Semantic representation diagnostics along pretraining checkpoints [ftip-00OZ]
✍️source

This bounded diagnostic records when a declared semantic representation becomes recoverable along a pretraining trajectory. It uses the spatial and temporal probing design of [gurnee2024space, Sections 2-4 and 7] as a measurement template. The result is a representation diagnostic, not a claim about a developmental mechanism, a deployment capability, or a post-training law.

The task family is fixed before inspecting checkpoints: the six coordinate task datasets in the source family (world, United States, and New York City locations; historical figures, cultural works, and news headlines). Each score uses held-out entities. An eligible model family supplies at least three ordered pretraining checkpoints, identical train, validation, and test entity splits and filtering at every checkpoint, and the same layer-wise linear ridge-probe protocol, including a predeclared ridge-regularization grid and validation selection rule. A family or checkpoint missing one of those fields is reported as missing rather than silently pooled.

For model family \(m\), checkpoint \(c\), task \(q\), and layer \(\ell \), let \(s^{\mathrm {val}}_{m,c,q,\ell }\) and \(s^{\mathrm {test}}_{m,c,q,\ell }\) be validation and held-out-test coefficients of determination for the fixed probe. Choose the layer on validation entities, then evaluate once on the test entities:

\[ \ell ^*_{m,c,q}=\arg \max _{\ell \in L_m}s^{\mathrm {val}}_{m,c,q,\ell },\qquad s_{m,c,q}=s^{\mathrm {test}}_{m,c,q,\ell ^*_{m,c,q}}. \]

A predeclared layer may replace the validation choice, but a test score is never used to select \(\ell ^*\); this keeps the emergence score from being a layer-selection envelope.

The primary absolute emergence threshold is \(\tau =0.50\) in this held-out score, chosen before the trajectory is inspected. The first crossing is

\[ e_{m,q}(\tau )=\min \{c:s_{m,c,q}\geq \tau \}, \]

with \(e_{m,q}(\tau )=\bot \) when no checkpoint crosses. A crossing is reported with the checkpoint token count, layer, a bootstrap uncertainty interval over held-out entities, and the probe seed; it is not smoothed into a claim about an unobserved training interval. Sensitivity must include \(\tau \in \{0.25,0.50,0.75\}\), model family, parameter scale, and checkpoint spacing.

The structural record contains held-out coordinate prediction, block and cross-entity generalization, prompt variants, and the layer at which the score peaks. These are content and representation probes. A function or deployment record is separate: it must specify an intervention or user task, an outcome metric, matched controls, and its own cost and failure accounting. A probe crossing alone cannot populate that record.

The source reports linear spatial and temporal representations across Llama-2 and Pythia scales, prompt sensitivity, cross-entity robustness, and individual neurons; see [gurnee2024space, Sections 2.1-2.3, 3.1-3.3, 4.1-4.2, and 5]. Those observations motivate the fields above, but every trajectory association reported by this protocol is noncausal. In particular, it makes no claim about which training environment caused the score, whether a representation is used by a deployed function, or what post-training will preserve.

Each trajectory record therefore retains the model family and size, checkpoint token count, task and entity split, probe class and seed, all layer scores, threshold grid, missingness, and the separately reported function or deployment assay. Comparisons are within the declared task family and model family before any cross-family summary. A change in checkpoint spacing or probe protocol is a new arm, not evidence of an earlier or later emergence.

Convention. Identifiability regimes for semantic representation diagnostics [ftip-00P1]

Zhou and Klindt study labels-to-latents and latents-to-labels data-generating directions. Their general assumptions include a fixed continuous invertible map \(g:Z\to X\), a continuous learned encoder with \(h=f\mathbin \circ g\), a row-centered log-linear posterior \(p(y|z)={\rm softmax}(W^*z)\), effective dimension \(n'={\rm rank}(W^*)\), and a global population cross-entropy optimum with linear readout \(U\). The source boundary for this note is the DGP construction and assumptions in Sections 3.1 to 3.2, Theorems 4.1 to 4.4, Sections 5.1 to 5.4, and Section 6, with Supplementary Assumptions 8.1 to 8.3, of [zhou2026complete, Theorems 4.1 to 4.4, Sections 3 to 5, and Section 6].

Source observation. When \(m\geq n'\), Theorem 4.1 makes the classification-relevant part of every optimum linear, \(h(z)=Az+\nu (z)\) with \(UA=W^*\); the null-readout term satisfies \(\nu (z)\in N(U)\) and may remain unconstrained. When \(m<n'\), Theorem 4.2 gives generic nonlinear, non-identifiable optima when \(\operatorname {supp}(p_Z)\) has nonempty interior \(\Omega \), \(p_Z\) is continuous and strictly positive on \(\Omega \), and every candidate \(A\) has \(N(A)\cap \Omega \ne \varnothing \). These are different capacity regimes, not two scores for one representation.

The stronger conclusions require additional structure. In the generative case, Assumption 8.2 takes \(Z=\mathbb R_+^n\) and adds a factorized exponential conditional, constant conditional entropy, a uniform class prior, exact capacity \(m=n=n'\), non-negative activations, fixed within-class variance, and an energy-minimizing global optimum. Theorem 4.3 then gives the scaled permutation \(h(z)=DP_\sigma z\). In the discriminative case, Assumption 8.3 uses \(A^*\in \mathbb R^{n\prime \times n}\) with full row rank and \(U^*\in \mathbb R^{|Y|\times n\prime }\), adds \(W^*=U^*A^*\), \(k\)-sparse latents, and \(\operatorname {spark}(A^*)>2k\). With \(m\geq n\prime \), let \(P_U\) be the orthogonal projector onto \(\operatorname {Row}(U)\); the classifier-visible encoder \(P_Uh(z)=Az\) has \(N(A)=N(A^*)\), so the unique \(k\)-sparse solution can recover \(z\). Axis alignment and sparse recovery are distinct routes through distinct assumptions.

Section 5 reports finite experiments rather than a universal law. Linear alignment uses held-out coefficient of determination for a linear decoder; axis alignment uses the mean correlation coefficient (MCC) after Hungarian matching recovered axes to true latents. Sparse recovery compares supervised Lasso, dense OLS, and unsupervised sparse dictionary learning. Cross-system correspondence uses the Maximum Matching Unit (MMU): for each source unit, the maximum absolute Pearson correlation with any unit in a declared comparison model and layer on a declared stimulus set. The synthetic experiments use ten random seeds; the real-data cross-system analysis does not inherit that seed count. Section 6 records limits from constant conditional entropy, non-negativity, class priors, and the assumed sparse data-generating process; signed latents do not inherit Theorem 4.3 directly.

Finite identifiability record. For a fixed model family, checkpoint, task split, and probe seed, record

\[ \mathcal D=\bigl (r,m,n,n',\mathcal A,[h]_{\sim _r},R^2_{\rm lin}, R^2_{\rm sparse},\operatorname {MCC}_{\rm axis}, \operatorname {MCC}_{\rm sparse},\operatorname {MMU}_{\rm cross},\mathsf E\bigr ). \]

Here \(r\) is one declared regime: sufficient-capacity linear, undercomplete nonlinear, generative axis-aligned, or discriminative sparse recovery. \(\mathcal A\) lists the causal direction, DGP, capacity, sparsity, and assumptions actually held. \(R^2_{\rm lin}\) is the held-out coefficient for linear latent alignment; \(R^2_{\rm sparse}\) names the decoder and target, with supervised Lasso and dense OLS kept as separate arms. \(\operatorname {MCC}_{\rm axis}\) records the matching and correlation convention for axis recovery, while \(\operatorname {MCC}_{\rm sparse}\) records the matching convention, sparsity level, and whether the decoder is supervised or an unsupervised dictionary. \(\operatorname {MMU}_{\rm cross}\) records the directed source and comparison model, layers, unit sets, stimulus set, and correlation rule. An unavailable or inapplicable field is \(\bot \).

The equivalence class identifies only the ambiguity allowed by the declared regime: \((A,\nu )\) with \(UA=W^*\), \(\operatorname {rank}(A)=n'\), and \(\nu (z)\in N(U)\) in the linear regime; no universal quotient, recorded as non-identifiable under the stated assumptions, in the undercomplete regime; positive diagonal scaling and permutation \((D,P_\sigma )\) in the axis-aligned regime; and the classifier-visible null-space class \(N(A)=N(A^*)\) in the sparse regime. A sparse decoder has permutation-and-scaling ambiguity only when that convention is declared and measured. No scalar summary is defined, and none of these fields alone establishes monosemanticity.

Source-grounded finite instance. In the compressed-sensing arm, the paper fixes \(n=16\), \(n'=m=8\), \(|Y|=64\), draws a Gaussian measurement matrix with normalized columns, and tests \(k\in \{1,2,3\}\) over ten seeds. For the \(k=1\) arm, the record declares discriminative sparse recovery and the \(W^*=U^*A^*\) and spark conditions; it reports \(R^2_{\rm sparse}\approx 1\) for supervised Lasso, \(R^2_{\rm sparse}\approx n'/n=0.5\) for OLS, and \(\operatorname {MCC}_{\rm sparse}\approx 0.86\) for unsupervised Lasso, with \(\operatorname {MCC}_{\rm axis}=\bot \) and \(\operatorname {MMU}_{\rm cross}=\bot \). The ten-seed estimate, decoder arm, and missing fields are part of \(\mathsf E\); the values are finite synthetic observations, not a deployment or capability claim.

FTIP deduction. This record makes the representation question conditional on capacity, causal direction, and an explicit equivalence class. It lets the existing checkpoint route report linear recoverability, axis alignment, sparse recovery, and cross-system matching without treating any one readout as a capability or deployment result. A finite observation can support a regime-specific comparison; it cannot identify a developmental mechanism or transfer beyond the declared task and assumptions.

Proposed protocol. Before inspecting a checkpoint, freeze the task family, latent or proxy labels, split, width ratio, causal regime, assumptions, decoder class, comparison model and layer for MMU, stimulus set, and held-out evaluator. Select layers or decoder settings on validation data only. Report the full record, seed repetitions, missingness, and assumption-violation arms; keep each \(R^2\), MCC, and MMU in its native units. For real model activations, treat latent recovery as a declared proxy assay and report the proxy construction with the same boundary.

A scoped local formalization search of Forest, formal-math, TauCetiReview, and TauCetiWorker found no declaration that packages this four-regime record with its separately typed readouts. Existing representation and evaluation declarations cover neighboring fields only, so this finite identifiability record remains a protocol proposal.

This protocol links the conceptual-discovery question in § [ftip-00MH] to the capability coordinates in Definition [ftip-00JM]; the panel of open comparison questions in Remark [ftip-00M2] transcludes this child.