Phonological contrast directions in frozen HuBERT-base embeddings collapse in separability with dysarthria severity, training-free and across languages

For each of 12 phonological contrasts (nasality, voicing, stridency, sonorance, manner, 4 vowel features), a difference-of-means direction is computed in 768-dimensional layer-12 HuBERT-base activations and separability along that direction is measured via d-prime, with no training on pathological data [muller-etal-2026-phonological-subspace-dysarthria] Healthy speakers show tight, well-separated phonological classes along the fitted direction (d-prime = 4.2), while severe dysarthria shows the class centroids remain distinguishable but within-class distributions widen and overlap substantially (d-prime = 1.1) [muller-etal-2026-phonological-subspace-dysarthria] Measured on 890 real speakers across 10 corpora and 5 languages using a single frozen facebook/hubert-base-ls960 checkpoint, with no per-language or per-corpus fine-tuning [muller-etal-2026-phonological-subspace-dysarthria]