Only 1 to 3 percent of an LLM's embedding dimensions carried all the linear signal for predicting human neural activity
AspireAI community · arXiv
Language models are used as computational stand-ins for human language processing, but which parts of their representations actually track the brain is unclear. Using ridge reduced-rank regression, this work isolates the brain-predictive subspace of LLM embeddings. Across three naturalistic language-listening datasets spanning intracranial electrophysiology and fMRI, and six model conditions covering autoregressive, bidirectional masked and randomly initialised language models, full linear encoding performance was recovered using only 1 to 3 percent of the original embedding dimensionality. Those subspaces generalised across held-out participants and, less well, across datasets. Principal component analysis needed substantially more dimensions to recover the same neural prediction, showing that the directions that predict brain activity are not the directions that carry the most variance in the model.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.