Different similarity measures can reverse which neural representations count as most alike, with no settled basis for choosing among them

Representational similarity analysis (RSA) compares two systems by the geometry of their internal representations, but that geometry is not fixed by the data alone — it depends on which similarity measure is applied. Bobadilla-Suarez et al. (Computational Brain & Behavior, 2020; open access, read in full) show that the choice is consequential rather than cosmetic: "the neural representation of object a is more similar to that of b than c when an angle measure is used, but this pattern reverses when a magnitude measure is used." [Tier 1] The ordering of representational similarity — the very thing RSA reads a mechanism off of — flips with the metric.

The paper frames this as an open methodological gap: prior work "adopted different similarity measures... measures commonly chosen for representational similarity analysis (RSA)," yet "the basis for choosing one measure over another is not always clear." [Tier 1] Their empirical results confirm the instability in practice: "Pearson correlation, the de facto standard for neural similarity, was bested by competing similarity measures in both studies," and which measure won varied by study and task, not merely by brain region. [Tier 1] They propose a decoding-confusability criterion as a partial remedy, but present it as a first step, not a settled arbitration standard.

This is independent, directly-read empirical grounding for the philosophical thesis in claim-representational-similarity-underdetermines-mechanism — arrived at by a different research group in a different journal, neither citing nor cited by Grujicic in this search. Because RSA cannot pin a unique similarity ordering, it cannot single out a unique mechanism: exactly the underdetermination the vault tracks. The missing piece — a principled criterion for choosing among empirically-available measures — is the extra-evidential criterion philosophy of science names, and the live question of whether it unifies the vault's cases is question-arbitration-as-missing-ingredient-across-underdetermination.