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arXiv · 2604.00443

Polysemanticity or Polysemy? Lexical Identity Confounds Superposition Metrics

Iyad Ait Hou·Rebecca Hwa
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Abstract

If the same neuron activates for both "lender" and "riverside," standard metrics attribute the overlap to superposition--the neuron must be compressing two unrelated concepts. This work explores how much of the overlap is due a lexical confound: neurons fire for a shared word form (such as "bank") rather than for two compressed concepts. A 2x2 factorial decomposition reveals that the lexical-only condition (same word, different meaning) consistently exceeds the semantic-only condition (different word, same meaning) across models spanning 110M-70B parameters. The confound carries into sparse autoencoders (18-36% of features blend senses), sits in <=1% of activation dimensions, and hurts downstream tasks: filtering it out improves word sense disambiguation and makes knowledge edits more selective (p = 0.002).

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Iyad Ait Hou, Rebecca Hwa. 2026-04-01. Polysemanticity or Polysemy? Lexical Identity Confounds Superposition Metrics. https://arxiv.org/abs/2604.00443

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