Barlow's 1961 chapter states the redundancy-reduction hypothesis explicitly, in his own words, as the operating principle of sensory relays

Read directly against the primary, Horace Barlow's 1961 chapter "Possible Principles Underlying the Transformations of Sensory Messages" does state the redundancy-reduction hypothesis explicitly and centrally — not as a later gloss, but in Barlow's own text. He lays out three hypotheses for what sensory relays do and gives the redundancy-reduction hypothesis the most space. His up-front statement of the third hypothesis: relays "recode sensory messages, extracting signals of high relative entropy from the highly redundant sensory input." Its formal statement follows: "The hypothesis is that sensory relays recede [OCR for recode] sensory messages so that their redundancy is reduced but comparatively little information is lost" (p. 225). And the Summary closes on it: "reduction of redundancy is an important principle guiding the organization of sensory messages and is carried out at relays in the sensory pathways" (p. 233).

This confirms, against the source itself, the load-bearing core the vault previously held only on secondary supports — the Barlow Twins paper's one-line eponym and capture-time search synthesis. It settles the central question in question-verify-barlow-1961-efficient-coding-primary and grounds the anchor note claim-barlow-1961-efficient-coding-removes-sensory-redundancy at Tier 1 for its core. It is the "why" beneath perceptual fading — unchanging, redundant input is discarded, so the eye must move to keep generating signal (claim-fixational-eye-movements-prevent-perceptual-fading) — and the 1961 ancestor the 2021 self-supervised loss reaches back for by name (claim-barlow-twins-loss-minimizes-embedding-redundancy). Two elaborations often bundled with this hypothesis — the "factorial code" label and center-surround receptive fields as its mechanism — are not in this chapter; see claim-barlow-1961-does-not-name-factorial-code and claim-barlow-1961-does-not-invoke-center-surround-as-the-redundancy-reducing-mechanism.