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Empirical Constraint: Neuronal firing exhibits extreme input degeneracy. Any subset of inputs arriving at ~100\u2013200 out of ~104 spines can trigger an action potential; ~140 inputs yield &gt;10300 possible effective input combinations. This degeneracy introduces ambiguity regarding the specific input combinations that contribute to associative encoding. Testable Mechanism: The semblance hypothesis proposes a spine-level conjunction code between co-active spines forming inter-postsynaptic functional links (IPLs) that operate independently of postsynaptic neuronal firing, and are proposed to maintain the fidelity of original associations during retrieval. Computational Validation: During retrieval, cue-induced reactivation of IPLs is hypothesized to depolarize inter-linked spines and generate semblances. Taking the depolarization of an inter-linked spine in the absence of direct presynaptic input as an \u201coperational semblance\u201d constituting the elemental substrate of associative recall, a computational model was implemented within a biologically realistic 1000 \u03bcm3 neuropil volume containing 1600 dendritic spines (density 1.6\/\u03bcm3; diameter 0.6\u20130.7 \u03bcm) as independent dendritic segments. Of 391 crossing dendritic sites, 151 have abutted spine pairs, using \u226430 nm inter-spine distance as a candidate eligibility criterion, motivated by reported nanoscale appositions. Spine density, diameter, and separation were parameterized from electron microscopy and confocal\/super-resolution imaging literature; coupling strength (0.7) was fixed and exploratory. IPL formation was implemented probabilistically across 20 learning trials (formation probability = 0.30 per trial at each of the 49 designated sites; reversal probability = 0.04), producing a gradual acquisition curve and trial-to-trial variance emerging naturally from the two-state Markov dynamics. Across 100 independent simulations, 49 of the 151 abutted pairs were designated as structurally eligible Bell\u2013Food convergence sites; of these, 44.7 \u00b1 2.1 developed active IPLs after 20 trials, generating operational semblances per retrieval (recall accuracy 36.93% \u00b1 1.71%; paired t-test, p &lt; 0.001; Cohen\u2019s d = 30.54, illustrative not calibrated). This work demonstrates that given a plausible convergence of associative pathways onto existing abutted spine pairs, the structural relations between spines in the cortical neuropil are compatible with the formation of a spine-level conjunction code as a potential substrate for associative learning.<\/jats:p>","DOI":"10.31083\/jin49226","type":"journal-article","created":{"date-parts":[[2026,9,29]],"date-time":"2026-09-29T16:16:31Z","timestamp":1790698591000},"source":"Crossref","is-referenced-by-count":0,"title":["Inter-Postsynaptic Functional Links for Associative Memory: Feasibility in a Realistic Neuropil Model"],"prefix":"10.31083","volume":"25","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-2721-6931","authenticated-orcid":true,"given":"Kunjumon I.","family":"Vadakkan","sequence":"first","affiliation":[{"name":"Neurosearch Center, Toronto, ON M5A 2N2, Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"16242","published-online":{"date-parts":[[2026,9,21]]},"reference":[{"key":"ref1","unstructured":"Hebb DO. 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