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  • Polysemanticity or Polysemy? Lexical Identity Confounds Superposition Metrics

arXiv:2604.00443v1 Announce Type: cross
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 2×2 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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