REVIEW 4 major objections 6 minor 47 references
A polar coordinate system represents syntax in large language models
T0 review · 4 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read In the activations of large language models, dependency syntax is laid out as a polar coordinate system: distance marks whether a relation exists, while relative angle marks its type and direction.
desk verdict A clean, useful extension of the Structural Probe showing that dependency type and direction are linearly decodable from angle, but the 'spontaneous' claim needs a label-permutation control. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the probed edge vector $B_P s_{ij}$, the image under a linear map of the difference between two contextualized word embeddings; this is what the paper calls the polar coordinate system. Its squared norm encodes whether two words are syntactically linked, and its direction encodes the relation's type and head. The map $B_P$ is the machinery: a single linear transformation trained on the sum of a distance objective and a contrastive angular objective, so that edges of the same dependency type are pulled into the same direction and edges of different types are pushed toward orthogonality. Readout then reduces to comparing each probed edge with type prototype vectors by cosine similarity, making the whole tree linearly decodable.
What would settle it
A decisive control would train the Polar Probe on the same model activations and corpus but with the dependency labels, or their head directions, randomly permuted during training; if the probe still identifies the true labels on a held-out test set, the claimed geometry would not be specifically about syntax, whereas chance-level performance would confirm that it is.
Extended reading notes
Core claim
The central claim is that a single linear readout of contextualized word embeddings can recover the full labeled and directed dependency tree, not just the unlabeled graph. Writing the edge embedding between words $i$ and $j$ as $s_{ij}=h_i-h_j$, the Polar Probe learns one linear map $B_P$ such that the predicted existence of a relation is $\hat d=\|B_P s_{ij}\|^2$ (Eq. 8), the predicted type is $\hat t=\arg\max_c |\cos(B_P s_{ij}, V_c)|$ where $V_c$ is the average probed vector for type $c$ (Eq. 9), and the head is $h_i$ when that cosine is nonnegative and $h_j$ otherwise (Eq. 10). The probe is trained jointly on a tree-distance loss and a contrastive angle loss, and the result is that same-type edges become collinear while different-type edges become orthogonal in the probed space. On the English Web Treebank, the probe reaches about 95% AUC for dependency-type classification versus 74% for the structural probe subspace, preserves the structural score for whether a dependency exists, and peaks at the same intermediate layer across three language models. The same angular code is recovered when a main clause is embedded in nested relative clauses, which the paper takes as evidence that the geometry is systematic across tree levels.
Load-bearing premise
The load-bearing premise is that the manual grammar annotations used as ground truth are the correct specification of syntactic relations, their types, and their head directions, because the polar coordinate system is only measured against those labels.
Editorial extensions
If this is right
- Dependency type and head direction are linearly readable from activation differences, completing the distance-only picture and removing the need for a separate labeled probe.
- The full labeled tree fits in about 128 dimensions, far smaller than the layer width, suggesting that syntax occupies a compact geometric subspace rather than being spread across all units.
- The code appears at the same intermediate layer across several families of models and sharpens with model scale and recency, so it is a reproducible property of trained representations rather than a quirk of one architecture.
- The same relation type is coded by the same direction whether it appears in a short sentence or inside a long-nested relative clause, supporting a coordinate system that is invariant to surface position.
- Because a probe trained on a randomly initialized model performs at chance, the polar geometry is inherited from what the model learned, not manufactured by the probe's objective.
Reading between the lines
- The paper does not test causality; a natural extension would be to rotate the angular coordinate of a probed edge and check whether the model's syntactic behavior shifts accordingly, which would show the code is used rather than merely correlational.
- The same distance-plus-angle scheme could be adapted to other structured domains the paper does not probe, such as semantic role relations, knowledge-graph edges, or object-feature binding in vision.
- Because the probe is supervised by one annotation scheme, an unsupervised variant would be needed to decide whether the polar geometry would be discovered from the activations alone or is partly imposed by the chosen labels.
- The explicit direction sign suggests a cross-linguistic test: if the same relation types point the same way in head-initial and head-final languages under the same annotation framework, the code would be a candidate universal syntactic primitive.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper introduces the Polar Probe, a linear transformation trained on LLM contextual embeddings with a joint objective that combines the Structural Probe distance loss (Eq. 3) and an angular contrastive loss (Eq. 4). The resulting probe represents each dependency edge as a vector B_P(h_i - h_j); the squared norm of this vector is claimed to encode the existence of a syntactic relation (Eq. 8), the absolute cosine to type-specific prototypes is claimed to encode the relation type (Eq. 9), and the sign of that cosine is claimed to encode head direction (Eq. 10). The authors evaluate the probe on the English Web Treebank across several LLMs (Llama-2, Mistral, BERT-large, GPT-2, Pythia), report that it outperforms the Structural Probe on labeled and directed attachment, analyze layer- and dimensionality-dependent performance, and present a small controlled nested-sentence dataset. They conclude that LLMs spontaneously learn a polar-coordinate geometry that explicitly represents labeled directed dependency trees.
Significance. If the geometry is genuinely intrinsic to LLM activations, the paper would provide a simple linear readout for labeled directed dependency trees, contributing to the long-standing debate about symbolic versus connectionist representations. The manuscript has several concrete strengths: evaluation on a held-out test set, a range of model families and sizes, the Random-LLM and No-Probe controls, and a dimensionality analysis that goes beyond the original Structural Probe. However, the load-bearing interpretation that the polar geometry is 'spontaneous' and 'explicitly represents' UD syntax is not yet fully supported: the probe is trained with UD labels, and the angular objective imposes the collinearity and orthogonality structure that is later measured. The quantitative comparison with the Structural Probe is not like-for-like, and the headline numerical claims are reported without confidence intervals or significance tests. With additional controls and clarifications, the central results could be made solid, but as presented the evidence is incomplete.
major comments (4)
- [§2.4, Eq. (11); §3 'Comparison with baselines'; §4 'Limitations'] The 'spontaneous' and 'explicitly represents' claims require a label-permutation control. The angular objective (Eq. 4) explicitly pushes same-type edges toward collinearity and different-type edges toward orthogonality, so measuring that structure in the probe output partly checks the training objective. The Random-LLM and No-Probe baselines do not establish that the true UD labels are privileged: a real LLM's activations contain abundant lexical, part-of-speech, and positional regularities (as the No-Probe 80% AUC shows), and a supervised linear probe could plausibly fit permuted labels to a similar degree. I request a control in which the same LLM activations are trained with randomly permuted relation labels (or permuted head/direction assignments), preserving label frequencies and edge distributions; without it, the evidence supports linear decodability of UD labels, not a spontaneous syntax-specific polar geometry.
- [§3 'Comparison with baselines'; Fig. 2.B; Fig. 3] The comparison with the Structural Probe on label accuracy is apples-to-oranges. The Structural Probe is trained only to minimize the distance objective (Eq. 3) and is not designed to encode relation type or direction, so lower label accuracy is expected and does not demonstrate that the Polar Probe's geometry is superior. To support the claimed 'nearly two folds' improvement, the authors should compare against a supervised linear readout trained on top of the Structural Probe's output embeddings, or against a linear classifier applied to raw activations; otherwise the baseline is not a fair control for label and direction accuracy.
- [§3, Fig. 2.B, Fig. 3, Fig. 6] The paper reports no confidence intervals, error bars, or significance tests for the headline AUC, balanced accuracy, or LAS scores. Given that the central quantitative claims include 'substantially outperforms' and 'becomes increasingly precise in the latest frontier models,' the authors should report variance across test splits, random seeds, or bootstrap resamples, and where appropriate perform significance tests for the differences between probes.
- [§2.1, Eq. (1); §2.3, Eqs. (4)-(7); §2.4, Eqs. (9)-(10)] The definition of the edge embedding s_i,j = h_i - h_j does not specify whether the ordered pair (w_i, w_j) is always (head, dependent), always (dependent, head), or in sentence order. This matters because Eq. (4) pushes all same-type edges toward positive collinearity and Eq. (7)/(10) reads head direction from the sign of the cosine relative to the prototype. If the ordering is not consistently head-to-dependent, the angular objective conflicts with direction encoding. The paper should clarify the ordering, and include a validation that the predicted direction correlates with UD gold heads independently of sentence position (e.g., by checking that swapping the order of the pair flips the predicted head).
minor comments (6)
- [§2.4, Eq. (11)] The loss notation is inconsistent: Eq. (4) defines L_A with the Angular Probe matrix B_A, but Eq. (11) uses L_A within the Polar Probe objective for B_P; the authors should state that B_A is replaced by B_P during joint training or introduce a separate notation.
- [Fig. 1 caption] The caption contains a typo: 'finds a a linear transform' should read 'finds a linear transform.'
- [§3 'Comparison with baselines'] The sentence 'The same relative results across probes are conserved for the Balanced Accuracy score' does not report the actual Balanced Accuracy numbers; please provide them in the text or table.
- [§2.5 'Controlled dataset'] The text says 'as shown in Fig: 5' but the reference should be to Figure 5 without the colon; also the dataset description would benefit from a table of the 100 sentences and their level counts for reproducibility.
- [Fig. 6] The y-axis label 'Structural and Label Score' is vague, the 'SOTA' point is not explained in the caption or text, and there is no legend for the dashed versus solid markers; please clarify.
- [§3 'Comparison with baselines'] The phase 'nearly two folds' is awkward; I suggest 'nearly two-fold' or 'roughly twice as accurate.'
Circularity Check
No load-bearing circularity: the Polar Probe is supervised but evaluated on held-out data; the 'spontaneous' framing is an interpretive overreach, not a derivation-level circularity.
full rationale
The derivation chain is not circular. The Polar Probe is trained on the UD-labeled training split through the joint objective (Eqs. 4 and 11) and then evaluated on held-out sentences (Section 2.7, Section 3). Its high test-set accuracy for dependency type and direction is therefore genuine generalization, not a restatement of the training objective. The polar structure visualized in Fig. 2C (same-type collinearity, cross-type orthogonality) does coincide with the Angular Probe's objective in Eq. 4, but because it is measured on the test set, it is evidence that the learned geometry transfers to unseen activations rather than a tautology. The paper itself acknowledges the supervised nature of the probe in the Limitations: 'the Polar Probe is based on a supervised task: we optimize a linear transformation that maximally retrieves a known syntactic structure from the neural activations.' This tempers the abstract's 'spontaneously' wording, but it is a framing concern, not a circular derivation. Self-citations (e.g., King and Dehaene 2014 for linear readouts; Lakretz et al. 2021b for the controlled dataset) are incidental and not load-bearing. The absence of a label-permutation control weakens the stronger 'spontaneous syntax-specific geometry' interpretation, especially since the No-Probe baseline already reaches AUC=80% on type labels, but that is an external validity and control-selection gap, not an internal circularity. Overall, the central empirical claim — that a linear polar readout can recover labeled, directed dependency trees on held-out data — rests on a train/test separation and does not reduce to its inputs by construction.
Assumptions & free parameters
free parameters (4)
- Probe matrix B_P (and angular/structural variants) =
learned linear map, not released
- lambda (loss weight) =
10.0
- Probe dimensionality k'' =
128
- Prototype vectors V_c =
centroids of 10,000 training edges per type
assumptions (4)
- domain assumption Universal Dependencies annotations (EWT) are the correct ground truth for syntactic trees, including head direction and relation labels.
- domain assumption A representation is defined as linearly readable information; a linear probe is the appropriate measurement.
- domain assumption Euclidean geometry and the difference embedding h_i - h_j capture relational information.
- standard math The Structural Probe's distance-based representation is valid and provides the existence signal.
invented entities (1)
-
Polar coordinate system for syntax in activation space
Cite this review
Pith. "Pith review of A polar coordinate system represents syntax in large language models." pith.science (2026). https://pith.science/paper/XAEXHRN5
@misc{pith2026241205571,
author = {Pith},
title = {Pith review of: A polar coordinate system represents syntax in large language models},
year = {2026},
howpublished = {\url{https://pith.science/paper/XAEXHRN5}},
note = {Machine review of arXiv:2412.05571}
}
read the original abstract
Originally formalized with symbolic representations, syntactic trees may also be effectively represented in the activations of large language models (LLMs). Indeed, a 'Structural Probe' can find a subspace of neural activations, where syntactically related words are relatively close to one-another. However, this syntactic code remains incomplete: the distance between the Structural Probe word embeddings can represent the existence but not the type and direction of syntactic relations. Here, we hypothesize that syntactic relations are, in fact, coded by the relative direction between nearby embeddings. To test this hypothesis, we introduce a 'Polar Probe' trained to read syntactic relations from both the distance and the direction between word embeddings. Our approach reveals three main findings. First, our Polar Probe successfully recovers the type and direction of syntactic relations, and substantially outperforms the Structural Probe by nearly two folds. Second, we confirm that this polar coordinate system exists in a low-dimensional subspace of the intermediate layers of many LLMs and becomes increasingly precise in the latest frontier models. Third, we demonstrate with a new benchmark that similar syntactic relations are coded similarly across the nested levels of syntactic trees. Overall, this work shows that LLMs spontaneously learn a geometry of neural activations that explicitly represents the main symbolic structures of linguistic theory.
Figures
Figures from the paper (6 more)
Reference graph
Works this paper leans on
-
[1]
Caucheteux, C., Gramfort, A., and King, J.-R. (2021). Disentangling syntax and semantics in the brain with deep networks. In International conference on machine learning , pages 1336--1348. PMLR
work page 2021
-
[2]
Caucheteux, C. and King, J.-R. (2022). Brains and algorithms partially converge in natural language processing. Communications biology , 5(1):134
work page 2022
-
[3]
Chen, B., Fu, Y., Xu, G., Xie, P., Tan, C., Chen, M., and Jing, L. (2021). Probing bert in hyperbolic spaces. arXiv preprint arXiv:2104.03869
arXiv 2021
-
[4]
A., Hewitt, J., and Manning, C
Chi, E. A., Hewitt, J., and Manning, C. D. (2020). Finding universal grammatical relations in multilingual BERT . In Jurafsky, D., Chai, J., Schluter, N., and Tetreault, J., editors, Proceedings of the 58th Annual Meeting of the Association for Computational Linguistics , pages 5564--5577, Online. Association for Computational Linguistics
work page 2020
-
[5]
Chomsky, N. (1957). Syntactic Structures . De Gruyter
work page 1957
-
[6]
Chomsky, N. (2014). The minimalist program . MIT press
work page 2014
-
[7]
Cinque, G. and Rizzi, L. (2009). The cartography of syntactic structures
work page 2009
-
[8]
Costa-juss \`a , M. R., Cross, J., C elebi, O., Elbayad, M., Heafield, K., Heffernan, K., Kalbassi, E., Lam, J., Licht, D., Maillard, J., et al. (2022). No language left behind: Scaling human-centered machine translation. arXiv preprint arXiv:2207.04672
arXiv 2022
Show all 47 references
-
[9]
Desai, K., Nickel, M., Rajpurohit, T., Johnson, J., and Vedantam, S. R. (2023). Hyperbolic image-text representations. In Krause, A., Brunskill, E., Cho, K., Engelhardt, B., Sabato, S., and Scarlett, J., editors, Proceedings of the 40th International Conference on Machine Lear...
2023
-
[10]
Devlin, J., Chang, M.-W., Lee, K., and Toutanova, K. (2019). Bert: Pre-training of deep bidirectional transformers for language understanding. pages 4171--4186. Association for Computational Linguistics
2019
-
[11]
Dhingra, B., Shallue, C., Norouzi, M., Dai, A., and Dahl, G. (2018). Embedding text in hyperbolic spaces
2018
-
[12]
DiCarlo, J. J. and Cox, D. D. (2007). Untangling invariant object recognition. Trends in cognitive sciences , 11(8):333--341
2007
-
[13]
Eisape, T., Gangireddy, V., Levy, R., and Kim, Y. (2022). Probing for incremental parse states in autoregressive language models. In Goldberg, Y., Kozareva, Z., and Zhang, Y., editors, Findings of the Association for Computational Linguistics: EMNLP 2022 , pages 2801--2813, Ab...
2022
-
[14]
Evanson, L., Lakretz, Y., and King, J.-R. (2023). Language acquisition: do children and language models follow similar learning stages? arXiv preprint arXiv:2306.03586
2023 arXiv
-
[15]
T., Campanelli, L., Li, J., Bhattasali, S., Pallier, C., and Brennan, J
Hale, J. T., Campanelli, L., Li, J., Bhattasali, S., Pallier, C., and Brennan, J. R. (2022). Neurocomputational models of language processing. Annual Review of Linguistics , 8:427--446
2022
-
[16]
and Manning, C
Hewitt, J. and Manning, C. D. (2019). A structural probe for finding syntax in word representations. pages 4129--4138. Association for Computational Linguistics
2019
-
[17]
Huang, Q., Smolensky, P., He, X., Deng, L., and Wu, D. (2017). Tensor product generation networks for deep nlp modeling. arXiv preprint arXiv:1709.09118
2017 arXiv
-
[18]
and Huth, A
Jain, S. and Huth, A. (2018). Incorporating context into language encoding models for fmri. Advances in neural information processing systems , 31
2018
-
[19]
Q., Sablayrolles, A., Mensch, A., Bamford, C., Chaplot, D
Jiang, A. Q., Sablayrolles, A., Mensch, A., Bamford, C., Chaplot, D. S., de las Casas, D., Bressand, F., Lengyel, G., Lample, G., Saulnier, L., Lavaud, L. R., Lachaux, M.-A., Stock, P., Scao, T. L., Lavril, T., Wang, T., Lacroix, T., and Sayed, W. E. (2023). Mistral 7b
2023
-
[20]
Joshi, A. K. and Schabes, Y. (1997). Tree-adjoining grammars. In Handbook of Formal Languages: Volume 3 Beyond Words , pages 69--123. Springer
1997
-
[21]
and Dehaene, S
King, J.-R. and Dehaene, S. (2014). Characterizing the dynamics of mental representations: the temporal generalization method. Trends in cognitive sciences , 18(4):203--210
2014
-
[22]
Kingma, D. P. and Ba, J. (2014). Adam: A method for stochastic optimization
2014
-
[23]
and Bandettini, P
Kriegeskorte, N. and Bandettini, P. (2007). Analyzing for information, not activation, to exploit high-resolution fmri. Neuroimage , 38(4):649--662
2007
-
[24]
Lakretz, Y., Dehaene, S., and King, J.-R. (2020). What limits our capacity to process nested long-range dependencies in sentence comprehension? Entropy , 22(4):446
2020
-
[25]
Lakretz, Y., Desbordes, T., King, J.-R., Crabb \'e , B., Oquab, M., and Dehaene, S. (2021a). Can rnns learn recursive nested subject-verb agreements? arXiv preprint arXiv:2101.02258
2021 arXiv
-
[26]
Lakretz, Y., Hupkes, D., Vergallito, A., Marelli, M., Baroni, M., and Dehaene, S. (2021b). Mechanisms for handling nested dependencies in neural-network language models and humans. Cognition , 213:104699
2021
-
[27]
Lakretz, Y., Kruszewski, G., Desbordes, T., Hupkes, D., Dehaene, S., and Baroni, M. (2019). The emergence of number and syntax units in. pages 11--20. Association for Computational Linguistics
2019
-
[28]
and Mare c ek, D
Limisiewicz, T. and Mare c ek, D. (2021). Introducing orthogonal constraint in structural probes. In Zong, C., Xia, F., Li, W., and Navigli, R., editors, Proceedings of the 59th Annual Meeting of the Association for Computational Linguistics and the 11th International Joint Co...
2021
-
[29]
Linzen, T., Dupoux, E., and Goldberg, Y. (2016). Assessing the ability of lstms to learn syntax-sensitive dependencies. Transactions of the Association for Computational Linguistics , 4:521--535
2016
-
[30]
M \"u ller-Eberstein, M., van der Goot, R., and Plank, B. (2022). Probing for labeled dependency trees. In Muresan, S., Nakov, P., and Villavicencio, A., editors, Proceedings of the 60th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers) , ...
2022
-
[31]
and Kiela, D
Nickel, M. and Kiela, D. (2017). Poincar\' e embeddings for learning hierarchical representations. In Guyon, I., Luxburg, U. V., Bengio, S., Wallach, H., Fergus, R., Vishwanathan, S., and Garnett, R., editors, Advances in Neural Information Processing Systems , volume 30. Curr...
2017
-
[32]
Nivre, J., Zeman, D., Ginter, F., and Tyers, F. (2017). Universal Dependencies . ACL Anthology
2017
-
[33]
Palangi, H., Huang, Q., Smolensky, P., He, X., and Deng, L. (2017). Grammatically-interpretable learned representations in deep nlp models. In Advances in Neural Information Processing Systems Workshop
2017
-
[34]
Pallier, C., Devauchelle, A.-D., and Dehaene, S. (2011). Cortical representation of the constituent structure of sentences. Proceedings of the National Academy of Sciences , 108:2522--2527
2011
-
[35]
Pasquiou, A., Lakretz, Y., Hale, J., Thirion, B., and Pallier, C. (2022). Neural language models are not born equal to fit brain data, but training helps. In ICML 2022-39th International Conference on Machine Learning , page 18
2022
-
[36]
Pasquiou, A., Lakretz, Y., Thirion, B., and Pallier, C. (2023). Information-restricted neural language models reveal different brain regions’ sensitivity to semantics, syntax, and context. Neurobiology of Language , 4(4):611--636
2023
-
[37]
Pedregosa, F., Varoquaux, G., Gramfort, A., Michel, V., Thirion, B., Grisel, O., Blondel, M., Prettenhofer, P., Weiss, R., Dubourg, V., Vanderplas, J., Passos, A., Cournapeau, D., Brucher, M., Perrot, M., and Duchesnay, E. (2011). Scikit-learn: Machine learning in P ython. Jou...
2011
-
[38]
Reddy, A. J. and Wehbe, L. (2021). Can fmri reveal the representation of syntactic structure in the brain? Advances in Neural Information Processing Systems , 34:9843--9856
2021
-
[39]
Robins, R. H. (2013). A Short History of Linguistics . Routledge
2013
-
[40]
Silveira, N., Dozat, T., de Marneffe, M.-C., Bowman, S., Connor, M., Bauer, J., and Manning, C. D. (2014). A gold standard dependency corpus for E nglish. In Proceedings of the Ninth International Conference on Language Resources and Evaluation (LREC-2014)
2014
-
[41]
Smolensky, P. (1987). Connectionist ai, symbolic ai, and the brain. Artificial Intelligence Review , 1:95--109
1987
-
[42]
Smolensky, P. (1990). Tensor product variable binding and the representation of symbolic structures in connectionist systems. Artificial Intelligence , 46:159--216
1990
-
[43]
T., Fernandez, R., Goldrick, M., and Gao, J
Smolensky, P., McCoy, R. T., Fernandez, R., Goldrick, M., and Gao, J. (2022). Neurocompositional computing: From the central paradox of cognition to a new generation of ai systems. AI Magazine , 43(3):308–322
2022
-
[44]
Soulos, P., McCoy, T., Linzen, T., and Smolensky, P. (2019). Discovering the compositional structure of vector representations with role learning networks. arXiv preprint arXiv:1910.09113
2019 arXiv
-
[45]
Tesnière, L. (1953). Esquisse d'une syntaxe structurale . Klincksieck
1953
-
[46]
Touvron, H., Martin, L., Stone, K., Albert, P., Almahairi, A., Babaei, Y., Bashlykov, N., Batra, S., Bhargava, P., Bhosale, S., Bikel, D., Blecher, L., Ferrer, C. C., Chen, M., Cucurull, G., Esiobu, D., Fernandes, J., Fu, J., Fu, W., Fuller, B., Gao, C., Goswami, V., Goyal, N....
2023
-
[47]
C., Pimentel, T., Saphra, N., and Cotterell, R
White, J. C., Pimentel, T., Saphra, N., and Cotterell, R. (2021). A non-linear structural probe. In Toutanova, K., Rumshisky, A., Zettlemoyer, L., Hakkani-Tur, D., Beltagy, I., Bethard, S., Cotterell, R., Chakraborty, T., and Zhou, Y., editors, Proceedings of the 2021 Conferen...
2021
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.