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Thomas Fel

@thomasfel.bsky.social
1.4K followers 363 following 36 posts

Explainability, Computer Vision, Neuro-AI.🪴 Kempner Fellow @Harvard. Prev. PhD @Brown, @Google, @GoPro. Crêpe lover. 📍 Boston | 🔗 thomasfel.me

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Reposted by Thomas Fel
Kempner Institute at Harvard University @kempnerinstitute.bsky.social · 04/12/2025
Are you at #NeurIPS2025? Check out the #KempnerInstitute’s Day 2 presentations! 💡 #AI #NeuroAI @cpehlevan.bsky.social @kanakarajanphd.bsky.social @thomasfel.bsky.social @andykeller.bsky.social @binxuwang.bsky.social @njw.fish @yilundu.bsky.social
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Kempner Institute at Harvard University @kempnerinstitute.bsky.social · 12/11/2025
🐇Into the Rabbit Hull — Part 1: A Deep Dive into DINOv2🧠 Our latest Deeper Learning blog post is an #interpretability deep dive into one of today’s leading vision foundation models: DINOv2. 📖Read now: bit.ly/4nNfq8D Stay tuned — Part 2 coming soon. #AI #VLMs #DINOv2
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Into the Rabbit Hull – Part I - Kempner Institute
This blog post offers an interpretability deep dive, examining the most important concepts emerging in one of today’s central vision foundation models, DINOv2. This blogpost is the first of a […]
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Thomas Fel @thomasfel.bsky.social · 06/11/2025
The Bau lab is on fire ! 😍
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Jennifer Hu @jennhu.bsky.social · 04/11/2025
Interested in doing a PhD at the intersection of human and machine cognition? ✨ I'm recruiting students for Fall 2026! ✨ Topics of interest include pragmatics, metacognition, reasoning, & interpretability (in humans and AI). Check out JHU's mentoring program (due 11/15) for help with your SoP 👇
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Ida Momennejad @neuroai.bsky.social · 27/10/2025
Pleased to share new work with @sflippl.bsky.social @eberleoliver.bsky.social @thomasmcgee.bsky.social & undergrad interns at Institute for Pure and Applied Mathematics, UCLA. Algorithmic Primitives and Compositional Geometry of Reasoning in Language Models www.arxiv.org/pdf/2510.15987 🧵1/n
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Thomas Serre @thomasserre.bsky.social · 24/10/2025
🧠 Thrilled to share our NeuroView with Ellie Pavlick! "From Prediction to Understanding: Will AI Foundation Models Transform Brain Science?" AI foundation models are coming to neuroscience—if scaling laws hold, predictive power will be unprecedented. But is that enough? Thread 🧵👇
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Thomas Fel @thomasfel.bsky.social · 16/10/2025
Thx a lot Naomi ! 🙌🥹
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Naomi Saphra @nsaphra.bsky.social · 16/10/2025
This is so cool. When you look at representational geometry, it seems intuitive that models are combining convex regions of "concepts", but I wouldn't have expected that this is PROVABLY true for attention or that there was such a rich theory for this kind of geometry.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
That concludes this two-part descent into the Rabbit Hull. Huge thanks to all collaborators who made this work possible — and especially to @binxuwang.bsky.social , with whom this project was built, experiment after experiment. 🎮 kempnerinstitute.github.io/dinovision/ 📄 arxiv.org/pdf/2510.08638
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
If this holds, three implications: (i) Concepts = points (or regions), not directions (ii) Probing is bounded: toward archetypes, not vectors (iii) Can't recover generating hulls from sum: we should look deeper than just a single-layer activations to recover the true latents
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
Synthesizing these observations, we propose a refined view, motivated by Gärdenfors' theory and attention geometry. Activations = multiple convex hulls simultaneously: a rabbit among animals, brown among colors, fluffy among textures. The Minkowski Representation Hypothesis.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
Taken together, the signs of partial density, local connectedness, and coherent dictionary atoms indicate that DINO’s representations are organized beyond linear sparsity alone.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
Can position explain this ? We found that pos. information collapses: from high-rank to a near 2-dim sheet. Early layers encode precise location; later ones retain abstract axes. This compression frees dimensions for features, and *position doesn't explain PCA map smoothness*
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
Patch embeddings form smooth, connected surfaces tracing objects and boundaries. This may suggests interpolative geometry: tokens as mixtures between landmarks, shaped by clustering and spreading forces in the training objectives.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
We found antipodal feature pairs (dᵢ ≈ − dⱼ): vertical vs horizontal lines, white vs black shirts, left vs right… Also, co-activation statistics only moderately shape geometry: concepts that fire together aren't necessarily nearby—nor orthogonal when they don't.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
Under the Linear Rep. Hypothesis, we'd expect Dictionary to be quasi-orthogonality. Instead, training drives atoms from near-Grassmannian initialization to higher coherence. Several concepts fire almost always the embedding is partly dense (!), contradicting pure sparse coding.
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Thomas Fel @thomasfel.bsky.social · 15/10/2025
🕳️🐇Into the Rabbit Hull – Part II Continuing our interpretation of DINOv2, the second part of our study concerns the *geometry of concepts* and the synthesis of our findings toward a new representational *phenomenology*: the Minkowski Representation Hypothesis
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Huge thanks to all collaborators who made this work possible, and especially to @binxuwang.bsky.social. This work grew from a year of collaboration! Tomorrow, Part II: geometry of concepts and Minkowski Representation Hypothesis. 🕹️ kempnerinstitute.github.io/dinovision 📄 arxiv.org/pdf/2510.08638
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Curious tokens, the registers. DINO seems to use them to encode global invariants: we find concepts (directions) that fire exclusively (!) on registers. Example of such concepts include motion blur detector and style (game screenshots, drawings, paintings, warped images...)
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Now for depth estimation. How does DINO know depth? It turns out it has discovered several human-like monocular depth cues: texture gradients resembling blurring or bokeh, shadow detectors, and projective cues. Most units mix cues, but a few remain remarkably pure.
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Another surprise here: the most important concepts are not object-centric at all, but boundary detectors. Remarkably, these concepts coalesce into a low-dimensional subspace within (see paper).
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
This kind of concept breaks a key assumption in interpretability: that a concept is about the tokens where it fires. Here it is the opposite—the concept is defined by where it does not fire. An open question is how models form such concepts.
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Let's zoom in on classification. For every class, we find two concepts: one fires on the object (e.g., "rabbit"), and another fires everywhere *except* the object -- but only when it's present! We call them Elsewhere Concepts (credit: @davidbau.bsky.social).
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
Assuming the Linear Rep. Hypothesis, SAEs arise naturally as instruments for concept extraction, they will be our companions in this descent. Archetypal SAE uncovered 32k concepts. Our first observation: different tasks recruit distinct regions of this conceptual space.
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Thomas Fel @thomasfel.bsky.social · 14/10/2025
🕳️🐇 𝙄𝙣𝙩𝙤 𝙩𝙝𝙚 𝙍𝙖𝙗𝙗𝙞𝙩 𝙃𝙪𝙡𝙡 – 𝙋𝙖𝙧𝙩 𝙄 (𝑃𝑎𝑟𝑡 𝐼𝐼 𝑡𝑜𝑚𝑜𝑟𝑟𝑜𝑤) 𝗔𝗻 𝗶𝗻𝘁𝗲𝗿𝗽𝗿𝗲𝘁𝗮𝗯𝗶𝗹𝗶𝘁𝘆 𝗱𝗲𝗲𝗽 𝗱𝗶𝘃𝗲 𝗶𝗻𝘁𝗼 𝗗𝗜𝗡𝗢𝘃𝟮, one of vision’s most important foundation models. And today is Part I, buckle up, we're exploring some of its most charming features. :)
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Thomas Fel @thomasfel.bsky.social · 12/10/2025
Really neat, congrats !
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Meenakshi Khosla @meenakshikhosla.bsky.social · 08/10/2025
Superposition has reshaped interpretability research. In our @unireps.bsky.social paper led by @andre-longon.bsky.social we show it also matters for measuring alignment! Two systems can represent the same features yet appear misaligned if those features are mixed differently across neurons.
arxiv.org
Superposition disentanglement of neural representations reveals hidden alignment
The superposition hypothesis states that a single neuron within a population may participate in the representation of multiple features in order for the population to represent more features than the ...
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Jessica Hullman @jessicahullman.bsky.social · 09/10/2025
For XAI it’s often thought explanations help (boundedly rational) user “unlock” info in features for some decision. But no one says this, they say vaguer things like “supporting trust”. We lay out some implicit assumptions that become clearer when you take a formal view here arxiv.org/abs/2506.22740
arxiv.org
Explanations are a means to an end
Modern methods for explainable machine learning are designed to describe how models map inputs to outputs--without deep consideration of how these explanations will be used in practice. This paper arg...
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Thomas Fel @thomasfel.bsky.social · 10/10/2025
Beautiful work !
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David Picard @davidpicard.eurosky.social · 08/10/2025
🚨Updated: "How far can we go with ImageNet for Text-to-Image generation?" TL;DR: train a text2image model from scratch on ImageNet only and beat SDXL. Paper, code, data available! Reproducible science FTW! 🧵👇 📜 arxiv.org/abs/2502.21318 💻 github.com/lucasdegeorg... 💽 huggingface.co/arijitghosh/...
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Greta Tuckute @gretatuckute.bsky.social · 04/10/2025
Check out @mryskina.bsky.social's talk and poster at COLM on Tuesday—we present a method to identify 'semantically consistent' brain regions (responding to concepts across modalities) and show that more semantically consistent brain regions are better predicted by LLMs.
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Deniz Bayazit @bayazitdeniz.bsky.social · 25/09/2025
1/🚨 New preprint How do #LLMs’ inner features change as they train? Using #crosscoders + a new causal metric, we map when features appear, strengthen, or fade across checkpoints—opening a new lens on training dynamics beyond loss curves & benchmarks. #interpretability
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Leshem (Legend) Choshen @EMNLP @lchoshen.bsky.social · 26/09/2025
Employing mechanistic interpretability to study how models learn, not just where they end up 2 papers find: There are phase transitions where features emerge and stay throughout learning 🤖📈🧠 alphaxiv.org/pdf/2509.17196 @amuuueller.bsky.social @abosselut.bsky.social alphaxiv.org/abs/2509.05291
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Tiago Pimentel @tpimentel.bsky.social · 14/07/2025
Mechanistic interpretability often relies on *interventions* to study how DNNs work. Are these interventions enough to guarantee the features we find are not spurious? No!⚠️ In our new paper, we show many mech int methods implicitly rely on the linear representation hypothesis🧵
Paper title "The Non-Linear Representation Dilemma: Is Causal Abstraction Enough for Mechanistic Interpretability?" with the paper's graphical abstract showing how more powerful alignment maps between a DNN and an algorithm allow more complex features to be found and more "accurate" abstractions.
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Sam Gershman @gershbrain.bsky.social · 28/09/2025
I was part of an interesting panel discussion yesterday at an ARC event. Maybe everybody knows this already, but I was quite surprised by how "general" intelligence was conceptualized in relation to human intelligence and the ARC benchmarks.
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Thomas Fel @thomasfel.bsky.social · 28/09/2025
Phenomenology → principle → method. From observed phenomena in representations (conditional orthogonality) we derive a natural instantiation. And it turns out to be an old friend: Matching Pursuit! 📄 arxiv.org/abs/2506.03093 See you in San Diego, @neuripsconf.bsky.social 🎉 #interpretability
arxiv.org
From Flat to Hierarchical: Extracting Sparse Representations with Matching Pursuit
Motivated by the hypothesis that neural network representations encode abstract, interpretable features as linearly accessible, approximately orthogonal directions, sparse autoencoders (SAEs) have bec...
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Malcolm Campbell @malcolmgcampbell.bsky.social · 19/09/2025
🚨Our preprint is online!🚨 www.biorxiv.org/content/10.1... How do #dopamine neurons perform the key calculations in reinforcement #learning? Read on to find out more! 🧵
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Isabel Papadimitriou @isabelpapad.bsky.social · 17/09/2025
Are there conceptual directions in VLMs that transcend modality? Check out our COLM oral spotlight 🔦 paper! We use SAEs to analyze the multimodality of linear concepts in VLMs with @chloesu07.bsky.social, @thomasfel.bsky.social, @shamkakade.bsky.social and Stephanie Gil arxiv.org/abs/2504.11695
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Thomas Fel @thomasfel.bsky.social · 17/09/2025
One interesting result: our "Bridge Score" points to concept pairs that connect vision & language. In the demo you can explore these bridges (links) and see how multimodality shows up ! :) with @isabelpapad.bsky.social, @chloesu07.bsky.social, @shamkakade.bsky.social and Stephanie Gil
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Thomas Fel @thomasfel.bsky.social · 17/09/2025
Check out our COLM 2025 (oral) 🎤 SAEs reveal that VLM embedding spaces aren’t just "image vs. text" cones. They contain stable conceptual directions, some forming surprising bridges across modalities. arxiv.org/abs/2504.11695 Demo 👉 vlm-concept-visualization.com
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Jennifer Hu @jennhu.bsky.social · 16/07/2025
Excited to announce the first workshop on CogInterp: Interpreting Cognition in Deep Learning Models @ NeurIPS 2025! 📣 How can we interpret the algorithms and representations underlying complex behavior in deep learning models? 🌐 coginterp.github.io/neurips2025/ 1/4
coginterp.github.io
Home
First Workshop on Interpreting Cognition in Deep Learning Models (NeurIPS 2025)
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Andrew Lampinen @lampinen.bsky.social · 02/05/2025
How do language models generalize from information they learn in-context vs. via finetuning? In arxiv.org/abs/2505.00661 we show that in-context learning can generalize more flexibly, illustrating key differences in the inductive biases of these modes of learning — and ways to improve finetuning. 1/
arxiv.org
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Harry Thasarathan @hthasarathan.bsky.social · 01/05/2025
Our work finding universal concepts in vision models is accepted at #ICML2025!!! My first major conference paper with my wonderful collaborators and friends @matthewkowal.bsky.social @thomasfel.bsky.social @Julian_Forsyth @csprofkgd.bsky.social Working with y'all is the best 🥹 Preprint ⬇️!!
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Kosta Derpanis @csprofkgd.bsky.social · 01/05/2025
Accepted at #ICML2025! Check out the preprint. HUGE shoutout to Harry (1st PhD paper, in 1st year), Julian (1st ever, done as an undergrad), Thomas and Matt! @hthasarathan.bsky.social @thomasfel.bsky.social @matthewkowal.bsky.social
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Gilles Louppe @glouppe.bsky.social · 29/04/2025
<proud advisor> Hot off the arXiv! 🦬 "Appa: Bending Weather Dynamics with Latent Diffusion Models for Global Data Assimilation" 🌍 Appa is our novel 1.5B-parameter probabilistic weather model that unifies reanalysis, filtering, and forecasting in a single framework. A thread 🧵
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Arno Solin @arnosolin.bsky.social · 29/04/2025
Have you thought that in computer memory model weights are given in terms of discrete values in any case. Thus, why not do probabilistic inference on the discrete (quantized) parameters. @trappmartin.bsky.social is presenting our work at #AABI2025 today. [1/3]
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Kempner Institute at Harvard University @kempnerinstitute.bsky.social · 28/04/2025
New in the Deeper Learning blog: Kempner researchers show how VLMs speak the same semantic language across images and text. bit.ly/KempnerVLM by @isabelpapad.bsky.social ,Chloe Huangyuan Su, @thomasfel.bsky.social, Stephanie Gil, and @shamkakade.bsky.social #AI #ML #VLMs #SAEs
bit.ly
Interpreting the Linear Structure of Vision-Language Model Embedding Spaces - Kempner Institute
Using sparse autoencoders, the authors show that vision-language embeddings boil down to a small, stable dictionary of single-modality concepts that snap together into cross-modal bridges. This resear...
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Martin Vinck @martinavinck.bsky.social · 10/04/2025
Firing rates in visual cortex show representational drift, while temporal spike sequences remain stable www.sciencedirect.com/science/arti... Great work by Boris Sotomayor and with @battaglialab.bsky.social
sciencedirect.com
Firing rates in visual cortex show representational drift, while temporal spike sequences remain stable
Neural firing-rate responses to sensory stimuli show progressive changes both within and across sessions, raising the question of how the brain mainta…
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Greta Tuckute @gretatuckute.bsky.social · 10/04/2025
PINEAPPLE, LIGHT, HAPPY, AVALANCHE, BURDEN Some of these words are consistently remembered better than others. Why is that? In our paper, just published in J. Exp. Psychol., we provide a simple Bayesian account and show that it explains >80% of variance in word memorability: tinyurl.com/yf3md5aj
tinyurl.com
APA PsycNet
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Satpreet (Sat) Singh @satpreetsingh.bsky.social · 07/04/2025
📽️Recordings from our @cosynemeeting.bsky.social #COSYNE2025 workshop on “Agent-Based Models in Neuroscience: Complex Planning, Embodiment, and Beyond" are now online: neuro-agent-models.github.io 🧠🤖
neuro-agent-models.github.io
🤖 Agent-Based Models in Neuroscience
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