Auditory attention improves scale-invariant neural fidelity to speech across three EEG datasets

Neural speech tracking is stronger for attended speech, yet its common correlation-based readout is scale invariant, so interpreting this effect only as response gain is incomplete. We tested whether attention improves representational fidelity, defined here as scale-invariant agreement between a speech envelope and its neural reconstruction. A leakage-resistant analysis evaluated held-out trials or story parts in three public electroencephalography datasets (52 participants). Fidelity was Fisher-transformed reconstruction-envelope correlation; projection slope quantified scale-dependent gain. In the spontaneous Auditory Attention Switching Dataset, nine odd- numbered participants were used for discovery and nine even-numbered participants for split- sample validation. Fidelity was higher for attended speech in the validation sample and exceeded 5,000 within-trial circular label shifts. The effect replicated under story-part-disjoint validation in KUL and trial-disjoint validation in DTU. A KUL crossover compared the same clean speech sources in attended and ignored states. Across 4,819 isolated spontaneous switches, fidelity did not differ from baseline before the report but shifted toward the newly reported target 0.25-1 s afterward. Passive keypresses altered nonspecific decoder energy. Gain advantages also occurred in all datasets. Selective attention was therefore evident in the scale-invariant preservation of target dynamics, while gain remained a complementary feature.