Inharmonicity enhances brain signals of attentional capture and auditory stream segregation

Harmonicity is an important feature for auditory perception as it influences pitch processing, memory and hearing in noisy environments. However, the neural substrates of processing harmonic and inharmonic sounds remain unclear. Here, we systematically manipulated the harmonicity of synthetic sounds by introducing random jittering to the frequencies above the fundamental. Using electroencephalography, we studied the spectral uncertainty induced by inharmonic sounds and the effect on markers of auditory prediction errors—mismatch negativity (MMN) and P3a— in a roving oddball paradigm. Inharmonic sounds with a constant jittering pattern generated similar MMN and stronger P3a responses than harmonic sounds. In contrast, MMN responses became undetectable when the jittering pattern changed between consecutive sounds, suggesting that prediction errors are weighted by sequential but not spectral uncertainty. Interestingly, inharmonic sounds generated an object-related negativity, a response associated with the segregation of auditory objects. Our results suggest that inharmonicity induces the segregation of the auditory scene into different streams, captures attention, and gives rise to specific neural processes that are independent from the predictive mechanisms underlying sequential deviance detection.