Task-irrelevant background broadband noise reduces redundancy in auditory-motor adaptation in speech production

Previous speech-production experiments with vowel formant perturbations show that lowering the low-pass filter cutoff frequency increases solution redundancy, defined here as the non-uniqueness of corrective motor-command solutions derived from auditory errors, thereby amplifying deviations in auditory-motor adaptation. Here we test whether task-irrelevant broadband background noise, common in everyday acoustic environments, influences adaptation across levels of solution redundancy. The lower cutoff condition (Fc = 3 kHz) was treated as high in solution redundancy because it provides fewer high-frequency constraints on possible motor-command solutions than the higher cutoff condition (Fc = 4 kHz). We observed an interaction between cutoff frequency and noise after learning had sufficiently progressed: under high solution redundancy, noise significantly reduced adaptation deviation, whereas no reliable effect was found under low solution redundancy. The same interaction appeared in the early after-effect phase, suggesting improved retention as solution redundancy decreases with noise. Unlike prior visuomotor and auditory-motor studies where task-dependent noise increases uncertainty and can reduce learning, task-irrelevant noise used here modulated adaptation in a redundancy-dependent manner. Overall, background broadband noise lowers effective solution redundancy under high-solution-redundancy conditions, potentially facilitating speech motor learning by optimizing motor command updates.