Non-linear relationships between auditory mismatch responses and the inharmonicity of complex sounds

Harmonicity is a feature of sound that is important for many aspects of auditory perception and previous research has shown that harmonicity modulates the brain’s mismatch responses to oddball sounds. Predictive processing accounts of perception suggest that the brain generates predictions about the incoming stimuli. Since inharmonic sound spectra contain more information, inharmonicity has been suggested to be involved in precision weighting, an important component of predictive processing theories. In this study we explored this issue by parametrically modulating the level of inharmonicity applied to synthetic sounds and recording mismatch responses (MMN and P3a) from healthy volunteers (N = 37) using electroencephalography. Our results show that a sigmoid function models the relationship between inharmonicity and MMN amplitude better than any linear or polynomial function. Furthermore, P3a amplitude has an inverted-U relationship with inharmonicity and peaks at inharmonicity levels just below the threshold for pitch discrimination. These results are consistent with the hypothesis that inharmonicity impairs F0 extraction above a certain threshold.