021-82622-5</ref> This non-spike mode is envisaged as operating ''exclusively within the brain proper'' — as advanced-thought mechanisms (in the higher vertebrates) — leaving the conventional “spike” signals to do all the intercommunication with the outside world, and do other routine tasks including Hebbian maintenance.
Surprisingly though, there has been some suggestion that a similar mode would have ''evolved independently'' in '''insects''' (thus accounting for their extraordinary performance-abilities despite their tiny brains). Indeed, as there is a case that the spines and antennas of moths etc. may be receiving infra-red signals direct from the environment,<ref>
Callahan, P.S. (1977) Tuning in to Nature. Routledge & Kegan Paul: London.</ref> (reviewed <ref>Traill, R.R. (2005c). How Popperian positivism killed a good-but-poorly-presented theory — Insect Communication by Infrared. ''Gen.Sci.J.'', https://www.gsjournal.net/Science-Journals/Research%20Papers/View/897</ref>), there is thus a further possibility that there might sometimes be a dedicated feed-in of these signals directly into the insect’s nervous systems (without the usually-expected ‘spike’ sensory mechanisms). That is merely conjectural at this stage, but it might offer scope for some easy-and-economical experimentation.
'''Yet another non-spike signal-mode''': There is also indirect evidence for a ''third'' signal-mode for the axon! This mode is ''very much slower'' but capable of carrying “''much bigger documents''” in the form of already-formatted ncRNA-schemas of the above-mentioned static coding — carried as [[axonal transport]] by [[kinesin]] within the axon<ref name="doi.org"/> (just like the known transport of [[mRNA]], with which it may have been confused in laboratory studies).
== See also ==
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