Hinton's position on backpropagation-in-the-brain reversed over fifteen years — the 2007 talk tried to rescue it with a temporal-derivative mechanism; the 2022 Forward-Forward paper abandoned it

Two Hinton texts, fifteen years apart, open with near-identical framing — a slide/section titled "What is wrong with back(-)propagation" — and reach opposite verdicts.

2007 ("How to do backpropagation in a brain," NIPS DL Workshop). Hinton treats the biological-implausibility objection as a failure of imagination: "Many neuroscientists think back-propagation is biologically implausible because they cannot see how neurons could possibly do it… Do you really believe that evolution could not find an efficient way…? (have faith!)" He then rescues it with a mechanism: a stack of RBMs trained by contrastive divergence, using temporal derivatives to stand in for error derivatives, so a neuron's activity can carry both its feature and an error signal — avoiding the "two different types of signal" problem. He flags this as "a big assumption about cortical representations" needing approximate ("functional") weight symmetry — the weight-transport-problem avant la lettre.

2022 (Forward-Forward, arXiv:2212.13345). "As a model of how cortex learns, backpropagation remains implausible despite considerable effort to invent ways in which it could be implemented by real neurons" — and the works he cites as that failed "effort" include Lillicrap et al. 2020 (claim-brain-approximates-backprop-core-principles-ngrad), his own co-authored paper. He abandons backprop-as-cortical-model for Forward-Forward, whose two-forward-pass, Boltzmann-lineage design descends directly from the 2007 mechanism's contrastive core.

The honest read (better than "Hinton flip-flopped"): a fifteen-year experiment run on himself — propose a mechanism, watch the field try to firm it up (feedback alignment, equilibrium prop, NGRAD), conclude the backward pass isn't there, switch to a design that needs no backward pass at all. The link is thematic, not cited: the 2022 reference list omits the 2007 talk. See backpropagation-gap, claim-hinton-biological-implausibility-four-objections.