[Note] On SAE Feature Geometry

SAE feature directions are likely "special" rather than "random". 

• Different SAEs seem to converge to learning the same features 
• SAE error directions increase model loss by a lot compared to random directions, indicating that the error directions are "special", which points to the feature directions also being "special" 
• Conversely, SAE feature directions increase model loss by much less than random directions

Re: the last point above, this points to singular learning theory being an effective tool for analysis. 

• Reminder: The LLC measures "local flatness" of the loss basin. A higher LLC = flatter loss, i.e. changing the model's parameters by a small amount does not increase the loss by much. 
• In preliminary work on LLC analysis of SAE features, the "feature-targeted LLC" turns out to be something which can be measured empirically and distinguishes SAE features from random directions

[Note] The Polytope Representation Hypothesis

This is an empirical observation about recent works on feature geometry, that (regular) polytopes are a recurring theme in feature geometry. 

Simplices in models. Work studying hierarchical structure in feature geometry finds that sets of things are often represented as simplices, which are a specific kind of regular polytope. Simplices are also the structure of belief state geometry. 

Regular polygons in models. Recent work studying natural language modular arithmetic has found that language models represent things in a circular fashion. I will contend that "circle" is a bit imprecise; these are actually regular polygons, which are the 2-dimensional versions of polytopes. 

A reason why polytopes could be a natural unit of feature geometry is that they characterize linear regions of the activation space in ReLU networks. However, I will note that it's not clear that this motivation for polytopes coincides very well with the empirical observations above.  

Interpretability needs a good proxy metric

I’m concerned that progress in interpretability research is ephemeral, being driven primarily by proxy metrics that may be disconnected from the end goal (understanding by humans). (Example: optimising for the L0 metric in SAE interpretability research may lead us to models that have more split features, even when this is unintuitive by human reckoning.) 

It seems important for the field to agree on some common benchmark / proxy metric that is proven to be indicative of downstream human-rated interpretability, but I don’t know of anyone doing this. Similar to the role of BLEU in facilitating progress in NLP, I imagine having a standard metric would enable much more rapid and concrete progress in interpretability. 

[Repro] Circular Features in GPT-2 Small

This is a paper reproduction in service of achieving my seasonal goals

Recently, it was demonstrated that circular features are used in the computation of modular addition tasks in language models. I've reproduced this for GPT-2 small in this Colab. 

We've confirmed that days of the week do appear to be represented in a circular fashion in the model. Furthermore, looking at feature dashboards agrees with the discovery; this suggests that simply looking up features that detect tokens in the same conceptual 'category' could be another way of finding clusters of features with interesting geometry.

Next steps:

1. Here, we've selected 9 SAE features, gotten the reconstruction, and then compressed this down via PCA. However, were all 9 features necessary? Could we remove some of them without hurting the visualization?

2. The SAE reconstruction using 9 features is probably a very small component of the model's overall representation of this token. What's in the rest of the representation? Is it mostly orthogonal to the SAE reconstruction, or is there a sizeable component remaining in this 9-dimensional subspace? If the latter, it would indicate that the SAE representation here is not a 'full' representation of the original model.

Thanks to Egg Syntax for pair programming and Josh Engels for help with the reproduction. 

[Proposal] Out-of-context meta learning as a toy model of steganography

Steganography; the idea that models may say one thing but mean another, and that this may enable them to evade supervision. Essentially, models might learn to "speak in code". 

In order to better study steganography, it would be useful to construct model organisms of steganography, which we don't have at the moment. How might we do this? I think out-of-context meta learning is a very convenient path. 

Out-of-context meta learning: The idea that models can internalise knowledge during training, and then use this knowledge at test-time to answer questions. E.g.

• Train the model on inputs of the form: "A <nonsense word> is <color>". 
• At test time, ask the model "What color is <nonsense world>?" 

This has been demonstrated to be possible in Krasheninnikov et al, 2024 as well as Berglund et al, 2024. 

This seems like a pretty effective path to creating model organisms of steganography. E.g.

• Train the model on re-definitions of existing words, e.g. "A cat is a vehicle for transport" 
• Test the model on whether it uses "cat" instead of "car" at test time. Or something in this vein. 

I probably won't work on this myself, but I'm pretty interested in someone doing this and reporting their results

[Note] Excessive back-chaining from theories of impact is misguided

Rough summary of a conversation I had with Aengus Lynch 

As a mech interp researcher, one thing I've been trying to do recently is to figure out my big cruxes for mech interp, and then filter projects by whether they are related to these cruxes. 

Aengus made the counterpoint that this can be dangerous, because even the best researchers' mental model of what will be impactful in the future is likely wrong, and errors will compound through time. Also, time spent refining a mental model is time not spent doing real work. Instead, he advocated for working on projects that seem likely to yield near-term value 

I still think I got a lot of value out of thinking about my cruxes, but I agree with the sentiment that this shouldn't consume excessive amounts of my time

[Note] Is adversarial robustness best achieved through grokking? 

A rough summary of an insightful discussion with Adam Gleave, FAR AI

We want our models to be adversarially robust. 

• According to Adam, the scaling laws don't indicate that models will "naturally" become robust just through standard training. 

One technique which FAR AI has investigated extensively (in Go models) is adversarial training. 

• If we measure "weakness" in terms of how much compute is required to train an adversarial opponent that reliably beats the target model at Go, then starting out it's like 10m FLOPS, and this can be increased to 200m FLOPS through iterated adversarial training. 
• However, this is both pretty expensive (~10-15% of pre-training compute), and doesn't work perfectly (even after extensive iterated adversarial training, models still remain vulnerable to new adversaries.) 
• A useful intuition: Adversarial examples are like "holes" in the model, and adversarial training helps patch the holes, but there are just a lot of holes. 

One thing I pitched to Adam was the notion of "adversarial robustness through grokking". 

• Conceptually, if the model generalises perfectly on some domain, then there can't exist any adversarial examples (by definition). 
• Empirically, "delayed robustness" through grokking has been demonstrated on relatively advanced datasets like CIFAR-10 and Imagenette; in both cases, models that underwent grokking became naturally robust to adversarial examples.  

Adam seemed thoughtful, but had some key concerns. 

• One of Adam's cruxes seemed to relate to how quickly we can get language models to grok; here, I think work like grokfast is promising in that it potentially tells us how to train models that grok much more quickly. 
• I also pointed out that in the above paper, Shakespeare text was grokked, indicating that this is feasible for natural language 
• Adam pointed out, correctly, that we have to clearly define what it means to "grok" natural language. Making an analogy to chess; one level of "grokking" could just be playing legal moves. Whereas a more advanced level of grokking is to play the optimal move. In the language domain, the former would be equivalent to outputting plausible next tokens, and the latter would be equivalent to  being able to solve arbitrarily complex intellectual tasks like reasoning. 
• We had some discussion about characterizing "the best strategy that can be found with the compute available in a single forward pass of a model" and using that as the criterion for grokking. 

His overall take was that it's mainly an "empirical question" whether grokking leads to adversarial robustness. He hadn't heard this idea before, but thought experiments / proofs of concept would be useful. 

[Note] On the feature geometry of hierarchical concepts

A rough summary of insightful discussions with Jake Mendel and Victor Veitch

Recent work on hierarchical feature geometry has made two specific predictions: 

• Proposition 1: activation space can be decomposed hierarchically into a direct sum of many subspaces, each of which reflects a layer of the hierarchy. 
• Proposition 2: within these subspaces, different concepts are represented as simplices. 

Example of hierarchical decomposition: A dalmation is a dog, which is a mammal, which is an animal. Writing this hierarchically, Dalmation < Dog < Mammal < Animal. In this context, the two propositions imply that: 

• P1: $x_{dog} = x_{animal} + x_{mammal | animal} + x_{dog | mammal } + x_{dalmation | dog}$, and the four terms on the RHS are pairwise orthogonal. 
• P2: If we had a few different kinds of animal, like birds, mammals, and fish, the three vectors $x_{mammal | animal}, x_{fish | animal}, x_{bird | animal}$ would form a simplex.   

According to Victor Veitch, the load-bearing assumption here is that different levels of the hierarchy are disentangled, and hence models want to represent them orthogonally. I.e. $x_{animal}$ is perpendicular to $x_{mammal | animal}$. I don't have a super rigorous explanation for why, but it's likely because this facilitates representing / sensing each thing independently. 

• E.g. sometimes all that matters about a dog is that it's an animal; it makes sense to have an abstraction of "animal" that is independent of any sub-hierarchy. 

Jake Mendel made the interesting point that, as long as the number of features is less than the number of dimensions, an orthogonal set of vectors will satisfy P1 and P2 for any hierarchy. 

Example of P2 being satisfied. Let's say we have vectors $x_{animal} = (0,1)$ and $x_{plant} = (1,0)$, which are orthogonal. Then we could write $x_{living_thing} = (1/sqrt(2), 1/ sqrt(2))$. Then $x_{animal | living_thing}, x_{plant | living_thing}$ would form a 1-dimensional simplex. 

Example of P1 being satisfied. Let's say we have four things A, B, C, D arranged in a binary tree such that AB, CD are pairs. Then we could write $x_A = x_{AB} + x_{A | AB}$, satisfying both P1 and P2. However, if we had an alternate hierarchy where AC and BD were pairs, we could still write $x_A = x_{AC} + x_{A | AC}$. Therefore hierarchy is in some sense an "illusion", as any hierarchy satisfies the propositions. 

Taking these two points together, the interesting scenario is when we have more features than dimensions, i.e. the setting of superposition. Then we have the two conflicting incentives:

• On one hand, models want to represent the different levels of the hierarchy orthogonally. 
• On the other hand, there isn't enough "room" in the residual stream to do this; hence the model has to "trade off" what it chooses to represent orthogonally. 

This points to super interesting questions: 

• what geometry does the model adopt for features that respect a binary tree hierarchy? 
• what if different nodes in the hierarchy have differing importances / sparsities?
• what if the tree is "uneven", i.e. some branches are deeper than others. 
• what if the hierarchy isn't a tree, but only a partial order? 

Experiments on toy models will probably be very informative here. 

[Proposal] Attention Transcoders: can we take attention heads out of superposition? 

Note: This thinking is cached from before the bilinear sparse autoencoders paper. I need to read that and revisit my thoughts here. 

Primer: Attention-Head Superposition

Attention-head superposition (AHS) was introduced in this Anthropic post from 2023. Briefly, AHS is the idea that models may use a small number of attention heads to approximate the effect of having many more attention heads.  

Definition 1: OV-incoherence. An attention circuit is OV-incoherent if it attends from multiple different tokens back to a single token, and the output depends on the token attended from. 

Example 2: Skip-trigram circuits. A skip trigram consists of a sequence [A]...[B] -> [C], where A, B, C are distinct tokens. 

Claim 3: A single head cannot implement multiple OV-incoherent circuits. Recall from A Mathematical Framework that an attention head can be decomposed into the OV circuit and the QK circuit, which operate independently. Within each head, the OV circuit is solely responsible for mapping linear directions in the input to linear directions in the output.  only the query token. Since it does not see the key token, it must compute a fixed function of the query. 

Claim 4: Models compute many OV-incoherent circuits simultaneously in superposition. If the ground-truth data is best explained by a large number of OV-incoherent circuits, then models will approximate having these circuits by placing them in superposition across their limited number of attention heads. 

Attention Transcoders 

An attention transcoder (ATC) is described as follows:

• An ATC attempts to reconstruct the input and output of a specific attention block
• An ATC is simply a standard multi-head attention module, except that it has many more attention heads. 
• An ATC is regularised during training such that the number of active heads is sparse. 
  • I've left this intentionally vague at the moment as I'm uncertain how exactly to do this. 

Remark 5: The ATC architecture is the generalization of other successful SAE-like architectures to attention blocks. 

• Residual-stream SAEs simulate a model that has many more residual neurons. 
• MLP transcoders simulate a model that has many more hidden neurons in its MLP. 
• ATCs simulate a model that has many more attention heads. 

Remark 6: Intervening on ATC heads. Since the ATC reconstructs the output of an attention block, ablations can be done by simply splicing the ATC into the model's computational graph and intervening directly on individual head outputs. 

Remark 7: Attributing ATC heads to ground-truth heads. In standard attention-out SAEs, it's possible to directly compute the attribution of each head to an SAE feature. That seems impossible here because the ATC head outputs are not direct functions of the ground-truth heads. Nonetheless, if ATC heads seem highly interpretable and accurately reconstruct the real attention outputs, and specific predictions can be verified via interventions, it seems reasonable to conclude that they are a good explanation of how attention blocks are working. 

Key uncertainties

Does AHS actually occur in language models? I think we do not have crisp examples at the moment. 

Concrete experiments

The first and most obvious experiment is to try training an ATC and see if it works. 

• Scaling milestones: toy models, TinyStories, open web text
• Do we achieve better Pareto curves of reconstruction loss vs L0 vs standard attention-out SAEs? 

Conditional on that succeeding, the next step would be to attempt to interpret individual heads in an ATC and determine whether they are interpretable. 

• It may be useful to compare to known examples of suspected AHS; however, direct comparison is difficult due to Remark 7 above. 

[Draft][Note] On Singular Learning Theory

Relevant links

• AXRP with Daniel Murfet on an SLT primer 
• Manifund grant proposal on DevInterp research agenda
• Daniel Murfet's post on "simple != short"
• Timaeus blogpost on actionable research projects
• DevInterp repository for estimating LLC

[Proposal] Do SAEs capture simplicial structure? Investigating SAE representations of known case studies

It's an open question whether SAEs capture underlying properties of feature geometry. Fortunately, careful research has elucidated a few examples of nonlinear geometry already. It would be useful to think about whether SAEs recover these geometries. 

Simplices in models. Work studying hierarchical structure in feature geometry finds that sets of things are often represented as simplices, which are a specific kind of regular polytope. Simplices are also the structure of belief state geometry. 

The proposal here is: look at the SAE activations for the tetrahedron, identify a relevant cluster, and then evaluate whether this matches the ground-truth.  

[Proposal] Is reasoning in natural language grokkable? Training models on language formulations of toy tasks. 

Previous work on grokking finds that models can grok modular addition and tree search. However, these are not tasks formulated in natural language. Instead, the tokens correspond directly to true underlying abstract entities, such as numerical values or nodes in a graph. I question whether this representational simplicity is a key ingredient of grokking reasoning. 

I have a prior that expressing concepts in natural language (as opposed to directly representing concepts as tokens) introduces an additional layer of complexity which makes grokking much more difficult. 

The proposal here is to repeat the experiments with tasks that test equivalent reasoning skills, but which are formulated in natural language. 

• Modular addition can be formulated as "day of the week" math, as has been done previously
• Tree search is more difficult to formulate, but might be phrasable as some kind of navigation instruction. 

I'd expect that we could observe grokking, but that it might take a lot longer (and require larger models) when compared to the "direct concept tokenization". Conditioned on this being true, it would be interesting to observe whether we recover the same kinds of circuits as demonstrated in prior work. 

[Proposal] Are circuits universal? Investigating IOI across many GPT-2 small checkpoints

Universal features. Work such as the Platonic Representation Hypothesis suggest that sufficiently capable models converge to the same representations of the data. To me, this indicates that the underlying "entities" which make up reality are universally agreed upon by models.

Non-universal circuits. There are many different algorithms which could correctly solve the same problem. Prior work such as the clock and the pizza indicate that, even for very simple algorithms, models can learn very different algorithms depending on the "attention rate". 

Circuit universality is a crux. If circuits are mostly model-specific rather than being universal, it makes the near-term impact of MI a lot lower, since finding a circuit in one model tells us very little about what a slightly different model is doing. 

Concrete experiment: Evaluating the universality of IOI. Gurnee et al train several GPT-2 small checkpoints from scratch. We know from prior work that GPT-2 small has an IOI circuit. What, if any, components of this turn to be universal? Maybe we always observe induction heads. But do we always observe name-mover and S-inhibition heads? If so, are they always at the same layer? Etc. I think this experiment would inform us a lot about circuit universality.